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Microstructure and mechanical properties of a newly developed low Young's modulus Ti–15Zr–5Cr–2Al biomedical alloy
Materials Science and Engineering C 72 (2017) 536–542
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Microstructure and mechanical properties of a newly developed low Young's modulus Ti–15Zr–5Cr–2Al biomedical alloy Pan Wang a,b,⁎, Lihong Wu a, Yan Feng c, Jiaming Bai b, Baicheng Zhang b, Jie Song b,⁎⁎, Shaokang Guan a a b c
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450002, PR China Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore State Key Laboratory of Solidification Processing, School of Materials, Northwestern Polytechnical University, 127# YouYi West Road, Xi'an 710072, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 3 November 2016 Accepted 24 November 2016 Available online 26 November 2016 Keywords: Titanium alloy Microstructure Mechanical properties Young's modulus Biomedical applications
a b s t r a c t The Ti–15Zr–5Cr–2Al alloy has been developed and various heat treatments have been investigated to develop new biomedical materials. It is found that the heat treatment conditions strongly affect the phase constitutions and mechanical properties. The as-cast specimen is comprised of β phase and a small fraction of α phase, which is attributed to the suppression of ω phase caused by adding Al. A high yield strength of 1148 ± 36 MPa and moderate Young's modulus of 96 ± 3 GPa are obtained in the as-cast specimen. Besides the β phase and α phase, ω phase is also detected in the air cooled and liquid nitrogen quenched specimens, which increases the Young's modulus and lowers the ductility. In contrast, only β phase is detected after ice water quenching. The ice water quenched specimen exhibits a good combination of mechanical properties with a high microhardness of 302 ± 10 HV, a large plastic strain of 23 ± 2%, a low Young's modulus of 58 ± 4 GPa, a moderate yield strength of 625 ± 32 MPa and a high compressive strength of 1880 ± 59 MPa. Moreover, the elastic energies of the ice water quenched specimen (3.22 MJ/m3) and as-cast specimen (6.86 MJ/m3) are higher than that of c.p. Ti (1.25 MJ/m3). These results demonstrate that as-cast and ice water quenched Ti–15Zr–5Cr–2Al alloys with a superior combination of mechanical properties are potential materials for biomedical applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The amazing combination of strength, toughness, ductility and corrosion resistance has made titanium and its alloys promising metals for biomedical applications [1]. Although the commercially pure Ti (c.p. Ti) and the Ti–6Al–4V alloy are still widely used for orthopedic applications, the lower mechanical strength for c.p. Ti and some long-term health problems caused by the release of V ion for Ti–6Al–4V alloy have prompted researchers to develop alternative alloys [1–3]. Therefore, many efforts have been devoted to develop V-free Ti alloy, such as Ti– Zr [4–6], Ti–Nb [7–12] and Ti–Cr alloy [13–16]. A recent work has shown that Ti–15Zr–xCr alloys exhibit high hardness, high strength, high elastic energy, but high Young's modulus [6]. Especially the Ti–15Zr–5Cr alloy, that has minimum Cr content to retain the β phase, exhibits very high Young's modulus and brittle fracture, which is caused by the appearance of the ω phase [6,14,17]. The ω ⁎ Correspondence to: Pan Wang, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450002, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Wang), [email protected], [email protected] (J. Song).
http://dx.doi.org/10.1016/j.msec.2016.11.101 0928-4931/© 2016 Elsevier B.V. All rights reserved.
phase is worth considering because its presence drastically affects the mechanical properties and increases the Young's modulus of the alloy [18,19]. Since Ti–Zr–Cr alloys are developed for dental implants and hard tissue replacements [6], a combination of low Young's modulus and high strength is necessary to avoid the stress-shielding effect and to bear heavy loading. Stress-shielding, which is caused by the difference between the Young's modulus of the replacements and that of the natural human bone, results in a bone degradation and absorption [20–22]. Therefore, a further investigation on the Ti–15Zr–5Cr alloy to develop new alloys with lower Young's modulus and high strength for biomedical applications is necessary. Al is by far the most important alloying element of titanium and some biomedical alloys have been developed by the addition of Al element [11,12,23–25]. Moreover, the addition of Al into some Ti alloys suppressed the formation of ω phase [24, 26,27]. It is speculated that ω phase could be suppressed and the Young's modulus could be lowered by the addition of Al element in the Ti–15Zr–5Cr alloy. For dental implants and hard tissue replacements, casting is a preferred technology to achieve very sophisticated geometrical parts. However, complex thermo-mechanical treatments, such as forging, rolling, extrusion, etc., which are always applied to modify the microstructure and in turn to optimize the mechanical properties [7,16,28–31], cannot
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be applied to these parts. Instead, simple heat treatments are available, besides the modification of compositions, to control the part's microstructure which will in turn affect its mechanical properties. Therefore, a newly developed Ti–15Zr–5Cr–2Al alloy was prepared and subjected to various simple heat treatments in the present study. The evolution of the microstructure and mechanical properties due to the addition of Al and heat treatments were investigated. Furthermore, the potential applications of the newly developed low Young's modulus Ti–15Zr–5Cr–2Al alloy were also discussed. 2. Experimental procedure The ingots of Ti–15Zr–5Cr–2Al (in wt.%) alloy were prepared using arc melting on a water-cooled copper hearth under a high-purity argon gas atmosphere. The detailed fabrication method can be found in Ref. [6]. Specimens for all measurements were cut from the ingots. In order to investigate the effect of heat treatment on the microstructure and mechanical properties, these specimens were divided into four groups. Three groups of them were put into the quartz tube with Ar atmosphere and subjected to solution treatment at 1173 K for 0.5 h. These specimens were mechanically polished with SiC paper of 1000# and cleaned with acetone and ethyl alcohol before heat treatment. Then, three cooling conditions corresponding to three cooling rates ((i) ice water quenching (WQ), (ii) liquid nitrogen quenching (LNQ) and (iii) air cooling (AC)) were applied in the present experiment. The cooling rates of these specimens for each heat treatment condition were WQ N LNQ N AC [32]. After heat treatment, the oxidized surface layer was smoothly removed by mechanical polishing using SiC paper of 2000#. Following that, electropolishing was carried out in a solution of 6% per chloric acid, 35% butanol, and 59% methanol at ~230 K. The phase constitutions of electropolished specimens were characterized using the X-ray diffraction (XRD) at room temperature with Cu Kα radiation operated at 45 kV and 40 mA with a low scanning rate, 0.2°/min. The microstructure of the specimens was examined using an optical microscope. For optical microscopy observation, the polished specimens were etched in an aqueous solution of hydrofluoric acid (HF, 10 vol.%) and nitric acid (HNO3, 20 vol.%). The microhardness of polished specimens was measured using a Vickers hardness tester (MMT-3; Matsuzawa, Tokyo, Japan), with a load of 500 g applied for 10 s. For each specimen, the average microhardness value was obtained by examining at least 12 indentations. Considering the target of this research is the development of new low Young's modulus Ti alloy for dental implants and hard tissue replacements, which are mainly under compression stress during performance, mechanical properties like Young's modulus, yield strength, compressive strength and ductility were thus evaluated by the compressive test using an Instron servo hydraulic dynamic testing system. The compressive test was conducted at room temperature in air at an initial strain rate of 1.67 × 10−3 s−1 and a strain gage was applied to measure the Young's modulus. The dimension of the compression specimen was 5 mm × 5 mm × 10 mm. All six faces of the specimen for compressive test were mechanical polished using SiC paper of 2000#. To minimize the measurement error of Young's modulus, the ends of each specimen were ensured to be flat and parallel within 0.0003 mm/mm, and perpendicular to the lateral surfaces within 2°. 3. Results and discussion 3.1. Microstructure Fig. 1 shows the XRD patterns of the Ti–15Zr–5Cr alloy [6] which is designated as 0Al in the following figures and Ti–15Zr–5Cr–2Al in different heat treatment conditions. Addition of Al changed the phase constitutions of the Ti–15Zr–5Cr alloy and the phase constitutions of the Ti– 15Zr–5Cr–2Al alloy were sensitive to the heat treatments. Ti–15Zr–5Cr alloy contained β phase and ω phase, whereas the as-cast Ti–15Zr–5Cr–
Fig. 1. XRD patterns of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions and the Ti–15Zr–5Cr alloy (0Al) [6].
2Al alloy contained β phase and α phase. There was no indication that ω phase peaks were included in the as-cast Ti–15Zr–5Cr–2Al alloy. This result implies that ω phase could be suppressed with the addition of 2 wt.% Al in the Ti–15Zr–5Cr alloy. It was reported that Al addition could suppress the formation of ω phase [24–26]. However, the microstructure with only low stability β phase was not obtained because of the precipitation of α phase. The low cooling rates during casting could cause the formation of α phase from diffusion [14]. Similar to the as-cast specimen, the AC and LNQ specimens exhibited a mixture of the β phase and α phase, though a small fraction of the ω phase was also detected with a low scanning rate, 0.2°/min. On the other hand, only the β phase was detected in the WQ specimen because of the fast cooling rate that suppressed the formation of α phase. Fig. 2 shows optical micrographs of the as-cast, AC, WQ and LNQ Ti– 15Zr–5Cr–2Al alloy. Typical mixed features consisting of α phase and β phase (untransformed areas) were observed in the as-cast (Fig. 2a), AC (Fig. 2b) and LNQ (Fig. 2d) specimens. Especially, the amount of α phase was significantly high in the AC specimen (Fig. 2b) and low in the LNQ (Fig. 2d) specimen. It should be mentioned that the α phase was mainly exhibited at the grain boundaries (Fig. 2d inset) in the LNQ specimen. This suggested a predominant nucleation site for the α phase from β phase. On the other hand, no trace of α phase was observed in the microstructure of the WQ (Fig. 2c) specimen, indicating that the microstructure was a complete β phase. These results agreed with the XRD results though the ω phase is too small to be observed by an optical microscope, which is in agreement with the previous report by Ho et al. [33].
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Fig. 2. Optical micrographs of the (a) as-cast, (b) AC, (c) WQ and (d) LNQ Ti–15Zr–5Cr–2Al alloy.
3.2. Mechanical properties Fig. 3 shows the microhardness values of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti were also shown here for comparison. The microhardness values of all the heat treatment conditions (302 HV–410 HV) were significantly higher than that of c.p. Ti (177 HV). The microhardness of as-cast specimen was lower than that of the Ti–15Zr–5Cr alloy, which was
Fig. 3. Microhardness of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti are shown for comparison [6].
attributed to the disappearance of ω phase. The highest microhardness value in the Ti–15Zr–5Cr alloy was mainly caused by the precipitation strengthening of the ω phase [6] since the ω phase had the highest hardness among all of the stable phases and metastable phases in Ti alloy [34]. The AC specimen exhibited the highest microhardness among all the studied specimens, which was due to the combined hardening effect of the ω phase and the α phase. On the other hand, the lowest microhardness was observed in the WQ specimen that was mainly a result of the absence of the second phase, though it was still 1.7 times greater than that of c.p. Ti. Fig. 4 shows the room temperature compressive stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. It is exhibited that the heat treatment strongly affected the yield strength, compressive strength, plastic strain and Young's modulus of the Ti–15Zr–5Cr–2Al alloy. The AC and LNQ specimens exhibited low ductility and high strength, whereas the as-cast specimen showed high strength and high ductility. Furthermore, the yield strength, compressive strength, plastic strain and Young's modulus of the specimens under different heat treatment conditions were revealed in Table 1. Ti–15Zr–5Cr alloy and c.p. Ti were also displayed here for comparison [6]. All the examined specimens manifested significantly higher yield strengths than c.p. Ti [6]. Although the addition of Al may have solid solution strengthening effect in the Ti–15Zr–5Cr alloy, the variation of phase constitutions has a dominant effect on the yield strength of the studied Ti–15Zr–5Cr–2Al alloy [25]. The yield strength was 625 MPa in the specimen with only the β phase (Figs. 1 and 2c). This value is still 1.6 times greater than that of c.p. Ti (393 MPa) and is higher than that of Ti–29Nb–13Ta–4.6Zr alloy [29] and Ti–24Nb–4Zr–7.6Sn [36] alloy which are well developed Ti alloys for biomedical applications. In contrast, the specimens that had the phase constitutions of β
Ti based implant alloys
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539 112 110
Ti-6Al-4V Ti-6Al-7Nb
104
c.p. Ti 80
Ti-15Mo-5Zr-3Al Ti-13Nb-13Zr
78
Ti-10Cr-0.2O
77 68
Ti-12Cr 60
Ti-29Nb-13Ta-4.6Zr
58
Ti-15Zr-5Cr-2Al
55
Ti-24Nb-4Zr-7.6Sn
55
Ti-29Nb-13Ta-7.1Zr 30
Bone
0
20
40
60
80
100
120
Young's Modulus, E/ GPa Fig. 5. Comparison of Young's moduli of Ti based implant alloys, Ti–6Al–4V [1], Ti–6Al– 7Nb [1], Ti–15Mo–5Zr–3Al [36], Ti–13Nb–3Zr [1], Ti–10Cr–0.2O [16], Ti–12Cr [37], Ti– 29Nb–13Ta–4.6Zr [29], Ti–24Nb–4Zr–7.6Sn [35], Ti–29Nb–13Ta–7.1Zr [1], c.p. Ti and Ti– 15Zr–5Cr–2Al (WQ) alloy.
Fig. 4. Stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions.
phase + α phase and/or ω phase exhibited high yield strength (1148 MPa–1250 MPa, which was about 2.9–3.2 times greater than that of c.p. Ti). On the other hand, the as-cast specimen and the WQ specimen exhibited higher ductility than the AC and LNQ specimens, in which the ω phase was detected. It is suggested that the ω phase enhances the strength and simultaneously induces embrittlement in the Ti alloy [14,17]. In contrast to the yield strength, no significant difference in the compressive strength was observed for the as-cast and AC specimens. The LNQ specimen exhibited the lowest compressive strength which was caused by the poor ductility due to the presence of the ω phase [13,14,34]. In contrast, the WQ specimen with only the β phase exhibited the highest compressive strength (1880 MPa), which resulted from significant work hardening effect under compression in the metallic materials with good ductility. Fig. 4(inset) demonstrates the Young's modulus obtained from the compressive stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under various heat treatment conditions. It is found that the WQ specimen (58 GPa) exhibited the lowest Young's modulus as compared to that of c.p. Ti (104 GPa), Ti–15Zr–5Cr alloy (129 GPa) and the other studied conditions (96–106 GPa). Fig. 5 shows the comparison of Young's modulus of WQ Ti–15Zr–5Cr–2Al alloy and the other Ti alloys developed for biomedical applications, such as Ti–6Al–7Nb [1], Ti–15Mo–5Zr–3Al [35], Ti–13Nb–3Zr [1], Ti–29Nb–13Ta–4.6Zr [29] and Ti–24Nb–4Zr– 7.6Sn [36]. It is clearly seen that the WQ Ti–15Zr–5Cr–2Al alloy has a lower Young's modulus than most of them and is comparable to that
Table 1 Mean and standard deviation of yield strength, compressive strength, plastic strain and Young's modulus of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti are shown for comparison [6]. All the results were obtained from at least five experimental values. Specimens Yield strength (0.2% offset) (MPa)
Compressive strength (MPa)
Plastic strain (%)
Compressive Young's modulus (GPa)
As-cast AC WQ LNQ 0Al c.p. Ti
1689.5 1674.5 1879.9 1301.3 1217.3 2193.5
24.0 ± 0.7 13.1 + 1.1 23.7 + 2.1 9.1 + 1.5 N/A N40.0
96.1 ± 3.2 106.6 ± 6.6 58.1 ± 3.5 96.2 ± 1.8 120.8 ± 9.5 104.3 ± 7.6
1147.6 ± 36.2 1249.6 ± 26.3 625.4 ± 31.8 1202.5 ± 13.8 1217.3 ± 6.6 393.0 ± 7.1
± ± ± ± ± ±
44.5 29.1 58.5 42.6 6.6 47.4
of Ti–29Nb–13Ta–7.1Zr (55 GPa) [1] and Ti–24Nb–4Zr–7.6Sn (55 GPa) [36]. The low Young's modulus in WQ Ti–15Zr–5Cr–2Al alloy can be explained by its phase stability, which will be discussed in detail in Section 4. Moreover, the Young's modulus of WQ Ti–15Zr–5Cr–2Al alloy is close to the Young's modulus of the cortical bone. This can be effective in the suppression of stress-shielding effect when the WQ specimen is used as biomedical implants [21,22,37]. The stress-shielding effect was caused by the difference in Young's modulus between the replacements and the natural human bone, which can cause bone degradation and absorption [21,24,37]. In addition, no significant difference was observed between the AC specimen and c.p. Ti, although the Young's moduli of the as-cast and LNQ specimens were lower than that of c.p. Ti. It should be mentioned that the Young's moduli of all the studied specimens were significantly lower than that of the Ti–15Zr–5Cr alloy, which was attributed to the suppression of the ω phase in the present study (Fig. 1). It was reported that the ω phase has the highest Young's modulus and a small fraction of the ω phase increases the Young's modulus drastically [5,19,29]. 3.3. Elasticity of the Ti–15Zr–5Cr–2Al biomedical materials For biomedical applications, it is important to investigate the elastic energy of the materials for ensuring a safe performance during service [6]. Fig. 6(a) shows the illustration of the elastic energy in a stress–strain curve and the elastic energy, δe, is expressed as [39]
δe ¼
σ 2y 1 εe σ y ¼ 2E 2
ð1Þ
where εe is the elastic strain and σy is the yield strength and E is Young's modulus. Fig. 6(b) shows the elastic energies, calculated using the Eq. (1), of the present studied alloy and the other metallic biomaterials [6, 40,41]. It is found that the heat treatment conditions strongly affected the elastic energy of the present studied alloy. Compared with the c.p. Ti, the Ti–6Al–4V, Mg alloy etc., specimens under all the studied heat treatment conditions exhibited superior elastic energies. These elastic energies, except that of the WQ specimen, were even higher than that of most of the other Ti alloys. However, the WQ specimen that exhibited high ductility and the lowest Young's modulus should also be considered as biomedical materials since its hardness, strength and elastic energy were still higher than that of c.p. Ti. On the other hand, the AC and LNQ specimens exhibited a higher Young's modulus and lower plastic strain than the as-cast specimen. Therefore, the as-cast and WQ specimens exhibited superior ductility and high elastic energy, which make them the promising candidates for biomedical applications.
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Fig. 6. (a) Illustration of elastic energy in a stress–strain curve and (b) elastic energy dependence of Young's modulus, comparing the present alloy and the other biomaterials.
4. Potential applications of the Ti–15Zr–5Cr–2Al biomedical materials Besides the casting of Ti–15Zr–5Cr–2Al alloy for dental or hard tissue replacements, the first potential application of this alloy is for single crystal implants by controlling the different crystal orientations. According to previous studies, the electron–atom (e/a) ratio has the dominant effect on the Young's modulus of the β phase Ti alloy system [8,10, 24]. The e/a is the average number of the valence electron per atom in the free atom configuration. Fig. 7 shows the comparison of the Young's modulus between the Ti–15Zr–5Cr–2Al alloy and the β–Ti–Cr–base alloys [5,6,15,16,38,42], as a function of e/a ratio. E of these alloys decreased with the decrease of e/a. In the present study, the e/a ratio for
Fig. 7. Comparison of Young's modulus between the Ti–15Zr–5Cr–2Al alloy and β–Ti–Cr alloys (Ti–Cr [15,16,37,42], Ti–15Zr–Cr [6] and Ti–30Zr–Cr [5]) on the basis of e/a ratio. The polycrystals Young's modulus of Ti–Cr [42] single crystals are calculated by using the independent elastic constants of the single crystals from the Hill approximation [19,24].
the Ti–15Zr–5Cr–2Al alloy was 4.06, which resulted in a relatively low Young's modulus. The e/a of the Ti–15Zr–5Cr–2Al alloy was calculated using the nominal compositions of Ti, Zr, Cr and Al on the basis of the electron configurations of Ti ([Ar]3d24s2), Zr ([Kr]4d25s2), Cr ([Ar]4d45s2) and Al ([Ne]3d23p1). Recently, Wang [25] reported that the Young's modulus at 〈100〉 direction, E100, decreased with the decrease of the e/a ratio. Moreover, the elastic anisotropy increased with the decrease of the e/a ratio and the E100/E reached to 0.49 in the Ti– 28Nb–7Al (in at.%) single crystal (e/a = 4.21) [25]. This implies that the Ti–15Zr–5Cr–2Al alloy with an e/a ratio of 4.06 may reach a lower E100/E than that of the Ti–28Nb–7Al (in at.%) single crystal. Accordingly, the E100 should be around 28.4 GPa if E100/E of Ti–15Zr–5Cr–2Al alloy reaches 0.49 or even lower. This level is comparable to the Young's modulus of the cortical one (20–40 GPa) [21] and can further reduce the stress-shielding effect if we focused on the control of crystallographic orientation of this alloy for implants [24]. The second potential application of this alloy is for spinal fixation devices, where a tunable Young's modulus is necessary to satisfy the conflicting requirements of the surgeons and patients. A high Young's modulus is an advantageous property from the viewpoint of surgeons to avoid a big spring back, while a low Young's modulus is desired from the viewpoint of the patient to prevent the stress-shielding effect. Therefore, some researchers have focused on the development of the new alloys with tunable Young's modulus, such as Ti–12Cr [38], Ti–(10–12)Cr–(0.2–0.6)O [15], Ti–30Zr–(Cr, Mo) [5] and Ti–29Nb– 13Ta–4.6Zr–(8, 16)Ti–(2, 4)Cr [29], by introducing the deformation-induce ω transformation. As pointed out by Li et al., the deformation-induced ω transformation was more easily obtained in the metastable β Ti alloy by the addition of Cr [29]. Therefore, the WQ Ti–15Zr–5Cr–2Al alloy has the possibility to be induced ω transformation by cold deformation. Further study to elucidate the application of spinal fixation devices is now in progress. The third potential application of this alloy is to develop high strength and moderate Young's modulus Ti alloys for biomedical applications. The strength of β phase Ti alloys could be modified by the addition of alloy elements and/or thermo-mechanical treatment [1,15,16,29, 40]. By optimizing the fraction, size and distribution of the α, α′ and ω phases, the thermo-mechanically treated Ti–15Zr–5Cr–2Al alloy could reach a strength of 1250 MPa and achieve a moderate Young's modulus, such as in a range of 70–90 GPa. In summary, the as-cast and WQ Ti–15Zr–5Cr–2Al alloys exhibited superior ductility and high elastic energy, making them promising candidates for dental implants and hard tissue replacements. In addition, three potential applications, namely, (i) low Young's modulus implants
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by focusing on crystallographic orientations, (ii) spinal fixation devices by focusing on deformation-induced ω transformation, and (iii) high strength and moderate Young's modulus Ti alloy implants by focusing on thermo-mechanical treatment, further make this alloy a promising biomedical alloy. Although the biocompatibility of Ti–15Zr–5Cr–2Al alloys in the present study was not mentioned, the Ti–15Zr–5Cr–2Al alloy was designed by considering of biocompatibility. The preliminary cell culturing results revealed that this alloy was not only biocompatible, but also conducive to cell attachment. Furthermore, biomedical Ti alloys containing Cr or Zr/Al elements which exhibit excellent biocompatibility has been reported in the literatures, such as Ti–12Cr [38], Ti–10Cr– 0.2O [16] and Ti–15Mo–5Zr–3Al [35]. Although we may predict the in vitro and in vivo behavior, we will assess and discuss the biomedical tests of this alloy, which are underway and are going to be published in a separate paper. 5. Conclusions The microstructure and mechanical properties of the as-cast and heat treated Ti–15Zr–5Cr–2Al alloy were investigated by XRD analysis, optical microscopy analysis, microhardness measurement and compressive test. The following conclusions can be drawn: (1) The as-cast Ti–15Zr–5Cr–2Al alloy was comprised of the β and α phases. After AC and LNQ, the additional ω phase was detected. On the other hand, the α phase disappeared and only the β phase was detected in the WQ specimen. (2) The microhardness of the as-cast Ti–15Zr–5Cr–2Al alloy was 385 ± 20 HV, whereas that of the WQ specimen was 302 ± 10 HV. The AC specimen exhibited the highest microhardness (410 ± 14 HV) because of the combined hardening effect of the ω and α phases. (3) The yield strength of the WQ specimen was 625 ± 32 MPa, whereas the specimens under the other conditions exhibited higher yield strength in the range of 1148–1250 MPa. (4) The as-cast and WQ specimens exhibited higher ductility than that of the AC and LNQ specimens. Moreover, the WQ specimen exhibited the highest compressive strength (1880 ± 59 MPa) and the lowest Young's modulus (58 ± 4 GPa). (5) The elastic energies of the specimens under all studied conditions, ranging between 3.22 and 7.52 MJ/m3, were higher than that of c.p. Ti (1.25 MJ/m3). (6) The Ti–15Zr–5Cr–2Al alloy has several potential applications, namely, (i) low Young's modulus implants by focusing on crystallographic orientations, (ii) spinal fixation devices by focusing on deformation-induced ω transformation, and (iii) high strength and moderate Young's modulus Ti alloy implants by focusing on thermo-mechanical treatment. Acknowledgments The authors are grateful for the helpful discussions with Dr. S.M.L. Nai at A*STAR Singapore Institute of Manufacturing Technology (SIMTech), and Dr. X.P. Tan, at Nanyang Technological University. The National Natural Science Foundation of China (No. 51171174) supported this study. References [1] M. Geetha, A. Singh, R. Asokamani, A. Gogia, Ti based biomaterials, the ultimate choice for orthopaedic implants – a review, Prog. Mater. Sci. 54 (2009) 397–425. [2] P. Wang, M.L.S. Nai, X. Tan, G. Vastola, R. Srinivasan, W.J. Sin, Recent progress of additive manufactured Ti-6Al-4V by electron beam melting, Austin, TX, USA, August 8–10, 27th Annual International Solid Freeform Fabrication Symposium—An Additive Manufacturing Conference, 2016, 2016, pp. 691–704. [3] S. Nag, R. Banerjee, H.L. Fraser, Microstructural evolution and strengthening mechanisms in Ti–Nb–Zr–Ta, Ti–Mo–Zr–Fe and Ti–15Mo biocompatible alloys, Mater. Sci. Eng. C 25 (2005) 357–362.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Microstructure and mechanical properties of a newly developed low Young's modulus Ti–15Zr–5Cr–2Al biomedical alloy Pan Wang a,b,⁎, Lihong Wu a, Yan Feng c, Jiaming Bai b, Baicheng Zhang b, Jie Song b,⁎⁎, Shaokang Guan a a b c
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450002, PR China Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore State Key Laboratory of Solidification Processing, School of Materials, Northwestern Polytechnical University, 127# YouYi West Road, Xi'an 710072, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 3 November 2016 Accepted 24 November 2016 Available online 26 November 2016 Keywords: Titanium alloy Microstructure Mechanical properties Young's modulus Biomedical applications
a b s t r a c t The Ti–15Zr–5Cr–2Al alloy has been developed and various heat treatments have been investigated to develop new biomedical materials. It is found that the heat treatment conditions strongly affect the phase constitutions and mechanical properties. The as-cast specimen is comprised of β phase and a small fraction of α phase, which is attributed to the suppression of ω phase caused by adding Al. A high yield strength of 1148 ± 36 MPa and moderate Young's modulus of 96 ± 3 GPa are obtained in the as-cast specimen. Besides the β phase and α phase, ω phase is also detected in the air cooled and liquid nitrogen quenched specimens, which increases the Young's modulus and lowers the ductility. In contrast, only β phase is detected after ice water quenching. The ice water quenched specimen exhibits a good combination of mechanical properties with a high microhardness of 302 ± 10 HV, a large plastic strain of 23 ± 2%, a low Young's modulus of 58 ± 4 GPa, a moderate yield strength of 625 ± 32 MPa and a high compressive strength of 1880 ± 59 MPa. Moreover, the elastic energies of the ice water quenched specimen (3.22 MJ/m3) and as-cast specimen (6.86 MJ/m3) are higher than that of c.p. Ti (1.25 MJ/m3). These results demonstrate that as-cast and ice water quenched Ti–15Zr–5Cr–2Al alloys with a superior combination of mechanical properties are potential materials for biomedical applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The amazing combination of strength, toughness, ductility and corrosion resistance has made titanium and its alloys promising metals for biomedical applications [1]. Although the commercially pure Ti (c.p. Ti) and the Ti–6Al–4V alloy are still widely used for orthopedic applications, the lower mechanical strength for c.p. Ti and some long-term health problems caused by the release of V ion for Ti–6Al–4V alloy have prompted researchers to develop alternative alloys [1–3]. Therefore, many efforts have been devoted to develop V-free Ti alloy, such as Ti– Zr [4–6], Ti–Nb [7–12] and Ti–Cr alloy [13–16]. A recent work has shown that Ti–15Zr–xCr alloys exhibit high hardness, high strength, high elastic energy, but high Young's modulus [6]. Especially the Ti–15Zr–5Cr alloy, that has minimum Cr content to retain the β phase, exhibits very high Young's modulus and brittle fracture, which is caused by the appearance of the ω phase [6,14,17]. The ω ⁎ Correspondence to: Pan Wang, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450002, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Wang), [email protected], [email protected] (J. Song).
http://dx.doi.org/10.1016/j.msec.2016.11.101 0928-4931/© 2016 Elsevier B.V. All rights reserved.
phase is worth considering because its presence drastically affects the mechanical properties and increases the Young's modulus of the alloy [18,19]. Since Ti–Zr–Cr alloys are developed for dental implants and hard tissue replacements [6], a combination of low Young's modulus and high strength is necessary to avoid the stress-shielding effect and to bear heavy loading. Stress-shielding, which is caused by the difference between the Young's modulus of the replacements and that of the natural human bone, results in a bone degradation and absorption [20–22]. Therefore, a further investigation on the Ti–15Zr–5Cr alloy to develop new alloys with lower Young's modulus and high strength for biomedical applications is necessary. Al is by far the most important alloying element of titanium and some biomedical alloys have been developed by the addition of Al element [11,12,23–25]. Moreover, the addition of Al into some Ti alloys suppressed the formation of ω phase [24, 26,27]. It is speculated that ω phase could be suppressed and the Young's modulus could be lowered by the addition of Al element in the Ti–15Zr–5Cr alloy. For dental implants and hard tissue replacements, casting is a preferred technology to achieve very sophisticated geometrical parts. However, complex thermo-mechanical treatments, such as forging, rolling, extrusion, etc., which are always applied to modify the microstructure and in turn to optimize the mechanical properties [7,16,28–31], cannot
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be applied to these parts. Instead, simple heat treatments are available, besides the modification of compositions, to control the part's microstructure which will in turn affect its mechanical properties. Therefore, a newly developed Ti–15Zr–5Cr–2Al alloy was prepared and subjected to various simple heat treatments in the present study. The evolution of the microstructure and mechanical properties due to the addition of Al and heat treatments were investigated. Furthermore, the potential applications of the newly developed low Young's modulus Ti–15Zr–5Cr–2Al alloy were also discussed. 2. Experimental procedure The ingots of Ti–15Zr–5Cr–2Al (in wt.%) alloy were prepared using arc melting on a water-cooled copper hearth under a high-purity argon gas atmosphere. The detailed fabrication method can be found in Ref. [6]. Specimens for all measurements were cut from the ingots. In order to investigate the effect of heat treatment on the microstructure and mechanical properties, these specimens were divided into four groups. Three groups of them were put into the quartz tube with Ar atmosphere and subjected to solution treatment at 1173 K for 0.5 h. These specimens were mechanically polished with SiC paper of 1000# and cleaned with acetone and ethyl alcohol before heat treatment. Then, three cooling conditions corresponding to three cooling rates ((i) ice water quenching (WQ), (ii) liquid nitrogen quenching (LNQ) and (iii) air cooling (AC)) were applied in the present experiment. The cooling rates of these specimens for each heat treatment condition were WQ N LNQ N AC [32]. After heat treatment, the oxidized surface layer was smoothly removed by mechanical polishing using SiC paper of 2000#. Following that, electropolishing was carried out in a solution of 6% per chloric acid, 35% butanol, and 59% methanol at ~230 K. The phase constitutions of electropolished specimens were characterized using the X-ray diffraction (XRD) at room temperature with Cu Kα radiation operated at 45 kV and 40 mA with a low scanning rate, 0.2°/min. The microstructure of the specimens was examined using an optical microscope. For optical microscopy observation, the polished specimens were etched in an aqueous solution of hydrofluoric acid (HF, 10 vol.%) and nitric acid (HNO3, 20 vol.%). The microhardness of polished specimens was measured using a Vickers hardness tester (MMT-3; Matsuzawa, Tokyo, Japan), with a load of 500 g applied for 10 s. For each specimen, the average microhardness value was obtained by examining at least 12 indentations. Considering the target of this research is the development of new low Young's modulus Ti alloy for dental implants and hard tissue replacements, which are mainly under compression stress during performance, mechanical properties like Young's modulus, yield strength, compressive strength and ductility were thus evaluated by the compressive test using an Instron servo hydraulic dynamic testing system. The compressive test was conducted at room temperature in air at an initial strain rate of 1.67 × 10−3 s−1 and a strain gage was applied to measure the Young's modulus. The dimension of the compression specimen was 5 mm × 5 mm × 10 mm. All six faces of the specimen for compressive test were mechanical polished using SiC paper of 2000#. To minimize the measurement error of Young's modulus, the ends of each specimen were ensured to be flat and parallel within 0.0003 mm/mm, and perpendicular to the lateral surfaces within 2°. 3. Results and discussion 3.1. Microstructure Fig. 1 shows the XRD patterns of the Ti–15Zr–5Cr alloy [6] which is designated as 0Al in the following figures and Ti–15Zr–5Cr–2Al in different heat treatment conditions. Addition of Al changed the phase constitutions of the Ti–15Zr–5Cr alloy and the phase constitutions of the Ti– 15Zr–5Cr–2Al alloy were sensitive to the heat treatments. Ti–15Zr–5Cr alloy contained β phase and ω phase, whereas the as-cast Ti–15Zr–5Cr–
Fig. 1. XRD patterns of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions and the Ti–15Zr–5Cr alloy (0Al) [6].
2Al alloy contained β phase and α phase. There was no indication that ω phase peaks were included in the as-cast Ti–15Zr–5Cr–2Al alloy. This result implies that ω phase could be suppressed with the addition of 2 wt.% Al in the Ti–15Zr–5Cr alloy. It was reported that Al addition could suppress the formation of ω phase [24–26]. However, the microstructure with only low stability β phase was not obtained because of the precipitation of α phase. The low cooling rates during casting could cause the formation of α phase from diffusion [14]. Similar to the as-cast specimen, the AC and LNQ specimens exhibited a mixture of the β phase and α phase, though a small fraction of the ω phase was also detected with a low scanning rate, 0.2°/min. On the other hand, only the β phase was detected in the WQ specimen because of the fast cooling rate that suppressed the formation of α phase. Fig. 2 shows optical micrographs of the as-cast, AC, WQ and LNQ Ti– 15Zr–5Cr–2Al alloy. Typical mixed features consisting of α phase and β phase (untransformed areas) were observed in the as-cast (Fig. 2a), AC (Fig. 2b) and LNQ (Fig. 2d) specimens. Especially, the amount of α phase was significantly high in the AC specimen (Fig. 2b) and low in the LNQ (Fig. 2d) specimen. It should be mentioned that the α phase was mainly exhibited at the grain boundaries (Fig. 2d inset) in the LNQ specimen. This suggested a predominant nucleation site for the α phase from β phase. On the other hand, no trace of α phase was observed in the microstructure of the WQ (Fig. 2c) specimen, indicating that the microstructure was a complete β phase. These results agreed with the XRD results though the ω phase is too small to be observed by an optical microscope, which is in agreement with the previous report by Ho et al. [33].
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Fig. 2. Optical micrographs of the (a) as-cast, (b) AC, (c) WQ and (d) LNQ Ti–15Zr–5Cr–2Al alloy.
3.2. Mechanical properties Fig. 3 shows the microhardness values of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti were also shown here for comparison. The microhardness values of all the heat treatment conditions (302 HV–410 HV) were significantly higher than that of c.p. Ti (177 HV). The microhardness of as-cast specimen was lower than that of the Ti–15Zr–5Cr alloy, which was
Fig. 3. Microhardness of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti are shown for comparison [6].
attributed to the disappearance of ω phase. The highest microhardness value in the Ti–15Zr–5Cr alloy was mainly caused by the precipitation strengthening of the ω phase [6] since the ω phase had the highest hardness among all of the stable phases and metastable phases in Ti alloy [34]. The AC specimen exhibited the highest microhardness among all the studied specimens, which was due to the combined hardening effect of the ω phase and the α phase. On the other hand, the lowest microhardness was observed in the WQ specimen that was mainly a result of the absence of the second phase, though it was still 1.7 times greater than that of c.p. Ti. Fig. 4 shows the room temperature compressive stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. It is exhibited that the heat treatment strongly affected the yield strength, compressive strength, plastic strain and Young's modulus of the Ti–15Zr–5Cr–2Al alloy. The AC and LNQ specimens exhibited low ductility and high strength, whereas the as-cast specimen showed high strength and high ductility. Furthermore, the yield strength, compressive strength, plastic strain and Young's modulus of the specimens under different heat treatment conditions were revealed in Table 1. Ti–15Zr–5Cr alloy and c.p. Ti were also displayed here for comparison [6]. All the examined specimens manifested significantly higher yield strengths than c.p. Ti [6]. Although the addition of Al may have solid solution strengthening effect in the Ti–15Zr–5Cr alloy, the variation of phase constitutions has a dominant effect on the yield strength of the studied Ti–15Zr–5Cr–2Al alloy [25]. The yield strength was 625 MPa in the specimen with only the β phase (Figs. 1 and 2c). This value is still 1.6 times greater than that of c.p. Ti (393 MPa) and is higher than that of Ti–29Nb–13Ta–4.6Zr alloy [29] and Ti–24Nb–4Zr–7.6Sn [36] alloy which are well developed Ti alloys for biomedical applications. In contrast, the specimens that had the phase constitutions of β
Ti based implant alloys
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539 112 110
Ti-6Al-4V Ti-6Al-7Nb
104
c.p. Ti 80
Ti-15Mo-5Zr-3Al Ti-13Nb-13Zr
78
Ti-10Cr-0.2O
77 68
Ti-12Cr 60
Ti-29Nb-13Ta-4.6Zr
58
Ti-15Zr-5Cr-2Al
55
Ti-24Nb-4Zr-7.6Sn
55
Ti-29Nb-13Ta-7.1Zr 30
Bone
0
20
40
60
80
100
120
Young's Modulus, E/ GPa Fig. 5. Comparison of Young's moduli of Ti based implant alloys, Ti–6Al–4V [1], Ti–6Al– 7Nb [1], Ti–15Mo–5Zr–3Al [36], Ti–13Nb–3Zr [1], Ti–10Cr–0.2O [16], Ti–12Cr [37], Ti– 29Nb–13Ta–4.6Zr [29], Ti–24Nb–4Zr–7.6Sn [35], Ti–29Nb–13Ta–7.1Zr [1], c.p. Ti and Ti– 15Zr–5Cr–2Al (WQ) alloy.
Fig. 4. Stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions.
phase + α phase and/or ω phase exhibited high yield strength (1148 MPa–1250 MPa, which was about 2.9–3.2 times greater than that of c.p. Ti). On the other hand, the as-cast specimen and the WQ specimen exhibited higher ductility than the AC and LNQ specimens, in which the ω phase was detected. It is suggested that the ω phase enhances the strength and simultaneously induces embrittlement in the Ti alloy [14,17]. In contrast to the yield strength, no significant difference in the compressive strength was observed for the as-cast and AC specimens. The LNQ specimen exhibited the lowest compressive strength which was caused by the poor ductility due to the presence of the ω phase [13,14,34]. In contrast, the WQ specimen with only the β phase exhibited the highest compressive strength (1880 MPa), which resulted from significant work hardening effect under compression in the metallic materials with good ductility. Fig. 4(inset) demonstrates the Young's modulus obtained from the compressive stress–strain curves of the Ti–15Zr–5Cr–2Al alloy under various heat treatment conditions. It is found that the WQ specimen (58 GPa) exhibited the lowest Young's modulus as compared to that of c.p. Ti (104 GPa), Ti–15Zr–5Cr alloy (129 GPa) and the other studied conditions (96–106 GPa). Fig. 5 shows the comparison of Young's modulus of WQ Ti–15Zr–5Cr–2Al alloy and the other Ti alloys developed for biomedical applications, such as Ti–6Al–7Nb [1], Ti–15Mo–5Zr–3Al [35], Ti–13Nb–3Zr [1], Ti–29Nb–13Ta–4.6Zr [29] and Ti–24Nb–4Zr– 7.6Sn [36]. It is clearly seen that the WQ Ti–15Zr–5Cr–2Al alloy has a lower Young's modulus than most of them and is comparable to that
Table 1 Mean and standard deviation of yield strength, compressive strength, plastic strain and Young's modulus of the Ti–15Zr–5Cr–2Al alloy under different heat treatment conditions. Ti–15Zr–5Cr alloy and c.p. Ti are shown for comparison [6]. All the results were obtained from at least five experimental values. Specimens Yield strength (0.2% offset) (MPa)
Compressive strength (MPa)
Plastic strain (%)
Compressive Young's modulus (GPa)
As-cast AC WQ LNQ 0Al c.p. Ti
1689.5 1674.5 1879.9 1301.3 1217.3 2193.5
24.0 ± 0.7 13.1 + 1.1 23.7 + 2.1 9.1 + 1.5 N/A N40.0
96.1 ± 3.2 106.6 ± 6.6 58.1 ± 3.5 96.2 ± 1.8 120.8 ± 9.5 104.3 ± 7.6
1147.6 ± 36.2 1249.6 ± 26.3 625.4 ± 31.8 1202.5 ± 13.8 1217.3 ± 6.6 393.0 ± 7.1
± ± ± ± ± ±
44.5 29.1 58.5 42.6 6.6 47.4
of Ti–29Nb–13Ta–7.1Zr (55 GPa) [1] and Ti–24Nb–4Zr–7.6Sn (55 GPa) [36]. The low Young's modulus in WQ Ti–15Zr–5Cr–2Al alloy can be explained by its phase stability, which will be discussed in detail in Section 4. Moreover, the Young's modulus of WQ Ti–15Zr–5Cr–2Al alloy is close to the Young's modulus of the cortical bone. This can be effective in the suppression of stress-shielding effect when the WQ specimen is used as biomedical implants [21,22,37]. The stress-shielding effect was caused by the difference in Young's modulus between the replacements and the natural human bone, which can cause bone degradation and absorption [21,24,37]. In addition, no significant difference was observed between the AC specimen and c.p. Ti, although the Young's moduli of the as-cast and LNQ specimens were lower than that of c.p. Ti. It should be mentioned that the Young's moduli of all the studied specimens were significantly lower than that of the Ti–15Zr–5Cr alloy, which was attributed to the suppression of the ω phase in the present study (Fig. 1). It was reported that the ω phase has the highest Young's modulus and a small fraction of the ω phase increases the Young's modulus drastically [5,19,29]. 3.3. Elasticity of the Ti–15Zr–5Cr–2Al biomedical materials For biomedical applications, it is important to investigate the elastic energy of the materials for ensuring a safe performance during service [6]. Fig. 6(a) shows the illustration of the elastic energy in a stress–strain curve and the elastic energy, δe, is expressed as [39]
δe ¼
σ 2y 1 εe σ y ¼ 2E 2
ð1Þ
where εe is the elastic strain and σy is the yield strength and E is Young's modulus. Fig. 6(b) shows the elastic energies, calculated using the Eq. (1), of the present studied alloy and the other metallic biomaterials [6, 40,41]. It is found that the heat treatment conditions strongly affected the elastic energy of the present studied alloy. Compared with the c.p. Ti, the Ti–6Al–4V, Mg alloy etc., specimens under all the studied heat treatment conditions exhibited superior elastic energies. These elastic energies, except that of the WQ specimen, were even higher than that of most of the other Ti alloys. However, the WQ specimen that exhibited high ductility and the lowest Young's modulus should also be considered as biomedical materials since its hardness, strength and elastic energy were still higher than that of c.p. Ti. On the other hand, the AC and LNQ specimens exhibited a higher Young's modulus and lower plastic strain than the as-cast specimen. Therefore, the as-cast and WQ specimens exhibited superior ductility and high elastic energy, which make them the promising candidates for biomedical applications.
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Fig. 6. (a) Illustration of elastic energy in a stress–strain curve and (b) elastic energy dependence of Young's modulus, comparing the present alloy and the other biomaterials.
4. Potential applications of the Ti–15Zr–5Cr–2Al biomedical materials Besides the casting of Ti–15Zr–5Cr–2Al alloy for dental or hard tissue replacements, the first potential application of this alloy is for single crystal implants by controlling the different crystal orientations. According to previous studies, the electron–atom (e/a) ratio has the dominant effect on the Young's modulus of the β phase Ti alloy system [8,10, 24]. The e/a is the average number of the valence electron per atom in the free atom configuration. Fig. 7 shows the comparison of the Young's modulus between the Ti–15Zr–5Cr–2Al alloy and the β–Ti–Cr–base alloys [5,6,15,16,38,42], as a function of e/a ratio. E of these alloys decreased with the decrease of e/a. In the present study, the e/a ratio for
Fig. 7. Comparison of Young's modulus between the Ti–15Zr–5Cr–2Al alloy and β–Ti–Cr alloys (Ti–Cr [15,16,37,42], Ti–15Zr–Cr [6] and Ti–30Zr–Cr [5]) on the basis of e/a ratio. The polycrystals Young's modulus of Ti–Cr [42] single crystals are calculated by using the independent elastic constants of the single crystals from the Hill approximation [19,24].
the Ti–15Zr–5Cr–2Al alloy was 4.06, which resulted in a relatively low Young's modulus. The e/a of the Ti–15Zr–5Cr–2Al alloy was calculated using the nominal compositions of Ti, Zr, Cr and Al on the basis of the electron configurations of Ti ([Ar]3d24s2), Zr ([Kr]4d25s2), Cr ([Ar]4d45s2) and Al ([Ne]3d23p1). Recently, Wang [25] reported that the Young's modulus at 〈100〉 direction, E100, decreased with the decrease of the e/a ratio. Moreover, the elastic anisotropy increased with the decrease of the e/a ratio and the E100/E reached to 0.49 in the Ti– 28Nb–7Al (in at.%) single crystal (e/a = 4.21) [25]. This implies that the Ti–15Zr–5Cr–2Al alloy with an e/a ratio of 4.06 may reach a lower E100/E than that of the Ti–28Nb–7Al (in at.%) single crystal. Accordingly, the E100 should be around 28.4 GPa if E100/E of Ti–15Zr–5Cr–2Al alloy reaches 0.49 or even lower. This level is comparable to the Young's modulus of the cortical one (20–40 GPa) [21] and can further reduce the stress-shielding effect if we focused on the control of crystallographic orientation of this alloy for implants [24]. The second potential application of this alloy is for spinal fixation devices, where a tunable Young's modulus is necessary to satisfy the conflicting requirements of the surgeons and patients. A high Young's modulus is an advantageous property from the viewpoint of surgeons to avoid a big spring back, while a low Young's modulus is desired from the viewpoint of the patient to prevent the stress-shielding effect. Therefore, some researchers have focused on the development of the new alloys with tunable Young's modulus, such as Ti–12Cr [38], Ti–(10–12)Cr–(0.2–0.6)O [15], Ti–30Zr–(Cr, Mo) [5] and Ti–29Nb– 13Ta–4.6Zr–(8, 16)Ti–(2, 4)Cr [29], by introducing the deformation-induce ω transformation. As pointed out by Li et al., the deformation-induced ω transformation was more easily obtained in the metastable β Ti alloy by the addition of Cr [29]. Therefore, the WQ Ti–15Zr–5Cr–2Al alloy has the possibility to be induced ω transformation by cold deformation. Further study to elucidate the application of spinal fixation devices is now in progress. The third potential application of this alloy is to develop high strength and moderate Young's modulus Ti alloys for biomedical applications. The strength of β phase Ti alloys could be modified by the addition of alloy elements and/or thermo-mechanical treatment [1,15,16,29, 40]. By optimizing the fraction, size and distribution of the α, α′ and ω phases, the thermo-mechanically treated Ti–15Zr–5Cr–2Al alloy could reach a strength of 1250 MPa and achieve a moderate Young's modulus, such as in a range of 70–90 GPa. In summary, the as-cast and WQ Ti–15Zr–5Cr–2Al alloys exhibited superior ductility and high elastic energy, making them promising candidates for dental implants and hard tissue replacements. In addition, three potential applications, namely, (i) low Young's modulus implants
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by focusing on crystallographic orientations, (ii) spinal fixation devices by focusing on deformation-induced ω transformation, and (iii) high strength and moderate Young's modulus Ti alloy implants by focusing on thermo-mechanical treatment, further make this alloy a promising biomedical alloy. Although the biocompatibility of Ti–15Zr–5Cr–2Al alloys in the present study was not mentioned, the Ti–15Zr–5Cr–2Al alloy was designed by considering of biocompatibility. The preliminary cell culturing results revealed that this alloy was not only biocompatible, but also conducive to cell attachment. Furthermore, biomedical Ti alloys containing Cr or Zr/Al elements which exhibit excellent biocompatibility has been reported in the literatures, such as Ti–12Cr [38], Ti–10Cr– 0.2O [16] and Ti–15Mo–5Zr–3Al [35]. Although we may predict the in vitro and in vivo behavior, we will assess and discuss the biomedical tests of this alloy, which are underway and are going to be published in a separate paper. 5. Conclusions The microstructure and mechanical properties of the as-cast and heat treated Ti–15Zr–5Cr–2Al alloy were investigated by XRD analysis, optical microscopy analysis, microhardness measurement and compressive test. The following conclusions can be drawn: (1) The as-cast Ti–15Zr–5Cr–2Al alloy was comprised of the β and α phases. After AC and LNQ, the additional ω phase was detected. On the other hand, the α phase disappeared and only the β phase was detected in the WQ specimen. (2) The microhardness of the as-cast Ti–15Zr–5Cr–2Al alloy was 385 ± 20 HV, whereas that of the WQ specimen was 302 ± 10 HV. The AC specimen exhibited the highest microhardness (410 ± 14 HV) because of the combined hardening effect of the ω and α phases. (3) The yield strength of the WQ specimen was 625 ± 32 MPa, whereas the specimens under the other conditions exhibited higher yield strength in the range of 1148–1250 MPa. (4) The as-cast and WQ specimens exhibited higher ductility than that of the AC and LNQ specimens. Moreover, the WQ specimen exhibited the highest compressive strength (1880 ± 59 MPa) and the lowest Young's modulus (58 ± 4 GPa). (5) The elastic energies of the specimens under all studied conditions, ranging between 3.22 and 7.52 MJ/m3, were higher than that of c.p. Ti (1.25 MJ/m3). (6) The Ti–15Zr–5Cr–2Al alloy has several potential applications, namely, (i) low Young's modulus implants by focusing on crystallographic orientations, (ii) spinal fixation devices by focusing on deformation-induced ω transformation, and (iii) high strength and moderate Young's modulus Ti alloy implants by focusing on thermo-mechanical treatment. Acknowledgments The authors are grateful for the helpful discussions with Dr. S.M.L. Nai at A*STAR Singapore Institute of Manufacturing Technology (SIMTech), and Dr. X.P. Tan, at Nanyang Technological University. The National Natural Science Foundation of China (No. 51171174) supported this study. References [1] M. Geetha, A. Singh, R. Asokamani, A. Gogia, Ti based biomaterials, the ultimate choice for orthopaedic implants – a review, Prog. Mater. Sci. 54 (2009) 397–425. [2] P. Wang, M.L.S. Nai, X. Tan, G. Vastola, R. Srinivasan, W.J. Sin, Recent progress of additive manufactured Ti-6Al-4V by electron beam melting, Austin, TX, USA, August 8–10, 27th Annual International Solid Freeform Fabrication Symposium—An Additive Manufacturing Conference, 2016, 2016, pp. 691–704. [3] S. Nag, R. Banerjee, H.L. Fraser, Microstructural evolution and strengthening mechanisms in Ti–Nb–Zr–Ta, Ti–Mo–Zr–Fe and Ti–15Mo biocompatible alloys, Mater. Sci. Eng. C 25 (2005) 357–362.
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