- Email: [email protected]
Deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy for biomedical applications
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy for biomedical applications S. Guoa,b,n, Q.K. Mengb, X.N. Chenga, X.Q. Zhaob,nn a
Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China School of Materials Science and Engineering, Beihang University, Beijing 100191, China
b
ar t ic l e in f o
abs tra ct
Article history:
The deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn (wt%) alloy subjected to
Received 14 April 2014
different thermo-mechanical treatments was discussed by the combining results from
Received in revised form
transmission electron microscope, tensile test and in-situ synchrotron X-ray diffraction.
8 June 2014
Visible “double yielding” behavior, which is characterized by the presence of stress-
Accepted 11 June 2014
plateau, was observed in the solution treated specimen. Upon a cold rolling treatment,
Available online 21 June 2014
the Ti–25Nb–2Mo–4Sn alloy performs nonlinear deformation because of the combined
Keywords:
effects of elastic deformation and stress-induced α″ martensitic transformation. After the
Metastable β Ti alloy
subsequent annealing, the β phase is completely stabilized and no stress-induced
Deformation behavior
martensitic transformation takes place on loading due to the inhibitory effect of grain
Martensitic transformation
boundaries and dislocations on martensitic transformation. As a result, the annealed
In-situ synchrotron X-ray diffraction
specimen exhibits linear elastic deformation.
1.
Introduction
Titanium and its alloys have become the most attractive biomedical implant materials owing to their light weight, high corrosion resistance, excellent biocompatibility and good mechanical properties, especially low modulus (Geetha et al., 2008; Niinomi, 2002, 1998; Ozaki et al., 2004). For example, (αþβ) type Ti–6Al–4V alloy, which exhibits an elastic modulus ( 108 GPa) only about half of that of the 316L stainless steel ( 200 GPa) or Co–Cr–Mo alloy ( 210 GPa), has been widely used in the field of orthopedic implant. However, the elastic modulus of Ti–6Al–4V is not low enough to match that of human bone ( 30 GPa), giving rise to so-called “stress
& 2014 Elsevier Ltd. All rights reserved.
shielding effect” (Ho et al., 1999). Furthermore, the release of V and Al ions from Ti–6Al–4V is easy to cause long-term health problems, such as Alzheimer disease, neuropathy and osteomalacia (Geetha et al., 2008). Therefore, in the past decades, substantial efforts have been paid to develop noncytotoxic metastable β-type Ti alloys with low elastic modulus (Elias et al., 2006; Jung et al., 2012; Laheurte et al., 2010). Previous investigations have shown that both elastic modulus and deformation behavior of β-type Ti alloys are closely related to the stability of β phase determined by alloy composition and microstructure (Elias et al., 2006; Kim et al., 2006; Matlakhova et al., 2005; Matsumoto et al., 2005; Nobuhito et al., 2005; Zhou and Luo, 2011). The elastic
n Corresponding author at: Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China. Tel.: þ86 511 88783268; fax: þ86 511 88797783. nn Corresponding author. Tel.: þ86 10 82338559; fax: þ86 10 82338200. E-mail addresses: [email protected] (S. Guo), [email protected] (X.Q. Zhao).
http://dx.doi.org/10.1016/j.jmbbm.2014.06.006 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
modulus of β-phase Ti alloys increases with increasing the β-phase stability (Abdel-Hady et al., 2006; Kim et al., 2004), while the deformation behavior of β-phase Ti alloys involves stress-induced martensitic transformation, twinning and slip, depending on the stability of β phase (Elias et al., 2006; Kim et al., 2006; Matsumoto et al., 2005; Nobuhito et al., 2005; Zhou and Luo, 2011). Since deformation behavior of β-type Ti alloys is closely associated with the β-phase stability that dominates the modulus of β phase, a great deal of interest has recently been triggered in the study of the deformation behavior of β-type Ti alloys used for biomedical applications (Hao et al., 2007; Kuramoto et al., 2006; Wang et al., 2009; Withey et al., 2010). The Ti–25Nb–2Mo–4Sn (all elemental concentrations in wt%) alloy, which consists entirely of non-cytotoxic elements, is a recently developed metastable β-type Ti alloy with low Young's modulus and high strength, suitable for biomedical applications. Upon a cold rolling plus annealing treatment, this alloy can exhibit ultimate tensile strength as high as 1113 MPa at room temperature, while retaining Young's modulus as low as 65 GPa (Guo et al., 2013, 2012). In addition, it was reported that the Ti–25Nb–2Mo–4Sn alloy can perform different deformation behavior, including “double yielding”, nonlinear deformation, and linear elastic deformation, depending on the thermo-mechanical treatment condition. This result suggests that the Ti–25Nb–2Mo–4Sn alloy could serve as a prototypical alloy for the study of deformation behavior of biomedical β-type Ti alloys. In the present paper, the deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy under different thermomechanical conditions was investigated in detail. On the basis of the combining results from transmission electron microscope, tensile test, and in-situ synchrotron X-ray diffraction, the relation between deformation behavior and microstructural evolution induced by thermo-mechanical treatment was discussed.
2.
Experimental procedure
A metastable β-type Ti–25Nb–2Mo–4Sn (wt%, hereafter denoted as Ti-2524) alloy was arc-melted in an argon atmosphere using nontoxic high purity Ti (99.99%), Nb (99.95%), Mo (99.95%) and Sn (99.95%). The arc-melted ingot was homogenized at 1273 K for 4 h in vacuum, and then forged at 1173 K to a billet. After forging, the billet was solution treated at 1073 K for 1 h in an evacuated quartz tube, followed by quenching into water ( 298 K). The specimens which were cut from the solution treated billet will be denoted as solution treated specimens hereafter. The solution treated billet was cold rolled to about 1 mm in thickness at a reduction of 70%. The resultant specimens will be subsequently referred to as cold rolled specimens. Parts of the cold rolled specimens were annealed at 748 K for 15 min. They will be called annealed specimen hereafter. Thin-foil specimens for transmission electron microscopy observations were prepared by mechanical grinding and ion milling (BAL-TEC RES 010). The microstructures of the Ti– 25Nb–2Mo–4Sn alloy were investigated by a JEM 2100F transmission electron microscope (TEM) operating at a voltage of
27
200 kV. Uniaxial tensile test was performed on an Instron8801 testing system at a strain rate of 1 10 3 s 1. Tensile specimens have a rectangular cross-section of 1 1.46 mm2 and a gage length of 30 mm, with the rolling direction parallel to the loading axis. In order for accurate strain measurement, a strain extensometer was used to record the value of strain. The in-situ synchrotron X-ray experiments were conducted on the 11-ID-C beam-line of Advanced Photon Source (APS) at Argonne National Laboratory (ANL). High-energy X-rays with a beam size of 0.4 mm 0.4 mm and wavelength of 0.10798 Å were used to obtain two-dimensional (2-D) diffraction patterns during loading. The 2-D diffraction patterns were calibrated, using Fit2d software and a cerium dioxide calibration sample to refine the sample-detector distance and detector misalignment, and output one-dimensional (1-D) patterns for analysis.
3.
Results and discussion
Fig. 1(a and b) shows a typical bright-field micrograph and the corresponding [110]β zone axis selected area diffraction (SAD) pattern of solution treated Ti-2524 specimen, respectively. In the bright-field image, coarse lath-shaped α″ martensite is visible within β matrix. The existence of α″ martensite can be evidenced from the SAD pattern shown in Fig. 1(b), where the reflections near the 1/2 {211}β positions indicate the presence of α″ martensite. In order to clearly reveal internal substructure of α″ martensite, a higher magnification image of a α″ martensite plate is shown in Fig. 1(c). Internal twins can be clearly observed in the α″ martensite plate, revealing the twined nature of the α″ martensite (i.e. twinning substructure can be formed in α″ martensite plates). The TEM observation suggests that the solution treated Ti-2524 specimen undergoes martensitic transformation from β to α″ upon quenching from the high-temperature β field. The tensile stress–strain curve of solution treated Ti-2524 specimen is shown in Fig. 2. The solution treated specimen exhibits notable “double yielding” deformation behavior characterized by the presence of stress-plateau. Similar stress-plateau has been widely reported in the solution treated β-type Ti alloys, such as Ti–Nb, Ti–Ta, and Ti–Nb–Mo alloys (Al-Zain et al., 2011; Kim et al., 2004; Nobuhito et al., 2005). It has been demonstrated that this stress-plateau is associated with the stress-induced α″ martensitic transformation that occurs within a narrow stress range (Al-Zain et al., 2011; Nobuhito et al., 2005). As shown in Fig. 1, the martensitic transformation from β to α″ is also observed in the solution treated Ti-2524 specimen. Accordingly, the stress-plateau in the solution treated Ti-2524 specimen is attributed to the stress-induced martensitic transformation occurring in a narrow stress range. Upon a severe cold rolling, there exist a lot of irregular dark areas caused by dislocation tangles in the Ti-2524 specimen, as shown in the bright-field image of cold rolled specimen in Fig. 3(a). In a severely deformed zone, dislocation cell, which is marked by a white arrow, can be clearly observed. Fig. 3(b) shows the corresponding SAD pattern of the cold rolled Ti-2524 specimen taken from the area of a circle with a radius of 350 nm. In comparison to the SAD
28
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
Fig. 2 – Tensile stress–strain curve of solution treated specimen.
Fig. 1 – Bright-field micrograph (a), the corresponding [110]β zone axis selected area electron diffraction (SAD) pattern (b) and a higher magnification image of a α″ martensite plate (c) of solution treated specimen. pattern of solution treated specimen in Fig. 1(b), it can be seen that the cold rolling does not exert significant effect on the phase constitution, and that the phase constitution of cold rolled specimen can still be identified as β matrix and α″ martensite. Nevertheless, the SAD pattern in Fig. 3(b) exhibits near-continuous diffraction rings which are different from
Fig. 3 – Bright-field micrograph (a) and the corresponding SAD pattern (b) of cold rolled specimen. the isolated diffraction spots of solution treated specimen. This result indicates that the cold rolling leads to significant grain refinement, regardless of the β phase and α″ martensite. Fig. 4(a) shows the tensile stress–strain curve of cold rolled specimen. In this curve, no visible stress-plateau can be observed. Instead, the cold rolled specimen performs remarkable nonlinear deformation behavior, exhibiting no apparent
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
29
Fig. 4 – Tensile stress–strain curves of cold rolled specimen: (a) single-step loading to 4.5% strain and (b) single-step loading to 2% strain. yielding point in the stress–strain curve. This nonlinear deformation behavior can be more clearly seen in the magnified stress–strain curve to 2% strain shown in Fig. 4(b). In order to clarify the origin of nonlinear deformation behavior, in-situ synchrotron X-ray diffraction was carried out on the cold rolled Ti-2524 specimen, and the results are shown in Fig. 5. The points at which the in-situ synchrotron X-ray diffraction experiments were conducted were marked by the diamond symbols on the stress–strain curve in Fig. 4(a). Fig. 5(a) shows the diffraction patterns for the 020α″, 110β, and 021α″ perpendicular to the loading direction during tensile loading. It can be seen that the intensity of 110β peak decreases with increasing macroscopic stress, while the intensities of 020α″ and 021α″ peaks increase with increase in macroscopic stress. This can be more clearly seen from the relative peak intensity versus macroscopic stress curves, as shown in Fig. 5(b). One can see from Fig. 5(b) that the relative intensity of 110β decreases with increasing macroscopic stress. At the same time, the relative intensities of 020α″ and 021α″ peaks increase with increasing macroscopic stress. Therefore, it is believed that the stress-induced α″ martensitic transformation in the cold rolled specimen takes place during the whole loading process. Obviously, this is different from the case of the solution treated Ti-2524 specimen where the stressinduced martensitic transformation occurs within a narrow
Fig. 5 – In situ synchrotron X-ray diffraction analysis of cold rolled specimen: (a) diffraction patterns for the 020α″, 110β, and 021α″ perpendicular to the loading direction during tensile loading, (b) relative peak intensity versus macroscopic stress curves, and (c) plots of the d-spacing for the 020α″, 110β, and 021α″ perpendicular to the loading direction versus macroscopic strain.
stress range (Fig. 2). A reasonable explanation is related to the inhomogeneous dislocation distribution in the cold rolled specimen. It has been recognized that microstructure factors, such as dislocation and precipitation, can play a significant
30
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
influence on the martensitic transformation start temperature (Hao et al., 2002; Guo et al., 2012). As shown in Fig. 3, the dislocations are not distributed uniformly in the cold rolled Ti2524 specimen. As a result, the critical stress for inducing the martensitic transformation varies largely in the different regions of the cold rolled specimen. Accordingly, the stressinduced martensitic transformation occurs over a wide stress range in the cold rolled specimen, which is different from that occurring within a narrow stress range in the solution treated specimen. Fig. 5(c) shows the plots of the d-spacing for the 020α″, 110β, and 021α″ perpendicular to the loading direction versus macroscopic strain. It can be observed that the d-spacing of 020α″, 110β, and 021α″ increases with increasing macroscopic strain to 3.5%, and then remains almost constant with further increase in macroscopic strain. Obviously, the first stage at which the d-spacing increases with increasing macroscopic strain is attributed to elastic deformation, and the second stage at which the d-spacing remains almost constant with increasing macroscopic strain is ascribed to plastic deformation. On the basis of the above results shown in Fig. 5, one can see that both stress-induced martensitic transformation and elastic deformation take place at the initial deformation stage (up to about 3.5% strain). Therefore, it can be concluded that the initial nonlinear deformation of the cold rolled specimen is associated with the combined effects of stress-induced martensitic transformation and elastic deformation. The bright-field micrograph of annealed Ti-2524 specimen is shown in Fig. 6(a). A few fine precipitates with a width of several nanometers, labeled by a white arrow, were observed in the annealed specimen. Additionally, the dark areas arising from dislocation tangles still existed within β matrix after the annealing at 748 K for 15 min, implying that this annealing treatment does not give rise to significant recrystallization. Indeed, the near-continuous diffraction rings clearly demonstrate that the grains of the annealed specimen are still very small, as shown in the SAD pattern in Fig. 6(b). According to this SAD pattern, the fine precipitates located within the β matrix were identified as α. In addition, no visible reflection rings attributable to α″ martensite were seen in this pattern, suggesting that the martensitic transformation from β to α″ does not take place during the cooling process after annealing. In other words, the Ti-2524 alloy exhibits higher β phase stability in the annealed condition as compared with the solution treated condition. A previous experimental study by the present authors has demonstrated that the improvement of β phase stability in the annealed Ti-2524 specimen is closely involved in the inhibitory effect of high-density grain boundaries and dislocations on martensitic transformation (Guo et al., 2012). Fig. 6(c) shows a dark field image recorded using the diffraction spot from the α-phase labeled by a circle in the SAD pattern. One can see that the α-precipitates exhibit lenticular morphology, with several nanometers in width and dozens of nanometers in length. Fig. 7 shows the tensile stress–strain curve of annealed specimen. By comparison with the stress–strain curve of cold rolled specimen in Fig. 4(a), it is evident that the nonlinear deformation behavior has disappeared upon the annealing at 748 K for 15 min. Consequently, it is reasonable to speculate
Fig. 6 – TEM images of annealed specimen: (a) bright-field micrograph, (b) SAD pattern, and (c) dark field image of α precipitates recorded using the diffraction spot from the α-phase labeled by a circle in the SAD pattern. that the β phase in the annealed specimen is fully stabilized and no stress-induced martensitic transformation takes place on tensile loading. In order to verify this speculation, in-situ synchrotron X-ray experiments were carried out on the annealed specimen, and the results are shown in Fig. 8. Fig. 8(a) shows the
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
Fig. 7 – Tensile stress–strain curve of annealed specimen.
31
the annealing treatment, the β phase is completely stabilized and no stress-induced martensitic transformation occurs on loading. It can also be seen from Fig. 8(a) that the d-spacing of 110β firstly increases with increasing macroscopic strain and then remains almost constant with further increase in macroscopic strain. This change of d-spacing of 110β is more distinctly visible in Fig. 8(b). Based on the results shown in Fig. 8(b), a demarcation point between elastic deformation and plastic deformation can be confirmed at about 2% strain. According to the results shown in Fig. 8, it is found that stress-induced martensitic transformation is fully suppressed and elastic deformation occurs at the initial deformation stage (up to about 2% strain). Consequently, the annealed Ti-2524 specimen performs linear deformation at the initial deformation stage.
4.
Conclusions
A systematic study has been made to investigate the deformation behavior of metastable β-type Ti-2524 alloy under different thermo-mechanical conditions using transmission electron microscope, tensile test, and in-situ synchrotron X-ray diffraction. The solution treated Ti-2524 specimen exhibits notable “double yielding” behavior, which is characterized by the existence of stress-plateau. Upon a cold rolling treatment, the Ti-2524 alloy exhibits remarkable nonlinear deformation due to the combined effects of elastic deformation and stress-induced martensitic transformation. After the subsequent annealing at 748 K for 15 min, the β phase stability is significantly improved because of the suppressive effect of high-density grain boundaries and dislocations on martensitic transformation. Therefore, the β phase is completely stabilized and no stress-induced martensitic transformation takes place on loading, giving rise to the linear elastic deformation of the annealed specimen.
Acknowledgments
Fig. 8 – In situ synchrotron X-ray diffraction analysis of annealed specimen: (a) diffraction patterns for the 110β perpendicular to the loading direction during tensile loading and (b) d-spacing for the 110β perpendicular to the loading direction versus macroscopic strain curve.
diffraction patterns for the 110β perpendicular to the loading direction during tensile loading. The first pattern was obtained prior to loading (at 0% strain), and then other patterns were obtained by elongating the specimen at a strain interval of 0.25%, until the strain reached 3%. It can be observed from Fig. 8(a) that there is no evidence of stressinduced α″ phase transformation, which appears as no α″ diffraction peak arises at about 2.44 Å, during the whole loading process. Therefore, it can be concluded that upon
The authors greatly appreciate the financial support from the National Natural Science Foundation of China (51271010), the National 973 Program of China (2012CB619403), and the Senior Intellectuals Fund of Jiangsu University (13JDG098). The use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357.
r e f e r e nc e s
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Geetha, M., Singh, A.K., Asokamani, R., Gogia, A.K., 2008. Ti based biomaterials, the ultimate choice for orthopaedic implants – a review. Prog. Mater. Sci. 54, 397–425. Guo, S., Bao, Z.Z., Meng, Q.K., Hu, L., Zhao, X.Q., 2012. A novel metastable Ti-25Nb-2Mo-4Sn alloy with high strength and low Young’s modulus. Metall. Mater. Trans. A 43, 3447–3452. Guo, S., Meng, Q.K., Hu, L., Liao, G.Y., Zhao, X.Q., Xu, H.B., 2013. Suppression of isothermal ω phase by dislocation tangles and grain boundaries in metastable β-type titanium alloys. J. Alloys Compd. 550, 35–38. Hao, Y.L., Li, S.J., Sun, S.Y., Zheng, C.Y., Yang, R., 2007. Elastic deformation behaviour of Ti-24Nb-4Zr-7.9Sn for biomedical applications. Acta Biomater. 3, 277–286. Hao, Y.L., Niinomi, M., Kuroda, D., Fukunaga, K., Zhou, Y.L., Yang, R., 2002. Young’s modulus and mechanical properties of Ti-29Nb13Ta-4.6Zr in relation to α´´ martensite. Metall. Mater. Trans. A 33, 3137–3144. Ho, W.F., Ju, C.P., Chern, J.H., 1999. Structure and properties of cast binary Ti-Mo alloys. Biomaterials 20, 2115–2122. Jung, T.K., Lee, H.S., Semboshi, S., Masahashi, N., Abumiya, T., Hanada, S., 2012. A new concept of hip joint stem and its fabrication using metastable TiNbSn alloy. J. Alloys Compd. 536, 582–585. Kim, H.Y., Hashimoto, S., Kim, J.I., Inamura, T., Hosoda, H., Miyazaki, S., 2006. Effect of Ta addition on shape memory behavior of Ti-22Nb alloy. Mater. Sci. Eng. C 417, 120–128. Kim, H.Y., Satoru, H., Kim, J.I., Hosoda, H., Miyazaki, S., 2004. Mechanical properties and shape memory behavior of Ti-Nb alloys. Mater. Trans. 45, 2443–2448. Kuramoto, S., Furuta, T., Hwang, J.H., Nishino, K., Saito, T., 2006. Plastic deformation in a multifunctional Ti-Nb-Ta-Zr-O alloy. Metall. Mater. Trans. A 37, 657–662.
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Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy for biomedical applications S. Guoa,b,n, Q.K. Mengb, X.N. Chenga, X.Q. Zhaob,nn a
Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China School of Materials Science and Engineering, Beihang University, Beijing 100191, China
b
ar t ic l e in f o
abs tra ct
Article history:
The deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn (wt%) alloy subjected to
Received 14 April 2014
different thermo-mechanical treatments was discussed by the combining results from
Received in revised form
transmission electron microscope, tensile test and in-situ synchrotron X-ray diffraction.
8 June 2014
Visible “double yielding” behavior, which is characterized by the presence of stress-
Accepted 11 June 2014
plateau, was observed in the solution treated specimen. Upon a cold rolling treatment,
Available online 21 June 2014
the Ti–25Nb–2Mo–4Sn alloy performs nonlinear deformation because of the combined
Keywords:
effects of elastic deformation and stress-induced α″ martensitic transformation. After the
Metastable β Ti alloy
subsequent annealing, the β phase is completely stabilized and no stress-induced
Deformation behavior
martensitic transformation takes place on loading due to the inhibitory effect of grain
Martensitic transformation
boundaries and dislocations on martensitic transformation. As a result, the annealed
In-situ synchrotron X-ray diffraction
specimen exhibits linear elastic deformation.
1.
Introduction
Titanium and its alloys have become the most attractive biomedical implant materials owing to their light weight, high corrosion resistance, excellent biocompatibility and good mechanical properties, especially low modulus (Geetha et al., 2008; Niinomi, 2002, 1998; Ozaki et al., 2004). For example, (αþβ) type Ti–6Al–4V alloy, which exhibits an elastic modulus ( 108 GPa) only about half of that of the 316L stainless steel ( 200 GPa) or Co–Cr–Mo alloy ( 210 GPa), has been widely used in the field of orthopedic implant. However, the elastic modulus of Ti–6Al–4V is not low enough to match that of human bone ( 30 GPa), giving rise to so-called “stress
& 2014 Elsevier Ltd. All rights reserved.
shielding effect” (Ho et al., 1999). Furthermore, the release of V and Al ions from Ti–6Al–4V is easy to cause long-term health problems, such as Alzheimer disease, neuropathy and osteomalacia (Geetha et al., 2008). Therefore, in the past decades, substantial efforts have been paid to develop noncytotoxic metastable β-type Ti alloys with low elastic modulus (Elias et al., 2006; Jung et al., 2012; Laheurte et al., 2010). Previous investigations have shown that both elastic modulus and deformation behavior of β-type Ti alloys are closely related to the stability of β phase determined by alloy composition and microstructure (Elias et al., 2006; Kim et al., 2006; Matlakhova et al., 2005; Matsumoto et al., 2005; Nobuhito et al., 2005; Zhou and Luo, 2011). The elastic
n Corresponding author at: Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China. Tel.: þ86 511 88783268; fax: þ86 511 88797783. nn Corresponding author. Tel.: þ86 10 82338559; fax: þ86 10 82338200. E-mail addresses: [email protected] (S. Guo), [email protected] (X.Q. Zhao).
http://dx.doi.org/10.1016/j.jmbbm.2014.06.006 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
modulus of β-phase Ti alloys increases with increasing the β-phase stability (Abdel-Hady et al., 2006; Kim et al., 2004), while the deformation behavior of β-phase Ti alloys involves stress-induced martensitic transformation, twinning and slip, depending on the stability of β phase (Elias et al., 2006; Kim et al., 2006; Matsumoto et al., 2005; Nobuhito et al., 2005; Zhou and Luo, 2011). Since deformation behavior of β-type Ti alloys is closely associated with the β-phase stability that dominates the modulus of β phase, a great deal of interest has recently been triggered in the study of the deformation behavior of β-type Ti alloys used for biomedical applications (Hao et al., 2007; Kuramoto et al., 2006; Wang et al., 2009; Withey et al., 2010). The Ti–25Nb–2Mo–4Sn (all elemental concentrations in wt%) alloy, which consists entirely of non-cytotoxic elements, is a recently developed metastable β-type Ti alloy with low Young's modulus and high strength, suitable for biomedical applications. Upon a cold rolling plus annealing treatment, this alloy can exhibit ultimate tensile strength as high as 1113 MPa at room temperature, while retaining Young's modulus as low as 65 GPa (Guo et al., 2013, 2012). In addition, it was reported that the Ti–25Nb–2Mo–4Sn alloy can perform different deformation behavior, including “double yielding”, nonlinear deformation, and linear elastic deformation, depending on the thermo-mechanical treatment condition. This result suggests that the Ti–25Nb–2Mo–4Sn alloy could serve as a prototypical alloy for the study of deformation behavior of biomedical β-type Ti alloys. In the present paper, the deformation behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy under different thermomechanical conditions was investigated in detail. On the basis of the combining results from transmission electron microscope, tensile test, and in-situ synchrotron X-ray diffraction, the relation between deformation behavior and microstructural evolution induced by thermo-mechanical treatment was discussed.
2.
Experimental procedure
A metastable β-type Ti–25Nb–2Mo–4Sn (wt%, hereafter denoted as Ti-2524) alloy was arc-melted in an argon atmosphere using nontoxic high purity Ti (99.99%), Nb (99.95%), Mo (99.95%) and Sn (99.95%). The arc-melted ingot was homogenized at 1273 K for 4 h in vacuum, and then forged at 1173 K to a billet. After forging, the billet was solution treated at 1073 K for 1 h in an evacuated quartz tube, followed by quenching into water ( 298 K). The specimens which were cut from the solution treated billet will be denoted as solution treated specimens hereafter. The solution treated billet was cold rolled to about 1 mm in thickness at a reduction of 70%. The resultant specimens will be subsequently referred to as cold rolled specimens. Parts of the cold rolled specimens were annealed at 748 K for 15 min. They will be called annealed specimen hereafter. Thin-foil specimens for transmission electron microscopy observations were prepared by mechanical grinding and ion milling (BAL-TEC RES 010). The microstructures of the Ti– 25Nb–2Mo–4Sn alloy were investigated by a JEM 2100F transmission electron microscope (TEM) operating at a voltage of
27
200 kV. Uniaxial tensile test was performed on an Instron8801 testing system at a strain rate of 1 10 3 s 1. Tensile specimens have a rectangular cross-section of 1 1.46 mm2 and a gage length of 30 mm, with the rolling direction parallel to the loading axis. In order for accurate strain measurement, a strain extensometer was used to record the value of strain. The in-situ synchrotron X-ray experiments were conducted on the 11-ID-C beam-line of Advanced Photon Source (APS) at Argonne National Laboratory (ANL). High-energy X-rays with a beam size of 0.4 mm 0.4 mm and wavelength of 0.10798 Å were used to obtain two-dimensional (2-D) diffraction patterns during loading. The 2-D diffraction patterns were calibrated, using Fit2d software and a cerium dioxide calibration sample to refine the sample-detector distance and detector misalignment, and output one-dimensional (1-D) patterns for analysis.
3.
Results and discussion
Fig. 1(a and b) shows a typical bright-field micrograph and the corresponding [110]β zone axis selected area diffraction (SAD) pattern of solution treated Ti-2524 specimen, respectively. In the bright-field image, coarse lath-shaped α″ martensite is visible within β matrix. The existence of α″ martensite can be evidenced from the SAD pattern shown in Fig. 1(b), where the reflections near the 1/2 {211}β positions indicate the presence of α″ martensite. In order to clearly reveal internal substructure of α″ martensite, a higher magnification image of a α″ martensite plate is shown in Fig. 1(c). Internal twins can be clearly observed in the α″ martensite plate, revealing the twined nature of the α″ martensite (i.e. twinning substructure can be formed in α″ martensite plates). The TEM observation suggests that the solution treated Ti-2524 specimen undergoes martensitic transformation from β to α″ upon quenching from the high-temperature β field. The tensile stress–strain curve of solution treated Ti-2524 specimen is shown in Fig. 2. The solution treated specimen exhibits notable “double yielding” deformation behavior characterized by the presence of stress-plateau. Similar stress-plateau has been widely reported in the solution treated β-type Ti alloys, such as Ti–Nb, Ti–Ta, and Ti–Nb–Mo alloys (Al-Zain et al., 2011; Kim et al., 2004; Nobuhito et al., 2005). It has been demonstrated that this stress-plateau is associated with the stress-induced α″ martensitic transformation that occurs within a narrow stress range (Al-Zain et al., 2011; Nobuhito et al., 2005). As shown in Fig. 1, the martensitic transformation from β to α″ is also observed in the solution treated Ti-2524 specimen. Accordingly, the stress-plateau in the solution treated Ti-2524 specimen is attributed to the stress-induced martensitic transformation occurring in a narrow stress range. Upon a severe cold rolling, there exist a lot of irregular dark areas caused by dislocation tangles in the Ti-2524 specimen, as shown in the bright-field image of cold rolled specimen in Fig. 3(a). In a severely deformed zone, dislocation cell, which is marked by a white arrow, can be clearly observed. Fig. 3(b) shows the corresponding SAD pattern of the cold rolled Ti-2524 specimen taken from the area of a circle with a radius of 350 nm. In comparison to the SAD
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Fig. 2 – Tensile stress–strain curve of solution treated specimen.
Fig. 1 – Bright-field micrograph (a), the corresponding [110]β zone axis selected area electron diffraction (SAD) pattern (b) and a higher magnification image of a α″ martensite plate (c) of solution treated specimen. pattern of solution treated specimen in Fig. 1(b), it can be seen that the cold rolling does not exert significant effect on the phase constitution, and that the phase constitution of cold rolled specimen can still be identified as β matrix and α″ martensite. Nevertheless, the SAD pattern in Fig. 3(b) exhibits near-continuous diffraction rings which are different from
Fig. 3 – Bright-field micrograph (a) and the corresponding SAD pattern (b) of cold rolled specimen. the isolated diffraction spots of solution treated specimen. This result indicates that the cold rolling leads to significant grain refinement, regardless of the β phase and α″ martensite. Fig. 4(a) shows the tensile stress–strain curve of cold rolled specimen. In this curve, no visible stress-plateau can be observed. Instead, the cold rolled specimen performs remarkable nonlinear deformation behavior, exhibiting no apparent
journal of the mechanical behavior of biomedical materials 38 (2014) 26 –32
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Fig. 4 – Tensile stress–strain curves of cold rolled specimen: (a) single-step loading to 4.5% strain and (b) single-step loading to 2% strain. yielding point in the stress–strain curve. This nonlinear deformation behavior can be more clearly seen in the magnified stress–strain curve to 2% strain shown in Fig. 4(b). In order to clarify the origin of nonlinear deformation behavior, in-situ synchrotron X-ray diffraction was carried out on the cold rolled Ti-2524 specimen, and the results are shown in Fig. 5. The points at which the in-situ synchrotron X-ray diffraction experiments were conducted were marked by the diamond symbols on the stress–strain curve in Fig. 4(a). Fig. 5(a) shows the diffraction patterns for the 020α″, 110β, and 021α″ perpendicular to the loading direction during tensile loading. It can be seen that the intensity of 110β peak decreases with increasing macroscopic stress, while the intensities of 020α″ and 021α″ peaks increase with increase in macroscopic stress. This can be more clearly seen from the relative peak intensity versus macroscopic stress curves, as shown in Fig. 5(b). One can see from Fig. 5(b) that the relative intensity of 110β decreases with increasing macroscopic stress. At the same time, the relative intensities of 020α″ and 021α″ peaks increase with increasing macroscopic stress. Therefore, it is believed that the stress-induced α″ martensitic transformation in the cold rolled specimen takes place during the whole loading process. Obviously, this is different from the case of the solution treated Ti-2524 specimen where the stressinduced martensitic transformation occurs within a narrow
Fig. 5 – In situ synchrotron X-ray diffraction analysis of cold rolled specimen: (a) diffraction patterns for the 020α″, 110β, and 021α″ perpendicular to the loading direction during tensile loading, (b) relative peak intensity versus macroscopic stress curves, and (c) plots of the d-spacing for the 020α″, 110β, and 021α″ perpendicular to the loading direction versus macroscopic strain.
stress range (Fig. 2). A reasonable explanation is related to the inhomogeneous dislocation distribution in the cold rolled specimen. It has been recognized that microstructure factors, such as dislocation and precipitation, can play a significant
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influence on the martensitic transformation start temperature (Hao et al., 2002; Guo et al., 2012). As shown in Fig. 3, the dislocations are not distributed uniformly in the cold rolled Ti2524 specimen. As a result, the critical stress for inducing the martensitic transformation varies largely in the different regions of the cold rolled specimen. Accordingly, the stressinduced martensitic transformation occurs over a wide stress range in the cold rolled specimen, which is different from that occurring within a narrow stress range in the solution treated specimen. Fig. 5(c) shows the plots of the d-spacing for the 020α″, 110β, and 021α″ perpendicular to the loading direction versus macroscopic strain. It can be observed that the d-spacing of 020α″, 110β, and 021α″ increases with increasing macroscopic strain to 3.5%, and then remains almost constant with further increase in macroscopic strain. Obviously, the first stage at which the d-spacing increases with increasing macroscopic strain is attributed to elastic deformation, and the second stage at which the d-spacing remains almost constant with increasing macroscopic strain is ascribed to plastic deformation. On the basis of the above results shown in Fig. 5, one can see that both stress-induced martensitic transformation and elastic deformation take place at the initial deformation stage (up to about 3.5% strain). Therefore, it can be concluded that the initial nonlinear deformation of the cold rolled specimen is associated with the combined effects of stress-induced martensitic transformation and elastic deformation. The bright-field micrograph of annealed Ti-2524 specimen is shown in Fig. 6(a). A few fine precipitates with a width of several nanometers, labeled by a white arrow, were observed in the annealed specimen. Additionally, the dark areas arising from dislocation tangles still existed within β matrix after the annealing at 748 K for 15 min, implying that this annealing treatment does not give rise to significant recrystallization. Indeed, the near-continuous diffraction rings clearly demonstrate that the grains of the annealed specimen are still very small, as shown in the SAD pattern in Fig. 6(b). According to this SAD pattern, the fine precipitates located within the β matrix were identified as α. In addition, no visible reflection rings attributable to α″ martensite were seen in this pattern, suggesting that the martensitic transformation from β to α″ does not take place during the cooling process after annealing. In other words, the Ti-2524 alloy exhibits higher β phase stability in the annealed condition as compared with the solution treated condition. A previous experimental study by the present authors has demonstrated that the improvement of β phase stability in the annealed Ti-2524 specimen is closely involved in the inhibitory effect of high-density grain boundaries and dislocations on martensitic transformation (Guo et al., 2012). Fig. 6(c) shows a dark field image recorded using the diffraction spot from the α-phase labeled by a circle in the SAD pattern. One can see that the α-precipitates exhibit lenticular morphology, with several nanometers in width and dozens of nanometers in length. Fig. 7 shows the tensile stress–strain curve of annealed specimen. By comparison with the stress–strain curve of cold rolled specimen in Fig. 4(a), it is evident that the nonlinear deformation behavior has disappeared upon the annealing at 748 K for 15 min. Consequently, it is reasonable to speculate
Fig. 6 – TEM images of annealed specimen: (a) bright-field micrograph, (b) SAD pattern, and (c) dark field image of α precipitates recorded using the diffraction spot from the α-phase labeled by a circle in the SAD pattern. that the β phase in the annealed specimen is fully stabilized and no stress-induced martensitic transformation takes place on tensile loading. In order to verify this speculation, in-situ synchrotron X-ray experiments were carried out on the annealed specimen, and the results are shown in Fig. 8. Fig. 8(a) shows the
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Fig. 7 – Tensile stress–strain curve of annealed specimen.
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the annealing treatment, the β phase is completely stabilized and no stress-induced martensitic transformation occurs on loading. It can also be seen from Fig. 8(a) that the d-spacing of 110β firstly increases with increasing macroscopic strain and then remains almost constant with further increase in macroscopic strain. This change of d-spacing of 110β is more distinctly visible in Fig. 8(b). Based on the results shown in Fig. 8(b), a demarcation point between elastic deformation and plastic deformation can be confirmed at about 2% strain. According to the results shown in Fig. 8, it is found that stress-induced martensitic transformation is fully suppressed and elastic deformation occurs at the initial deformation stage (up to about 2% strain). Consequently, the annealed Ti-2524 specimen performs linear deformation at the initial deformation stage.
4.
Conclusions
A systematic study has been made to investigate the deformation behavior of metastable β-type Ti-2524 alloy under different thermo-mechanical conditions using transmission electron microscope, tensile test, and in-situ synchrotron X-ray diffraction. The solution treated Ti-2524 specimen exhibits notable “double yielding” behavior, which is characterized by the existence of stress-plateau. Upon a cold rolling treatment, the Ti-2524 alloy exhibits remarkable nonlinear deformation due to the combined effects of elastic deformation and stress-induced martensitic transformation. After the subsequent annealing at 748 K for 15 min, the β phase stability is significantly improved because of the suppressive effect of high-density grain boundaries and dislocations on martensitic transformation. Therefore, the β phase is completely stabilized and no stress-induced martensitic transformation takes place on loading, giving rise to the linear elastic deformation of the annealed specimen.
Acknowledgments
Fig. 8 – In situ synchrotron X-ray diffraction analysis of annealed specimen: (a) diffraction patterns for the 110β perpendicular to the loading direction during tensile loading and (b) d-spacing for the 110β perpendicular to the loading direction versus macroscopic strain curve.
diffraction patterns for the 110β perpendicular to the loading direction during tensile loading. The first pattern was obtained prior to loading (at 0% strain), and then other patterns were obtained by elongating the specimen at a strain interval of 0.25%, until the strain reached 3%. It can be observed from Fig. 8(a) that there is no evidence of stressinduced α″ phase transformation, which appears as no α″ diffraction peak arises at about 2.44 Å, during the whole loading process. Therefore, it can be concluded that upon
The authors greatly appreciate the financial support from the National Natural Science Foundation of China (51271010), the National 973 Program of China (2012CB619403), and the Senior Intellectuals Fund of Jiangsu University (13JDG098). The use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357.
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