grain boundary analysis of ht9 steelcreep curve can be divided into three stages: primary creep,...
Post on 10-Mar-2020
2 Views
Preview:
TRANSCRIPT
GRAIN BOUNDARY ANALYSIS OF HT9 STEEL
UNDER CREEP TEST
By
ZHE LENG
A thesis submitted in partial fulfillment of
the requirements for the degree of
Master of Material Science and Engineering
WASHINGTON STATE UNIVERSITY
School of Mechanical and Materials Engineering
AUGUST 2010
II
To the Faculty of Washington State University:
The members of the Committee appointed to examine the thesis of ZHE
LENG find it satisfactory and recommend that it be accepted.
_____________________________
David P Field, Ph.D., Chair
_____________________________
Hussein M. Zbib, Ph.D.
_____________________________
Jow-Lian Ding, Ph.D.
III
Acknowledgements
First and foremost, I would like to express my deepest gratitude to my supervisor, Dr.
David Field, a respectable, responsible and resourceful professor, who has provided
me with valuable guidance in every stage of the writing of this thesis. Without his
enlightening instruction, impressive kindness and patience, I could not have
completed my thesis. His keen and vigorous academic observation enlightens me not
only in this thesis but also in my future study.
Secondly, I would like to give my thanks to Dr. Ding, for the access to the testing
frame.
Finally, my thanks go to Dr. Alankar and my colleagues at WSU for their valuable
suggestions.
IV
GRAIN BOUNDARY ANALYSIS OF HT9 STEEL
UNDER CREEP TEST
Abstract
By Zhe Leng, M. S.
Washington State University
August 2010
Chair: David P. Field
Ferritic/martensitic steels are attractive materials for use as components in nuclear
reactors because of their high strength and good swelling resistance. Grain boundary
specific phenomena (such as segregation, voiding, cracking, etc) are prevalent in these
materials so grain boundary character is of primary importance. Certain types of
boundaries are more susceptible to thermal creep damage whereas others tend to resist
damage. If more damage resistant boundaries can be introduced into the structures,
this will result in steel that is more resistant to the processes of degradation that
prevail in high-temperature environments. In this study, the grain boundary structure
in HT9 steel was characterized by electron backscatter diffraction to identify
boundaries that are resistant or susceptible to damage in extreme environments. It is
found that intergranular damage is mitigated by a high fraction of low angle
boundaries, and certain kinds of grain boundaries, such as the 3 boundary, are more
favored by intergranular cracks.
V
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ......................................................................................... III
ABSTRACT ................................................................................................................. IV
LIST OF FIGURES ................................................................................................... VII
CHAPTER
1. INTRODUCTION ........................................................................................... 1
1.1 General ................................................................................................... 1
1.2 Objective ................................................................................................ 3
2. BACKGROUND AND LITERATURE REVIEW .......................................... 4
2.1 Review of creep phenomenon ................................................................ 4
2.2 Creep mechanism ................................................................................... 6
2.3 Grain boundary analysis ........................................................................ 8
2.4 EBSD observation ................................................................................ 11
2.4.1 Crystal orientation map ............................................................. 11
2.4.2 Image quality (IQ) map ............................................................. 12
2.4.3 Kernel average misorientation map .......................................... 13
3. MATERIAL AND EXPERIMENT PROCEDURE ..................................... 15
3.1 Material and microstructure ............................................................... 15
3.2 Creep test ........................................................................................... 16
4. EXPERIMENT RESULTS .......................................................................... 19
4.1 Microstructures of material before creep testing ............................... 19
VI
4.2 Microstructures of materials
after short term creep testing............................................................ 22
4.3 Microstructures of materials
after long term creep testing ............................................................ 30
4.4 The distribution of grain boundary planes
for ∑3 boundary ............................................................................... 36
5. DISCUSSION ............................................................................................ 39
6. SUMMARY AND CONCLUSION ........................................................... 43
7. FUTURE WORK ....................................................................................... 44
8. REFERENCE ............................................................................................. 45
VII
LIST OF FIGURES
Fig.1 The illustration of Ashby map ..................................................................... 2
Fig.2 Wedge crack formed at the triple junction in association with grain
boundary sliding......................................................................................... 3
Fig.3 Schematic creep curves for constant tensile load ........................................ 5
Fig.4 Effect of Applied Stress on a Creep Curve at Constant Temperature .......... 6
Fig.5 Schematic illustration of dislocation creep involving both climb and glide
of dislocations ............................................................................................ 7
Fig.6 Illustration of diffusion creep ...................................................................... 8
Fig.7 Illustration of different oriented grain boundary plane segments ................ 9
Fig.8 Schematic illustration of the principle of five parameter stereology
method ...................................................................................................... 10
Fig.9 Schematic illustrations showing the location of twist and tilt boundaries
for ∑3 boundaries .................................................................................. 11
Fig.10 Electron backscatter diffraction pattern from Si ........................................ 12
Fig.11 The orientation map of HT9 steel after heat treatment .............................. 12
Fig.12 The image quality map of HT9 steel after heat treatment ......................... 13
Fig.13 Selection of pixels for calculating KAM ................................................... 14
Fig.14 Dimension of tensile bars used for the creep test ...................................... 15
Fig.15 Various structures composing the microstructure of 9 to 12 pct Cr
Steels ........................................................................................................ 16
Fig.16 The creep test frame .................................................................................. 17
VIII
Fig.17 Empirical relationship of creep life as a function temperature and stress . 18
Fig.18 EBSD orientation map of HT9 steel .......................................................... 20
Fig.19 The grain size distribution of HT9 steel .................................................... 21
Fig.20 The misorientation distribution map for HT9 steel ................................... 22
Fig.21 SEM images of HT9 steel fracture surface ................................................ 23
Fig.22 SEM micrograph of Crack on the surface of HT9 steel ............................ 24
Fig.23 The orientation map and the image quality map of HT9 steel after creep
test ............................................................................................................ 24
Fig.24 The misorientation distribution map for HT9 steel after short term creep
test ............................................................................................................ 25
Fig.25 Image quality map and orientation map of HT9 steel after short term creep
testing ....................................................................................................... 26
Fig.26 The misorientation angle distribution of damaged boundaries ................. 27
Fig.27 Misorientation distribution function associated with the cracked grain
boundaries ................................................................................................ 28
Fig.28 Various pole figures of the crystallites along the crack ............................. 29
Fig.29 Kernel average misorientation map of HT9 .............................................. 29
Fig.30 The misorientation distribution map for HT9 steel after long term creep
test ............................................................................................................ 30
Fig.31 SEM images of the high density void area of HT9 steel after long term
creep test .................................................................................................. 31
Fig.32 SEM images and orientation map of the Cracks on the surface of HT9
IX
steel after long term creep test ................................................................. 31
Fig.33 The misorientation angle distribution of damaged boundaries along the
crack ......................................................................................................... 33
Fig.34 Axis/angle misorientation distribution plot for the crack .......................... 33
Fig.35 Various pole figures of the crystallites along the crack ............................. 34
Fig.36 The misorientation angle distribution of boundaries beside the voids ...... 35
Fig.37 Axis/angle misorientation plot of the voids in the high voids density
area ........................................................................................................... 35
Fig.38 Pole figures of the crystallites besides the void ......................................... 36
Fig.39 Distribution of boundary plane normals in HT9 steel after heat
treatment .................................................................................................. 37
Fig.40 Distribution of boundary plane normals in HT9 steel after short term creep
test ............................................................................................................ 37
Fig.41 Distribution of boundary plane normals in HT9 steel after long term creep
test ............................................................................................................ 38
Fig.42 Axis/angle misorientation plot of HT9 steel after the long term
creep ......................................................................................................... 40
Fig.43 The relation between the dimension of the function and the number of data
that give a reliable result .......................................................................... 42
1
1. INTRODUCTION
1.1 General.
The expected increasing demand for energy in the twenty-first century has boosted the
world-wide cooperation to consider ways to meet energy needs while maintaining and
improving the environment. With the supplies of fossil fuel fast running out, the
search is on for a plentiful and reliable replacement. No matter what we decide to use,
wind power, solar energy, the tide and waves, it seems that nuclear power will play a
major role. Indeed, it is already an important economical source of electricity too, but
it also poses many difficult problems. Principal amongst these are the problems of
developing materials sufficiently resistant to the extreme environments produced by
the reactor core, such as high temperatures, large stresses, chemically reactive
environments and so on.
For structural applications in reactors, high-chromium(9-12 wt%) ferritic/martensitic
steels have several advantages such as good creep strength and excellent swelling
resistance in comparison with austenitic stainless steels [1,2].
During their service life, one of the major damage types is from thermal creep, a
continuing, time-dependent plastic deformation of materials subjected to constant
stress at elevated temperature [3]. Results from experimental and theoretical study
show that the creep of crystalline solids is the result of thermal activated migration of
dislocation, grain-boundary diffusion, and the diffusion of vacancies, the mechanism
2
that contribute to the creep includes: dislocation glide, dislocation creep, diffusion
creep and grain boundary sliding [4]. The dominant creep mechanism is highly
dependent on the applied stress and temperature, as is shown in the deformation
mechanism map below [5]:
Fig.1 Schematic illustration of a deformation mechanism map [5]
Among these mechanisms, two are grain boundary related: grain boundary sliding and
grain boundary diffusion creep (coble creep), thus the grain boundary character plays
an important role in the creep damage. Actually, it is already observed that creep
cavities frequently nucleate on grain boundaries [6,7,8,9], and grain boundary sliding
is suggested as the driving force for creep void nucleation as is shown in Fig.2. Voids
nucleate at stress concentration areas such as triple points or second phase boundaries,
and the growth of these voids result from the grain boundary diffusion. Finally, the
grain boundary cavities coalesced to form intergranular cracks [10,11,12]. Grain
boundaries with certain crystallographic relationships are more favored by the cracks
3
and voids. In most cases the ∑3 boundary is more resistant to crack formation than
random boundaries [13-15, 16,17], however, in the case of void formation it did not
has such a great effect[18].
Fig.2 Wedge crack formed at the triple junction in association with grain boundary
sliding[7].
As a typical ferritic/martensitic stainless steel, HT9 steel has been in use in nuclear
reactors as pressure vessels or pressure tubes for many years [19]. Although the creep
properties of HT9 steel have already been studied [20-24], none of the literature
concerns creep damage with respect to grain boundaries of HT9 steel. It is therefore
necessary to inspect the damaged grain boundaries and determine grain boundary
character for boundaries which are susceptible to damage and those which are more
resistant. If more damage resistant boundaries are induced in HT9 steel by grain
boundary engineering, the creep resistant ability will surely be improved, thus HT9 is
still one of the best options for structure material.
4
1.2 Objective.
The main objective of this study was to determine the high temperature creep strength
and the resistance to environmental attack of certain boundary types by characterizing
the grain boundaries that are damaged under the creep test and by identifying the
boundaries that are susceptible to damage. For this purpose, samples which had been
crept at elevated temperatures are examined and quantitatively characterized for
damage and grain boundary structure.
5
2. BACKGROUND AND LITERATURE REVIEW
2.1 Review of creep phenomenon.
At high temperatures, stresses well below the material’s yield strength can result in
permanent deformation over a period of time. This time dependent progressive
deformation of the material at constant stress is called creep. The engineering creep
curve of a material can be obtained by applying a constant load to the sample at
constant temperature, and the resulting strain is determined as a function of time. A
creep curve can be divided into three stages: primary creep, secondary creep and
tertiary creep. A schematic of this type of curve is shown in Fig.3.
Fig.3 Schematic creep curves for constant tensile load [25].
Primary creep is the period with a decreasing creep rate. Dislocations are being
generated continuously in this stage and with increasing time, more and more
dislocations are present. The dislocations produce increasing interference with each
6
other’s movement, thus causing the creep rate to decrease. The secondary creep is a
period of constant creep rate. In this stage, a situation arises where the number of
dislocations being generated is exactly equal to the number of dislocations being
annealed out. This dynamic equilibrium results in the metal to creep at a constant rate,
thus this stage is also called steady-state creep. During the tertiary creep stage, the
creep rate increases and the specimen fails due to localized necking of the specimen.
This includes void and micro crack formation at high diffusivity regions such as grain
boundaries.
Fig.4 shows the effect of applied stress on the creep curve at constant temperature, it
can be observed that only for certain combination of stress and temperature, the three
creep curve with well-defined stages can be identified. The higher the stress, the
greater the observed creep rate.
Fig.4 Effect of Applied Stress on a Creep Curve at Constant Temperature [26]
2.2 Creep mechanism
As is already stated, the creep is contributed to by several mechanisms, and the
7
dominant mechanism is highly dependent on the applied stress and temperature.
For dislocation creep, it refers to deformation controlled by dislocation slip in the
grain lattice as shown in Fig.5. The dislocation motion process involves both glide on
slip planes and climb over obstacles. At low stresses, there is a linear dependence of
strain rate on the applied stresses, it is known as Harper-Dorn creep, and this is
governed by climb-controlled dislocation motion. At intermediate stresses, power-law
creep is observed and the creep can be controlled by both glide and climb mechanism.
For solid-solutions, creep is controlled by glide step and for pure metals, creep is
controlled by climb. At high stresses, the creep process is a glide controlled instead of
climb-controlled, and a simple power law is not enough to predict the creep rate.
Fig.5 Schematic illustration of dislocation creep involving both climb and glide of
dislocations[27].
Diffusion creep mechanism is significant at low stress and high temperature, and it is
grain size dependent. Under the driving force of applied stress shown in Fig.6, atoms
8
diffuse from the sides of the grains to the tops and bottoms. The grain becomes longer
as the result of applied stress, and the process will be faster at high temperature as
there are more vacancies. For diffusion paths through the grains, the atoms have a
slower jump frequency, but more paths, it is called Nabarro-Herring creep, and for the
diffusion paths along the grain boundaries, the jump frequency is higher, but fewer
paths exist, it is called Coble creep.
Fig.6 Illustration of diffusion creep[27]
Grain boundary sliding occurs during creep because grain boundaries are imperfectly
bonded and thus weaker than the ordered crystalline structure of grains. Grain
boundary sliding is prevalent at lower stresses and it helps to initiate intergranular
fracture by facilitating cracks and voids on the grain boundaries.
2.3. Grain boundary analysis
A grain boundary is the interface between two grains in a polycrystalline material, and
9
it is one of the major factors that determine the physical, mechanical, electrical, and
chemical properties of polycrystalline materials.
Normally, the macroscopic description of grain boundary geometry is derived through
mathematical means using crystal orientations determined from crystallographic
diffraction patterns. Five independent parameters are necessary to describe or
characterize a grain boundary, the misorientation between the two adjacent lattices is
described by three parameters (two parameters describe the rotation axis and one
parameter describes the rotation angle about the axis), and the orientation of the grain
boundary plane normal is described by the remaining two. Since a grain boundary
with a fixed misorientation usually consists of many differently oriented boundary
plane segments, as shown in Fig.7, it can be expressed as the grain boundary plane
distribution at a grain boundary with specific misorientation. This is done by
partitioning the boundary into small sections, each with a given plane normal
orientation (as shown in Fig.7).
Fig.7 Illustration of different oriented grain boundary plane segments[28].
The stereological procedure to extract boundary plane data from a single section is
10
summarized as follows: electron backscatter diffraction (EBSD) mapping is used to
obtain both grain orientations and to reconstruct the boundary segment traces. This
provides four out of the five independent parameters to describe the grain boundary.
Only the boundary inclination angle is unknown, however, since the boundary normal
must be perpendicular to the boundary trace on the single traction, thus these normals
can be represented as a great circle on the stereographic projection, as shown in Fig.8.
If a sufficiently large number of observations are made, the true boundary plane
normal will accumulate more than the false planes, and form a peak in the distribution.
The background which contains false planes is finally removed based on the
assumption of random sampling [28,29].
Fig.8 Schematic illustration of the principle of the five parameter stereology method
[28,29].
From the distribution of grain boundary plane plot, it is easy to identify the boundary
type: twist or tilt. Twist boundaries have their boundary normal parallel to the
11
misorientation axis while tilt boundaries are 90 degrees from the misorientation axis,
The triangle mark and the dark line in Fig.9 represent the twist ∑3 boundaries and
tile ∑3 boundaries, respectively[30].
Fig.9 Schematic illustrations showing the location of twist and tilt boundaries for ∑3
boundaries [30].
2.4 EBSD observation
2.4.1 Crystal orientation map
Electron backscatter diffraction (EBSD) is a technique used to examine the
crystallographic orientation of many materials. It is used in various fields of materials
science of crystalline or polycrystalline materials such as recrystallization, grain
boundary analysis and anything for which the orientation of a crystallite, or the
distribution of crystallite lattice orientations, must be known.
An electron backscatter diffraction pattern is formed when many different planes
diffract different electrons to form Kikuchi bands as seen in Fig.10. Each of these
bands corresponds to a different diffracting plane in the lattice. The bands present in
12
the pattern are related to the underlying crystal phase and orientation of the lattice,
such as Fig.11. For the single phase material studied, grain boundaries were defined
by an orientation difference exceeding 5 degrees.
Fig.10 Electron backscatter diffraction pattern from Si[31].
Fig.11 The orientation map of HT9 steel after heat treatment
13
2.4.2 Image quality map
Image quality (IQ) describes the quality of an electron backscatter diffraction pattern
and it dependence on the material and its condition. For the grain boundaries, when
the electron beam is situated near a grain boundary, the diffraction volume may
contain both crystal lattices separated by the boundary. Thus, the diffraction pattern
will be composed of a mix of both patterns, leading to a lower quality pattern which is
a dark line in the IQ map. The dark dots in Fig.12 represent the voids in the sample.
Fig.12 The image quality map of HT9 steel after heat treatment
2.4.3 Kernel average misorientation map
Kernel average misorientation (KAM) is the average orientation differences between
the neighboring pixels of a measurement target pixel within a grain. From the KAM
14
maps, the information of local deformation in each crystal will be obtained. Fig.13
contains a schematic illustration of how KAM is determined [32]:
Fig.13 Selection of pixels for calculating KAM[32].
15
3. EXPERIMENTAL PROCEDURES
3.1 Material and microstructure
The material used in the present investigation is HT9 steel, a nominally
Fe–12Cr–1Mo–0.5W–0.5Ni–0.25V–0.2C steel alloy. The chemical composition of
HT9 steel is shown in Table 1. Samples were machined in the form of tensile bars
with a gage length of 24 mm and a cross section of 6 mm x 4 mm as shown in Fig.14.
The samples were austenitized at 1050C for 1 hour to dissolve the precipitates and
carbides in the austenite phase, and air cooled after austenitizing. The resulting
martensitic structure was tempered at 750C for 2 hours in order to achieve high
ductility and toughness. This condition is typical for structural components used in the
reactor such as those discussed in Chapter 1.
Table 1. Nominal chemical composition of HT9 steel( wt-%)
Alloy C S Mn Cr Mo W V others
HT9 0.20 0.4 0.60 12.0 1.0 0.50 0.25 0.50
Fig. 14 - Dimension (mm’s) of tensile bars used for the creep test. Specimen thickness
is 4 mm.
16
HT9 steel is a low carbon steel and has a ferritic-martensitic structure, which consists
of tempered martensite lath forming subgrains in a ferrite matrix. The multi-grain
structure of HT9 steel is illustrated in Fig.15. The high temperature strength results
from the subgrain structure produced by the martensitic transformation and the
precipitation of carbides and carbo-nitrides [33].
Fig.15 - Various structures composing the microstructure of 9 to 12 pct Cr steels [34]
3.2 Creep test
In the experiments performed in this investigation, creep tests were conducted using a
creep testing frame as shown in Fig.16. The temperature was set to 600C and the
stress required for failure after a given length of time was determined according to the
empirical formula proposed by Amodeo [35], as is shown below:
17
Fig.17 is the corresponding stress and creep life chart. From this figure, it can be seen
that there is a linear relationship between the log scale of ruptured stress and the creep
life.
Fig.16 – The creep test frame
Sample
Furnace
Weight
18
Fig.17 Empirical relationship of creep life as a function temperature and stress
For creep tests with failure times of about 100 hours and 1000 hours, the
corresponding stresses calculated by the formula are 250 MPa and 160 MPa. The first
sample broke after 87 hours and the second sample did not break after 1000 hours
testing. The tested sample was air cooled because it was unrealistic to remove the
specimen from the grips when they were hot. The specimens were then sectioned and
polished down to 0.3 µm manually, followed by a vibratory polish with 0.05 µm
colloidal silica solution.
19
4. RESULTS
4.1 Microstructures of material before creep testing.
Fig.18 shows the orientation maps of HT9 steel at different states: before heat
treatment and after heat treatment. Fig.19 contains the corresponding grain size
distributions before and after heat treatment. As can be seen, after the heat treatment
there is an obvious increase in grain size from 1.4 to 5.7 microns, and the packet
structures are evident that are described in Ch. 3 and shown schematically in Fig.15.
(a)
20
(b)
Fig.18 EBSD orientation map of HT9 steel a) before heat treatment, b) after heat
treatment
(a)
21
(b)
Fig.19 The grain size distribution of HT9 steel a) before heat treatment and b) after
heat treatment
Fig.20 shows the misorientation angle histogram of HT9 steel before and after heat
treatment. From the plots, it can be seen that the low angle boundaries
(misorientation<10 degrees) dominate in these two states, and there is a small increase
in the fraction of low angle boundaries after heat treatment.
22
(a)
(b)
Fig.20 - The misorientation distribution map for HT9 steel a) before heat treatment, b)
after heat treatment, the fractions of low angle boundaries are a) 0.51, b) 0.63
4.2 Microstructures of materials after short term creep testing.
Fig.21 shows secondary electron images of the fracture surface of HT9 steel. There is
23
a high density of voids in the fracture surface and the growth of these voids and the
necking decrease the load bearing cross section, and finally result in failure. The
dimpled microstructure indicates void coalescence of HT9 steel at high temperature
and a rather ductile failure that is evidence for having failed in the regime of
power-law creep.
Fig.22 shows secondary electron images of a crack seen on the polished surface of the
HT9 steel specimen. The orientation map and image quality map in Fig.23 show
another crack occurs at a grain boundary. The black line in Fig.23 corresponds to the
crack on the surface. In addition, it is obvious that the subgrains have elongated in the
loading direction.
Fig.21 - SEM images of HT9 steel fracture surface (micron bars are 100 µm and
20 µm)
24
Fig.22 - SEM micrograph of Crack on the surface of HT9 steel
(a) (b)
Fig.23 a) The orientation map and b) the image quality map of HT9 steel after creep
test
Crack
25
The misorientation distribution map for HT9 steel after short term creep test is shown
in Fig.24. This histogram indicates that the fraction of low angle boundaries decreases
greatly after the creep test (as compared to before testing), but the low angle
boundaries still dominate the distribution.
Fig.24 - The misorientation distribution map for HT9 steel after short term creep test.
The fraction of low angle boundaries (<10°) is 0.47.
In Fig. 25, two cracks were observed in the image. Misorientation measurements were
conducted along the cracks in order to determine which kind of grain boundaries are
damaged in the structure. Such measurements were performed on all observed cracks
from each polished section.
26
(a) (b)
Fig.25 a) Image quality map and b) orientation map of HT9 steel after short term
creep testing
Fig. 26 shows the misorientation angle distribution along these two cracks. As can be
seen, most of the damaged boundaries are high angle boundaries, and the fraction of
damaged low angle boundaries is 0.12. Compared with the high fraction of low angle
boundaries before and after creep testing, the large number of high angle boundaries
that are damaged indicates that the crack initiation and propagation is not a random
process. It prefers the high angle boundaries, whereas the low angle boundaries are
more resistant to thermal creep damage [36-37].
Crack
Crack
27
Fig.26 The misorientation angle distribution of damaged boundaries.
The misorientation distribution function of misorientations associated with the
cracked grain boundaries is plotted in Fig.27. This figure shows the distribution in the
axis-angle space since the misorientation can be expressed by the rotation required to
bring two neighboring crystals into coincidence and the crystal axis about which to
rotate. Thus in the following plots, as the misorientation increases, the crack prefers
<111> as the misorientation axis. This is different from the FCC structures, according
to the literature [15,18], wherein some coincidence site lattice boundaries (such as ∑
3) are more resistant to creep damage regardless of the high misorientation angle. In
the present case (BCC structure), the crack tends to prefer boundaries with a
misorientation of 60 degrees about the <111> axis, which are classified as the
well-known ∑3 boundaries.
28
Fig.27 - Misorientation distribution function associated with the cracked grain
boundaries.
The corresponding pole figures of the crystals along the crack are shown in Fig. 28. In
these plots, it is apparent that cracks are likely to form near grains of certain
orientations. However, additional geometrical information, such as the grain boundary
normal vector [28], is needed to enable conclusions about the crack preference. If one
assumes that the grain boundary normal lies in the plane of the sample surface, the
intergranular crack is likely to grow along the grain boundaries which are parallel to
the <100> axes. It is unlikely that cracks will form for the grain boundary normal
directions parallel to the <101> axis.
Fig. 29 contains the kernel average misorientation map of HT9 steel after heat
treatment and after the creep testing. The creep tested material has a higher fraction of
high angle misorientations, and most of the high angle KAM positions are located at
the cracks or at the grain boundaries, which indicate the highly deformed region there.
Cracks tend to form along regions of high deformation, particularly if the neighboring
material is less deformed.
29
Fig. 28 – Various pole figures of the crystallites along the crack.
Fig.29 - Kernel average misorientation map of HT9 a)before short term creep test,
and b) after short term creep test (color scales are in degrees).
30
4.3 Microstructures of materials after long term creep testing.
From the misorientation distribution map for HT9 steel after long term creep in Fig.
30, it can be seen that the low angle boundaries still dominate a large proportion.
Fig.30 - The misorientation distribution map for HT9 steel after long term creep test.
The fractions of low angle boundaries are 0.6.
Although the steel that was tested at lower stress for longer times did not fail, some
high void density areas and cracks could still be seen on the polished surface, as
shown in Fig.31 and Fig.32.
31
Fig.31 - SEM images of the high density void area of HT9 steel after long term creep
test
Fig.32 - SEM images and orientation map of the Cracks on the surface of HT9 steel
after long term creep test
32
The same methods as those followed for analyzing the materials that failed during
short term creep tests were again followed for analysis of the long term creep
specimens.
The misorientation angle histogram and the axis/angle misorientation distribution
plots of the crack are shown in Fig.33 and Fig.34. These figures indicate that the crack
still prefers the high angle boundaries, and the crack seldom occurs at the grain
boundaries with misorientation angles less than 50 degrees. For the failed grain
boundaries with misorientations greater than 50 degrees, the axis <111> seems to be
more preferred by the crack than other plane orientations, this is similar to the results
observed for the short term creep experiments.
The pole figures of the crystallites along the crack are different between the short term
and long term creep tests. For example, in Fig. 35, making the same assumption that
the grain boundary normal lies in the plane of the sample surface, then the
intergranular crack is likely to grow along the grain boundaries which are parallel to
the <101> and <111> axes in the long term creep (in contrast to the observations of
cracks along the apparent {001} type GB planes seen for the short term creep tests).
33
Fig.33 The misorientation angle distribution of damaged boundaries along the crack
Fig.34 Axis/angle misorientation distribution plot for the crack
34
Fig. 35 – Various pole figures of the crystallites along the crack.
The analysis of grain boundaries that contained voids was carried out on the high
density voided area of each polished section. In this case, the high angle boundary is
still the place easy for the void formation as is shown in Fig. 36, which is the same as
the crack formed in the short term and long term creep tests. However, in the
axis/angle misorientation distribution plots shown in Fig. 37, it can be seen that for
the grain boundaries with misorientation between 25 degrees and 45 degrees, the
formation of the voids is a random process, which means that the probability of void
formation is independent of misorientation axis. For voids forming at the grain
boundaries with misorientations larger than 50 degrees, the <111> axis preference still
exists.
35
Fig.36 The misorientation angle distribution of boundaries beside the voids
Fig.37 Axis/angle misorientation plot of the voids in the high voids density area
Fig. 38 shows the pole figures of the crystallites beside the voids. There is a weak
preference of the voids to specific crystallite orientations, and by comparing all the
pole figures above, it could be find that the pole figure for the crystallites in different
cases are all different, since the grain boundary normal is still unknown, additional
36
information is needed in order to give a more reliable conclusion.
Fig. 38 –Pole figures of the crystallites besides the void
4.4 The distribution of grain boundary planes for ∑3 boundary
From the result above, it seems that the ∑3 boundaries are preferentially susceptible
to the damage, so it is instructive to analyze the types of ∑3 boundaries that are
present in the distribution. Fig.39, Fig.40 and Fig.41 are the distribution of grain
boundary plane normals for ∑3 boundary of HT9 steel at different state: after heat
treatment, after short term creep test and after long term creep test.
37
Fig.39 Distribution of boundary plane normals in HT9 steel after heat treatment
Fig. 40 Distribution of boundary plane normals in HT9 steel after short term creep test
38
Fig.41 Distribution of boundary plane normals in HT9 steel after long term creep test
From those figures, it can be concludes that ∑3 tilt boundary is favored after heat
treating, with only a small portion of ∑3 boundary having twist boundary character,
and after the short term creep test, the ∑3 tilt boundary is still favored after the test,
but the portion of twist boundary increases, after the long term creep test, the ∑3 tilt
boundary still dominates, but it is more dispersed than it was before creep testing.
39
5. DISCUSSION
The result above confirms the fact that creep resulted in plastic deformation and creep
damage in the metal during heating. The cavities in Fig.21 and Fig.31 revealed
evidence of creep damage, and the coalescence of the cavities form intergranular
cracks, thus the grain boundary plays a major role in the failure process.
Various papers [38,39] show that low angle boundaries, which have misorientation
angles of less than 10 degrees, display improved creep strength relative to general or
high angle boundaries. This can be also observed from the present results, since most
of the creep damage occurs at the high angle boundaries, as is shown in Fig.26, Fig.33
and Fig.36. This is because of the low diffusivity and the greater resistance of grain
boundary sliding of those low angle boundaries, making it hard for the voids to form
and grow at these low angle boundaries. Thus these kinds of boundaries are more
resistant to creep damage.
In Refs.[39,40,41,42,43,44], all ∑3 boundaries were found to be crack resistant,
because the increased structure order results in smaller diffusivity and greater
resistance to grain boundary sliding compared with that of low angle boundaries. This
is not the case in every situation and sometimes it is not a factor that affects void
formation at all. In the long term creep tests, according to the axis/angle
misorientation plot of HT9 steel after the long term creep shown in Fig.42, it could be
seen that most of the grain boundaries after long term creep seems to have rotation
axis<111> preference. If the crack occurs at the high angle boundaries randomly, the
40
axis/angle misorientation plot of the crack after the creep should be similar to the
axis/angle misorientation plot of HT9 steel after the long term creep. However,
according to Fig.34, the rotation axis of most of the damaged boundaries are not
<111> exactly. There is a deviation of these damaged boundaries from the ideal ∑3
boundaries, thus, they are still high angle boundaries which are likely to crack. To
some extent, the rotation axes <111> for those high angle boundaries are helpful to
alleviate the probability of intergranular crack. But this kind of effect did not happen
for the short term creep, since most of the damaged boundaries have misorientation
with rotation axes<111>, this may be because of the high stress, which is the only
different factor between these two sets of experiments.
Fig.42 Axis/angle misorientation plot of HT9 steel after the long term creep
Grain boundary phenomena are not only dependent on misorientation between the
grains adjacent at the grain boundary, but also depend on the grain boundary plane
orientation. The damaged ∑3 boundary type is also a factor that should be
considered, in Fig.39, Fig.40 and Fig.41, tilt ∑3 boundaries are dominant, but they
41
are dispersed after creep, so the ∑3 tilt boundaries are deformed during the creep,
they are not so stable under such a high stress.
Figure 37 shows that creep voids which form during the long term creep have no
misorientation axis preference for the voids forming at the grain boundaries with
misorientation between 25 to 45 degrees. There is a higher probability for void
formation at grain boundaries with a higher misorientation, likely because the high
angle boundaries have a higher diffusivity which is beneficial for void growth. The
rotation axis preference of the voids may come from the fact that most of the grain
boundaries in the tested sample have <111> axis preference when the misorientation
is greater than 45 degrees. Since the exact ∑3 boundaries alleviate the probability of
creep damage the boundaries with misorientation greater than 45 degrees and rotation
axes near <111> have a higher probability for void formation. The coalescence of
those voids result in micro-cracks. This is also the reason why the misorientation
distribution of micro-cracks shown in Fig.34, have the same rotation axis preference
as the misorientation distribution of voids in Fig.37.
In this study, approximate 35,000 data points were used to characterize the boundary
structure. This includes the observation of hundreds of grain boundaries and a
relatively small number of cracked boundaries. This calls into question the statistical
reliability of the observation. According to H.J. Bunge [45], more data are needed to
obtain a reliable result. Since 5 parameters are necessary to characterize the grain
boundary, almost 105 data points are needed to give a statistically reliable result. The
result obtained at this point was from observation of a relatively low population of
42
grain boundary and may not be representative, thus future work is needed to confirm
the result.
Fig.43 The relation between the dimension of the function and the number of data that
give a reliable result
0
10
20
30
40
0 4 8 12 16 20
Dimensionality of the Function
log
(no
. d
ata
)
High resolution
Low resolution
43
6. SUMMARY AND CONCLUSION
Creep samples of HT9 steel were subjected to short term and long term creep tests at
high temperatures. The samples were observed using EBSD to characterize the
damaged grain boundaries. It is found that, for both short term and long term creep
tests, the low angle boundaries are more resistant to crack formation and propagation.
For these tested HT9 steels, the ∑3 boundaries were also susceptible to preferential
creep damage under high stress. The primary difference observed is that the crack
seldom occurs at grain boundaries with misorientations less than 50 degrees in the
long term creep and ∑3 boundaries in the long term creep test were helpful to
alleviate the probability of creep damage. Most of the ∑3 boundaries in the sample
are tilt boundaries, and during the creep test, the tilt ∑3 boundaries disperse. This
demonstrates that the ∑3 tilt boundary in this material is not a stable boundary.
In the long term creep tests, the voids formed at the grain boundaries with
misorientations between 25 degrees and 45 degrees show no misorientation axis
preference, but for the voids formed at the grain boundaries with misorientation
greater than 45 degrees, the rotation axis near <111> is preferred.
At 600C and stress conditions imposed in our experiments, HT9 steel is a ductile
material and the metal goes through plastic deformation before failure. These regions
of high plastic deformation tend to crack, particularly if the neighboring regions are
less prone to the same deformation.
44
7. FUTURE WORK
Since this thesis has illustrated that there is grain boundary character influence on
intergranular creep damage of HT9 steel, logically, the next step is to apply grain
boundary design and control to engineer the overall grain boundary character
distribution of HT9 steel. Since it is already illustrated that the low angle boundary is
more resistant to the creep damage and ∑3 boundaries seem to be more resistant to
the crack under lower stress, then increasing the fraction of low angle boundaries and
decreasing the weak boundaries have a beneficial effect on the creep strength.
Although it is concluded that high angle boundaries are more susceptible to creep
damage, other kinds of investigations should be conducted. Damaged grain boundary
normals should also be focused on because it is also a very important parameter that
determines the grain boundary creep strength.
The creep test was conducted at a very high stress in this case, future creep tests are
needed under lower stress conditions closer to the actual service conditions which the
pressure vessel steels might encounter.
45
8. REFERENCE
[1] R.L. Klueh and D.R. Harries, ASTM, West Conshihocken (2001) p. 1.1.
[2] S.H. Kim, W.S. Ryu and I.H. Kuk: J. Kor. Nucl. Soc. 31 (1999) 561.
[3] Treatise on Material Sci & Tech, Vol 6, Ed. R.J.Arsenault, Academic Press, 1975
[4] Dieter, G.E., Mechanical Metallurgy, McGraw Hill, 1988
[5] Harold J Frost, Deformation-Mechanism Maps: The Plasticity and Creep of
Metals and Ceramics 1982.
[6] A.Srinivasan, J. Swaminathan, M.K.Gunjan, U.T.S.Pillai, B.C.Pai: Material
Science and Engineering A 527(2010) 1395-1403.
[7] M.E.Kassner, T.A.Hayes: International Journal of Plasticity 19(2003) 1715-1748
[8] A.Wisniewski, J.Beddoes: J Material Sci(2008) 43:5350-5357.
[9] David P.Field, Brent L. Adams and Junwu Zhao: Textures and Microstructures,
1991, Vols14-18, pp.977-982.
[10] J.Ahmad, J.Purbolaksono, L.C.Beng: Engineering Failure Analysis 17(2010)
328-333
[11] C.R.Blanchard, R.A.Page: Journal of Materials Science 33(1998) 5037-5047.
[12] C.R.Blanchard, R.A.Page: Journal of Materials Science 33(1998) 5049-5058.
[13] Field, D.P. and Adams, B.L, Metall. Trans, 23A (1992) 2515.
[14] Thomas Lillo, James Cole, Megan Frary, and Scott Schlegel, Metall. Mater.
Trans 40A, (2009) 2803.
[15] A-F Gourgues, Matls. Sci. Tech, 18 (2002) 119.
46
[16] D.P.Field, B.L.Adams and J.Zhao : Textures Microstruct. 1991. 14-18. 977-982.
[17] D.P.Field, B.L.Adams: Metall. Trans. A. 1992. 23A 2515-2516.
[18] T.Lillo, J.Cole, M.Frary and S.Schlegel, Metallurgical and Materials transactions
A, Vol. 40A Dec. 2009, 2803-2811.
[19] Han-Jou Chang, Ji-Jung Kai, Chuen-Horng Tsai, J. Nucl. Matls, 212-215 (1994)
574-578
[20] R.L.Klueh: International Materials Reviews 2005 Vol. 50 No.5 287
[21] Mychailo B.Toloczko, Benjamin R.Grambau: ASTM special technical
publication 2001 557-567.
[22] Raymond J.Puigh: ASTM special technical publication 1985 7-18.
[23] Sung Ho kim, Woo Seog Ryu: Journal of the Korean Nuclear Society Vol.31
No.6 561-571.
[24] Toshio FUJITA: ISIJ International Vol.32(1992), No.2 175-181.
[25] Courtney, Thomas H., Mechanical Behavior of Materials, McGraw Hill, 1999.
[26] Junhui Li, Abhijit Dasgupta, IEEE TRANSACTIONS ON RELIABILITY, Vol.
42, No. 3, 1993 SEPTEMBER
[27] Nieh, T.G. et al, Superplasticity in metals and Ceramics, Cambridge University
Press 1997.
[28] G.S.Rohrer and V.Randle, Electron Backscatter Diffraction in Material Science.
Chapter 16, P215-229.
[29] V.Randle: Journal of Microscopy, Vol.230 Pt 3 2008, pp406-413.
[30] Tricia A.Bennett, Chang-Soo Kim, Gregory S.Rohrer and Anthony D. Rollett:
47
Five parameter grain boundary character distribution in Fe-1%Si.
[31] Scott D.Sitzman: Proc. of SPIE Vol.5392, pp78-90.
[32] Kazunari Fujiyama,Keita Mori, Daisuke Kaneko, Hirohisa Kimachi, Takashi
Saito, Ryuichi Ishii, Takehisa Hino: International Journal of Pressure Vessels and
Piping 86 (2009) 570–577.
[33] G.Gupta, P. Ampornrat, X. Ren, K. Sridharan, T.R. Allen, G.S.Was, J. Nucl. Matls,
361 (2007) 160.
[34] B. Fournier, M. Sauzay, F. Barcelo, E. Rauch, A. Renault, T. Cozzika, L. Dupuy,
and A. Pineau, Metall. Mater. Trans 40A (2009) page 330-341.
[35] Robert J. Amodeo and Nasr M. Ghoniem, J. Nucl. Matls, 122-123 (1984) 91.
[36]T.Watanabe, S.Tsurekawa, S.Kobayashi and S.Yamaura, Material Science and
Engineering A(2005), 410-411, 140-147
[37] C.J.Boehlert and S.C.Longanbach, J. Material. Res, Vol. 23, No.2, Feb 2008.
500-506
[38] Gary S.Was: Metallurgical and Materials Transactions A Vol. 28A, October 1997
2101-2112
[39] Gary S.Was, Douglas C.Crawford: grain boundary misorientation effects on
creep and cracking in Ni-based alloys, JOM Feb, 2008
[40] Pan, Y, Adams,B.L, Olson.T, and Panayotou, N, Acta metall.mater., 1994,
42,1785
[41] G.Palumbo, K.T.Aust, E.M.Lehockey, U.Erb and P.Lin: Scripta materialia, Vol.38,
No.11, pp.1685-1690, 1998
48
[42] G. S. Was: Scripta materialia, Vol.35, No.1, pp.1-8,1996.
[43] T. Watanable, Metall. Trans. A. 1983. 14 A. 531-545
[44] T. Watanable, Res. Mech. 1984. 11. P47
[45] H. J Bunge, Proc. ICOTOM 10, 1993
top related