Materials Science and Engineering C 46 (2015) 387–393

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Materials Science and Engineering C

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Microstructure, mechanical properties and superelasticity of biomedicalporous NiTi alloy prepared by microwave sintering

J.L. Xu a,b,⁎, L.Z. Bao a, A.H. Liu b, X.J. Jin a, Y.X. Tong c, J.M. Luo a, Z.C. Zhong a, Y.F. Zheng d,⁎a School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR Chinab Jiangsu Provincial Key Lab for Interventional Medical Devices, Huaiyin Institute of Technology, Huaian 223003, PR Chinac Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, PR Chinad State Key Laboratory for Turbulence and Complex System and Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China

⁎ Corresponding authors.E-mail addresses: jlxu@nchu.edu.cn (J.L. Xu), yfzheng@

http://dx.doi.org/10.1016/j.msec.2014.10.0530928-4931/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 April 2014Received in revised form 8 October 2014Accepted 21 October 2014Available online 23 October 2014

Keywords:Porous NiTi alloyMicrowave sinteringSuperelasticityPorosityMechanical properties

Porous NiTi alloys were prepared by microwave sintering using ammonium hydrogen carbonate (NH4HCO3) asthe space holder agent to adjust the porosity in the range of 22–62%. The effects of porosities on the microstruc-ture, hardness, compressive strength, bending strength, elastic modulus, phase transformation temperature andsuperelasticity of the porous NiTi alloys were investigated. The results showed that the porosities and averagepore sizes of the porous NiTi alloys increased with increasing the contents of NH4HCO3. The porous NiTi alloysconsisted of nearly single NiTi phase, with a very small amount of two secondary phases (Ni3Ti, NiTi2) whenthe porosities are lower than 50%. The amount of Ni3Ti and NiTi2 phases increased with further increasing ofthe porosity proportion. The porosities had few effects on the phase transformation temperatures of the porousNiTi alloys. By increasing the porosities, all of the hardness, compressive strength, elastic modulus, bendingstrength and superelasticity of the porous NiTi alloys decreased. However, the compressive strength and bendingstrength were higher or close to those of natural bone and the elasticmodulus was close to the natural bone. Thesuperelastic recovery strain of the trainedporousNiTi alloys could reach between 3.1 and 4.7% at the pre-strain of5%, even if the porositywas up to 62%.Moreover, partial shapememory effectwas observed for all porosity levelsunder the experiment conditions. Therefore, themicrowave sinteredporousNiTi alloys could be a promising can-didate for bone implant.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the past few decades, nearly equiatomic nickel–titanium alloys(NiTi, nitinol) have been considered as excellent biomaterialswith poten-tial use in human hard tissue repair and replacement due to their uniqueproperties, such as shape memory effect, superelasticity, good impactresistance and damping, high corrosion resistance and excellent biocom-patibility [1–4]. However, one of the most critical issues frequently en-countered in hard tissue replacement applications is “stress shielding”generated from the large mismatch of elastic modulus between thehard tissue (b20 GPa) and the implant materials (N100 GPa), whichmay lead to the resorption of the hard tissue, loosening of the implantsand finally, the failure of implantation [5–7]. To solve this problem, intro-ducing pores into the bulkmaterials and forming the porousmaterials arethemost efficientmethods except for developing new lower elasticmod-ulus biomaterials [8–13]. Porous structure could not only provide the ad-justable elastic modulus and improve the biomechanical compatibility ofthe implants, but also allow the ingrowth of new bone tissue and vascu-larization and afirmfixation of the implants could be obtained. Therefore,

pku.edu.cn (Y.F. Zheng).

porousNiTi alloys have attractedmany attentions as implants for hard tis-sue repair and replacement recently, such as maxillofacial and dental im-plants, cervical and lumbar vertebral replacements, joint replacements,bone plates, bone tissue engineering, spine fracture fixation, and anchor-age and repair [8,9].

Previously several powder metallurgical methods had beenemployed to fabricate the porous NiTi alloys, including conventionalsintering (CS) [14], hot isostatic pressing (HIP) [15], self-propagatinghigh-temperature synthesis (SHS) [16] and spark plasma sintering(SPS) [17]. In recent years microwave sintering technique has emergedas a new sintering method for ceramics, semiconductors, metals andcomposites [18–20]. Microwave sintering is a process described asfollows: the materials, coupled with microwaves, absorb the electro-magnetic energy volumetrically, which transforms into heat up tosintering temperature when the densification and alloying are eventu-ally realized [18,19]. As a consequence, compared with conventionalsintering, the microwave sintering technique possesses many intrinsicadvantages, such as reduced energy consumption, rapid heating rates,reduced sintering times, enhanced element diffusion processes and im-proved physical andmechanical properties of the sinteredmaterials [18,19]. Recently, Tang et al. [21] and our preliminary work [22] reportedthat the porous NiTi alloys could be prepared by microwave sintering,

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but the sintered porous NiTi alloys exhibited low porosities andirregular pore size as well as containing many undesired secondaryphases (Ni3Ti and NiTi2). In this paper, the biomedical porous NiTialloys possessing different porosities and regular pore sizes withfew second phases were prepared by microwave sintering andspace holder technique. At the same time, the effects of porositieson the microstructure, mechanical properties, phase transformationtemperatures and superelasticity of porous NiTi alloys were also in-vestigated systematically.

2. Material and methods

Commercially available Ni carbonyl powders (particle size ~2 μm,purity N99.7%) and Ti powders (particle size ~10 μm, purity N99.9%),with a nominal atomic ratio of 50.8 to 49.2, were used to prepare porousNiTi alloy in this experiment. The −120 mesh sieved pure ammoniumhydrogen carbonate (NH4HCO3) particles were mixed into the Ni–Tipowders as the space holder agent to adjust the porosities with the con-tents of 0, 10 wt.%, 20 wt.%, and 30 wt.%, respectively. The mixedNi(CO)4-Ti-NH4HCO3 powders were blended in a planetary ball mill(QM-3SP4, Nanjing University Instrument Plant) at the speed of200 r/min for 2 h. The blended powders were cold-pressed into greensamples (Ф 20mm×15mmand6mm×6mm×50mm) through a uni-axial pressure of 260 MPa for 30 s. The green compact samples were putinto an alumina crucible with SiC particles covering the green sample.Then the alumina crucible was put inside amullite fiber cotton insulationbarrel. The schematic diagram of the insulation barrel setup is shown inFig. 1. Finally, the insulation barrel was put into a 2.45 GHz 5 kW contin-uously adjustable microwave equipment (NJZ4-3, Nanjing Juequan co.,Ltd.). The green compact samples were sintered by microwave heatingat a rate of 20–30 °C/min to 1000 °C for 15min. During the sintering pro-cess, the microwave sintering chamber was filled with high purity argongas flow (99.999%) and a Reytek infrared pyrometerwas used tomeasurethe temperature of the sintered samples.

The porous structure of the porousNiTi alloyswas investigated by anoptical microscope (DM1500, Shenzhen Hipower). The average poresizes of the porous samples were analyzed by the software of image-pro-plus 6.0 and the general porosity (P) was tested by Archimedesdrainage method, calculated by the following formula:

P ¼ 1– ρ=ρ0ð Þ ð1Þ

where ρ and ρ0 represent the density of the sintered porous NiTi alloyand the theoretical density of solid NiTi alloy, respectively; ρ/ρ0 is therelative density. In this experiment, the theoretical density (ρ0) was6.45 g/cm3 [23].

Fig. 1. Schematic diagram of the insulation barrel setup.

The phase composition of the porous NiTi alloys was identified byX-ray diffraction (XRD, Bruker D8 FOCUS). The phase transformationbehavior of the porous NiTi alloys was characterized by using a Perkin-Elmer Diamond differential scanning calorimeter (DSC) with a heating/cooling rate of 20 °C/min, and the phase transformation temperaturesextracted from DSC curves were obtained by tangent method usingPyris software. Rockwell hardness of the porousNiTi alloyswasmeasuredby HRB-150A Hardness tester with a load of 100 kg for 3 s. Compressiontest was carried out at an ambient temperature of 25 °C with a constantrate of 0.05 mm/min on Instron WDW-50 testing machine to obtain thecompressive strength and elastic modulus of the porous NiTi alloys andwith a constant rate of 0.5%/min on Instron 3365 testing machine to in-vestigate the superelasticity of the porous NiTi alloys. At the same time,the three loading–unloading cycle compressive tests under the pre-strain of 5% were carried out (the different pre-strains had been tried,but 5%of thepre-strainwas thebest to distinguish the superelastic behav-ior of the porous NiTi alloys with different porosities). The compressivesamples were machined into a cylindrical solid with a dimension of Ф5 mm × 10 mm (L/D = 2.0, ASTM E9-09). The bending tests of the rect-angular porous NiTi alloys (5 mm × 5 mm × 45 mm) were carried outat ambient temperature of 25 °C with a constant rate of 0.05 mm/minon Instron WDW-50 testing machine. The bending strength (σf) of theporous NiTi alloys could be calculated by the following formula:

σ f ¼ 3FL=2bh2

where F is the maximum loading during testing procedure, L is the spanbetween two supports and b and h represent the breadth and height ofthe samples, respectively. In this test, the span Lwas 30 mm.

3. Results and discussion

Fig. 2 shows the optical micrographs of the porous NiTi alloysprepared by microwave sintering with different contents of NH4HCO3. Itcould be seen that the number of pores distributed over the surfaceof the porous NiTi alloys increased with increasing the contents ofNH4HCO3. The pores of the porous NiTi alloys without adding NH4HCO3

were isolated and the connectivity among the pores was gradually en-hanced as the increase of the NH4HCO3 contents. The average pore sizesof the porous NiTi alloys also increased with increasing the contents ofNH4HCO3, shown in Fig. 3. The average pore size of the porous NiTialloy without adding NH4HCO3 was only 26 μm, while it increased from120 μm to 178 μm with increasing the NH4HCO3 contents from 10 wt.%to 30 wt.%. Especially, the pore size of the sample prepared with 10%NH4HCO3 (120 μm) was consistent with the size of the sieved NH4HCO3

particles (−120 mesh, ~125 μm). There existed a geometrical heredityeffect of space-holder NH4HCO3 particles on the pore shape and sizeof the porous NiTi alloys [24]. By further increasing the contents ofNH4HCO3, the number of pores increased and some of them might con-nect together, resulting in the increase of the pore sizes, even higherthan the size of NH4HCO3 particles.

The porosities and densities of the porous NiTi alloys prepared bymicrowave sintering with different NH4HCO3 contents are shown inFig. 4. The porosities of the porous NiTi alloys increased with increasingthe contents of NH4HCO3, while the densities decreased. The porosity ofthe porous NiTi samples without adding NH4HCO3 was only 22%, and itincreased from 41% for 10 wt.% NH4HCO3 sample to 62% for 30 wt.%NH4HCO3 sample. On the other hand, the density decreased from5.03 g/cm3 (0 NH4HCO3 sample) to 2.41 g/cm3 (30 wt.% NH4HCO3

sample), which was lower than those of aluminum and its alloys(~2.7 g/cm3) and very close to the density of human bone (1.8–2.1 g/cm3) [25]. According to the references [26,27], the ideal boneimplant materials should have the porosity in the range of 30–90%and the optimal pore size of 100–400 μm. Therefore, the porousNiTi alloy fabricated by microwave sintering had suitable porosityand pore size to become a promising candidate for bone implant.

Fig. 2. Optical micrographs of the porous NiTi alloys prepared by microwave sintering with different contents of NH4HCO3: (a) 0; (b) 10 wt.%; (c) 20 wt.%; and (d) 30 wt.%.

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Fig. 5 shows the XRD patterns of the porous NiTi alloys prepared bymicrowave sintering with different porosities. The porous NiTi alloysconsisted of nearly single B2 NiTi phasewith few other impurity phasesas the porosities lower than 50%, while the diffraction peaks of the un-desired secondary phases Ni3Ti and NiTi2 increased by further increas-ing the porosities. The microwave sintering process, including thegreatly enhanced diffusion of the atoms under the microwave fieldand the quickly derivedmicrowave heating from absorbing the electro-magnetic energy volumetrically (which were fundamentally differentfrom the conventional heating derived from the conduction, radiationand convection) [18,19], could accelerate the alloying of the NiTigreen compact sample and facilitate its complete reaction, resulting inthe formation of nearly single NiTi phase with few secondary phases.After the thermal decomposition of NH4HCO3 occurred, the excessivepores were left inside the NiTi green compact sample, whichwould im-pede thediffusion of the atoms on a large scale and form the local Ti-richregion and Ni-rich region, resulting in the increase of Ni3Ti phase andNiTi2 phase as the porosities higher than 50%. However, the weak dif-fraction intensity of the Ni3Ti and NiTi2 indicated that their contents

Fig. 3. The average pore sizes of the porous NiTi alloys prepared by microwave sinteringwith different NH4HCO3 contents.

inside the porous NiTi alloys were limited, much lower than otherporous NiTi alloy prepared in the previous references [21–23,28,29]. Itis difficult to obtain single NiTi phase for porous NiTi alloy preparedthrough one-step powder metallurgy methods including CS, HIP, SHSand SPS [8,30,31]. Therefore, it can be concluded that the microwavesintering method is beneficial to reduce, even eliminate the secondaryphases (Ni3Ti and NiTi2) for the preparation of porous NiTi alloys,which can improve the thermomechanical properties, corrosion resis-tance and biocompatibility of the porous NiTi alloys [32–34].

Fig. 6 shows the effect of porosities on Rockwell hardness of the po-rousNiTi alloys. The Rockwell hardness of the porous NiTi alloys abrupt-ly decreased by increasing the porosities, meanwhile its standarddeviation gradually increased. The hardness of sample with porosity of22% could reach 66HRB,while it decreased by 71% for the sample poros-ity of 62% (19HRB). The increase of porosities resulted in the decrease ofthe support force of porewalls,which inevitably decreased the hardnessof the samples.

The compressive stress–strain curves of the porous NiTi alloys withdifferent porosities are shown in Fig. 7 and the compressive strength

Fig. 4. The porosities and densities of the porous NiTi alloys prepared by microwavesintering with different NH4HCO3 contents.

Fig. 5.XRDpatterns of the porous NiTi alloys prepared bymicrowave sinteringwith differentporosities.

Fig. 7. Compressive stress–strain curves of the porous NiTi alloyswith different porosities.

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and elastic modulus of the porous NiTi alloys extracted from the stress–strain curves are shown in Fig. 8. An initial stage with low slope ap-peared in the compressive curves that may be related to experimentalinstrument and procedure error. By ignoring the initial stage, it couldbe seen from Fig. 7 that the compressive process could be divided intothree regions as follows [35]: (1) a linear elastic deformation region,where the slope could be considered as the elastic modulus of thesample; (2) a plastic yield deformation region, where a peak stress ap-peared, considered as the compressive strength of the sample; and(3) a densification and rupture region, where the walls of the poreswould collapse and the rupture of samples has occurred. It could beclear that the compressive strength and elastic modulus of the porousNiTi alloys decreased by increasing the porosities. The compressivestrength and elastic modulus of the sample with porosity of 22% couldreach 880 MPa and 7.5 GPa, respectively. By increasing the porosities,they decreased from319MPa and 3.91GPa for the samplewith porosityof 41% to 69 MPa and 1.1 GPa for the sample with porosity of 62%,respectively. The elasticmodulus of the porous NiTi alloyswhich rangedfrom 7.5 GPa to 1.1 GPa was very close to the elastic modulus of naturalbone (3–20 GPa for cortical bone and 0.05–0.5 GPa for cancellous bone)[36]. The compressive strength of the porous NiTi alloys which rangedfrom 880MPa to 69MPawas higher or close to the compressive strengthof natural bone (100–230MPa for cortical bone and 2–12MPa for cancel-lous bone) [36].

The bending strength is another important mechanical property forthe bone implantmaterials besides the compressive strength and elasticmodulus. The bending strength of the porous NiTi alloys with differentporosities is shown in Fig. 9. The bending strength of the porous NiTi

Fig. 6. Rockwell hardness of the porous NiTi alloys with different porosities for 100 kgf.

alloys almost linearly decreased from 371.4 MPa for the sample withporosity of 22% to 74.0 MPa for the sample with porosity of 62% byincreasing the porosities, all of whichwas higher or close to the bendingstrength of natural cortical bone (50 ~ 150MPa) [36]. Therefore, by onlyconsidering the compressive strength, elastic modulus and bendingstrength, the porous NiTi alloy fabricated by microwave sinteringcould be a promising candidate for the hard tissue repair and replace-ment implant.

The effect of porosities on the phase transformation behavior of theporousNiTi alloys is shown in Fig. 10 and the phase transformation tem-peratures are listed in Table 1. Two exothermic peaks were observedduring the cooling process, in which the transformations of B2 phaseto R phase and R phase to B19′ phase might occur. On the other hand,only one endothermic peak was detected during the heating process,in which B19′ phase transformed into B2 phase. This result was consis-tentwith the reference [21,23]. According to theDSC curves and Table 1,it was clear that the porosities had few effects on the phase transforma-tion temperatures of the porous NiTi alloys. In general, the phase trans-formation behavior of the dense NiTi alloysmainly depends on the alloycomposition and heat treatment process. In this paper, the compositionand heat treatment process of the porous NiTi alloys with different po-rosities were the same. Therefore, they had the same phase transforma-tion behavior and the same transformation temperatures. On the otherhand, the porosities had few effects on the phase transformation tem-peratures of the porous NiTi alloys.

The stress–strain curves of loading–unloading compressive tests ofthe porous NiTi alloy with different porosities at pre-strain of 5% areshown in Fig. 11 and the experimental results are shown in Table 2. It

Fig. 8.Relationship between porosity and compressive strength and elasticmodulus of theporous NiTi alloys.

Fig. 9. Relationship between porosity and bending strength of the porous NiTi alloys.

Table 1Phase transformation temperatures of the porous NiTi alloys with different porosity.

Porosity (%) Cooling process Heating process

Rs (°C) Rf (°C) Ms (°C) Mf (°C) As (°C) Af (°C)

22 18.0 7.3 7.1 −9.0 43.4 49.841 20.1 7.9 7.2 −9.1 43.4 50.750 21.4 7.7 7.5 −8.5 40.2 50.862 19.4 7.6 7.5 −7.5 42.1 50.6

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could be seen that the maximum stress and superelastic recoverystrains of the porous NiTi alloys decreased by increasing the porosities,while the residual strains increased. Each sample was compressed tothe strain of 5% at room temperature, and was then unloaded. Afterunloading, the residual strains were 0.44%, 1.65%, 1.84% and 2.72% forthe samples with porosity of 22%, 41%, 50% and 62%, respectively. Onthe other hand, the superelastic recovery strains of the samples couldbe up to 4.56%, 3.35%, 3.16% and 2.28%, respectively. If the compressedsamples were heated to 80 °C, higher than the Af, the residual strainsof the samples could be recovered to 100%, 75.1%, 63.2% and 37.7%, re-spectively due to their shapememory effect, which formed thememory

Fig. 10. DSC curves of the porous NiTi alloys with different porosities: (a) heating and(b) cooling.

recovery strains. Lastly, the total of strain recovery of the porous NiTi al-loys, including superelastic recovery strains and memory recoverystrains, could be obtained and shown in Table 2. The strain recovery ofthe porous NiTi alloys also decreased by increasing the porosities.

In order to investigate the effect of training on the superelasticity ofthe porous NiTi alloys, three loading–unloading cycles were carried out.The cycling stress–strain curves of the porous NiTi alloys with differentporosities are shown in Fig. 12. After three loading–unloading cycles, allof the stress–strain curves turned into closed loops, in other words, thesamples exhibited nearly complete superelasticity. The superelastic re-covery strains of the porous NiTi alloys could reach 4.7%, 4.4%, 4.0%and 3.1% for the samples with the porosity of 22%, 41%, 50% and 62%, re-spectively. The results were higher than those of the above-mentionedvalues, indicating that training could greatly improve the superelasticityof the porous NiTi alloy, consistent with other references [21,37]. How-ever, the superelasticity of the trained porous NiTi alloys was also lowerthan the pre-strain of 5%. It is well known that the dense bulk NiTi alloycan recover up to 8% strain in uniaxial deformation by a reversiblestress-inducedmartensitic transformation [1,2]. However,when the po-rous NiTi alloy was deformed to 5%, the certain local strain might behigher than 8% due to the stress concentration generated from thelarge number of pores or other defects inside the porous NiTi alloy.Therefore, the porous NiTi alloy is difficult to possess the actually com-plete superelasticity or shape memory effect even with training.

The porous NiTi alloys exhibiting excellent superelasticity (N3%) cangreatly match the natural bone which has a recoverable strain around2% [1,4,37]. This mechanical characteristic of the porous NiTi alloy isincomparable for other metallic biomaterials, such as Ti, Ti–6Al–4V,stainless steels, and Co based alloys. Moreover, the unique propertyshould obtain easier deployment of porous NiTi into the implantationsite. With the combination of the porosity, pore size, relatively purephase composition, mechanical properties, shape memory effect andsuperelasticity, it could be concluded that the porous NiTi alloys pre-pared by microwave sintering using ammonium hydrogen carbonateas the space holder agent should become a promising candidate forbone implant.

Fig. 11. Stress–strain curves of loading–unloading compressive tests of the porous NiTialloy with different porosities at the pre-strain of 5%.

Table 2Results of loading–unloading compressive tests of the porous NiTi alloys with differentporosities.

Porosity(%)

Maximumstress(MPa)

Superelasticrecoverystrain (%)

Residualstrain (%)

Memoryrecoverystrain (%)

Strainrecovery(%)

22 191.1 4.56 0.44 0.44 5.0041 169.5 3.35 1.65 1.24 4.5950 114.8 3.16 1.84 1.17 4.3362 44.3 2.28 2.72 1.03 3.31

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4. Conclusions

(1) Porous NiTi alloys were successfully prepared by microwavesintering and space holder technique. The porosities of the po-rous NiTi alloys increased from 22% to 62% by increasing the con-tents of NH4HCO3 and the corresponding average pore sizeincreased from 26 μm to 178 μm.

(2) The porous NiTi alloys consisted of nearly single NiTi phase withfew secondary phases as the porosities lower than 50%, while theNi3Ti phase and NiTi2 phase increased by further increasing theporosities.

(3) The porosities had few effects on the phase transformation tem-peratures of the porous NiTi alloys. Partial shape memory effectwas observed for all porosity levels and the porosities had ad-verse effect on the shape recovery.

(4) By increasing the porosities, all of the hardness, compressivestrength, elastic modulus, bending strength and superelasticityof the porous NiTi alloys decreased. The superelasticity of the po-rous NiTi alloys could be improved through training.

Fig. 12. The cycling stress–strain curves of the porous NiTi alloys w

Acknowledgments

The authors gratefully acknowledge the financial support of theproject from the National Natural Science Foundation of China(51101085), the National Natural Science Foundation of JiangxiProvince (20114BAB216014), the Foundation of Jiangsu ProvincialKey Laboratory for Interventional Medical Devices (JR1416) andthe Science and Technology Plan Projects of Jiangsu Province(BE2011726).

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