application of high entropy alloys in stent implants/67531/metadc...rajiv mishra, committee member ....
TRANSCRIPT
APPROVED:
Aleksandra Fortier, Major Professor Rajiv Mishra, Committee Member Kyle Horne, Committee Member Yong X. Tao, Chair of the Department of
Mechanical and Energy Engineering Costas Tsatsoulis, Dean of the College of
Engineering Victor Prybutok, Vice Provost of the
Toulouse Graduate School
APPLICATION OF HIGH ENTROPY ALLOYS IN STENT IMPLANTS
Karthik Alagarsamy
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
May 2017
Alagarsamy, Karthik. Application of High Entropy Alloys in Stent Implants. Master of
Science (Mechanical and Energy Engineering), May 2017, 95 pp., 10 tables, 68 figures, 78
numbered references.
High entropy alloys (HEAs) are alloys with five or more principal elements. The
outstanding properties of HEA includes higher strength/hardness, superior wear resistance, high
temperature stability, higher fatigue life, good corrosion and oxidation resistance. Even though
much research has been done to understand the characteristic of HEAs, there is little research on
how the HEAs can be applied for commercial uses. This work discusses the application of high
entropy alloys in biomedical applications.
The coronary heart disease, the leading cause of death in the United States kills more than
350,000 persons/year and it costs $108.9 billion for the nation each year in spite of significant
advancements in medical care and public awareness. A cardiovascular disease affects heart or
blood vessels (arteries, veins and capillaries) or both by blocking the blood flow. As a surgical
interventions, stent implants are deployed to cure or ameliorate the disease. However, the high
failure rate of stents has lead researchers to give special attention towards analyzing stent
structure, materials and characteristics. Many works related to alternate material and/or design
are carried out in recent time.
This paper discusses the feasibility of CoCrFeNiMn and Al0.1CoCrFeNi HEAs in stent
implant application. This work is based on the speculation that CoCrFeNiMn and Al0.1CoCrFeNi
HEAs are biocompatible material. These HEAs are characterized to determine the microstructure
and mechanical properties. Computational modeling and analysis were carried out on stent
implant by applying CoCrFeNiMn and Al0.1CoCrFeNi HEAs as material to understand the
structural behavior.
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Copyright 2017
by
Karthik Alagarsamy
iii
ACKNOWLEDGEMENTS
I express my gratitude to Dr. Aleksandra Fortier and Dr. Rajiv Mishra for providing me
all of the opportunities, support, advice, and technical guidance throughout my research. Thanks
to Dr. Rajiv Mishra for generously funding me throughout the year. I’d also need to thank Dr.
Nilesh Kumar and Dr. Mageshwari Komarasamy for their technical guidance. Thanks to Hua
Yang for her research which helped me a lot to develop my thesis. Thanks to Dr. Kong Fanrong
for helping me with ANSYS simulations. Thanks to all my fellow lab mates for helping and
encouraging me on my research.
Thanks to Dr. Sundeep Mukherjee and Dr. Santanu Das for helping us with Nano
indentation technique.
Thanks to the University for the Facilities that is encouraging research activities.
Thanks to my friends and colleagues for helping me when I am in trouble.
I really appreciate and thank my parents and family members for their trust and care for
me.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ........................................................................................................... iii
LIST OF TABLES ......................................................................................................................... vi
LIST OF FIGURES ...................................................................................................................... vii
CHAPTER 1 INTRODUCTION .....................................................................................................1
1.1 Background of High Entropy Alloys .......................................................................1
1.2 Core Effects of HEAs ..............................................................................................2
1.2.1 High Entropy Effect .....................................................................................2
1.2.2 Severe Lattice Distortion Effect ...................................................................2
1.2.3 Sluggish Diffusion Effect ............................................................................4
1.2.4 Mechanical Properties ..................................................................................5
1.3 Cardiovascular Diseases and Stent Implants ...........................................................7
1.4 Types of Stents .......................................................................................................11
1.4.1 Self-Expanding Stents ................................................................................11
1.4.2 Balloon Expansion Stents ..........................................................................11
1.4.3 Bare-Metal Stent (BMS) ............................................................................13
1.4.4 Drug-Eluting Stent (DES) ..........................................................................14
1.4.5 Biodegradable Stent (BDS)........................................................................16
1.5 Basic Stent Characteristics .....................................................................................19
1.6 Arterial Structure and Classification of Arteries ...................................................20
1.7 Atherosclerosis, Peripheral Arteries (PA) and Stents-Vessel Interaction ..............23
1.8 Conventional Stent Materials and Their Properties ...............................................26
1.9 Need for Alternative Material ................................................................................29
CHAPTER 2 EXPERIMENTAL APPROACH TO DETERMINE PROPERTIES .....................31
2.1 Outline of Experimental Approach ........................................................................31
2.1.1 Tensile Testing ...........................................................................................31
2.1.2 Hardness Testing ........................................................................................33
2.1.3 Nano-Indentation Testing ..........................................................................34
2.1.4 Fatigue Analysis.........................................................................................35
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2.1.5 Optical Microscopy ....................................................................................37 CHAPTER 3 DETERMINATION OF MECHANICAL PROPERTIES OF HIGH ENTROPY ALLOYS ........................................................................................................................................38
3.1 Determination of Mechanical Properties of CoCrFeNiMn High Entropy Alloy ...38
3.1.1 Introduction ................................................................................................38
3.1.2 Experimental Details and Thermo-Mechanical Processing .......................39
3.1.3 Results and Discussion ..............................................................................39
3.2 Determination of Mechanical Properties of Al0.1CoCrFeNi High Entropy Alloy .49
3.2.1 Introduction ................................................................................................49
3.2.2 Experimental Details and Thermo-Mechanical Processing .......................50
3.2.3 Results and Discussion ..............................................................................50
3.3 Comparison of mechanical properties of HEA with conventional stent materials 57
3.3.1 Comparison of CoCrFeNiMn with SS 316L .............................................59
3.3.2 Comparison of Al0.1CoCrFeNi with SS 316L ............................................59 CHAPTER 4 COMPUTATIONAL MODELLING OF STENT IMPLANT PROCEDURE AND COMPARISON OF MATERIALS ...............................................................................................60
4.1 Introduction ............................................................................................................60
4.2 Approach ................................................................................................................62
4.2.1 Modelling ...................................................................................................62
4.2.2 Computational Simulation .........................................................................63
4.3 Results ....................................................................................................................69
4.4 Discussion ..............................................................................................................81 CHAPTER 5 CONCLUSION AND FUTURE WORK ................................................................83
5.1 Conclusion .............................................................................................................83
5.2 Future works ..........................................................................................................85
5.2.1 FEM Simulation of Stent with Artery Load ..............................................85
5.2.2 Fatigue Analysis of CoCrFeNiMn .............................................................86
5.2.3 Analysis of Corrosion Properties and Biocompatibility of HEA...............86 REFERENCES ..............................................................................................................................87
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LIST OF TABLES
Table 1. Summary of Current Stents Used Worldwide ........................................................... 18
Table 2. Tensile properties of as cast HEA, annealed at 700 C and 900 C for one hour 41
Table 3. Diffusion lengths of elements at different time and temperature ........................... 45
Table 4. Mechanical properties of the as-cast and cold worked+ annealed Al0.1CoCrFeNi
HEA ........................................................................................................................................................... 52
Table 5. Number of cycles (N) survived by the sample when subjected to different stress
amplitudes (S) ........................................................................................................................................... 56
Table 6. Mechanical properties of conventional stent material and CoCrFeNiMn and
Al0.1CoCrFeNi HEAs [60, 61, 72] ......................................................................................................... 58
Table 7. Dimensions of the stent model. .................................................................................. 62
Table 8. Mechanical properties of SS 316L and Al0.1CoCrFeNi and CoCrFeNiMn HEAs
added to ANSYS Workbench library .................................................................................................... 64
Table 9. Displacement boundary in cylindrical coordinate system condition for the balloon
.................................................................................................................................................................... 67
Table 10. Comparison of results generated from computational analysis. .......................... 82
LIST OF FIGURES
Figure 1 : BCC lattice of (a) pure metal with (b) high entropy alloy with severe lattice
distortion [2].................................................................................................................................... 3
Figure 2: Variation in potential energy between two sites in pure metal, Fe-Cr-Ni and
CoCrFeNiMn0.5 HEA [8]. ............................................................................................................... 4
Figure 3: Plastic deformation based tetrahedron showing the superior propertied of HEAs
among conventional alloys (a) Yield strength limited design (b) Toughness limited design (c)
Creep limited design and (d) Fatigue limited design. [14] ............................................................. 6
Figure 4: Schematic representation of artery with atherosclerosis-plaque inside the arterial
wall [18] .......................................................................................................................................... 8
Figure 5: Surgical interventions for treating atherosclerosis diseases in arteries (a)
Atherectomy (b) Balloon angioplasty (c) Stent angioplasty [21] ................................................. 10
Figure 6: Self-expanding stent that expands when the constrain is removed [22] ........... 11
Figure 7: Deployment of balloon expansion stent with the inflation and deflation of balloon
[23] ................................................................................................................................................ 12
Figure 8: Left: BMS stent, Right: BMS stent with inflated balloon [24, 27] ................... 14
Figure 9: Drug eluting stent [27] ...................................................................................... 14
Figure 10: a) The NEVO cobalt chromium stent, which has an open-cell design and unique
reservoirs that contain a biodegradable polymer and sirolimus mix that b) completely biodegrades
within 90 days [26]. ...................................................................................................................... 15
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Figure 11: a) The Igaki-Tamai BDS Stent with gold markers b) metabolism cycle of
PLLA[26]. ..................................................................................................................................... 17
Figure 12: Stent Fabrication Techniques .......................................................................... 19
Figure 13: Stent Geometry Terminology a) rings (hoops) b) connectors c) strut dimensions
[21]. ............................................................................................................................................... 19
Figure 14: Various stent designs [21] ............................................................................... 20
Figure 15: Schematic representation of distinct layers of arterial wall left: top view of layers
in arterial wall [40] right: cross-sectional view of layers in arterial wall [42] ............................. 20
Figure 16: Anatomy of the femoropopliteal (FP) artery. CFA, common femoral artery;
PFA, profunda femoral artery; SFA, superficial femoral artery; DGA, descending genicular artery;
ATA, anterior tibial artery [32] ..................................................................................................... 22
Figure 17: Blood flows through a normal artery (A). In peripheral artery disease,
atherosclerotic plaque narrows the artery and impedes blood flow (B). During angioplasty to
restore blood flow, a stent maybe inserted to keep the artery open (C). Stent implant restores
normal blood flow [43] ................................................................................................................. 23
Figure 18: Left: As stents are placed into the artery, the artery ability to bend and compress
is reduced. The adjacent unstented artery bends more, possibly resulting in kinking at the margin
of the stent [45]; right: Stenting leads to different wall stresses and hemodynamics within the
implantation region [33] ............................................................................................................... 24
Figure 19: Custom- made mini-tensile test assembly ...................................................... 32
Figure 20: : Picture of a mini- tensile sample ................................................................... 32
Figure 21: Vickers’s hardness tester ................................................................................ 33
Figure 22: The load- displacement plot generated during the nano-indentation testing ... 34
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Figure 23: Custom made fatigue analysis setup ............................................................... 36
Figure 24: : Sample for fatigue analysis. (a) Picture of fatigue sample before and after
testing, (b) Dimensions of the fatigue sample .............................................................................. 34
Figure 25: Optical microscopy images of CoCrFeNiMn alloy in (a) as received, (b)
annealed 700 C for one hour and (c) annealed 900 C for one hour ........................................... 40
Figure 26: Engineering stress-strain curve of CoCrFeNiMn alloy in three conditions .... 41
Figure 27: Optical microscopy image of CoCrFeNiMn HEA annealed at 1000 C for 48
hours (a) Inhomogeneous microstructure with traces of dendrites and homogenized region (b)
Inhomogeneous microstructure with precipitates (c) Precipitates formed during annealing. ...... 43
Figure 28: Engineering stress- strain plot of 1000 C- 48 hour annealed CoCrFeNiMn HEA
....................................................................................................................................................... 44
Figure 29: Optical microscopy images at (a) 1000 C Annealed, (b) 1000 C Annealed +
50% rolled and (c) 1000 C Annealed + 50% rolled + 1000 C Annealed .................................. 46
Figure 30: Stress-plastic strain plot of annealed-rolled-annealed CoCrFeNiMn HEA .... 47
Figure 31: Load-displacement plot at 1000 mN load ....................................................... 48
Figure 32: Picture of the alloy showing large pores, cracks and cast defects .................. 49
Figure 33: Optical microscopy images of as-cast Al0.1CoCrFeNi high entropy alloy
showing coarse granular microstructure. ...................................................................................... 50
Figure 34: Optical microscopy image of cold rolled and annealed Al0.1CoCrFeNi HEA
showing twins ............................................................................................................................... 51
Figure 35: Engineering stress-strain curve of Al0.1CoCrFeNi alloy in as-cast and cold rolled
+ annealed condition. .................................................................................................................... 53
Figure 36: Load-displacement plot at 10000 µN load ...................................................... 54
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Figure 37: Number of cycles at (a) 160 MPa, (b) 200 MPa, (c) 210 MPa, (d) 220 MPa and
(e) 250 MPa................................................................................................................................... 55
Figure 38: S-N curve of Al0.1CoCrFeNi HEA .................................................................. 57
Figure 39: Inverted hexagonal honeycomb (a), V-type (b) and chiral(c) auxetic cellular
structure [69] ................................................................................................................................. 60
Figure 40: Spiral stent with un-welded junction of two wires [70] and helical stent[71] 61
Figure 41: Scanning electron micrographs (magnificationX18) show stents of 4-rhombus
(A) and 8 rhombus (B) designs after balloon expansion [74]. ...................................................... 61
Figure 42: Stent structure includes arrangement of strut members .................................. 61
Figure 43: CAD model of 8 rhombus unit stent .............................................................. 63
Figure 44: Reduced stent model used for axisymmetric structural analysis ..................... 65
Figure 45: The reduced stent and balloon model used for axisymmetric structural analysis
....................................................................................................................................................... 65
Figure 46: Frictionless contact surfaces ............................................................................ 66
Figure 47: Meshed stent and artery used for structural analysis. ...................................... 67
Figure 48: Graph showing the displacement boundary condition steps ........................... 68
Figure 49: Free faces and faces with frictionless supports used in axisymmetric analysis
....................................................................................................................................................... 68
Figure 50: SS 316L: Maximum equivalent von Mises stress when the balloon is fully
expanded (a) and residual stress retained by the stent after deflation of balloon (b) .................... 70
Figure 51: SS 316L: Stress variation of stent (green) and balloon (red) with respect to time
....................................................................................................................................................... 70
x
Figure 52: SS 316L: (a) Maximum total deformation at fully expanded condition and (b)
total deformation due to recoiling when the balloon deflates ....................................................... 71
Figure 53: SS 316L: Total deformation variation of stent (green) and balloon (red) with
respect to time. .............................................................................................................................. 72
Figure 54: SS 316L: (a) Maximum displacement along X at fully expanded balloon and (b)
displacement along X after recoiling ............................................................................................ 73
Figure 55: SS 316L: Displacement of stent along X direction with respect to time ........ 73
Figure 56: CoCrFeNiMn: (a) Maximum equivalent von Mises stress when the balloon is
fully expanded and (b) residual stress retained by the stent after deflation of balloon ................ 74
Figure 57: CoCrFeNiMn: Stress variation of stent (green) and balloon (red) with respect to
time ............................................................................................................................................... 75
Figure 58: CoCrFeNiMn: (a) Maximum total deformation at fully expanded condition and
(b) total deformation due to recoiling when the balloon deflates ................................................. 76
Figure 59: CoCrFeNiMn: Total deformation variation of stent (green) and balloon (red)
with respect to time. ...................................................................................................................... 76
Figure 60: CoCrFeNiMn: Maximum displacement along X at fully expanded balloon (a)
and displacement along X after recoiling ..................................................................................... 77
Figure 61: CoCrFeNiMn: Displacement of stent along X direction with respect to time 77
Figure 62: Al0.1CoCrFeNi: (a) Maximum equivalent von Mises stress when the balloon is
fully expanded and (b) residual stress retained by the stent after deflation of balloon ................ 78
Figure 63: Al0.1CoCrFeNi: Stress variation of stent (green) and balloon (red) with respect
to time ........................................................................................................................................... 79
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Figure 64: Al0.1CoCrFeNi: (a) Maximum total deformation at fully expanded condition and
(b) total deformation after the balloon deflates ............................................................................. 80
Figure 65: Al0.1CoCrFeNi: Total deformation variation of stent (green) and balloon (red)
with respect to time. ...................................................................................................................... 80
Figure 66: Al0.1CoCrFeNi: (a) Maximum displacement along X at fully expanded balloon
and (b) displacement along X after recoiling ................................................................................ 81
Figure 67: Al0.1CoCrFeNi: Displacement of stent along X direction with respect to time
....................................................................................................................................................... 81
Figure 68: Model of stent with balloon and artery ........................................................... 85
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1
CHAPTER 1
INTRODUCTION
1.1 Background of high entropy alloys
Conventional alloy are based on one principal element. One or more alloying elements of
small proportions are added to the principal element to achieve desired properties. Hence these
alloys forms a family based on the principal element. For example. Stainless steel is an alloy of
iron containing chromium (12–14%), molybdenum (0.2–1%), nickel (less than 2%), and carbon
(about 0.1–1%). These alloys are used in our day to day life for centuries. Scientists have
developed new form of materials like smart materials, shape memory alloys etc. to meet different
applications. In recent times, the new concept of multi principal element was demonstrated and
the concept of high entropy alloys was formed.
High entropy alloys (HEAs) are new class of multi-component alloys with five or more
principal elements each contributing 5 to 35 atomic percentage. The concept of HEA was first
introduced in 1996 but extensive research started after 2004 when Yeh and Cantor published
papers related to HEAs [1]. These elements are also referred as multi-principal elements,
equimolar alloys, equiatomic atomic ratio alloys, substitutional alloys and multi component alloys
by different researchers. The alloys are named as high entropy alloys as their solution have higher
entropy of mixing when compared to conventional alloys. As per Boltzmann hypothesis, the
entropy of mixing (ΔSmix) increases with the number of alloying elements when added in
equimolar. The configurational entropy (ΔSconfig) which is the major fraction of mixing entropy
(ΔSmix) is given as [2] ΔSconfig = -R ΣiXi ln (Xi) where Xi is the atomic fraction of element i and R is
2
the gas constant. With the increase in number of equiatomic alloying element the configurational
entropy increases. Any equiatomic alloy with ΔSconfig≥ 1.5R is considered as high entropy alloy and
5 or more elements are needed to achieve this condition. Alloys with ΔSconfig between 1R-1.5R are
called medium entropy alloys and those with ΔSconfig lesser that 1R are called low entropy alloys.
1.2 Core effects of HEAs
The four core effects of high entropy alloy determines the characteristic of these alloy. They
are high entropy effect, severe lattice distortions, sluggish diffusion and cocktail effect [3].
1.2.1 High entropy effect
One can guess that the concept of multi principal element will lead to complex phase diagrams
with multiple phases and intermetallic compounds resulting in brittle microstructures. On a
contrary, the solid solution phases exhibits simple microstructures and reduced phases. This is due
to the higher entropy effect which make these alloys exceptionally superior compared to other
conventional alloys. As the phase formation highly depends of the mixing enthalpies of the atom
pairs, a large positive or negative enthalpy does not alloy formation of random solid solution even
with the high entropy effect. As per Miracle et al. [4] criteria for the element to form random solid
solution, the atomic size difference must be below 3.8% or the enthalpy of mixing must be between
5 KJ mol-1 and -5 KJ mol-1 to get single phase high entropy alloy [3].
1.2.2 Severe lattice distortion effect
The lattice distortion is largely due to the size difference of the elements, crystal structure
and variation in cohesive energy between the binary atom pairs. Figures 1 (a) and (b) show the
difference between a BCC crystal structure of a pure metal and a high entropy alloy. The high
entropy alloy exhibits severe lattice distortion as the neighboring elements are different with
3
extremely varying sizes. This effect is the major factor influencing the properties of the random
solid solution.
Figure 1. BCC lattice of (a) pure metal with (b) high entropy alloy with severe lattice
distortion [2]
This lattice distortion in multi-principal element alloys is quantitatively measured using lattice
distortion parameter (g) which depends on the mean and square mean values of the lattice
geometrical parameter. For example the lattice distortion parameter for FeCoCrNi is 0.0085 and it
increases drastically to 0.0210 when Al is added. Chang et al. [5] found that both the lattice
distortion strain energy and cohesive energy increases with the number of elements. Recent work
by Mishra et al. [6] highlights the importance of lattice distortion/strain on the deformation
mechanisms, especially the slip and twinning formation in HEA. Various studies were done
recently to understand the relationship between lattice distortion and deformation mechanisms.
Kumar et al. [7] have calculated the lattice strain for various HEAs and has found that the HEAs
containing Al has large lattice strain when compared to HEAs without Al. This is because of the
4
large size of Al that leads to size mismatch resulting in increase in lattice strain. This effect
increases the strength of the material.
1.2.3 Sluggish diffusion effect
Vacancy in high entropy alloy is surrounded by different kind of atoms when
compared to vacancy in conventional alloys. The fluctuation in lattice potential energy between
the lattice sites are large in HEAs due to the varying neighboring elements and severe lattice
distortion. The lattice with low potential energy acts as an atom trap and reduces the diffusion rate.
The frequency of the forward and reverse atom jumps will have different energy barriers due to
the varying potential energy. In case of pure metals no difference in energy barrier is seen. Figure
2 compares the variations in potential energy between two sites in case of pure metal and HEA. In
case of pure metal there is no difference in energy between L and M sites. Whereas in cases on
high entropy alloy, the difference MD is seen between L and M sites. This is due to the lattice
distortion. This sluggish diffusion affect the diffusion related characteristics like recrystallization,
grain growth, precipitate coarsening and creep behaviors.
Figure 2. Variation in potential energy between two sites in pure metal, Fe-Cr-Ni and
CoCrFeNiMn0.5 HEA [8]
5
1.2.4 Mechanical properties
Kumar et al. [9] studied the Hall- Petch relationship in Al0.1CoCrFeNi HEA in which a
large lattice friction stress was noted. This was related to the lattice distortion which is inherent in
high entropy alloys. The Hall- Petch constant of HEA was noted to be higher than FCC and HCP
metals and alloys and was comparable with BCC, which usually shows higher Hall- Petch
constants. This denotes that these HEA exhibits higher strength and hardness. Refractory HEAs
like Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 are found to exhibit superior high temperature
compressive properties and can be potentially used in gas turbine engines [10]. Similarly hot
hardness of AlCoCrFeMo0.5NiX (X=0-1.5) was found to be larger than conventional super alloys
[11]. High entropy alloys are also found to have superior cryogenic fracture toughness [12].
Hemphill et al. [13] have observed that Al0.5CoCrFeNi HEA exhibit superior resistance to fatigue
failure and very high endurance limit. As a result of the multi-principal element composition and
core effects, HEAs show unique properties such as improved strength, superior wear resistance,
high temperature strength, improved fatigue and good corrosion resistance [1]. Figure 3 compares
various mechanical properties of conventional alloys with HEAs. Most HEAs can be easily
machined and requires no special processing techniques. These combination of superior properties
which are not noticed much in conventional alloys, makes the HEAs more attractive in many field
of engineering.
6
Figure 3: Plastic deformation based tetrahedron showing the superior propertied of
HEAs among conventional alloys (a) yield strength limited design (b) toughness limited design
(c) creep limited design and (d) fatigue limited design. [14]
Number of studies have been reported on microstructural characteristics, mechanical and
electronic properties of various combinations of HEAs but very few studies have been conducted
on implementation of HEAs for commercial application [1, 15]. These HEAs with extraordinary
properties when applied on commercial applications will benefit the end user to a greater extent.
7
1.3 Cardiovascular diseases and stent implants
Cardiovascular diseases are the major cause of death in the United States with
approximately 600,000 deaths per year where coronary heart disease alone kills about 350,000
every year. It costs $108.9 billion for the United States each year for health care services,
medications, and lost productivity [16]. Even though there is an appreciable advancements in
medical care and public awareness about the importance of cardio health, cardiovascular diseases
(CVD) are leading cause of death and disability to the country. Cardiovascular diseases (CVDs)
include high blood pressure (HBP), peripheral artery disease (PAD), coronary heart disease
(CHD), heart failure (HF) and stroke. Nearly 2400 peoples die due to these diseases each day in
the country, which is approximately 1 death every 37 seconds [16]. The major cause of CVD are
Atherosclerosis, syphilis, atheroma, congenital defects, obesity, smoking, hypertension, trauma,
hereditary conditions, hemodynamic and arterial biomechanical factors. A cardiovascular disease
affects heart and blood vessels like arteries, veins and capillaries. It can occur on blood vessels
reaching selected heart, brain, kidney, limbs etc. and can cause pain and failure over a period of
time. The symptoms of CVD depends upon the location of the disease. Symptoms can be noted
when the situation worsen. General symptoms are pulsing sensation, difficult in swallowing, pain,
hoarseness, coughing etc. Pains are caused due to rupturing of the arterial wall or increase of
loading on the arterial wall. Numbness can be felt in case of CVD on vessels reaching limbs or
hands. Generally these types of diseases are detected only through scanning techniques such as
Computed Tomography (CT), X-rays or Ultrasonography, Angiography and also by Magnetic
Resonance Imaging (MRI). CT scans can precisely detecting the location and shape of the disease.
This research work focuses mainly on treatments of arteries affected by atherosclerosis.
Atherosclerosis is the accumulation of plaque formed from cells, lipids, connective tissue, calcium
8
and other substances inside the inner lining of the arterial wall as shown in Figure 4 below which
leads into narrowing of the arterial wall and causes abnormal blood flow [18].
Figure 4: Schematic representation of artery with atherosclerosis-plaque inside the
arterial wall [18]
According to the University of Maryland Medical Center (UMMC) atherosclerosis is
commonly associated with aging. About 80 to 90 percent of individuals over the age of 30 have
some degree of atherosclerosis [19]. Atherosclerosis can occur in any artery in the body, including
arteries in the heart, brain, arms, legs, pelvis and kidneys that lead to serious health complications,
heart attack, stroke or even death depending on the location [19, 20].
Arteries are the blood vessels that carry oxygen and nutrients from the heart to the rest of
the body. The buildup of plaque reduces the internal diameter of the arteries and narrows it, thereby
limits the amount of flow of oxygen-rich blood to organs of the human body. Generally plaques
formed due to atherosclerosis can behave in different ways. It can stay within the walls of the
artery, grow to a certain size and stop [20]. Generally when the plaque invades less than 50 % of
9
the inner arterial wall, symptoms may not be noted. However, when plaque grows and blocks 50%
or more of the internal artery, significant blood flow obstruction will occur causing pains and other
symptoms. Usual symptoms are pain on the chest or legs depending on the location of the plaque.
In worst cases the plaques can rupture, break and forms series of blood clots inside an artery which
can lead to thrombosis disease [20].
The best way to avoid these diseases is to prevent the plaque growth by taking some
strategies such as following proper healthy food habits, exercises, avoid smoking etc. However,
these cannot be effective in advanced stages of atherosclerosis diseases which are cured only by
medical treatments. In addition to medications there are several types of surgical interventions as
follows:
Atherectomy is a minimally invasive surgical method of removing plaque burden within
the vessel as shown in Figure 5a [21].
Balloon Angioplasty is a technique where an empty and collapsed balloon mounted on a
guide wire called balloon catheter is passed into the narrowed artery location and then
inflated using air or water at a pressure 6 to 20 atm to a fixed size. The balloon expands the
inner blood clot or plaque deposits and the surrounding muscular wall. It opens up the
blood vessel for improved flow. The balloon is then deflated and withdrawn as shown in
Figure 5b [21]. Complications arises due to closing of the vessel after few days or weeks.
In order to overcome these issues and to avoid future surgeries, a mesh tube called stent is
used along with the balloon to scaffolds the narrow artery. It is expandable mesh tube that
can remain inside the vessel even after surgery and prevents the plaque from future
obstruction the artery.
10
Stent Angioplasty or stent implantation is the most preferred surgical technical where a
metallic mesh tube called stent is mounted on a catheter along with a balloon and inserted
into the narrow blood vessel containing plaque to counteract the effects associated with
plaque growth inside the artery as shown in Figure 5c [21]. After catheter delivery system
reaches the plaque affected area, the stent is expanded either using balloon or it expands
by itself depending on the type of stent. The expanded stent compresses the plaque by
exerting a radial force on the blood vessel to keep it open.
(a) (b)
(c)
Figure 5: Surgical interventions for treating atherosclerosis diseases in arteries (a)
Atherectomy (b) Balloon angioplasty (c) Stent angioplasty [21]
11
1.4 Types of stents
Stents can be broadly classified based on their deployment as balloon expanding stents and
self-expanding stents. Though neither of these stents are considered to be superior, they are
different and are appropriate for specific circumstances.
1.4.1 Self-expanding stents
Figure 6 shows the self-expanding stent. These stents are manufactured at the size quite larger
than the artery i.e. the diameter of these stents are bit bigger than the artery diameter. They are
crimped to smaller diameter using suitable constrain until they reach the narrow artery site. Once
they reach the site, constrains are removed so that the stent self-expands and opens the plaque
affected narrow artery. As the stent has to self-expand, it should always stay elastic. Thus the
material must possess higher yield strength.
Figure 6: Self-expanding stent that expands when constrain are removed [22]
1.4.2 Balloon expansion stents
They are the widely used stent type in the market. These stents are manufactured in crimped state.
They are expanded at the narrow artery site using a secondary expansion mechanism. Balloons are
used to expand the crimped stents. Deflated balloons which are placed inside the crimped stents
are inflated by air or water to expand the stent. When the catheter carrying balloon and stent
assembly reaches the narrow artery site, the balloon is inflated. When the balloon expands, it
expands the stent and this setup opens the narrow artery. The stent material and its design are very
12
significant as they are chosen in such a way that the stent reaches plastic state due to expansion.
Thus when the balloon is inflated, the stent expands and undergoes plastic deformation. Figure 7
shows the operation of a balloon expansion stent. By the time the balloon and stent fully opens the
artery, the stent reaches plasticity. Once the artery is fully open for normal blood flow, the balloon
is deflated and removed leaving the plastically deformed stent scaffolding the plague from further
blockage. As the stent has to reach plasticity with the expansion of balloon, the material has to
possess low yield strength or the crimped stent diameter has to be low. Manufacturing stent with
very low diameter increases manufacturing complexities and thereby makes it costly.
Figure 7: Deployment of balloon expansion stent with the inflation and deflation of
balloon [23]
Based on the construction and functionality, stents are classified into bare-metal stent
(BMS), drug eluting stent (DES), bio absorbable stent, dual therapy stent (combination of both
drug and bioengineered stent) [24, 25, 26].
13
1.4.3 Bare-Metal Stent (BMS)
These stents does not possess any coating of drug. It looks like a mesh tube of thin wire as shown
in Figure 8. These stents are often made from Stainless steel 316L (Low carbon Stainless steel)
and they are widely used in cardiac arteries. Other commercially used bare metal stent materials
are Nitinol (alloy of Nickel and Titanium) and cobalt chromium. These stents are flexible so as to
expand and accommodate into the arterial wall. The main purpose of these stent is to hold the inner
wall of the narrow artery in its newly compressed position without changing the diameter. The
diameter of the stents is chosen depending upon the vessel size, condition and the type of disease
and it ranges from 2 mm to 4 mm. Similarly the axial length ranges from 8 mm-38 mm depending
upon the length of the disease. The stents are material dependent as the strength and flexibility are
its major criteria. Material and design are the major parameters that determines the success or
failure of the surgery. They also determine other important characteristics like the life of the stent,
radiopaque and biocompatible. Radiopaque property helps the physicians to visualize the stent
while implanting by using a fluoroscope. For example, cobalt chromium stent material is more
radiopaque than stainless steel which makes it easy for physicians to visualize the stent during
implantation. The main disadvantage of using BMS is the occurrence of restenosis which means
recurrence of stenosis or re-narrowing of the blood vessel. The bare metal stents can also decreased
the elastic recoil effect after the balloon angioplasty surgery thereby reducing the restenosis
considerably after the surgery. The major cause of stent failure is the laterally stents fracture that
occurs because of the biomechanical environment of the vessels. Mainly stents used to treat
peripheral artery disease (PAD) suffers huge failure rate as the stent is affected by movement of
the patient such as sitting, running, walking, standing etc. Approximately 50% of stent failures are
noted in patients with PAD which leads to restenosis [27].
14
Figure 8: -Left: BMS stent, Right: BMS stent with inflated balloon [24, 27]
1.4.4 Drug-Eluting Stent (DES)
These stents when placed in the narrowed, diseased peripheral and coronary arteries slowly
releases drugs as shown in Figure 9. It prevents cell proliferation and heal the traumatized area in
the vessel thereby prevents restenosis [24-27]. Food and Drug Administration (FDA) has approved
drug eluting stents as a safe form of stent after examining the stent and its operations. The clinical
trials of DES shows superior performance to BMS in treatment of narrow arteries. It also reduced
the number of major adverse cardiac events (MACE). MACE, includes situations such as
myocardial infarction, the need for a repeated revascularization procedures or death.
Figure 9: Drug eluting stent [27]
15
DES consists of three main layers:
Stent Platform: The basic stent platform will be a bare metal stent which provides the basic
structure [24, 27]. It gives the basic mechanical strength and can undergo expansion, flexibility
and it is radio-opaque. Example: Cobalt chrome alloy is structurally stronger, thus it can be
designed thinner. It possess high radio-opaque properties and it is less corrosive and allergenic.
Coating: They are polymers in contact with the artery walls that holds and releases the
drug. These coatings are biodegradable coatings that releases the drugs as shown in Figure 10 [26].
Figure 10: a) The NEVO cobalt chromium stent, which has an open-cell design and
unique reservoirs that contain a biodegradable polymer and sirolimus mix that b) completely
biodegrades within 90 days [26]
These polymers are generally dip or spray coated. There can be one or more layers
depending upon the requirement. In multilayer case the first layer is adhesion, next layer holds the
drug and third coat can slow down the release of the drug and increase its effectiveness [24, 27].
Drug: The function of drug is to mainly reduce the neointimal growth and prevent
restenosis. Neointimal Hyperplasia is the proliferation of the smooth muscle cells caused when the
16
inner wall of the artery ruptures due to high stress. The high stress may be due to faulty surgery or
plague growth over period of time. When the inner wall ruptures the lipids are released causing
series of clots and proliferation of smooth muscles making the disease more severe. Hence the
growth of plague has to be suppressed internally. In DES immunosuppressive and anti-
proliferative drugs are used. Drugs like sirolimus and paclitaxel are the commonly used drugs [24,
27].
1.4.5 Biodegradable Stent (BDS)
These stents are similar to BMS and DES in structure and geometry but they are made of material
that the body can resorb after certain amount of time anywhere from 3 months up to 2 year. These
BDS has several potential advantages over bare or drug-coated metallic stents such as reductions
in stent thrombosis as the drug elution and vessel scaffolding are provided until the vessel heals.
The non-endothelialized stent struts or drug polymers are present long term [26]. Physiologically,
the absence of a rigid metallic casing facilitates the return of vessel vasomotion, adaptive shear
stress, late luminal enlargement, and late expansive remodeling. BDS can eliminate the thought
of having “an implant in their bodies for the rest of their lives” in the patients [27, 29]. In general
BDS are composed of polymer alloys (Figure 11 a). Polymers with a different chemical
composition and subsequent bio absorption time are chosen depending on the type and size of
plague formed. Poly-lactic–acid (PLLA) is the widely used BDS and it is commercially used in
numerous clinical items, such as resorbable sutures, soft-tissue implants, orthopedic implants,
dialysis media etc. [27, 29]. The PLLA is metabolized in the body over a period of time
(approximately 12 to 18 months) by Krebs cycle into small, inert particles of carbon dioxide and
water, which are then phagocytized by macrophages (Figure 11 b) [28, 30].
17
a) b)
Figure 11: a) The Igaki-Tamai BDS Stent with gold markers b) metabolism cycle of
PLLA [26]
Despite BDS seems promising stent with its biodegradable characteristics, they have major
disadvantages as polymer are used as the backbone to a coronary stent. The disadvantages includes
lack of radio-opacity, reduced radial force compared to stainless steel, and reduced ability to
deform. BDS were first implanted in animals as early as 1980; however, despite the impressive
results of these early stents like minimal thrombosis, moderate intimal hyperplasia and limited
inflammatory response, the technology failed to develop [26, 31]. This was due to an inability to
manufacture an ideal polymer that could limit inflammation and restenosis [26, 32].
Table 1 summarizes the stents sold in markets by different manufacturers that have been
reviewed and implemented into patient care [25, 26, 33]. Figure 12 shows the varieties of
combination of stent parameters such as material, form, fabrication, geometry and type of coatings
can influence the stent performance. Stent can be mesh structure, coil, slotted tube, ring, or multi-
design. Laser machining is the most common technique for fabricating of metallic stents.
Photochemical etching, EDM, and water-jet cutting are other methods used in stent fabrication
[34]. Fabrication techniques are selected based on the desired stent design and characteristics.
18
Table 1 – Summary of Current Stents Used Worldwide [78]
Stent Type
Sold on
the
Market
Companies Stent platform
Material Name
Drug
eluting
time
Locati
on
used
Metallic stents
with durable
polymers coating
4
Medtronic, Boston
Scientific, Elixir
Medical
Cobalt chromium, or
platinum chromium
Endeavor
Resolute(ZES),
Elixir DESyne,
TAXUS
Element(PES),
PROMU
< 90 days USA
Metallic stents
with
biodegradable
polymers coating
12
Sahajanand Medical, JW
Medical System, Cordis,
Biosensors,
Terumo,Devax Inc,
OrbusNEich,
Boston Scientific,
Elixir Medical, Xtent
Cobalt chromium,
platinum chromium,
stainless steel, or
nitinol
XTENT,
SYNERGY,
Combo, NEVO,
Biomatrix,
NOBORI
<90 days all
Polymer free
metallic stent 4
Minvasys, Biosensors,
MIV Therapeutics,
Translumina
Cobalt chromium, or
stainless steel
AmazoniaPax,
BioFREEDOM,
VESTAsync,
Yukon
Avg. of 30
days all
Metallic stents
coating 12
OrbusNeich, Medtronic,
Sahajanand Medical,
Hexacath, Abbott
Vascular
Cobalt chromium, or
stainless steel
Genous Stent,
R-stent, Blazer,
Azule,
Cornnium,
Catania
N/A
it is Bare
Metal
Stent
all
Biodegradable 10
OrbusNeich, Kyoto
Medical JP, REVA
Medical,
AbbottVascular,
Biotronik
Poly-L-lactic acid,
3Xlactide polymers,
Tyrosine-derived
polycarbonate,
Polymer+salicylate,
Magnesium alloy
Igaki-Tamai,
BVS, REVA,
IDEAL, AMS
4-36
months
absorption
time
all
Bifurcation(Ballo
on expandable) 10
TriReme Medical,
Invatec, Abbott
Vascular, Minvasys,
Boston Scientific,
Tryton Medical
Cobalt chromium, or
stainless steel
Antares, Ivatec
TwinRail,
Tryton, Petal,
Niloe Croco
N/A all
Self-expanding 5 Devax, Cappella,
Stentys Nitinol
Axxess,
Sigeguard,Bosto
n Scientific
N/A all
19
Figure 12: Stent Fabrication Techniques [78]
However, the advancement and variety of stents available, restenosis (i.e. reoccurrence of
restricted blood flow) still occurs in patients with 50% stent failure rate [28, 35 -37].
1.5 Basic stent characteristics
Stents are succession of rings that are connected. Radial support are provided by rings or
hoops (Figure 13a) and the longitudinal stability is provided by the connectors (Figure 13b) that
hold rings together. The mechanical characteristics of stents are defined by the number of crowns,
strut (Figure 13c) and connector [38]. The number of crowns and cells changes with the respect to
the size of the vessel. Stent diameter varies from 3.5 – 4 mm for large vessels and from 2.5 – 3
mm for small vessels. Several variations of stent designs are given in Figure 14.
a) b) c)
Figure 13: Stent Geometry Terminology a) rings (hoops) b) connectors c) strut
dimensions [21]
20
Figure 14: Various stent designs [21]
1.6 Arterial structure and classification of arteries
Blood vessels consist of three distinct layers: intima, media and adventitia (Figure 15)
[39].
Figure 15: Schematic representation of distinct layers of arterial wall left: top view of
layers in arterial wall [40] right: cross-sectional view of layers in arterial wall [42].
The inner wall or intima is a thin endothelial layer that has a layer of very thin (∼80nm)
basal lamina of a net-like type IV collagen. The endothelial cells elongates in the direction of the
blood flow and act as a semipermeable membrane that allows nutrients and chemical signals to
pass through so that it can reach the cells in the vessel wall from the bloodstream. The intima plays
a key role in communicating and regulating the pressure change with the media. Additionally, the
intima produces NO (nitric oxide) which relaxes smooth muscle cells in the media. But due to its
21
small thickness, the intima is usually neglected when considering the different layer contributions
to the mechanical resistance of the vessel wall. The next perforated sheet of elastic layer called
internal elastic lamina separates the intima from media [39]. The media is formed by smooth
muscle cells (SMC) of relatively high thickness that are embedded in an extracellular plexus of
elastin and collagen (mainly types I and III) and an aqueous ground substance that also contains
proteoglycans. Depending on the internal arrangement of the smooth muscle cells in the media,
the arteries can be distinguished into elastic arteries and muscular arteries. The elastic arteries are
the large-diameter vessels close to the heart that includes aorta, the main pulmonary artery, carotid
and iliac arteries. They form the histological feature called lamellar unit which sandwiches smooth
muscle cells and thin elastic laminae. Elastic arteries have concentric ring-like structures tied
together by radially oriented collagen. In case of muscular arteries, the media is a single thick ring
of smooth muscle cell [39]. The outermost layer of the vessel wall is the adventitia. Dense type I
collagen fibers are found along with scattered fibroblasts, elastin and nerves. Vasa vasorum, an
intramural network of arterioles, capillaries and venules are found in the medium and large arteries
that helps in proper interchange of O2, CO2, nutrients and metabolites. The nerves in the adventitia
allows supply of nutrients to the smooth muscle in the outer media by the diffusion of
neurotransmitters. The fibroblasts regulates the connective tissues for type I collagen production.
The fibers straighten during higher pressures which proves that adventitia serves as a protective
sheath, preventing rupture of the vessel due to high pressure [39].
Arteries classification - Arteries are classified into elastic arteries and muscular arteries.
Elastic arteries includes conducting arteries, including aorta, brachiocephalic, common carotid,
subclavian, vertebral, pulmonary and common iliac. Muscular arteries are distributing arteries,
including brachial artery, radial artery, popliteal, common hepatic artery [39, 40]. Arteries to
22
supply blood and oxygen to the tissues of legs, arms, and lower body. Circulation problems occurs
in these areas of the body when the arteries hardens. Coronary arteries are smaller and thinner
arteries typically near the heart and the larger arteries called peripheral arteries are seen on legs,
arms and lower body. The largest artery in the body being the femoro-popliteal artery (FPA) with
nearly 50cm in length that includes the superficial femoral artery (SFA) and popliteal artery (PA)
as shown in Figure 16 [24, 32, 42].
.
Figure 16: Anatomy of the femoropopliteal (FP) artery. CFA, common femoral artery;
PFA, profunda femoral artery; SFA, superficial femoral artery; DGA, descending genicular
artery; ATA, anterior tibial artery [32].
23
1.7 Atherosclerosis, peripheral arteries (PA) and stents-vessel interaction
Atherosclerosis specifically with coronary artery disease is much familiar among people
but the cause and best management for peripheral artery disease (PAD) are less known. PAD are
also treated similar to coronary artery disease. Unfortunately the assessment and management of
PAD in patients with diabetes mellitus are unclear. PAD is the reduction in blood flow to the legs
or, less often, to the arms due to atherosclerotic plaque or blood clots (Figure 17). PAD leads to
painful walking and slows healing of injuries. In the worst cases, it can result in the loss of a toe,
foot, or leg.
Figure 17: Blood flows through a normal artery (A). In peripheral artery disease,
atherosclerotic plaque narrows the artery and impedes blood flow (B). During angioplasty to
restore blood flow, a stent maybe inserted to keep the artery open (C). Stent implant restores
normal blood flow [43]
Generally atherosclerosis occurs at sites of complex geometry e.g., along the outer portions
of the bifurcation in artery and only within specific points within the vessel. Due to every day
body function, parts of the legs are exposed to multiaxial deformations with up to 60% rotation
24
and 20% contraction as the leg is bent from an extended position [47] due to which stent deployed
in the intersection of the femoral and popliteal arteries is exposed to multiaxial displacements. This
occurs due to the musculoskeletal motion as well as bending, torsion, flexure, tension, and
compression (Figure 18 left) [32, 45]. As stent implantation leads to artery strengthening which
restricts the natural artery curvature, it lead to various wall stresses and hemodynamics changes
within the implantation region (Figure 18 left and right) [33].
Figure 18: left: As stents are placed into the artery, the artery ability to bend and
compress is reduced. The adjacent unstented artery bends more, possibly resulting in kinking at
the margin of the stent [45]; right: Stenting leads to different wall stresses and hemodynamics
within the implantation region [33]
These high stresses can lead to fracture of the stent, which can further cause in-stent
restenosis issues. Technologies such as: Doppler ultrasound, magnetic resonance angiography, and
computed tomography angiography can provide detailed image of the stented arteries with
quantitative characterization of the disease. The Doppler ultrasound also provides information on
blood-flow velocity and turbulence [46]. It has been found that that several mechanical factors
such as pulsatile or steady blood flow velocity and the corresponding wall shear stress in the
coronary artery can lead to cell remodeling and decrease of mechanical strength in the arterial wall
[47, 48]. These factors lead to specific alternating and steady stress state in the arterial wall such
as elongation, compression, bending, tortuosity and the like that produce a fatigued environment.
Especially, in case of PAD in addition to internal factors, external factors such as movement of
25
legs while walking, climbing, seating down, standing up also contribute to increase of stress-strain
state in the artery carrying the stent implant. Further, in addition to impose stress-strain state Berry
et al. 2002 [48] found that stented arteries also have a mechanical mismatch at the interface
between the arterial luminal wall and the stent. This mismatch leads to the development of
circumferential stresses between the stent and the artery, causing vascular stretch, and changes in
hemodynamics [48] as shown in Figure 18. Thus understating the biomechanical state of the artery
and mechanical properties along with the deformation that the stent undergoes under the influence
of the arterial forces is much needed. Researches has been done on the biomechanical behavior of
the arterial wall using computational fluid dynamics (CFD) analysis to quantify the variation of
wall shear stress of the contained blood flow [49]. Further, CFD has been used to characterize
wall shear stress development for canine femoral arteries, using non-Newtonian simulation
models, analyzing resultant changes in blood flow velocity, wall shear stresses on the sensitive
endothelial cell layer, and also the role of incorporated stents in regulating shear development in
these regions was demonstrated [50]. Finite element analysis has been used to simulate the
operation by applying many features to the stent design such as: smaller size, balloon crimping
characteristics, flexibility during navigation, less recoil following expansion, and techniques to
improve reliability. Using FEA the interaction of the stent with the arterial vessel, stent induced
vessel injury as well as stent fatigue can be understood [51 - 56]. Further, FEA modeling can
provide a method to evaluate the potential “risk” regions for high stress formation within the
plaque and artery. However, studies are mostly focused on the coronary arterial stents and its
hemodynamics after arterial stenting. Many researches has been done on coronary artery stents,
but peripheral stenting, specifically SFA with high stent failure rates is a less studied field. Recent
literature review summarizes heterogeneous designs that include different physiologic settings
26
from young to mature participants and with and without disease, and also on cadavers to analyze
produced results specific to anatomic location within the SFA/PA [57, 58]. Further, a compilation
of stent improvements for treating atherosclerosis in SFA/PA shows evidence of increased
durability, finer surface finishes, increased flexibility, and longer lengths. Yet stents still fracture
at measurable rate and clinical significance of stent fracture remains controversial. A direct
relationship between stent fracture and clinical outcomes, such as restenosis, has not been
established; however, the uniqueness of the biomechanical environment of the SFA compared to
other vascular beds was clearly understood [57]. Stent desired characteristics includes flexibility,
ability to track, high radial strength, circumferential coverage, low surface area, and hydrodynamic
compatibility [59]. Each reported stent design has advantages and disadvantages but no stent
incorporates all of the cited characteristics. In other words there is no ideal stent. However, if one
quantifies the mechanical behavior of stents with their specific characteristics and match with the
diseased artery conditions, then there is no need for a standard off-the shelf ideal stent. This is
because, the stent will be able to sustain this biomechanical artery environment for each patient
individually that will lead to improved personalized treatments for patients with PAD and reduced
stent failures especially cases that lead to death. This project focuses the failure of stents in
materials perspective to propose as alternative materials that could possible reduce the failure.
1.8 Conventional stent materials and their properties
The conventional stent materials includes stainless steel, nitinol, cobalt chromium, titanium
alloys etc. [60]. These materials are biocompatible and corrosion resistant that does not mix or
dissolve in blood or any other body fluids.
27
Stainless steels: Most commonly used alloy is Stainless steel 316L, grade 2 which has less than
0.030% of carbon to avoid in vivo corrosion. The “L” represents low carbon content. SS 316L
consist of iron (60-65%) alloyed with major amount of chromium (17-19%) and nickel (12-14%),
small amount of nitrogen, manganese, molybdenum, phosphorus, silicon and sulfur [60]. The
alloying materials improves the surface and bulk microstructure. Chromium is added to make the
steel corrosion resistant by forming strong adherent Cr2O3 on the surface. At the same time the
chromium tends to stabilize the ferritic BCC phase which is weaker than the austenitic FCC phase.
Silicon and molybdenum are also ferrite stabilizers. To overcome these tendency of ferrite
formations, nickel is added which stabilizes the austenitic phase. Similarly when carbon content
exceeds 0.03%, it forms carbides like Cr23C6 which are dangerous. These carbides are capable of
reducing the chromium based oxides which leads to more corrosion and corrosion assisted
fractures. The SS 316L is a single phase FCC. The grain size of the stainless steel 316L is 100µm
or less. In annealed case the young’s modulus is 190 GPa with yield and ultimate strength 221
MPa and 483 MPa respectively. It possess good cold working capability. Usually the SS 316L are
30% cold worked to increase yield, ultimate tensile and fatigue strength compared to annealed
state. Cold rolling reduces the ductility which does not affect much in implant applications.
Further, stainless steel 316L is cheap, easily machinable and readily available in the market. This
makes the material more suitable for the implant application.
Cobalt chromium alloys: The cobalt bases alloys consist of Co-Cr-Mo with 58-69% Co and 26-
30% Cr and other alloying elements such as nickel, tungsten, titanium etc. The main characteristics
of Co-Cr alloy is their excellent resistant to corrosive environment [60]. It is due to the bulk
composition and surface oxide (Cr2O3). Some of the Co-Cr alloy are ASTM F75, F90, F799 and
28
multiphase F562. The as-cast F75 alloys possess dendritic microstructure with Co- rich matrix and
carbide of Cr or Mo in the inter-dendrites. It also possess large grain size which results in
undesirable low yield strength. It also forms micro pores and cracks due to shrinkage during
solidification of casting. To overcome these problem, powder metallurgical methods are used to
improve the alloy microstructure and mechanical properties. Thus sintering is done on the forged
powder material produces high yield and better ultimate strength. The young’s modulus of as-cast
or annealed F75 Co-Cr alloy is 210 GPa with yield strength of 448-517 MPa and tensile strength
of 655-889 MPa. By powder metallurgy the yield strength increases to 841 MPa and the tensile
strength increases to 1277 MPa. The ASTM F799 alloy is a thermomechanical treated F75 alloy
that is hot forged at 800C after casting. It exhibit HCP microstructure. The fatigue, yield and
ultimate strength of ASTM F799 is almost double that of F75. ASTM F90 is an alloy containing
tungsten and nickel which improve the machinability and fabrication properties. The yield strength
and tensile strength of F90 are 488-678 MPa and 951-1220 MPa respectively. The ASTM F562 or
MP35N consist of Co (29-38.3%) and Ni (33-37%) with significant amount of Cr and Mo. The
alloy has multiphase and possess cold worked matrix, solid solution strengthening and
precipitation hardening which makes this alloy the strongest material for the implant application.
Titanium based alloy [60]: CP titanium and Ti-6Al-4V are most commonly used implant
material. Though Al is bit toxic to human body the TiO2 layer provides the necessary corrosion
resistance to the alloy. The alloy possess low young’s modulus of 110 GPa with yield strength of
485 MPa and the tensile strength is 760 MPa.
29
Nitinol: Nitinol (55% Ni and 45 % Ti) is shape memory allot with super elastic property [61]. It
can undergo deformation at one temperature and can recover its original un-deformed shape when
heated above the transformation temperature. These interesting properties makes nitinol a
promising candidate for stent implant application. A collapsed stent can be inserted into the artery,
when the body temperature warms the stent, the stent can expand and reach its un-deformed state
there by opens the narrow artery. Nitinol is more applicable for self-expanding stents due to this
nature. The yield strength is largely temperature dependent. It varies form 70-600 MPa and the
ultimate strength reaches up to 1070 MPa. The young’s modulus varies from 41-75 GPa [61, 62].
It a flexible material with good corrosion resistance properties. The main disadvantage of nitinol
is that it is hard to manufacture as a small change in composition causes a huge difference in the
mechanical properties.
1.9 Need for alternative material
Even though there are variety of options in case of materials and recent advancement in
technologies, the 50% failure rates of stents is a huge challenge for the scientists and therapists.
With CVD being the leading cause of death in the nation, the need for better stent is very important
for the scientific and medical community. As design and material of the stent are the only two
parameters that can be controlled or modified, intense researches are done on these parameters to
develop an optimum design and a suitable material which ensures a longer stent life. With the
recent development in high-entropy alloys which exhibits promising properties, it can possibly be
the future of stent materials. This thesis work checks the compatibility of high-entropy alloys for
stent implant application by experimentally determining and comparing the mechanical properties
of CoCrFeNiMn and Al0.1CoCrFeNi high entropy alloys with the conventional stent implant
materials. These high-entropy alloys are chosen based on the conventional stent and implant
30
materials. As Co-Cr alloys, stainless steel, Ni-Ti alloys, Ti-6Al-4V and Fe-Mn are the most
commonly used biocompatible implant materials, the high-entropy alloy containing these elements
are assumed by the authors to be potentially biocompatible however, biocompatibility evaluation
is beyond the scope of this paper. Thus this work is based on a speculation that both CoCrFeNiMn
and Al0.1CoCrFeNi high entropy alloys are biocompatible.
CoCrFeNiMn HEA: This equiatomic alloy was first produced by Cantor and team. He found that
the alloy is highly stable with single phase solid solution showing FCC crystal structure [63]. It is
a soft and ductile alloy which exhibits good mechanical properties. In these HEAs the
configurational entropy is higher than fusion entropy. Literatures says that the CoCrFeNiMn HEA
has low stalking fault energy and exhibits twins. It exhibits dendritic microstructure and undergoes
sluggish diffusion [63]. The mechanical properties and the microstructural characteristics are
explained elaborately in the rest of this work.
Al0.1CoCrFeNi HEA: This alloy consist of 2.44 at. % of Al and 24.4 at. % of Co, Cr, Fe and Ni.
The alloy is a single phase FCC. Literatures says that this alloy exhibit low stalking fault energy
and forms annealing twins [14]. The mechanical properties and the microstructural characteristics
are explained elaborately in the rest of this work.
In this thesis, Chapter 2 introduces the experimental approach used to characterize and
determine the mechanical properties of the proposed materials. In Chapter 3, the mechanical
properties of CoCrFeNiMn and Al0.1CoCrFeNi HEAs found experimentally are symmetrized and
compared with the conventional stent materials. In Chapter 5 the computational modelling and
analysis of stent is discussed. Finally, in Chapter 5, the conclusion of this thesis is listed and future
work is discussed as well.
31
CHAPTER 2
EXPERIMENTAL APPROACH TO DETERMINE PROPERTIES
2.1 Outline of experimental approach
CoCrFeNiMn and Al0.1CoCrFeNi high entropy alloys was subjected to various thermo-
mechanical treatments such as annealing, cold rolling, hot rolling and their combinations. The
mechanical properties such as yield strength, ultimate strength, young’s modulus, hardness and
fatigue life of the thermo-mechanically treated high entropy alloys are determined experimentally
using tensile testing, hardness testing, Nano-indentation and fatigue analysis tools. The
microstructure of the alloy after different thermo-mechanical processing conditions were
examined using optical microscopy. Samples for these tests were prepared using CNCs and other
cutting techniques. All samples were polished in order to eliminate surface defects.
2.1.1 Tensile testing
General information: Figure 19 shows the tensile test sample used for the test. The test was
conducted using a custom made mini-tensile machine which is capable of loading up to 500 lb.
The strain rate can range from 10-2 to 10-6 s-1. The tensile testing can be done in room temperature
or elevated temperature with a help of furnace. The load cell and the LVDT sensors in the tensile
testing machine are used to measure the load and displacements. The sensed values are collected
and recorded using a LABVIEW software.
32
Figure 19: Custom- made mini-tensile test assembly
Sample preparation: Three tensile testing samples in every thermo-mechanical processed
condition were prepared by milling using CNCs with appropriate codes. The samples are prepared
with 5 mm gauge length, 1.23 mm width and 1.10 mm thickness. A mini tensile sample is shown
in the Figure 20. The samples are milled and polished using silicon carbide polishing paper of grid
sizes from 180 to 1200, then polished using 1 m dimond suspension on a vel cloth and finally
using 0.05 m colloidal silica suspension on a final-P cloth to achieve mirror finish.
Figure 20: Picture of a mini- tensile sample
33
Testing: The samples are mounted on the specimen grip assembly. These tests were
conducted at room temperature without any furnace. The strain rate was maintained to be 10-3 s-1.
The displacement and load measured using LVDT and load cell were recorded in the computer
and processed using LABVIEW interphase. The data acquired during the test were then processed
and the stress -strain curves are plotted using ORIGIN software. From the stress-strain curve, yield
and ultimate strength can be obtained. The average results obtained from three samples were
calculated and considered as the final result.
2.1.2 Hardness testing
General information: Figure 21 shows the Vickers micro hardness machine used to
measure the hardness. The machine that is capable of applying loads ranging from 25 to 500 gf
was used. The machine can apply load for designated time period. The indents are measured by
measuring the distance between the corners of the diamond indent using the eye piece. The
machine computes the hardness automatically with the applied load and the dimension of the
indent.
Figure 21: Vickers’s hardness tester
34
Sample preparation: The samples are milled and polished using silicon carbide polishing
paper of grid sizes from 180 to 1200, then polished using 1 m dimond suspension on a vel cloth
and finally using 0.05 m colloidal silica suspension on a final-P cloth to achieve mirror finish.
Testing procedure: Hardness was measure by applying 300 gf load for 10 seconds.
Hardness was measured at 10 different places and their average is considered.
2.1.3 Nano-indentation testing
General information: This is a kind of indentation hardness measuring test used to measure
certain mechanical properties by making an indent at nanoscale using highly precise tip by
applying loads in micro newton. The real time load-displacement information can be collected
and processed to derive the mechanical property of the material. The displacement or depth of
penetration and the applied load are plotted while loading and unloading. This plot can be used to
determine hardness and young’s modulus of the material. Figure 22 shows the load and
displacement curve generated from the Nano indentation test. As the slope of the unloading curve
(dP/dh) gives the young’s modulus of the material.
Figure 22: The load- displacement plot generated during the Nano-indentation testing
35
Sample preparation: The samples are milled and polished using silicon carbide polishing
paper of grid sizes from 180 to 1200, then polished using 1 m dimond suspension on a vel cloth
and finally using 0.05 m colloidal silica suspension on a final-P cloth to achieve mirror finish. It
is made sure that the samples are perfectly polished and cleaned so that it does not damage the tip
of indentation machine.
Testing procedure: In this work, 10000 µN load was applied on the sample for 10 seconds.
The test was done at three different spots and the average was considered.
2.1.4 Fatigue analysis
General information: Figure 23 shows the custom made fatigue analysis machine. Fatigue
analysis is done to determine the endurance limit of the material. Endurance limit is the stress
below which the material does not fail or has infinite life. Generally endurance limit is calculated
for 107 cycles. The samples are subjected to cyclic load until the sample fails. This test is conducted
at different stress and the corresponding number of cycles survived is noted. The values are plotted
in S-N (Stress vs Number of cycles) curve. The stress corresponding to 107 cycle is considered as
the endurance limit.
36
Figure 23: Custom made fatigue analysis setup
Sample preparation: Figure 24 shows the sample and the dimensions used for fatigue
analysis. The samples are milled and polished to mirror finish.
Figure 24: Sample for fatigue analysis. (a) Picture of fatigue sample before and after
testing and (b) dimensions of the fatigue sample
Testing procedure: The prepared samples were mounted between the rocking arm and
stationary block. The stress ratio and the load are fixed by adjusting the rotary cam and their
assembly. The samples are subjected to bending load with stress ratio of -1. The fatigue analysis
was done at 160 MPa, 180 MPa, 200 MPa, 210 MPa, 220 MPa and 250 MPa.
37
2.1.5 Optical microscopy
General information: A Zeiss optical microscope is used to determine the microstructural
characteristics of a material. The microscope has a magnification ranging from 5 to 100 X. The
microscope is integrated with a camera and computer using which the images are captured.
Sample preparation: The samples are milled to desired size and polished using silicon
carbide polishing paper of grid sizes from 180 to 1200. It was then polished using 1 m dimond
suspension on a vel cloth and finally using 0.05 m colloidal silica suspension on a final-P cloth
to achieve mirror finish. The polished samples are etched with aqua regia solution (3 parts of
Hydrochloric acid and 1 part of Nitric acid) for 10-12 seconds.
Testing procedure: The etched sample is viewed through the optical microscope at different
magnifications and the images are gathered. Appropriate legends and scales were included to
quantitatively understand and compare the grain sizes.
38
CHAPTER 3
DETERMINATION OF MECHANICAL PROPERTIES OF HIGH ENTROPY ALLOYS
3.1 Determination of mechanical properties of CoCrFeNiMn high entropy alloy
3.1.1 Introduction
The mechanical properties such as yield strength, ultimate strength, young’s modulus and
hardness of the CoCrFeNiMn HEA was determined using tensile testing, Nano indentation and
Vickers hardness testing. The microstructural arrangements were examined using optical
microscopy. Based on the results, decisions were made to trailer the properties by thermo-
mechanical processing. Various thermo-mechanical processing was done to achieve the desired
property and microstructure.
3.1.2 Experimental details and thermo-mechanical processing
The CoCrFeNiMn HEA was supplied by Sophisticated Alloys. The properties of the alloy
at as-cast condition was determined by the above mentioned testing methods. The tensile testing
was done at a strain rate on 10-3 s-1 at room temperature. The hardness was measured by applying
300 gf load for 10 seconds and the Nano indentation testing was done at 10000 µN for 10 seconds.
All the results along with the microstructural characteristics found by optical microscope was
39
considered for the consequent thermo-mechanical processing. Based on the results from as cast
condition, the alloys was subjected to four thermo-mechanical processing. The processes carried
out for every condition were independent and as follows: (1) as cast HEA annealed at 700 °C for
one hour, (2) as cast HEA annealed at 900 °C for one hour, (3) as cast HEA annealed at 1000 °C
for 48 hours and (4) as cast HEA annealed at 1000 °C for 4 hours, rolled to 50% thickness (from
5.8 mm to 2.9 mm) then annealed again at 1000 °C for 4 hours. The reason for these processing
are discussed in the results and discussions.
3.1.3 Results and discussion
3.1.3.1 As cast, 700°C- 1 hour and 900°C- 1 hour annealed conditions
Optical Microscopy: The as-cast CoCrFeNiMn HEA was polished and etched in aqua regia
solution for 10 seconds. Figure 25(a) shows the optical microscopy images of the as-cast
CoCrFeNiMn HEA samples. The as received HEA in cast condition was found to exhibit
dendritic microstructure under optical microscope. Salishchev et al. have also reported that
CoCrFeNiMn exhibits dendritic microstructure with Co, Cr and Fe rich dendrite and Ni and Mn
rich inter-dendrite [64]. In order to dissolve the inhomogeneity in the microstructure and to
understand the behavior of the material with annealing, the HEA was annealed at 700 °C for one
hour and annealed at 900 °C for one hour. Optical microscopy was done on the annealed samples.
Figure 25b and 25c shows the optical microscopy image of the HEA samples annealed at 700 °C
and 900 C respectively. It was clear that the annealed alloy exhibited similar dendritic
microsturcture like as-cast alloy without any changes. This proves the slow diffusion kinetics or
sluggish diffusion reported in the CoCrFeNiMn HEA [65].
40
Figure 25: Optical microscopy images of CoCrFeNiMn alloy in (a) as received, (b)
annealed 700 C for one hour and (c) annealed 900 C for one hour
Hardness testing: The hardness of the as-cast HEA was found to be 144.3 HV0.3 whereas that of
900 C for one hour annealed is 155.7 HV0.3 and 700 C for one hour annealed is 169.2 HV0.3.
Schuh et al. have reported that this unusual increase in hardness may be due to the formation of
nanostructured multiphase embedded in the matrix. The maximum hardness was reached when
annealed at 450 C for one hour after which it decreases due to the formation of NiMn and Cr-
rich second phases and FeCo rich third phase [66].
Tensile testing: Table 2 summarizes the tensile test results of the as cast, 700 C and 900 C for
one hour annealed CoCrFeNiMn high entropy alloy.
41
Table 2.Tensile properties of as cast HEA, annealed at 700 C and 900 C for one hour
Condition Yield strength
MPa
Ultimate strength
MPa
Uniform
Elongation (%)
As received (cast condition) 208±4 447±8 51±2
Annealed 700 C-1 hour 235±3 505±6 44±3
Annealed 900 C-1 hour 224±4 515±7 51±2
From the tensile result it is seen that the yield and ultimate strength of the HEA at 700 C-
one hour annealed condition is unusually higher compared to the as-received condition. This might
be due to the second phase and third phase formation [66]. Figure 26 shows the engineering stress-
strain plot for each case.
Figure 26: Engineering stress-strain curve of CoCrFeNiMn alloy in three conditions
42
3.1.3.2 1000°C- 48 hour annealed conditions
As dendritic microstructures lead to microstructural inhomogeneity, it is always preferable to
eliminate the dendrites by heat treatment. To eliminate the dendritic microstructure the as-cast
CoCrFeNiMn HEA was annealed at 1000 C for 48 hours keeping the sluggish diffusion under
consideration. Tensile testing, hardness testing, and optical microscopy were also done to
understand the behavior upon this condition.
Optical microscopy: From optical microscopy it was found that the material undergoes
inhomogeneous diffusion. Figure 27 shows the inhomogeneous diffusion. Figure 27a shows the
alloy that has undergone complete diffusion of inter-dendrites in some areas and precipitates were
formed in other areas as shown in Figures 27b and 27c. Dendrites of smaller inter-dendrite spacing
was noted in throughout the material which indicates that the homogenization is incomplete. This
indicates that either temperature or the time is insufficient to completely homogenize the material.
43
Figure 27: Optical microscopy image of CoCrFeNiMn HEA annealed at 1000 C for 48
hours (a) inhomogeneous microstructure with traces of dendrites and homogenized region (b)
inhomogeneous microstructure with precipitates (c) precipitates formed during annealing
Hardness testing: Since the microstructure was varying drastically, the hardness was measured
using Vickers micro-hardness testing was found to also vary from 144 HV in homogenized area
to 220 HV in precipitate rich area.
Tensile testing: The tensile testing of 1000 C for 48 hour annealed CoCrFeNiMn HEA resulted
in average yield stress, ultimate stress and uniform elongation as 216±4 MPa, 475±7 MPa and
50±2% respectively. It is noted that the results are close to results from as-received condition.
Therefore, it can be understood that the precipitates have least influence in this case. Figure 28
shows the engineering stress-strain plot of 1000 C for 48 hour annealed CoCrFeNiMn HEA.
44
Figure 28: Engineering stress- strain plot of 1000 C- 48 hour annealed CoCrFeNiMn
HEA
Calculation of homogenization time and temperature: It was understood that the composition
is unusual and the HEA undergoes sluggish diffusion. To homogenize the CoCrFeNiMn alloy the
diffusion of the elements between dendrites and inter-dendrites were studied. For homogenization
to occur, the inter-dendrites has to completely dissolve into the dendritic space. Fick’s law of
diffusion and Arrhenius equation concepts were used to calculate the temperature and time
required to diffuse the inter-dendrites into the dendritic space. According to which the diffusion
length is x is given by
x = √𝐷𝑡 2
D=𝐷𝑜𝑒(−𝑄
𝑅𝑇) 3
45
where t represents time in seconds, R is gas constant=8.31JK−1 mol−1, Do is the Pre exponential
factor in m2/s and Q is the Activation energy in kJ [65]. The value of pre exponential factor is
considered as 19.7x10-4 m2/s which is equal to Do of Nickel as it has the highest Do value in the
HEA alloy content [65]. The activation energy for the alloy is 321.7 kJ [65]. The spacing of the
inter-dendrites was measured using optical microscopy measure tool. The spacing was found to
vary between 10 µm to 32 µm. Therefore for homogenization of CoCrFeNiMn HEA the diffusion
length must be greater than 32 µm. Table 3 shows the diffusion length (x) at various temperatures
and time.
Table 3: Diffusion lengths of elements at different time and temperature
Temperature(C) Time (hours) D (m/s2) X(µm)
900 1 9.15E-18 0.182
900 4 9.15E-18 0.363
900 10 9.15E-18 0.574
900 24 9.15E-18 0.889
900 48 9.15E-18 1.26
1000 24 1.22E-16 3.25
1000 48 1.22E-16 4.6
1100 24 1.12E-15 9.84
1100 48 1.12E-15 13.9
1200 24 7.6E-15 25.6
1200 48 7.6E-15 36.2
Since heat treatment at 1200 C may lead to oxidation, alternate techniques were used to
homogenize the HEA.
46
3.1.3.3 Annealing-Rolling-Annealing technique
This is an alternative technique in which material is annealed at 1000 C for 4 hours, rolled
to 50% thickness from 5.8 mm to 2.9 mm at a rate of 0.2 mm reduction per pass and then annealed
again at 1000 C for 4 hours. The initial annealing softens the material and partially diffuses the
dendritic microstructure, the cold rolling/working reduces the inter-dendritic gaps and also
removes pores and cast defects and the final annealing diffuses the close inter-dendrites and
homogenize the material completely.
Optical microscopy: Figure 29 shows the optical microscopy images at each stages of the
technique. At the end, the inter-dendrites were fully diffused and homogeneous microstructure was
found.
Figure 29: Optical microscopy images at (a) 1000 C Annealed, (b) 1000 C Annealed +
50% rolled and (c) 1000 C Annealed + 50% rolled + 1000 C Annealed
47
Figure 29a shows that at 1000 C for 4 hours annealed condition the HEA shows dendritic
microstructure. It shows that the temperature and time is insufficient to homogenize the material.
But after 50% of cold rolling the inter-dendritic space reduces as seen in Figure 29 b. It is clear
that the HEA still shows dendritic microstructure but with reduced inter-dendritic spacing. The
effect of rolling was seen through the lines along the rolling direction. Figure 29 c shows the
homogenized microstructure with no dendrites. Large number of twins were also noted in this
condition. Mridha et al. have also found that presence of annealing twins is due to low stacking
fault energy [67].
Hardness testing: The average hardness of the HEA after homogenization is found to be 157.2
HV0.3
Tensile testing: The average yield stress, ultimate stress and uniform elongation are found to be
273±5 MPa, 591±8 MPa and 44±2% respectively. Figure 30 shows the engineering stress-strain
plot of the homogenized HEA.
Figure 30: Stress-plastic strain plot of annealed-rolled-annealed CoCrFeNiMn HEA
48
Nano indentation: The Young’s modulus was measured using Nano-indentation machine by
applying 10000 µN load for 10 seconds on the homogenized HEA. The force and displacement
(depth of penetration) were plotted and the Young’s modulus was found from the slope of the
curve during unloading. The average Young’s modulus was found to be 189 GPa. Figure 31 shows
the Load-displacement plots.
0 100 200 300 400
0
2000
4000
6000
8000
10000
Loa
d (
N)
Displacement (nm)
Figure 31: Load-displacement plot at 10000 µN load
Since young’s modulus (E) is a property of atomic distances at particular temperature,
it is understood that E is not affected much by any thermo-mechanical processing. Thus the
young’s modulus of CoCrFeMnNi is assumed to be 189 GPa in all processing conditions.
Pores and cast defects: The CoCrFeNiMn high entropy alloy received from the supplier had
plenty of pores and cast defects. The alloy was subjected to series of hot rolling, cold rolling and
heat treatment processes to get rid of the pores. Unfortunately all the efforts to eliminate these
defects failed. Figure 32 shows the large visible pores and cracks on the alloy.
49
Figure 32: Picture of the alloy showing large pores, cracks and cast defects
These pores affects certain mechanical properties such as ultimate strength and fatigue
life to a larger extent. As fatigue analysis of the material with these pores and defects leads to
faulty results, fatigue analysis was not carried out on this alloy. In future, the samples will be
subjected to hot forging or any other similar operations to eliminate the pores. Fatigue analysis
will be carried out on the defect free material.
3.2 Determination of mechanical properties of Al0.1CoCrFeNi high entropy alloy
3.2.1 Introduction
The mechanical properties such as yield strength, ultimate strength, young’s modulus,
hardness and fatigue life of the Al0.1CoCrFeNi HEA was determined using tensile testing, Nano
indentation, Vickers hardness testing and fatigue analysis experiments. The microstructural
arrangements were examined using optical microscopy. Based on the results, decisions were made
to trailer the properties by thermo-mechanical processing. Thermo-mechanical processing was
done to achieve the desired property and microstructure.
50
3.2.2 Experimental details and thermo-mechanical processing
The Al0.1CoCrFeNi HEA with nominal composition, at. % Al-2.44 and rest 24.4 each was
supplied by The University of Tennessee, Knoxville. The properties of the alloy at as-cast
condition was determined by the above mentioned testing methods. The tensile testing was done
at a strain rate on 10-3 s-1 at room temperature. The hardness was measured by applying 300 gf
load for 10 seconds and the Nano indentation testing was done at 10000 µN for 10 seconds. All
the results along with the microstructural characteristics found by optical microscope was
considered for the consequent thermo-mechanical processing. Based on the results from as cast
condition, the alloys was subjected cold rolling and heat treatment to alter the microstructure.
3.2.3 Results and discussion
3.2.3.1 As cast alloy
Optical Microscopy: The as-cast Al0.1CoCrFeNi HEA was polished and etched in aqua regia
solution for 10 seconds. Figure 33 shows the optical microscopy images of the as-cast
Al0.1CoCrFeNi HEA. The as received HEA in cast condition was found to exhibit coarse granular
microstructure under optical microscope.
Figure 33: Optical microscopy images of as-cast Al0.1CoCrFeNi high entropy alloy
showing coarse granular microstructure
51
Hardness testing: The average hardness of the as-cast Al0.1CoCrFeNi HEA was found to be 138.2
HV0.3. The low hardness was due to the coarse granular cast microstructure in the alloy.
Tensile testing: From the tensile test of as-cast Al0.1CoCrFeNi, the yield strength, ultimate
strength and uniform elongation was found to be 140±3 MPa, 370±6 MPa and 65±1% respectively
(Table 4) . The low strength was due to the coarse granular cast microstructure.
3.2.3.2 Cold rolled and annealed condition:
The as-cast HEA was cold rolled to 60% and then annealed at 1000C for 24 hours inorder
to eliminate cast microstructure and to improve the stength
Optical microscopy: The cold rolled and annealed Al0.1CoCrFeNi HEA was found to have
homogeneous microstructure with annealing twins. These annealing twins are formed due to low
stalking fault energy of the material. They are capable of restricting the deformation there by
improves the strength of material. Figure 34 shows the optical microscopy image of cold rolled
and heat treated Al0.1CoCrFeNi HEA.
Figure 34: Optical microscopy image of cold rolled and annealed Al0.1CoCrFeNi HEA showing
twins
52
Hardness testing: The average hardness of the cold rolled and annealed Al0.1CoCrFeNi HEA was
found to be 143.2 HV0.3. This improvement in hardness is due to the twins formed formed in the
alloy.
Tensile testing: From the tensile test of cold rolled and annealed Al0.1CoCrFeNi, the yield
strength, ultimate strength and uniform elongation was found to be 212±4 MPa, 570±6 MPa and
50±3% respectively (Table 4) . This improvement in strength was due to the twins formed as a
result of cold working and annealing. Table 4 compares the mechanical properties of the as-cast
and cold worked and annealed Al0.1CoCrFeNi HEA. Figure 35 shows the engineering plastic stress
vs engineering strain plot.
Table 4: Mechanical properties of the as-cast and cold worked+ annealed Al0.1CoCrFeNi HEA.
Condition Yield strength
MPa
Ultimate strength
MPa
Uniform
elongation (%)
As-cast 140 370 65
Cold rolled and annealed 212 570 50
53
Figure 35: Engineering stress-strain curve of Al0.1CoCrFeNi alloy in as-cast and cold
rolled + annealed condition
Nano indentation: The Young’s modulus was measured using the nano indentation machine by
applying 10000 µN load for 10 seconds on the cold rolled and annealed Al0.1CoCrFeNi HEA. The
force and depth were plotted and the Young’s modulus was found from the slope of the curve
during unloading. The average Young’s modulus was found to be 203 GPa. Figure 36 shows the
Load-displacement plots.
54
0 50 100 150 200 250 300 350 400
0
2000
4000
6000
8000
10000
Loa
d (
N)
Displacement (nm)
Figure 36: Load-displacement plot at 10000 µN load
Since young’s modulus (E) is a property of atomic distances at particular temperature,
it is understood that E is not affected much by any thermo-mechanical processing. Thus the
young’s modulus of Al0.1CoCrFeNi is assumed to be 203 GPa in all processing conditions.
Fatigue analysis: The fatigue response of the material was computed by S-N curve. The curve is
obtained by plotting the various stress amplitudes against their corresponding number of cycles of
failure. The Al0.1CoCrFeNi HEA was subjected to six different stress amplitudes and the
corresponding life was calculated. Five mini fatigue samples were prepared from the rolled and
annealed coupon, polished and then mounted between the rocking arm and stationary block. The
stress ratio was maintained as -1 for all cases. The samples were tested at 160 MPa, 200 MPa, 210
MPa, 220 MPa and 250 MPa. The stress amplitude was set by adjusting the rotary cam assembly.
Figure 37 shows the life of the Al0.1CoCrFeNi at each stress amplitude.
55
a) b)
c) d)
e)
Figure 37: Number of cycles at (a) 160 MPa, (b) 200 MPa, (c) 210 MPa, (d) 220 MPa and (e)
250 MPa
56
It is understood from Figure 37 that the sample suvives more than 10 million cycles when
subjected to 160 MPa and 200 MPa were as the life is less than 10 million when subjected to 210
MPa. This means that the sample has an endurance limit more that 200 MPa and less than 210
MPa. The stress dropped when the sample starts. This is due to the crack propogation. The point
at which the curve started dropping is considered as the life of the sample at that particular stress
amplitude. Table 5 shows the number of cycles survived by the sample when the sample is
subjected to different stress amplitude.
Table 5: Number of cycles (N) survived by the sample when subjected to different stress
amplitudes (S)
S-N curve was plotted with the stress amplitudes and corresponding number of cycles.
Figure 38 shows the S-N curve generated for the Al0.1CoCrFeNi HEA. A stright line is drawn
around the plot which can be considered as the S-N curve for the alloy. In case of 160 MPa and
200 MPa, the tests were stopped once the samples survived 107 cycles and they are considered to
have infinite life.
Stress amplitude, S (MPa) Number of cycles, N
160 >10000000
200 >10000000
210 6140932
220 4834967
250 1911764
57
Figure 38: S-N plot of Al0.1CoCrFeNi HEA
From the S-N curve, it is plot that the endurance limit of Al0.1CoCrFeNi HEA is between
200 MPa to 210 MPa. From the graph the appoximate value of endurance limit is 202 MPa.
3.3 Comparison of mechanical properties of HEA with conventional stent materials
The mechanical properties of the HEAs found experimentally are compared with the
properties of all conventional stent materials even though this work targets only on the most
commonly used stainless steel 316L. Table 6 show the mechanical properties such as yield
strength, ultimate strength, Young’s modulus and uniform elongation of the conventional stent
material and CoCrFeNiMn and Al0.1CoCrFeNi HEAs.
58
Table 6: Mechanical properties of conventional stent material and CoCrFeNiMn and
Al0.1CoCrFeNi HEAs [60, 61, 72]
Material
Yield
strength
(MPa)
Ultimate
strength
(MPa)
Young’s
modulus
E (GPa)
Uniform
elongation (%)
Cobalt chromium alloys 241-310 793-860 210-232 20-50
Titanium and Titanium alloys 485-1030 550-1100 110-116 10-15
Nitinol (Super elastic and shape memory) 70-600 1070 41-75 -
Stainless steel 316L 190 490 193 40
CoC
rFeN
iMn
As cast 208 447 189 51
Annealed at 700C- 1 hour 235 490 189 44
Annealed at 900C- 1 hour 200 500 189 51
Annealed at 1000C- 48
hours
216 475 189 50
1000C 4 hours annealed -
cold rolled-1000C 4 hours
annealed
273 590 189 44
Al 0
.1C
oC
rFeN
i As cast 140 370 203 65
60% cold rolled and
1000C 24 hours annealed
212 570 203 50
59
The comparison of mechanical proeprties of HEAs with stainless steel 316L shows that the
properties of HEAs are either close or superior to SS 316L.
3.3.1 Comparison of CoCrFeNiMn with SS 316L
From table 6 it is noted that the mechanical properties of as-cast and heat treated
CoCrFeNiMn HEA is close to that of SS 316L. In case of heat treated-cold rolled and heat treated
condition, the HEA exhibit higher yield and ultimate strength. This is because of the cold work
that has lead to strain hardening. In case of as cast and heat treated conditions the material exhibits
decent yield and ultimate strength inspite of cast defects and pores. In case of a defect-free
CoCrFeNiMn HEA, the alloy may exhibit higher ultimate strength. The HEA when properly cast
under a controlled inert gas environment will prevent pores and cast defects and can show superior
ultimate strength. As fatigue life is directly proportional to the ultimate strength, defect-free
CoCrFeNiMn is expected to possess very high fatigue life. Thus these preliminary results show
that CoCrFeNiMn has a potential to be base metal material for stent implants.
3.3.2 Comparison of Al0.1CoCrFeNi with SS 316L
From table 6 it is noted that the cold rolled and annealed Al0.1CoCrFeNi HEA shows
superior mechanical properties compared to SS 316L. The yield strength that determines the plastic
and elastic behavior is similar to SS 316L but the ultimate strength is higher than that of the
commercial stainless steel stent material. At the same time the fatigue limit of SS 316L for a stress
ratio of -1 is 183.9 MPa [73] where as the fatigue limit of Al0.1CoCrFeNi HEA is approximately
202 MPa. Thus these results proves that the mechanical properties of Al0.1CoCrFeNi HEA is
superior compared to SS 316L.
60
CHAPTER 4
COMPUTATIONAL MODELLING OF STENT IMPLANT PROCEDURE AND
COMPARISON OF MATERIALS
4.1 Introduction:
A stent in artery is subjected to significant mechanical loads due to the repetitive cyclical
pressure loads and displacement of the artery wall. The arteries expands and contracts slightly as
the pressure changes with the heart beat for long-term in-vivo service [68]. The structure of the
stent has to withstand these loads while scaffolding the narrow artery. Many different models and
structures like auxetic cellular model, honeycombs, helical and rhombus structures are studied.
Figure 39 shows the inverted hexagonal honeycomb, V-type and chiral auxetic cellular structures
that are used to construct stent. Figure 40 shows the most commonly used spiral and helical stent
models.
Figure 39: Inverted hexagonal honeycomb (a), V-type (b) and chiral(c) auxetic cellular
structure [69]
61
Figure 40: Spiral stent with un-welded junction of two wires [70] and helical stent [71]
This study is based on Hua Yung’s thesis on study on mechanical performance of stent
implant using theoretical and numerical approach in which she focused on the stability behavior
of generalized structure of stent with rhombus structure (shown in Figure 41, Figure 42) and
derived several property parameters relating to structure design [78]. A rhombus structure was
selected as it is most commonly used and made by some main stream manufacturers.
Figure 41: Scanning electron micrographs show stents of 8-strut (A) and 12-strut (B)
designs after balloon expansion [74]
Figure 42: Stent structure includes arrangement of strut members
62
As the mechanical properties of the two high entropy alloys are determined experimentally,
the one to one comparison of these HEAs with stainless steel 316L was done. To understand the
behavior of these materials in stent implant application, the operation was simulated
compuationally by applying stainless steel 316L and high entropy alloy properties as the material
properties for the model. ANSYS Workbench is used to analyze the stent model.
4.2 Approach
4.2.1 Modelling
By referring the basic stent model with 12 strut design (Figure 41) the stent with 16 strut
and 8 rhombus unit was modelled using SolidWorks. Table 7 shows the dimensions of the stent
that was modelled. Figure 43 shows the CAD model of the stent.
Table 7. Dimensions of the stent model
Parameters Values
Length 15 mm
Outer diameter 3.0 mm
Strut width 0.05 mm
Strut thickness 0.05 mm
Longest diagonal of the rhombus 1.620 mm
Shortest diagonal of the rhombus 1.1775 mm
63
Figure 43: CAD model of 8 rhombus unit stent- 16 struts
4.4.2 Computational simulation
The model is analyzed in ANSYS Workbench. This simulation is considered as a static
structural analysis.
Material models
The stents are modeled as linear elastic-plastic materials with bilinear isotropic hardening.
Since the work is more concerned towards the properties of the stent, the balloon and artery are
considered to be hyper-elastic materials.
The simulation was done by applying mechanical properties of stainless steel 316 L, CoCrFeNiMn
and Al0.1CoCrFeNi HEA by adding their material properties to ANSYS Workbench materials
library. Certain mechanical properties such tangent modulus, density, Poisson’s ratio were
calculated or referred from literatures. The tangent modulus which is the slope of the stress strain
curve above the yield strength was calculated for the HEAs. In case of SS 316L the tangent
modulus was considered as 0.01E [75]. The density which is mass divided by volume was also
calculated by weighing samples and measuring their dimensions. The density and Poisson ratio
64
of SS 316L was found to be 8000 kg/m3 and 0.3 respectively [76]. The Poisson ratio was assumed
to be 0.3 for the HEAs. Table 8 shows the material and their mechanical properties that are added
to the ANSYS Workbench library.
Table 8. Mechanical properties of SS 316L and Al0.1CoCrFeNi and CoCrFeNiMn HEAs added
to ANSYS Workbench library
Material Yield
strength
MPa
Ultimate
strength
MPa
Young’s
modulus
GPa
Tangent
modulus
GPa
Density
kg/m3
Poisson’s
ratio
SS 316L 190 490 193 1.93 8000 0.3
CoCrFeNiMn 216 475 189 0.643 7954 0.3
Al0.1CoCrFeNi 212 570 203 0.843 7950 0.3
The balloon was considered as hyperelastic Mooney-Rivlin 2 parameter model. The
material constants C10, C01 and incompressibility parameter D1 was assigned as 1.06 MPa,
0.710MPa and 0 Pa-1 respectively [77].
Geometry
The cylindrical stent was reduced by suppressing three out of the four quadrants and was
considered as an axisymmetric model. The model was further reduced along the length to reduce
the complexity of the analysis and also to reduce the computational time. The final stent model
was left with 4 rhombus units as shown in the Figure 44.
65
Figure 44: Reduced stent model used for axisymmetric structural analysis
The balloon was modelled along the inner radius of the stent using Workbench Geometry
tool. The balloon with radius 1.45 mm was extruded to 5 mm in such a way that the balloon covers
the entire inner face of the stent. Figure 45 (a) shows the design and dimension of the balloon along
with the stent. The SS 316L, CoCrFeNiMn and Al0.1CoCrFeNi material properties were applied
for the stent and the hyperelastic property was applied to the balloon. A cylindrical coordinate
system was created for the axisymmetric model. Figure 45 (b) shows isometric view of the reduced
stent and balloon model along with the cylindrical coordinate system.
(a) (b)
Figure 45: The reduced stent and balloon model used for axisymmetric structural analysis.
66
Contact parameters
A frictionless contact was defined between the upper surface of the balloon and inner
surface of the stent. Figure 46 shows the contact faces of the stent and balloon.
Figure 46: Frictionless contact surfaces
Meshing
The model is meshed in the ANSYS Workbench Model tool. An element size of 0.04 mm
was assigned for stent body and 0.05 mm for the balloon body in all three cases. Mapped face
mesh was assigned on the top surface of the balloon. Figure 47 shows the meshed stent and artery.
Since the mesh size was maintained constant in all three cases, mesh refinement and
convergence were not done. It was assumed that the mesh convergence was not important for one
to one comparison of HEAs with SS 316L.
67
Figure 47: Meshed stent and artery used for structural analysis
Boundary conditions
The analysis was simulated as a step controlled model as it involves two steps. Step 1:
Inflation of balloon that expands the stent and deforms it plastically and Step 2: Deflation of
balloon leaving the stent in its deformed state. A displacement boundary condition was assigned
to the inner surface of the artery. The balloon was deformed from 0 to 0.75 mm in the first step
and then from 0.75 mm to 0 in the second step. Figure 48 shows the graph explaining the
deformation boundary condition assigned to the balloon. Table 9 shows the displacement boundary
condition applied for the inner surface of the balloon in the cylindrical coordinate system.
Table 9: Displacement boundary in cylindrical coordinate system condition for the balloon
Steps Time [s] X [mm] Y [mm] Z [mm]
1
0 0
0 0
1 0.75
2 2 0 = 0 = 0
68
Figure 48: Graph showing the displacement boundary condition steps
Frictionless supports were applied on the 6 out of 8 faces around the corners of the stents
to arrest the stent to rotate along Y direction and to arrest the displacement along Z direction. These
supports just allows the stent to expand along X direction. Figure 49 shows the faces on which the
frictionless support was applied.
Figure 49: Free faces and faces with frictionless supports used in axisymmetric analysis
69
Solution
The equivalent von-Mises stress, total displacement and directional deformation of a point
along X direction were determined in each cases. The results in form of graphs and graphical
images were generated.
4.3. Results
All three models were analyzed for maximum equivalent von-Mises stress, residual stress,
maximum total deformation and directional displacement before and after recoil.
Stainless steel 316L
Figure 50 shows the maximum equivalent von-Mises stress (at time 1) and residual stress
(at time 2). The stent experiences maximum equivalent von Mises stress of 410 MPa when the
balloon expands completely as shown in Figure 50(a). As the maximum equivalent stress is greater
than the yield strength, the material reaches plasticity and does not regains is original shape when
the balloon deflates. Thus when the balloon deflates, unloading occurs, the stress experienced by
the stent drops and the contact between the stent and balloon breaks. The stent undergoes some
amount recoiling and retains a maximum of 177 MPa in a form of residual stress as shown in
Figure 50 (b). Figure 51 shows the variation of stress during the simulation. The green line shows
the stress experienced by the stent which reaches maximum at time=1 when the balloon fully
expands and the stress drops when the when the balloon deflates.
70
(a) (b)
Figure 50: SS 316L: (a) Maximum equivalent von Mises stress when the balloon is fully
expanded and (b) residual stress retained by the stent after deflation of balloon
Figure 51: SS 316L: Stress variation of stent (green) and balloon (red) with respect to time
71
Figure 53 shows the maximum total deformation and total deformation after recoiling
during inflation and deflation of balloon. When the balloon is fully expanded, the unconstrained
edges of the stent model undergoes a maximum total deformation of 1.156 mm and the minimum
total deformation is 0.745 mm as shown in Figure 52 (a). The total deformation deceases due to
recoiling as the balloon deflates as shown in Figure 52 (b). Figure 53 shows the variation of total
deformation with respect to time. The green line show the total deformation variation which
reaches maximum at time=1 and drops a bit due to recoiling. The red line shows the total
deformation of balloon.
(a) (b)
Figure 52: SS 316L: (a) Maximum total deformation at fully expanded condition and (b) total
deformation due to recoiling when the balloon deflates
72
Figure 53: SS 316L: Total deformation variation of stent (green) and balloon (red) with respect
to time
The directional deformation of a point along X direction in the stent gives a clear idea about
how the stent expands and recoils. Figure 55 shows the maximum displacement (at time=1) and
displacement after recoil along the X direction (at time=2). The stent reaches a maximum
displacement of 0.75325 mm along X direction at fully expanded balloon condition as shown in
Figure 54 (a). When the balloon deflates the stent recoils and reaches 0.72567 mm as shown in
Figure 54 (b). Therefore the stent recoils about 0.028 mm (3.7%) along X direction. Figure 55
graphically explains the displacement of the stent along the X direction.
73
(a) (b)
Figure 54: SS 316L: (a) Maximum displacement along X at fully expanded balloon and (b)
displacement along X after recoiling
Figure 55: SS 316L: Displacement of stent along X direction with respect to time
74
CoCrFeNiMn
Figure 56 shows the maximum equivalent von Mises stress (at time 1) and residual stress
(at time 2). The stent experiences maximum equivalent von Mises stress of 323 MPa when the
balloon expands completely as shown in Figure 56(a). The stent reaches plastic state during
loading. During unloading the stent undergoes some amount recoiling and retains a maximum of
193 MPa in a form of residual stress as shown in Figure 56 (b). Figure 57 shows the variation of
stress during the simulation. The green line shows the stress experienced by the stent which reaches
maximum at time=1 when the balloon fully expands and the stress drops when the when the
balloon deflates.
(a) (b)
Figure 56: CoCrFeNiMn: (a) Maximum equivalent von Mises stress when the balloon is fully
expanded and (b) residual stress retained by the stent after deflation of balloon
75
Figure 57: CoCrFeNiMn: Stress variation of stent (green) and balloon (red) with respect to time
Figure 58 shows the maximum total deformation and total deformation after recoiling when
the balloon innflates and deflates. When the balloon is fully expanded, the unconstrained edges of
the stent model undergoes a maximum total deformation of 1.161 mm and the minimum total
deformation is 0.745 mm as shown in Figure 58(a). The total deformation deceases due to recoiling
as the balloon deflates as shown in Figure 58(b). Figure 59 shows the variation of total deformation
with respect to time. The green line show the total deformation variation and which reaches
maximum at time=1 and drops a bit due to recoiling. The red line shows the total deformation of
balloon.
76
(a) (b)
Figure 58: CoCrFeNiMn: (a) Total maximum deformation at fully expanded condition and (b)
total deformation after the balloon deflates
Figure 59: CoCrFeNiMn: Total deformation variation of stent (green) and balloon (red) with
respect to time
Figure 60 shows the maximum displacement (at time=1) and displacement after recoil
along the X direction (at time=2). The stent reaches a maximum displacement of 0.75369 mm
along X direction at fully expanded balloon condition as shown in Figure 60(a). When the balloon
77
deflates the stent recoils and reaches 0.72927 mm as shown in Figure 60(b). Therefore the stent
recoils about 0.024 mm (3.2%) along X direction. Figure 61 graphically explains the displacement
of the stent along the X direction.
(a) (b)
Figure 60: CoCrFeNiMn: (a) Maximum displacement along X at fully expanded balloon and (b)
displacement along X after recoiling
Figure 61: CoCrFeNiMn: Displacement of stent along X direction with respect to time
78
Al0.1CoCrFeNi
Figure 62 shows the maximum equivalent von-Mises stress (at time 1) and residual stress
(at time 2). The stent experiences maximum equivalent von Mises stress of 345 MPa when the
balloon completely expands as shown in Figure 62(a). The stent reaches plastic state during
loading. During unloading the stent undergoes some amount recoiling and retains a maximum of
191 MPa in a form of residual stress as shown in Figure 62(b). Figure 63 shows the variation of
stress during the simulation. The green line shows the stress experienced by the stent which reaches
maximum at time=1 when the balloon fully expands and the stress drops when the when the
balloon deflates.
(a) (b)
Figure 62: Al0.1CoCrFeNi: (a) Maximum equivalent von Mises stress when the balloon is fully
expanded and (b) residual stress retained by the stent after deflation of balloon
79
Figure 63: Al0.1CoCrFeNi: Stress variation of stent (green) and balloon (red) with respect to time
Figure 64 shows the maximum total deformation and total deformation after recoiling
during inflation and deflation of the balloon. When the balloon is fully expanded, the
unconstrained edges of the stent model undergoes a maximum total deformation of 1.1596 mm
and the minimum total deformation was 0.745 mm as shown in Figure 64 (a). The total deformation
deceases due to recoiling as the balloon deflates as shown in Figure 64 (b). Figure 65 shows the
variation of total deformation with respect to time. The green line show the total deformation
variation and which reaches maximum at time=1 and drops a bit due to recoiling. The red line
shows the total deformation of balloon.
80
(a) (b)
Figure 64: Al0.1CoCrFeNi: (a) Maximum total deformation at fully expanded condition and (b)
total deformation after the balloon deflates
Figure 65: Al0.1CoCrFeNi: Total deformation variation of stent (green) and balloon (red) with
respect to time
The directional deformation of a point along X direction in the stent gives a clear idea about
how the stent expands and recoils. Figure 66 shows the maximum displacement (at time=1) and
displacement after recoil along the X direction (at time=2). The stent reaches a maximum
displacement of 0.75358 mm along X direction at fully expanded balloon condition as shown in
Figure 66 (a). When the balloon deflates the stent recoils and reaches 0.72978 mm as shown in
81
Figure 66 (b). Therefore the stent recoils about 0.0238 mm (3.1%) along X direction. Figure 67
graphically explains the displacement of the stent along the X direction.
(a) (b)
Figure 66: Al0.1CoCrFeNi: (a) Maximum displacement along X at fully expanded balloon and (b)
displacement along X after recoiling
Figure 67: Al0.1CoCrFeNi: Displacement of stent along X direction with respect to time
4.4 Discussion
Table 10 compares the results generated from the computational analysis of all three
materials. From the comparison it was found that the stainless steel experiences maximum stress
82
while loading and it also possess higher recoils percentage. From the results, it is evident that the
Stainless steel 316L is flexible compared to the HEAs but not by a larger margin which means that
HEAs have potential to be used as an alternate material for stainless steel 316L.
Table 10: Comparison of results generated from computational analysis
Material
Maximum von
Mises stress
MPa
Maximum
residual stress
MPa
Maximum total
deformation
mm
Recoil
percentage
%
Stainless steel
316L
410 177 1.156 3.7
CoCrFeNiMn 323 193 1.161 3.2
Al0.1CoCrFeNi 345 191 1.596 3.1
83
CHAPTER 5
CONCLUSION AND FUTURE WORK
5.1 Conclusion
The feasibility of applying CoCrFeNiMn and Al0.1CoCrFeNi high entropy alloys in stent
implants was discussed. The mechanical properties such as yield strength, ultimate strength,
Young’s modulus and hardness of CoCrFeNiMn and Al0.1CoCrFeNi high entropy alloys are
determined experimentally after subjecting it to various thermo-mechanical processing. The
microstructural characteristics were examined using optical microscopy. It was found that the as-
cast CoCrFeNiMn HEA exhibited dendritic microstructure and requires very high temperature and
longer time span to homogenize due to its sluggish diffusion. Annealing, rolling and annealing
technique is used to homogenize the CoCrFeNiMn HEA. Coarse granular microstructure was
noticed in case of as-cast Al0.1CoCrFeNi HEA. Twins are noticed when the HEA was cold rolled
and annealed. The mechanical properties determined experimentally were compared with
commercially available stent materials and it was found that the CoCrFeNiMn and Al0.1CoCrFeNi
HEAs have potential to be materials for stent implant applications. The large amount of pores and
cast defects were noted in CoCrFeNiMn HEA. The yield and ultimate strength of the
CoCrFeMnNi HEA with pores and cast defects were almost close to the yield and ultimate strength
84
of clinical standard, defect free stainless steel 316L. The fatigue analysis for CoCrFeNiMn HEA
was not conducted due to the cast defects and imperfections. It can be understood that the
CoCrFeNiMn HEA will exhibit higher ultimate strength when it is defect-free and it will have a
superior fatigue life. Fatigue analysis on Al0.1CoCrFeNi HEA was done and the endurance limit
was found to be 202 MPa for 107 cycle life. The endurance limit of Al0.1CoCrFeNi HEA was found
to be higher than that of stainless steel 316L (183 MPa). Thus by comparing the mechanical
properties, it is clear that both Al0.1CoCrFeNi and CoCrFeNiMn HEA when used as stent base
material will possibly exhibit increased in fatigue life and decrease the failure rates of stent
implants. From these experimental analysis it is clear that the high entropy alloys have potential
to find application as implant materials.
The computational FEM analysis was done using ANSYS Workbench. A stent with
rhombus structural unit and a hyperelastic balloon was modelled for the analysis. The mechanical
properties of stainless steel 316L and experimentally determined mechanical properties of the high
entropy alloys were assigned to the stent model. Static structural analysis was carried out in two
steps to simulate the inflation and deflation of balloon. The equivalent von Mises stress, total
deformation and recoiling characteristics of the HEAs and stainless steel 316L were determined
and compared.
From the computational FEM analysis it was found that the mechanical behavior of the
HEAs are same as stainless steel 316 L when subjected to similar loading.
Thus from the experimental and computational analysis along with the comparison of
HEAs and stainless steel 316L, it was found that the HEAs has great potential to be used as stent
materials. Though some mechanical properties were closely matching, the endurance limit of
85
HEAs are found to be superior to stainless steel. This infers that the fatigue life of the stent will
improve considerably when high entropy alloy are used as base material.
5.2 Future works
In future, we will concentrate more on computational fatigue analyze of stent when
subjected cyclic loads. The work will also address the biocompatibility of the high entropy alloy.
Various thermo-mechanical processing techniques will be tried to remove pores and cast defects
in CoCrFeNiMn HEA.
5.2.1 FEM Simulation of stent with artery load
The stent when implanted in artery undergoes cyclic load due to the systolic and diastolic
pressure caused during heartbeat. In future, stent model will be created with balloon and artery as
shown in Figure 68. Structural and fatigue analysis will be conducted by considering these the
balloon expansion and cyclic pressure/load acting on the stent due to heart beat. The analysis will
be conducted on all three material and the results will be compared to understand the life of stent
in each cases.
Figure 68: Model of stent with balloon and artery
86
5.2.2 Fatigue analysis of CoCrFeNiMn
The fatigue analysis will be conducted on CoCrFeNiMn HEA after removing all the pores
and cast defects in the alloy. Hot forging or friction stir processing will be considered to remove
the defects and imperfections. The fatigue analysis results will be compared with the other alloys.
5.2.3 Analysis of corrosion properties and biocompatibility of HEA
The corrosion properties of the CoCrFeNiMn and Al0.1CoCrFeNi high entropy alloys will
be analyzed in a simulated body fluid to understand the corrosion resistance of the HEA. Wear
resistance will also be evaluated using pin on disk type friction wear test system.
87
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