Materials and Design 34 (2012) 82–89

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Materials and Design

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Titanium-cenosphere syntactic foam made through powder metallurgy route

D.P. Mondal a,⇑, J. Datta Majumder b, Nidhi Jha a, Anshul Badkul a, S. Das a, Aruna Patel a, Gaurav Gupta a

a Advanced Materials and Processes Research Institute (CSIR), Bhopal 462 064, Indiab Department of Metallurgical and Material Engineering, Indian Institute of Technology, Kharagpur 721 302, India

a r t i c l e i n f o

Article history:Received 1 April 2011Accepted 21 July 2011Available online 4 August 2011

Keywords:B. FoamsC. Powder metallurgyE. Mechanical

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.07.055

⇑ Corresponding author. Tel.: +91 755 2417652; faxE-mail addresses: dpmondal@ampri.res.in, mondal

a b s t r a c t

Attempts have been made to use cenosphere as space holder to make Ti-cenosphere syntactic foamthrough powder metallurgy route. The cold compaction pressure was varied between 75 and 125 MPain order to examine its effect on density, cenosphere crushing and strength of Ti-cenosphere foam. Thecold compacted pallets were sintered at a temperature of 1100 �C for 2 h. Microstructural characteristics,density vis-a-vis porosity, compressive deformation behaviour and modulus of elasticity of the sinteredsamples were examined. It was noted that the porosity varies in the range of 51–70% depending on thepressure applied during cold compaction. A numerical model has been proposed to correlate density offoam with the extent of cenosphere crushing, density of cenosphere and density of titanium. The yieldstrength and modulus of elasticity of sintered foam samples are found to be a strong function of relativedensity.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium foams are finding wide range of applications in hightemperature energy absorptions, catalysts, crash worthiness, lightweight sandwich structures [1], high temperature heat exchangers[2], bone scaffolds and bone implantations [3–8]. However, mostsignificant efforts have been made to prepare open cell Ti-foam com-patible to the characteristics of bones for its use as bone implants.Chino and Dunand [5] prepared Ti-foam with 57–67% porositythrough freeze casting of aqueous slurry of Ti-powder. It is alsoreported by these investigators that the size of Ti-powder plays animportant role to the synthesis of Ti-foam. The strength of thesefoam ranges between 40 and 60 MPa which are equivalent to thatof human bone. Taylor et al. [6] examined that the sintering and den-sification kinetics also depends on the addition of foreign particleslike TiC in the Ti-powder mixture. Pompe et al. [7] made attemptto prepare Ti-foam with functionally graded characteristics to makemore closely compatible with bone tissues. The ingrowth behaviourof Ti-foam implants into hard tissue could be improved with bioac-tive polymer or hydroxyapetite coating. Frosch et al. [8] assess theosseointegration of Ti-foam through coating with autologous osteo-blasts. They conducted this study in adult chinchilla bastard rabbits.The coating with autologous osteoblasts accelerates and enhancesthe osseointegration of Ti-foam implants.

It is observed that attempts have been made by several investi-gators to synthesize Ti-foam using different techniques [9], andthese foams are futuristic material for bone implants. Different

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methods include controlled compaction and sintering of irregularTi-powders with and without ZrO2 addition through plasma rotat-ing electrode process as reported by Oh et al. [10], spark plasma sin-tering of loose Ti-powder compact as reported by Sakamoto et al.[11], sintering of hot compacted Ti–SiC fibre monotapes at temper-ature between 800 and 900 �C [12], entrapment of argon gas in acold compacted Ti-powder followed by liquid stage sintering [13],sintering of leachable salt and Ti-powder compacts followed by dis-solution of salts [3–8,14,15] and sintering of Mg and Ti-powdercompacts [16,17], followed by removal of Mg by heating at con-trolled atmosphere at temperature higher than the melting pointof Mg. Dizlek et al. [3] prepared Ti6Al4 V foam with 60% porosityand pore size between 315 and 500 lm using space holder tech-nique through powder metallurgy route. They have used bothmonomodal and bimodal particle distributions for preparation offoam. It is further observed by them that bimodal particle distribu-tion provides higher strength and stiffness as compared to mono-modal particle distribution. According to these investigations,strength and stiffness increases with increase in sintering tempera-ture. The strength and stiffness achieved by these investigators sat-isfied the strength and stiffness requirement of bone replacement.Wen et al. [17] also adopted the space holder technique for makingpure Ti-foam with porosity up to 80% using powder metallurgyroute. The strength and stiffness of the foam varies with fractionof porosity. It is observed by them that the strength and stiffnessof this Ti-foam is compatible with that of human bone, and thus,they recommended the use of Ti-foam for bone implants. Dunand[14] reviewed in detailed the different techniques for making Ti-foams. Bing and Dunand [15] made successfully open cell Ti-foamusing NaCl salt as space holder through compaction and sintering

(a)

(b)

Fig. 1. (a) Microstructure and (b) size distribution of Ti-powder.

D.P. Mondal et al. / Materials and Design 34 (2012) 82–89 83

technique. They achieved variable porosity and strength by varyingNaCl content and its size, compaction pressure and sintering tem-perature. Kearns et al. [16] and Wen et al. [17] used Mg metal pow-der as space holder in varying amount to make Ti-foam and Ti-alloyfoam with porosity in the range of 40–75%. Kearns et al. [16]achieved compressive strength of 35 MPa for porosity of 75%. Wenet al. [17] achieved strength of 40–116 MPa for porosity of 45–70%. Shen et al. also made an attempt for numerical modelling ofpore size and its distribution in Ti-foams [18] for examining its suit-ability for bone implants. Closed cell Ti-foams are prepared mainlyby liquid stage sintering process [13,14]. In this process, Ti-powdercompacts entrapped with pressurized argon gas were subjected tosintering under high vacuum and inert atmosphere. During sinter-ing, entrapped gases expanded and cause the porosity in the Ti-ma-trix. It was reported that through this process, one can get Ti-foamhaving the maximum porosity in the range of 30–60%[13,14,19,20]. In order to improve strength and stiffness, attemptshave been made to produce Ti-alloy foam such as Ti–Mn [21] andTi–Al–V alloy [22] foams. Syntactic metal foam can be made byusing ceramic and/or metallic microballoons as space holdersthrough powder metallurgy route or liquid metallurgy route. Thesemicroballoons have high strength and can withstand high compac-tion pressure, but these materials are relatively costlier. Ceno-spheres primarily obtained in coal base thermal power plants arespherical in shape and hollow in nature, and these materials primar-ily consist of alumina–silicates (mullite and sillimanite) phases[23,24]. As a result, there is a scope to use these materials as spaceholders for making syntactic metal foam. Several investigators[25–27], in addition to the present authors [23], have examinedthe feasibility of using cenosphere as space holders to make alumin-ium syntactic foam through liquid metallurgy route [23,24]. To thebest of our knowledge, no attempt has been made to prepare metalsyntactic foam particularly Ti-foam using cenosphere as spaceholder especially through powder metallurgy route. It may be dueto the fact that the cenosphere particles are very fragile in natureand its cell wall thickness is very thin (maximum 10% of its diame-ter). The strength of cenosphere shells is as high as �500 MPa [23].But, because of porous structure and very thin in nature, the shell ofthe cenospheres may be crushed at much lower applied pressure.Hence, for the synthesis of Ti-cenosphere syntactic foam, there isa need to control applied pressure to avoid crushing of the ceno-sphere shells during cold compaction. However, these cenospheresare very cheap and abundantly available in thermal power plants. Asa result, the use of cenosphere would certainly reduce the cost of Ti-foam as it could be produced through simple technique. The presentinvestigation aims at exploring the possibility of using cenospheresas space holders to make Ti-cenosphere syntactic foam through pre-cise control of cold compaction pressure and finally to characterizethese foams in terms of density, microstructure, modulus of elastic-ity and yield strength.

2. Experimental procedure

2.1. Raw material

Ti-powder of�325 mesh size having angular shape and purity of99.99% with average size of 25 ± 5 lm is obtained from Alfsar, UK.The microstructure of Ti-powder is shown in Fig. 1a. The size distri-bution of these powders is shown in Fig. 1b. The cenospheres used asspace holders are obtained from Cenosphere India Ltd., Kolkata, In-dia. The microstructures and size distribution of cenosphere parti-cles are shown in Fig. 2a and b, respectively. It is evident from themicrostructure and the size distribution that the average size of cen-osphere used is �90.1 ± 5.5 lm. The cenospheres are characterizedin detailed in terms of chemical composition and constituentsphases, and the results are reported elsewhere [21,24].

2.1.1. Selection of compositionThe weight fraction of Ti and cenosphere in the Ti-cenosphere

mixture was considered on the basis of the density of Ti and cen-osphere taking into consideration of the following equation:

dTif ¼ ½ðWTi þWcenoÞ�=½ðWTi=dTiÞ þ ðWceno=dcenoÞ� ð1Þ

where dTif is the density of Ti-foam having no porosity other thanthe porosity due to cenosphere; dTi and dceno are the density of Tiand cenosphere, respectively; and WTi and W ceno are the weightfraction of Ti and cenosphere in the mixture. The consideration ofTi/Cenosphere weight ratio of 2:1 thus gives a density of Ti-foam�1.12 gm/cc when dTi = 4.5 gm/cc and dceno = 0.45gm/cc. Densityof Ti was calculated through liquid displacement technique,whereas the density of cenospheres was calculated from its packdensity. The cenospheres primarily contain mullite [25]. The den-sity of crushed cenosphere, as measured using liquid displacementtechnique, comes to be 2.65 gm/cc. Using the above relation (Eq.(1)), the density of Ti-mullite composite comes to be �3.65 gm/ccconsidering Wceno = Wmull. The relative density of Ti-foam with Ti/cenosphere weight ratio of 2:1 comes to be 0.31. It thus states thatif the cenospheres and Ti particles are compacted to fully densecondition then the syntactic foam of Ti-cenosphere so made willhave porosity of �69%. Here, no cenosphere crushing and porosityother than cenosphere are considered. However, in practice, therewill be a possibility of having porosity in excess of empty space pro-vided by cenospheres, as fully dense compaction is not possible. Butbecause of fragile nature of cenosphere, there is possibility of ceno-sphere crushing which would lead to reduce the porosity. Consider-ing these facts, to get porosity in the range of 51–70% underdifferent applied pressure, Ti/cenosphere weight ratio is fixed at2:1.

02468

1012141618202224

(a)

(b)

Fig. 2. (a) Microstructure and (b) size distribution of cenosphere.

84 D.P. Mondal et al. / Materials and Design 34 (2012) 82–89

2.1.2. Selection of applied loadThe applied pressure for compaction of Ti-cenosphere mixture

was selected on the basis of strength of cenosphere shells, volumefraction of Ti-powder and cenospheres particles, strength of ceno-sphere and Ti and following Ashby’s relation [29]. The load will beshared by both Ti and cenosphere. In practice, while loading is per-formed to compact loose powder within the die, the particles willinitially move quite freely. But after attaining effective packing, theload is expected to be shared by the constituent phases accordingto their volume fraction, and thus, the load/pressure partitioningby the constituent elements in the packed particle mixture couldbe expressed by following relation:

ðPcÞ ¼ rTiV fTi þ rcenoV fceno ¼ rTiV fTi þ V fceno � Crcsð1� V fcpÞn ð2Þ

where rTi and rceno are the yield strength and plateau strength of Tiand cenosphere, respectively; rcs is the strength of cenosphereshell; VfTi, Vfceno and Vfcp are the volume fraction of Ti, cenosphereand the porosity fraction in cenosphere, respectively, in the fullypacked mixture. C is an empirical constant. Pc is the appliedpressure.

Thus, the load shared by cenosphere would be (Pc � rTiVfTi). Foroptimum packing and minimum crushing, the following conditionmust be satisfied:

ðPc � rTiV fTiÞ=V fceno 6 rceno 6 Crcsð1� V fcpÞn ð3Þ

The value of Pc comes to be �68 MPa if we consider rTi � 400 MPa[30], n = 2.19 [21], C = 0.75 [21], rceno � 600 MPa [21], VfTi � 1/6 andVfceno � 5/6. In addition, we require some amount of frictional stressbetween die surface and punch. On the basis of these aspects, ap-plied pressure of 75 MPa was selected. The applied pressure wasvaried in the range of 75–125 MPa in order to examine the effect

of applied pressure on cenosphere crushing and porosity on theTi-cenosphere syntactic foam.

2.1.4. Sintering of palletsThe cold compacted pallets are characterized in terms of its

dimensions and density. The pallets are then dried at 200 �C for2 h to remove moisture. Dried pallets are then heated at 400 �Cfor 2 h followed by heating at 600 �C for 2 h for debinding. Afterdebinding, the samples are sintered at a temperature of 1100 �Cfor 2 h in argon atmosphere. Prior to sintering, the pallets werevacuum sealed in quartz tubes.

2.1.5. Characterization of sintered palletsThe sintered pallets are examined in terms of its density or

porosity, phase analysis, microstructures and compressive defor-mation behaviour. For microstructural examination, the sinteredpallets are polished using standard metallographic practice andetched with Keller’s reagent (20 ml of distilled H2O, 20 ml ofHNO3 (70%), 20 ml of HCl (38%) and 20 ml of HF (40%)). The micro-structures of the etched pallets are examined in a Scanning Elec-tron Microscope (Model: JEOL, JSM-5600). The same instrumentwas used for EDX analysis of the sintered pallets for elementalanalysis. The density of the samples is measured through Archime-des principle and the porosity is calculated from its theoretical andmeasured density. The compressive deformation behaviour of thesamples (dimensions of 6 mm width and length 9 mm) is carriedout using an Universal Testing Machine (Instron Model: 8801) ata strain rate of 0.01/s.

3. Results and discussions

3.1. Materials and microstructures

It was observed from the microstructures that the Ti particlesare angular in shape and their average size was 25 ± 5 lm. Theseparticles have the tendency to agglomerate. On the other hand,cenosphere particles were spherical in shape and their average sizewas 92 ± 5 lm. This demonstrates that cenosphere particles arelarger in size. Thus, in this mixture, there is a possibility that coarsecenosphere particles would be surrounded by the finer Ti particles.The cenosphere particles are hollow in nature and the cell wall isvery thin (�8–10 lm). As a result there is a possibility that a frac-tion of cenospheres might be crushed during cold compaction, andthe fraction of cenospheres crushed depends on the applied pres-sure used during cold compaction. Therefore, the density of coldcompacted cenosphere Ti mixture would depend strongly on theapplied pressure. Three sets of samples were compacted at eachof the four different applied pressures to optimize the applied loadfor minimizing cenosphere crushing to a great extent. The densityof cold compacted pallets as a function of applied pressure isshown in Table 1. It is evident from this table that the density ofcold compacted pallets after drying even at cold compaction pres-sure of 75 MPa (1.38 gm/cc) is considerably greater than the den-sity (1.12 gm/cc) calculated for the Ti-foam using Eq. (1). This isprimarily attributed to (i) the presence of PVA added as organicbinder and (ii) crushing of a fraction of cenosphere. In fact, a largefraction of cenospheres are very weak because of their coarser sizeand thinner shell. It is also noted that the density of cold compac-tion is increasing almost linearly with applied pressure. This obser-vation demonstrates that the presence of PVA in the compacts isone of the important reasons for getting higher density. However,after sintering, PVA get burnt, and thus, the density of sinteredcompact would be governed by Ti and cenosphere only. It is evi-dent from the Table 1 that the density of compacted pallets, evenwhen compacted at 75 MPa, is around 1.25 gm/cc. This is also con-siderably higher than the calculated density of 1.12 gm/cc (from

Table 1Variation in Density, porosity and Young’s modulus of Ti-cenosphere syntactic foamwith applied pressure.

ColdcompactionPressure(MPa)

Density (gm/cc) Porosity(%)

Young’smodulus(GPa)

Yieldstrength(MPa)

After coldcompaction

Aftersintering

75 1.38 1.25 62 25.05 21.290 1.47 1.35 58 28.3 31.6

110 1.60 1.48 53 34.2 48.3125 1.63 1.53 51 41.4 57.7

Table 2Predicted density of pallets after sintering from Eq. (4) for varying level of crushedcenosphere (fcc).

Fraction of cenosphere crushed Density of pallets (gm/cc)

0.10 1.220.15 1.280.20 1.340.25 1.400.30 1.48

D.P. Mondal et al. / Materials and Design 34 (2012) 82–89 85

Eq. (1)). This strongly confirms that a considerable fraction of cen-ospheres get crushed even at an applied pressure of 75 MPa.

In order to calculate the fraction of cenosphere crushed, Eq. (1)is modified as follows:

dTif ¼ ½ðWTi þWcenoÞ�=½ðWTi=dTiÞ þ fðWceno=dcenoÞð1� fccÞþ ðfcc � V fcshÞg� ð4Þ

where fcc is the fraction of cenosphere crushed and Vfcsh is the vol-ume fraction of cenosphere shell in cenosphere. The density (dTif)of Ti-cenosphere sintered compact calculated as a function of frac-tion of cenosphere crushing (fcc) for WTi:Wceno = 2:1 andVfcsh = 0.1using Eq. (4) is reported in Table 2. The comparison ofdensity value of the pallets after sintering from Tables 1 and 2confirms that around 10–12% of cenosphere get crushed when apressure of 75 MPa was applied during compaction. Similarly, forcompaction, pressure of 90, 110 and 125 MPa around 20%, 30%and 35% of cenosphere respectively get crushed. Thus, overallreduction in density was noted to be around 0.30 gm/cc due to

(a) (

(c)

Fig. 3. Microstructure of Ti-cenosphere syntactic foam sintered at 1100 �C: (a) while coldof (a).

increase in pressure from 75 to 125 MPa. Table 2 also shows thatthe predicted density of Ti-cenosphere syntactic foam increaseswith increase in applied pressure during cold compaction. A mar-ginal variation in density of the foam between the experimentaland predicted values may be attributed to the fact that angularTi particles and cenosphere particles may change their packingarrangement with the variation in applied pressure and packingbecomes more compact with increase in applied pressure. Butthe tendency of crushing of cenosphere particles increases signifi-cantly with increase in applied pressure above 75 MPa. It is there-fore better to control pressure below 75 MPa during coldcompaction for making Ti-cenosphere syntactic foam, but signifi-cantly lower pressure (<75 MPa) does not give sufficient particleto particle contact, and thus, the cold compaction pallets becomefragile or could not be handled for further processing.

The porosity within the Ti-cenosphere syntactic foam is calcu-lated from the following relations:

%porosity ¼ 1� ½dtif=dtimul� ð5Þ

where dtimull is the density of Ti-mullite composite (from Eq. (1))�3.65 gm/cc and dtif is the density of Ti-syntactic foam (from Eq.

b)

compacted at 75 MPa, (b) while cold compacted at 125 MPa, and (c) magnified view

(a)

(b)

(c)

Fig. 4. EDX analysis of Ti-cenosphere syntactic foam: (a) entrapped particles in Ti-matrix, and (b) dark region primarily of cenosphere, and (c) bright region indicatingthe presence of Ti and O.

86 D.P. Mondal et al. / Materials and Design 34 (2012) 82–89

(4)). The porosity calculated using Eqs. (1),(4) and (5) is alsoreported in Table 1. If the Ti-cenosphere mixture is fully compacted,it will produce dense Ti-mullite composite. Hence, for calculation ofporosity, experimentally observed densities of Ti-cenosphere foamsand theoretically calculated density of Ti-mullite dense compositehave been used.

The microstructure of Ti-cenosphere syntactic foam after sinter-ing is shown in Fig. 3. Fig. 3a represents the microstructure of Ti-cenosphere syntactic foam when sintered at 1100 �C after coldcompaction at a pressure of 75 MPa. It is noted that cenosphereparticles are surrounded by the network of Ti particles. But themicrostructure of Ti-cenosphere syntactic foam when compactedat a pressure of 125 MPa and sintered at 1100 �C exhibits irregularregions of cenospheres (mixture of crushed cenosphere and uncru-shed cenosphere) surrounded with Ti-network (Fig. 3b). The highermagnification micrograph of Ti-network in Ti-cenosphere syntacticfoam (when cold compacted at 75 MPa) is shown in Fig. 3c, whichalso reveals that a fraction of cenospheres are crushed and frag-mented into fine particles and after sintering get embedded intothe Ti- matrix. It further reveals a few micro-indentation markswhich are taken for assessing hardness of this region. This figurealso demonstrates that these networks also contain fine porositiesindicating inefficient bonding between Ti particles. The entrap-ment of crushed cenosphere into Ti-network is confirmed throughEDX analysis as shown in Fig. 4a. Fig. 4b shows the EDX analysis ofdark regions (marked A in Fig. 3c), indicating that these regions areprimarily consisting of cenosphere particles. While polishing, someamount of Ti gets spread over cenospheres and thus shows its pres-ence in minor quantity in the EDX of Fig. 4b. The EDX of bright re-gion is shown in Fig. 4c, indicating the presence of Ti and oxygen.Stochiometric calculation from the Ti and oxygen concentrationstates that around 10% of Ti had been oxidized. This is attributedto the fact that Ti is highly reactive to oxygen at higher tempera-ture and sintering is carried out at a vacuum level of 10�3 mbarwhere there was a reasonably good amount of oxygen to oxidizeTi partially at a temperature of 1100 �C. The other possibility is thata little fraction of mullite might react with Ti to form Titaniumoxide, as it has been stated earlier that a fraction of crushed ceno-sphere gets embedded in Ti-network. This may be the reason forthe presence of aluminium and some more oxygen. The XRD pat-tern of sintered Ti-cenosphere syntactic foam is shown in Fig. 5.It indicates the presence of Ti, TiO2, mullite and quartz. This furtherconfirms that a fraction of Ti get oxidized during sintering.

The microhardness of the Ti-network is noted to be significantlyhigher (859.25 HV) than the reported hardness values of Ti [28].This is because of the entrapment of Titanium Oxide and crushedfine cenosphere shells in the Ti-matrix.

The engineering stress–strain behaviour of Ti-syntactic foam un-der compressive loading at different cold compaction pressure isshown in Fig. 6. It is noted that the yield stress of the foam variesin the range of 21–58 MPa. It may be further noted that after reach-ing yield stress, the stress value decreases with strain. It does notshow any clear plateau region as observed in other foams[2,3,16,28]. But, like other foams, the oscillating behaviour of varia-tion in stress after yielding is noted. However, these curves demon-strate that these foams can withstand yield stress of�21–58 MPa. Itis also evident from this figure that the yield stress increases withdecrease in porosity. Considering the results of Table 1, it could benoted that yield stress increases with increase in compaction pres-sure vis-a-vis density (Table 1).

The yield strength and Young’s modulus of Ti-cenosphere syn-tactic foam are plotted as a function of relative density (relativedensity = dtif/dtimul) in Figs. 7 and 8 respectively. It is interestinglynoted from Fig. 7 that the yield strength of these foam follows apower law relationship with the relative density as proposed byAshby for cellular materials [29]:

rtif ¼ 665:0½dtif=dtimul�3:14 ð6Þ

The yield strength of fully compacted Ti + 15 wt% crushed cen-osphere mixture after sintering was measured from the compres-sive stress–strain curve. It is noted that the yield strength of thismaterial, rTimul, is �450 ± 15 MPa. Thus, the above relation couldbe written as:

rtif ¼ 1:47rTimul½dtif=dtimul�3:14 ð7Þ

The value of coefficient and the exponent is quite large and be-yond the limit of their reported values.

However, the yield strength of the investigated foam materialcould be calculated using load partitioning concept [31] for alu-minium cenosphere syntactic foam following the relation as men-tioned below:

rtif ¼ Crtimul 1� V fceno 1� fccð Þf gn� �

þ Crcs 1� V fcp� �nV fceno 1� fccð Þ

h ið8Þ

where Vfcp is the fraction of voids in the cenosphere and rcs isthe strength of cenosphere shell. The calculated values of rtif

using Eq. (9) considering C = 0.75[21], n = 2.19[21], fcv = 0.9 [21],

D.P. Mondal et al. / Materials and Design 34 (2012) 82–89 87

rtimul = 450 MPa and rcs = 600 MPa for different fcc depending onthe applied pressure are compared with the experimentally ob-tained yield strength in Table 3. It is evident from this table thatthe experimental values are in good agreement with the calcu-lated values. This demonstrates that the above equation couldalso be used to predict the strength of Ti-cenosphere syntacticfoam with reasonably good accuracy.

The modulus of the sintered samples was also measured usingDEEPA sonic modulus equipment, and the measured values arealso reported in Table 1. It is noted that the Young’s modulus ofthese foam varies in the range of 25–42 GPa. These values of yieldstrength and modulus are in good agreement with the reportedvalues [2,3,13–15,24].

The Young’s modulus of this foam also increases with increasein relative density following power law relationship as shown inFig. 8. This demonstrates that modulus of these foam also followsthe characteristics behaviour of cellular and/or porous materials.It may be because of the weak bonding between Ti–Ti particlesand Ti particle-cenosphere shell. The weak bonding might be dueto oxidation of Ti particles or the presence of crushed cenosphereparticles between Ti particles and lower sintering temperatureused to avoid excessive oxidation.

The Young’s Modulus of these foams follows the following rela-tionship with relative density (dtif/dtimul) as shown below:

ETif ¼ 148:5ðdtif=dtimulÞ1:87 ð9Þ

where ETif is the Young’s modulus of the foam. It is understood thataround 10–35% cenosphere get crushed during cold compactiondepending on the applied pressure. As a result, the matrix exhibitsaround 10–35% crushed cenosphere shells, which primarily consistof mullite. Considering the modulus of Ti = 120 GPa and of ceno-sphere shells, i.e., mullite = 220 GPa, and crushed cenosphere �25%in the matrix, the modulus of the matrix comes to �145 GPa follow-ing simple rule of mixture. Thus, the above relation can be written as:

ETif=ETimul ¼ 1:02ðdtif=dtimulÞ1:87 ð10Þ

The exponent comes out to be 1.87 and the proportionality con-stant �1.02. The value of constant is noted to be in upper side. Thevalue of exponent is in line with the reported values for the foammaterial. This also strongly demonstrates that Ti-cenosphere com-pacts so made follow the characteristics of foam material.

0102030405060708090

100110120130140150160170180190200210220230240250

2-Thet

10 20 30

Ln

(Cou

nts)

Fig. 5. XRD pattern of sintered Ti

The fracture surface of the Ti-cenosphere foam clearly showsthe weak bonding between Ti particles (arrow marked in Fig. 9).Fig. 9 also reveals crushed cenospheres surrounded by Ti particles(region marked with ‘C’). Improved properties and clear plateau re-gion could be observed when the bonding between Ti–Ti particlescould be improved by sintering at relatively higher temperaturewith precise control of sintering atmosphere to avoid oxidationof particles and to facilitate more diffusion among the particles.These aspects shall be studied in detailed and communicated later.

However, the present study demonstrates that Ti-cenospheresyntactic foam with reasonably good strength can be preparedusing cenosphere as space holder through powder metallurgyroute with proper control of cold compaction pressure. In the pres-ent study, the optimum cold compaction pressure is �75 MPa(Table 1). Higher compaction pressure will lead to greater possibil-ity of cenosphere crushing and lower density of Ti-cenosphere syn-tactic foam.

Saliman et al. calculated elsewhere [2] the values of the mostimportant performance indices for different foam materials charac-terizing their performance for given applications. It was predictedby them that Ti-foams are promising materials for light weightstructural applications like light weight beams or plates, lightweight shock absorbers, heat insulators, doors and frames in aero-space, military armour, high temperature packaging, etc., wherespecific strength and stiffness at elevated temperature is primerequirement. But the cost of Ti-foam as mentioned by them is 4–5times greater than that of solid material, and it makes their use lim-ited only to the strategic applications like defence and aerospace.The use of cenosphere as space holder, as used in the present study,will make the process relatively simple and to be very cost effective.This is because of the fact that the cenosphere is a by-product ofthermal power plant and hence are very cheap. The processrequires simple facilities like moulds, a low capacity press and heattreatment furnace. Additionally, use of cenosphere fills a consider-able extent of vacant spaces in the powder compact and reducesnominal surface area of Ti particles exposed for reacting with anygas. As a result, furnaces with lower order of vacuum may also beused in sintering of Ti-cenosphere pallets. The foam so made, thusin fact, is expected to be cheaper.

The normalized performance indices like (ry)1/2/q and (E)1/2/q(used for grading the materials for structural applications in placeof standard one) for synthesized Ti-cenosphere syntactic foam

a -Scale

40 50 60 70

-cenosphere syntactic foam.

y = 665.08x3.5594

R2 = 0.9458

0

10

20

30

40

50

60

70

Yiel

d st

ress

(MPa

)

0.35 0.38 0.41 0.44 0.47 0.5Relative density

Fig. 7. Variation in yield stress as a function of relative density.

Table 3Comparison between the Yield strength (rTif) of Ti-cenosphere syntactic foam fromEq. (9) and experimental value for varying level of cenosphere crushing.

Fraction of cenosphere crushed(fcc)

rTif (MPa) from Eq.(9)

rTif (MPa)(Exp)

0.1 18.8 21.20.2 32.9 31.60.3 51.9 48.30.35 63.3 57.7

y = 148.57x1.871

R2 = 0.9442

20

25

30

35

40

45

0.35 0.38 0.41 0.44 0.47 0.5

Youn

gs m

odul

us (G

Pa)

Relative density

Fig. 8. Variation in Young’s modulus as a function of porosity fraction.

0

10

20

30

40

50

60

70

0 0.05 0.1 0.15 0.2

Engg

Str

ess

(Mpa

)

Engg Strain

53% porosity

58% porosity

51% porosity

62% porosity

53%

58%

51%

62%

Fig. 6. Compressive stress–strain curves of Ti-cenosphere syntactic foam (com-pacted at different pressure and sintered at 1100 �C).

Fig. 9. Microstructure showing fracture surface of Ti-cenosphere syntactic foam.

88 D.P. Mondal et al. / Materials and Design 34 (2012) 82–89

with respect to those of dense Ti and Ti-alloys are calculated in linewith the reported literature [2]. Here, ry, E and q are the yieldstrength, Young’s modulus and density of the respective materials.Considering ry, E and q, for Ti to be 400 MPa, 120 GPa and 4.5 gm/cc, respectively, the normalized performance indices of (ry)1/2/qfor Ti-cenosphere syntactic foam comes to be in the range of�0.85–1.1. The performance indices (E)1/2/q for the synthesizedTi-cenosphere foams were found to be in the range of 1.6–1.77.These calculations forecast that this Ti-cenosphere is an excellentmaterial or a new champion material [2] for the replacement ofTi dense metal and alloys. The possibility of making Ti-cenospheresyntactic foam at relatively lower cost and excellent values of per-formance indices make these Ti-cenosphere foams a promisingmaterials for applications for shock absorber, energy absorber,vibration control, packaging, thermal insulation, light weight

structure, especially which are subjected to high temperature evenin general engineering and automobiles applications. Ti-ceno-sphere foams made with hydroxyapetite coated cenosphere couldbe explored for biomedical applications at much affordable cost.Works in these directions are in progress and will be communi-cated later.

4. Conclusions

Following conclusions could be made from this study:

1. Cenospheres can be used as space holders to make Ti-cenospheresyntactic foam. But cold compaction pressure has to be appliedcautiously. An applied pressure of 75 MPa will be sufficient dur-ing cold compaction to minimize crushing of cenosphere shell,getting a porosity of �62%.

2. The crushing of cenosphere, porosity fraction and density ofTi-cenosphere syntactic foam are strong function of appliedpressure.

3. A fraction of crushed and fragmented cenosphere shells getsembedded or mixed with Ti, leading to higher hardness of Ti-matrix.

4. Ti-cenosphere cold compacted pallets can be sintered at1100 �C to get reasonably good yield strength and modulusbut do not give any clear plateau region. This may be due toinsufficient sintering at lower sintering temperature and oxida-tion of Ti particles during sintering. The strength and modulusof sintered Ti-cenosphere syntactic foam is a function of coldcompacted pressure vis-a-vis its density.

D.P. Mondal et al. / Materials and Design 34 (2012) 82–89 89

5. Ti-cenosphere could be excellent and new champion materialsfor replacement of Ti and its alloys for structural, shock andenergy absorbing applications.

6. The porosity fraction as low as 52% can be achieved in Ti-ceno-sphere syntactic foam under the present experimental condi-tion. The pores are observed primarily due to the presence ofunbroken cenosphere.

7. The strength of Ti-cenosphere syntactic foam could be pre-dicted with reasonably good accuracy using the followingequation:

rtif ¼ Crtimul 1� V fceno 1� fccð Þf gn� �

þ Crcs 1� V fcp� �nV fceno 1� fccð Þ

h i

where fcc is expressed as a function of applied load. The strengthof Ti-cenosphere syntactic foam could be predicted using Ti/cen-osphere weight ratio, their density and the strength of Ti andcenosphere powders or cenosphere cells.

8. The modulus of Ti-cenosphere could also be expressed with thefollowing power law relationshipETif=ETimul ¼ 1:02ðdtif=dtimulÞ1:87

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