journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
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Research Paper
Ti–Mo alloys employed as biomaterials: Effects of composition and aging heat treatment on microstructure and mechanical behavior Flavia F. Cardosoa, Peterson L. Ferrandinib, Eder S.N. Lopesa, Alessandra Cremascoa, Rubens Carama,n a
University of Campinas, School of Mechanical Engineering, Campinas, SP 13083-860, Brazil Sao Paulo State University, School of Engineering, Guaratingueta, SP, Brazil
b
art i cle i nfo
ab st rac t
Article history:
The correlation between the composition, aging heat treatments, microstructural features
Received 19 June 2013
and mechanical properties of β Ti alloys is of primary significance because it is the
Received in revised form
foundation for developing and improving new Ti alloys for orthopedic biomaterials.
17 October 2013
However, in the case of Ti–Mo alloys, this correlation is not fully described in the literature.
Accepted 26 November 2013
Therefore, the purpose of this study was to experimentally investigate the effect of
Available online 3 December 2013
composition and aging heat treatments on the microstructure, Vickers hardness and
Keywords:
elastic modulus of Ti–Mo alloys. These alloys were solution heat-treated and water-
Titanium alloys
quenched, after which their response to aging heat treatments was investigated. Their
Aging heat treatment
microstructure, Vickers hardness and elastic modulus were evaluated, and the results
Vickers hardness
allow us to conclude that stabilization of the β phase is achieved with nearly 10% Mo when
Elastic modulus
a very high cooling rate is applied. Young's modulus was found to be more sensitive to phase variations than hardness. In all of the compositions, the highest hardness values were achieved by aging at 723 K, which was attributed to the precipitation of α and ω phases. All of the compositions aged at 573 K, 623 K and 723 K showed overaging within 80 h. & 2014 Published by Elsevier Ltd.
1.
Introduction
Ti alloys are successfully employed in the area of orthopedic biomaterials due to their enhanced biocompatibility, high biocorrosion resistance and high strength-to-weight ratio (Lütjering and Williams, 2003; Niinomi, 2003). In recent years, research into metallic biomaterials has focused on the β Ti alloys produced from nontoxic elements, particularly using Mo as the alloying element (Zhou and Luo, 2011; Oliveira n
et al., 2007). In addition to superior mechanical strength, orthopedic biomaterials employed to replace hard tissues must present low elastic modulus values to avoid the stress-shielding phenomenon (Geetha et al., 2009). This phenomenon is caused by elastic modulus mismatch and leads to the insufficient loading of bone adjacent to an implant (Ahn and Grodzinsky, 2009). The effect deserves attention because it can result in bone mass loss, osteoporosis and sometimes bone failure (Carter et al., 1989). To
Correspondence to: Rua Mendeleiev, 200, Campinas 13083-860, SP, Brazil. Tel.: þ55 19 35213314; fax: þ55 19 32893722. E-mail addresses: [email protected], [email protected] (R. Caram).
1751-6161/$ - see front matter & 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jmbbm.2013.11.021
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journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
prevent the occurrence of stress-shielding, low-elasticmodulus implants are desirable. The microstructural features and hence the mechanical properties of β Ti–Mo alloys depend directly on alloy composition and the processing routes to which the alloys are subjected (Flower et al., 1974; Davis et al., 1979a). It is well known that solution heat-treating β Ti alloys at high temperatures and then cooling them rapidly may lead to microstructures composed of martensite and β phase (Ferrandini et al., 2007; Mantani et al., 2004). The two metastable phases, i.e., orthorhombic martensite (α″) and the bcc phase, display low elastic modulus values and mechanical strength (Lee et al., 2002). The effect of Mo content on the structure and properties of Ti–Mo alloys cast in graphite molds was investigated by Ho et al. (1999), who found that the crystal structure and phase morphology were composition-sensitive. Samples containing 6 wt% Mo resulted in acicular orthorhombic martensite. As the Mo content was increased, β phase became the only dominant phase. However, no details were provided about the cooling rates applied to the samples during the casting processes. A strengthening effect can be achieved in β Ti alloys by applying suitable heat treatment processes, causing the transformation of metastable phases into α phase (Mantani and Tajima, 2006). The precipitation of α phase, which is tougher and more rigid than orthorhombic martensite and β phase, can be controlled by submitting rapidly quenched β Ti alloys to aging heat treatment. However, the aging of β Ti alloys may also lead to the precipitation of the undesirable ω phase (Terauchi et al., 1981; Sukedai et al., 1992, 2003; Ho, 2008). Although ω phase precipitation significantly increases an alloy's mechanical strength, it also causes severe brittleness. Moreover, the presence of ω phase in a β phase matrix may play an important role in acting as a nucleation substrate for the formation of α phase, producing a fine and uniform distribution of α phase in the β phase matrix (Prima et al., 2006). The phase transformation of orthorhombic martensite by isothermal aging in Ti–8Mo (wt%) alloy was investigated by Mantani et al. (2004). The researchers' results indicated that orthorhombic martensite remained after aging at 723 K for 9.0 ks and was fully decomposed into α and β phases after aging at 923 K for 9.0 ks. In another interesting study, the sequence of phase transformation of Ti–12Mo (wt%) alloy was investigated by electrical resistivity and dilatometric measurements (Sun et al., 2010). It was found that under heating, samples of Ti–12Mo alloy showed the transformation of metastable β into ω and α precipitates, leading to very high tensile strength. The correlation between composition, aging heat treatments, microstructural features and mechanical properties of β Ti alloys is of primary significance because it is the foundation for developing and improving new Ti alloys for orthopedic biomaterials. However, in the case of Ti–Mo alloys, this correlation is not fully described in the literature. Therefore, the purpose of this study was to experimentally investigate the effect of composition and aging heat treatments on the microstructure, Vickers hardness and elastic modulus of Ti–Mo alloys. These alloys were solution heat-treated and
water-quenched, after which their response to aging heat treatments was investigated.
2.
Experimental procedure
Ingots of different compositions of the Ti–Mo system were arc melted using a water-cooled copper hearth and nonconsumable tungsten electrode in an argon atmosphere. The ingots were flipped and re-melted 5 times to ensure their chemical homogeneity. They were then homogenized at 1273 K for 24 h and hot rolled (initially heated to 1273 K) to obtain 2 mm thick plates. The nominal compositions of the 2 mm plates (wt%) were 3, 5, 6, 7.5, 8, 8.5, 9, 9.5, 10 and 15 wt% Mo (samples 3, 5, 6 and so on). The plates were solution heattreated for 1 h at 1273 K under inert atmosphere, waterquenched, aged at different temperatures and then air cooled to room temperature. The measured cooling rate provided by water quenching (WQ) was 160 K/s. In order to carry out the aging heat treatment, the samples were sealed in quartz ampoules in an argon atmosphere and inserted in a furnace heated at proper temperatures. Samples were conventionally prepared for metallography and etched in Kroll solution (3 vol% HF, 6 vol% HNO3 and 91 vol% H2O). The samples were analyzed by optical microscopy (OM) (Olympus BX60M). Vickers hardness was determined by applying a load of 1000 gf for 15 s, and the results presented herein are the averages of five measurements. Young's modulus was determined dynamically by the standard through-transmission technique, using coupled longitudinal and shear transducers with an active diameter of 6.35 mm. X-ray diffraction (XRD) measurements were taken with a Rigaku DMAX 2200 diffractometer and were performed at room temperature, using Cu-Kα radiation and operating at 40 kV/30 mA.
3.
Results and discussion
3.1.
Water-quenched samples
Initially, the aim of the experiments was to evaluate the effect of the Mo content on the phase formation in water-quenched samples, particularly the formation of hexagonal (α′) and orthorhombic (α″) martensites as well as the suppression of α″ and stabilization of β phase at room temperature. Samples containing 3 to 15 wt% Mo were solution heattreated and water-quenched, and their hardness, elastic modulus and microstructure were investigated. Fig. 1 presents the Vickers hardness values of water-quenched samples as a function of composition. In principle, the smaller strains involved in the formation of α′ and α″ martensites lead to low-hardness structures. Sample 3 presented a hardness value of HV 292, and samples 5 and 6 presented similar values. Davis et al. (1979b) reported that water-quenched Ti–4Mo (wt%) samples contained a thin layer of β phase between individual martensite plates due to some diffusion that occurred during quenching. An increase in hardness was observed when the Mo content reached 8 wt%. This high hardness is attributed to ω phase precipitation, which was
33
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
enabled by the amount of β phase that formed during quenching. For high-Mo-content alloys, the stabilization of β phase resulted in a decrease in hardness. Fig. 2 depicts optical microscopy images of waterquenched samples 3, 5, 6 and 7.5, and Fig. 3 presents their X-ray diffraction patterns. According to Davis et al. (1979b), Ti–4Mo (wt%) alloy presents α′ martensite, whereas Ti–6Mo (wt%) presents α″ martensite (orthorhombic martensite). Ho et al. (1999) reported the coexistence of α′ and α″ martensites in Ti–6Mo (wt%) alloy. Similar results were obtained for a different metallic system, Ti–Nb. Lee et al. (2002) reported the presence of both α′ and α″ martensites in Ti–15Nb (wt%) alloy. It should be emphasized that samples 3 and 5 presented only α′ martensite and sample 6 presented both α′ and α″ martensites. When the Mo content was increased to 7.5%, the microstructure presented a small volume fraction of β phase,
which should lead to some reduction in hardness, as opposed to what was observed for alloy 6. The results of X-ray diffraction analyses do not confirm the occurrence of β phase in sample 7.5 due to its small volume fraction. However, sample 7.5 presented higher hardness, which is explained by ω phase precipitation. Micrographs of water-quenched samples 8, 9.5, 10 and 15 are shown in Figs. 4 and 5 presents their X-ray diffraction patterns. The sample with Mo content of 8% exhibited α″, β and ω phases, while the one with Mo content of 9.5% exhibited α″ and β phases and possibly the ω phase. The amount of α″ phase decreased with increasing Mo content, and the hardness values revealed the strong influence of the ω phase. The microstructure of water-quenched sample 10 was also evaluated. According to the literature, the " "
" " "
650
120
600
80
500 Vickers Hardness Elastic Modulus
60
450 400
40 350 20
' "
'
Intensity (a.u.)
550
Vickers Hardness (HV)
Young´s Modulus (GPa)
100
"
"
"
" "
" " "
Ti-7.5Mo
" " "
Ti-6Mo
'
Ti-5Mo
" " "
"
"
" '
' " " "
' '
'
' '
'
300
'
'
'
'
'
'
'
Ti-3Mo
250
0 2
4
6
8
10
12
14
30
16
Fig. 1 – Hardness and Young's modulus of water-quenched Ti–Mo alloys.
40
50
60
70
80
90
Angle, 2
Mo content (wt. %)
Fig. 3 – X-ray diffraction patterns of water-cooled Ti–3Mo, Ti–5Mo, Ti–6Mo and Ti–7.5Mo (wt%).
Ti-3Mo
Ti-5Mo
Ti-6Mo
Ti-7.5Mo
Fig. 2 – Optical microscopy images of water-cooled Ti–3Mo, Ti–5Mo, Ti–6Mo and Ti–7.5Mo (wt%) samples.
34
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Ti-8Mo
Ti-9.5Mo
Ti-10Mo
Ti-15Mo
Fig. 4 – Optical microscopy images of water-cooled Ti–8Mo, Ti–9Mo, Ti–10Mo and Ti–15Mo (wt%) samples.
Intensity (a.u.)
Ti-15Mo
"
Ti-10Mo
"
Ti-9.5Mo
" " " " 30
" 40
" "
" " 50
60
70
Ti-8Mo 80
90
Angle, 2
volume fraction of α′ phase decreased in response to higher Mo contents; according to Lee et al. (2002), α′ martensite is stiffer than α″ martensite. Sample 3, which showed α′ martensite, presented an elastic modulus of 91 GPa, whereas sample 5 showed an elastic modulus of 80 GPa and sample 6 (α′ and α″ martensite) presented an elastic modulus of 75 GPa. As mentioned previously, samples 7.5 (82 GPa) and 8 (89 GPa) exhibited small amounts of β phase, which allowed for ω precipitation. Again, examination of the values of the elastic modulus reveals that they show a much clearer trend than the Vickers hardness values. Samples 8.5 to 10 presented high elastic modulus values, as expected because they displayed the presence of ω phase, and sample 15 presented an elastic modulus of 75 GPa, which is consistent with the β phase structure.
Fig. 5 – X-ray diffraction patterns of water-cooled Ti–8Mo, Ti–9.5Mo, Ti–10Mo and Ti–15Mo (wt%).
3.2.
addition of 10 wt% Mo suffices to retain β at room temperature (Ho et al., 1999). In this work, water-quenched sample 10 presented a structure consisting of β phase with ω precipitation. However, a small volume fraction of α″ martensite was detected by X-ray diffraction. The Vickers hardness values observed for the water-quenched samples are consistent with the phases observed. Sample 15 presented an entirely β phase structure and the lowest hardness value (HV 284). The Young's modulus of the water-quenched samples was determined, and the variation thereof is also illustrated in Fig. 1. Note that the elastic modulus is more effective than hardness in indicating changes in phase equilibrium. The
Alloy samples 5, 7.5, 10 and 15 were aged at 523 K, 573 K, 623 K and 723 K for periods of up to 100 h. Vickers hardness and X-ray diffraction analyses were performed to identify the constituent phases at the end of the heat treatments. Fig. 6 shows that sample 5 presented hardness plateaus despite the increase in aging temperature. When aged at 523 K, sample 5 showed a negligible increase in hardness after 40 min, which then leveled off, and finally, after 16 h, the increase in hardness was slightly more pronounced. Fig. 7 shows the X-ray diffraction pattern of sample 5 aged at 523 K. The figures shows the presence of α/α′ phase only. The X-ray diffraction patterns of α phase and α′ hexagonal martensite are very similar, and therefore, it is difficult to distinguish
Aging heat treatment
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journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
450
400
350
300
0.01
0.1
1
10
100
Time (h)
Fig. 6 – Vickers hardness values of Ti–5Mo alloy as a function of aging time.
Intensity (a.u.)
Ti-5Mo
"
"
"
"
723 K
"
623 K
"
573 K
"
"
' ' 30
' ' 40
50
'
' 60
70
'
' 80
'
523 K ' 90
Angle, 2
Fig. 7 – X-ray diffraction patterns of Ti–5Mo alloy aged at different temperatures.
between these phases. Eventually, aging at such a temperature may have initiated the precipitation of α phase after a long aging period, causing the hardness to increase. No evidence of the presence of β and ω phases was observed in the diffraction pattern. The sample aged at 573 K showed similar behavior, but the first increase in hardness occurred after 10 min of aging, with a more pronounced increase occurring after 2 h. A decrease in hardness was observed after 40 h of aging. As expected, the hardness values obtained were always higher and the periods of time involved shorter than those obtained with aging at 523 K. The X-ray diffraction results show that at the end of the heat treatment, sample 5 presented α and ω phases and evidence of the precipitation of an orthorhombic symmetry phase, with an X-ray diffraction pattern similar to that of the orthorhombic martensite phase, α″. This result is somewhat surprising, considering the fact that orthorhombic martensite is not expected to be formed in low-Mo-content Ti alloys. Orthorhombic martensite, α″, is formed when β phase is rapidly cooled to low temperatures from high temperatures
or during the deformation of β phase at room temperature (Wood, 1963). In spite of the fact that this precipitation is very intriguing, a number of studies have also reported the formation of an orthorhombic symmetry phase during aging heat treatments of Ti alloys. Malinov and co-authors applied high-resolution synchrotron X-ray diffraction to investigate phase transformation in Ti–6Al–4V alloy and found that samples that were water quenched from high temperature and aged showed evidence of the precipitation of an orthorhombic symmetry phase (Malinov et al., 2002). Ivasishin et al. (2005) investigated the aging of some Ti alloys and observed that in the early stages of β decomposition in VT22 and TIMETAL-LCB alloys, an orthorhombic symmetry phase was formed. Settefrati et al. (2011) investigated phase transformation in Ti-5553 alloy using high-energy X-ray diffraction. The researchers found that an orthorhombic symmetry phase is formed during aging and decomposition of the β metastable phase. Finally, in a recent study, Aeby-Gautier et al. (2013) investigated the isothermal aging of Ti17 and Ti-5553 alloys and found that an orthorhombic symmetry phase forms at the very beginning of β phase decomposition. In sample 5, the presence of an orthorhombic symmetry phase is consistent with the pronounced decrease in hardness at the end of the aging treatment. With regard to the aging heat treatments at 623 K and 723 K, the hardness vs. aging time curves confirm that hardness begins to increase in less time, which is consistent with the expected C-curve behavior. In addition, the highest hardness values were reached in less time at higher aging temperatures. According to the X-ray diffraction patterns, the sample aged at 623 K presented α, the orthorhombic symmetry phase and ω phases and possibly a small volume fraction of β phase, whereas the sample aged at 723 K presented α and β phases and a reduced amount of the orthorhombic symmetry phase. The orthorhombic phase appears to be partially decomposed at 723 K, and no evidence of ω phase was observed at this temperature. The hardening effect observed at 723 K was stronger than that observed at 623 K.
500
Vickers Hardness (kgf/mm2)
Vickers Hardness (kgf/mm2)
Ti-5Mo
523 K 573 K 623 K 723 K
500
450
523 K 573 K 623 K 723 K
Ti-7.5Mo
400
350
300
0.01
0.1
1
10
100
Time (h)
Fig. 8 – Vickers hardness values of Ti–7.5Mo alloy as a function of aging time.
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journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Fig. 8 shows the hardness vs. aging time curves for sample 7.5. An increase in hardness after 8 h of aging at 523 K is visible, albeit not pronounced, as expected. It is likely that a small amount β phase was precipitated, followed by the isothermal precipitation of ω phase. Neither phase was identified by X-ray diffraction due to the phases' low volume fraction. As presented in Fig. 9, the X-ray diffraction results show the presence of α″ phase, which is consistent with the quenched condition of the alloy. When aged at 573 K, the alloy presented a plateau between 20 min and 1 h, steadily increasing thereafter up to HV 371 at an aging time of 40 h and followed by overaging. The amount of ω phase precipitated during aging at 573 K was sufficient to be detected by X-ray diffraction. Thus, α″, β and ω phases were identified. When aged at 623 K, sample 7.5 presented the same plateau behavior, albeit more pronounced because higher hardness values were obtained. X-ray diffraction showed evidence of the presence of α phase, as well as α″, β and ω phases. The hardness vs. aging time curve obtained for sample 7.5 aged at 723 K indicates that the higher aging temperature enabled more intense precipitation, resulting in considerably higher hardness values. Unlike those for sample 5, which presented α and β phases when aged at 723 K, the X-ray diffraction results for sample 7.5 showed α, α″, β and ω phases, Ti-7.5Mo
723 K
Intensity (a.u.)
"
"
623 K
"
"
" "
"
" "
"
"
" " "
" "
"
"
"
" " 30
40
50
60
" " "
"" 70
" " 80
573 K
"
523 K 90
Angle, 2
Fig. 9 – X-ray diffraction patterns of Ti–7.5Mo alloy aged at different temperatures.
indicating that the ω phase solvus temperature for sample 7.5 is higher than 723 K. Fig. 10 shows the hardness vs. aging time curves obtained for sample 10. The hardening that occurred in response to 523 K aging was unexpectedly high and is attributed to ω phase precipitation. After 100 h of aging, the X-ray diffraction results shown in Fig. 11 revealed the presence of ω and β phases but not α phase, indicating that the nucleation and growth of α phase from ω phase particles requires more time. The same degree of hardness was attained with aging at 573 K. However, after 80 h of aging, X-ray diffraction showed the presence of α and β phases but not ω phase. A lower and more coherent hardness was attained after aging at 623 K, which according to the X-ray diffraction results, was due to the precipitation of α phase. Although hardness values should be analyzed carefully, the hardness vs. aging time curve indicates that the hardness began to increase after 40 min of aging at 623 K and that overaging occurred after 30 h. On the other hand, after 1 h of aging at 573 K, the hardness began to increase, with overaging occurring after 40 h. Again, this is evidence of C-curve behavior. The more intense precipitation that occurred in response to aging at 723 K led to the highest hardness value, with overaging occurring after 4 h. X-ray diffraction analysis revealed the presence of α and β phases. In Fig. 12, the hardness vs. aging time curves of sample 15 aged at 523 K and 573 K show very similar values. However, the latter temperature exhibits a clear tendency for overaging after 40 h, whereas the former presents more constant values between 16 h and 56 h and a slight decrease in hardness after 80 h of aging. The X-ray diffraction results show the presence of β and ω phases after 80 h of aging, as well as minor evidence of α phase precipitation, as indicated in Fig. 13. The same behavior was observed in response to aging at 623 K. The incubation period was approximately the same as that observed during aging at 523 K and 573 K. However, the more intense precipitation that occurred at a higher temperature (623 K) led to higher hardness values and overaging after 16 h of heat treatment. After 40 h of aging, the X-ray diffraction results indicated the presence of β and ω phases and a small amount of α phase. The rapid hardening observed Ti-10Mo
Ti-10Mo
723 K Intensity (a.u.)
Vickers Hardness (kgf/mm2)
450
523 K 573 K 623 K 723 K
400
623 K
573 K
350
523 K 0.01
0.1
1
10
100
Time (h)
Fig. 10 – Vickers hardness values of Ti–10Mo alloy as a function of aging time.
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40
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90
Angle, 2
Fig. 11 – X-ray diffraction patterns of Ti–10Mo alloy aged at different temperatures.
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Vickers Hardness (kgf/mm2)
Ti-15Mo
523 K 573 K 623 K 723 K
500
450
400
350
strong peaks of α phase, whereas the hardness values revealed pronounced overaging. These findings lead to the conclusion that hardening was due mostly to ω phase precipitation, followed by α phase precipitation, which started to coarsen after 12 h of aging. Precipitates of ω phase acted as nucleation sites for α phase and were completely depleted after 12 h of heat treatment, as indicated in Fig. 14. In addition, a clear decrease in hardness was observed after 12 h of aging.
4.
300
0.01
0.1
1
10
100
Time (h)
Fig. 12 – Vickers hardness values of Ti–15Mo alloy as a function of aging time.
37
Conclusions
Samples of Ti–Mo alloys were solution heat-treated and water-quenched, after which their response to aging heat treatments was investigated. Their microstructure, Vickers hardness and elastic modulus were evaluated, and the results allow us to conclude that:
Ti-15Mo
Intensity (a.u.)
723 K
623 K
573 K
523 K 30
40
50
60
70
80
90
Angle, 2
Fig. 13 – X-ray diffraction patterns of Ti–15Mo alloy aged at different temperatures. Ti-15Mo - aging at 723 K
Intensity (a.u.)
12 h
(a) Martensitic transformation in dilute Ti–Mo alloys results in the formation of hexagonal martensite, α′. As the solute content reached 6% Mo, martensite transformation led to the formation of orthorhombic martensite α″. The coexistence of α′ and α″ martensites in the microstructure of the rapidly cooled Ti–6Mo (wt%) alloy was observed. (b) As the Mo content was increased, rapidly cooling from temperatures in the β phase region produces microstructures with α′; α′þα″; α″þβþω and βþω phases. The precipitation of ω phase, which was enabled by the retained β phase, resulted in higher hardness and elastic modulus values, which was followed by a decrease in those values due to an increase in the β volume fraction. Stabilization of the β phase was achieved at a composition near 10% Mo when high cooling rates were applied; (c) Young's modulus was found to be more sensitive than hardness to phase variations; (d) The aging of solution heat-treated and water-quenched samples of alloy 5 resulted in the precipitation of an unexpected orthorhombic symmetry phase; (e) In all of the compositions, the highest hardness values were achieved with aging at 723 K, which was attributed to the precipitation of α and ω phases. All of the compositions aged at 573 K, 623 K and 723 K showed overaging within 80 h.
4h 5 min
Acknowledgements WQ 78.0
78.5
79.0
79.5
80.0
80.5
81.0
81.5
82.0
Angle, 2
Fig. 14 – X-ray diffraction patterns (low scanning speed) of water-cooled Ti–15Mo sample aged at 723 K for 5 min, 4 h and 12 h.
The authors gratefully acknowledge the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development) for their financial support of this work.
r e f e r e nc e s in response to aging at 723 K revealed that ω phase precipitation was intense during the first 5 min of aging. After 28 h of aging, the X-ray diffraction patterns indicated the presence of
Aeby-Gautier, E., Settefrati, A., Bruneseaux, F., Appolaire, B., Denand, B., Dehmas, M., Geandier, G., Boulet, P., 2013.
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Isothermal α´´ formation in β metastable titanium alloys. J. Alloy Compd. 577, S439–S443. Ahn, A.C., Grodzinsky, A.J., 2009. Relevance of collagen piezoelectricity to “Wolff’s Law”: a critical review. Med. Eng. Phys. 31, 733–741. Carter, D.R., Orr, T.E., Fyhrie, D.P., 1989. Relationship between loading history and femoral cancellous bone architecture. J. Biomech. 22, 231–244. Davis, R., Flower, H.M., West, D.R.F., 1979a. Martensitic transformations in Ti–Mo alloys. J. Mater. Sci. 14, 712–722. Davis, R., Flower, H.M., West, D.R., 1979b. The decomposition of Ti–Mo alloy martensites by nucleation and growth and spinodal mechanisms. Acta Metall. Mater. 27, 1041–1052. Ferrandini, P.L., Cardoso, F.F., Souza, S.A., Afonso, C.R., Caram, R., 2007. Aging response of the Ti–35Nb–7Zr–5Ta and Ti–35Nb–7Ta alloys. J. Alloy Compd. 433, 207–210. Flower, H.M., Henry, S.D., West, D.R.F., 1974. The β⇄α transformation in dilute Ti–Mo alloys. J. Mater. Sci. 9, 57–64. Geetha, M., Singh, A.K., Asokamani, R., Gogia, A.K., 2009. Ti based biomaterials, the ultimate choice for orthopaedic implants—a review. Prog. Mater. Sci. 54, 397–425. Ho, W.-F., Ju, C.P., Chern Lin, J.H., 1999. Structure and properties of cast binary Ti–Mo alloys. Biomaterials 20, 2115–2122. Ho, W.-F., 2008. Effect of omega phase on mechanical properties of Ti–Mo alloys for biomedical applications. J. Med. Biol. Eng. 28, 47–51. Ivasishin, O.M., Markovsky, P.E., Semiatin, S.L., Ward, C.H., 2005. Aging response of coarse- and fine-grained β titanium alloys. Mater. Sci. Eng. A 405, 296–305. Lee, C.M., Ju, C.P., Chern Lin, J.H., 2002. Structure–property relationship of cast Ti–Nb alloys. J. Oral Rehabil. 29, 314–322. Lu¨tjering, G., Williams, J.C., 2003. Titanium. Springer, Germany, p. 28. Malinov, S., Sha, W., Guo, Z., Tang, C.C., Long, A.E., 2002. Synchrotron X-ray diffraction study of the phase transformations in titanium alloys. Mater. Charact. 48, 279–295. Mantani, Y., Takemoto, Y., Hida, M., Sakakibara, A., Tajima, M., 2004. Phase transformation of α´´ martensite structure by aging in Ti-8 mass % Mo alloy. Mater. T. JIM 45, 1629–1634.
Mantani, Y., Tajima, M., 2006. Phase transformation of quenched α” martensite by aging in Ti–Nb alloys. Mater. Sci. Eng. A 438– 440, 315–319. Niinomi, M., 2003. Fatigue performance and cyto-toxicity of low rigidity titanium alloy, Ti–29Nb–13Ta–4.6Zr. Biomaterials 24, 2673–2683. Oliveira, N.T.C., Aleixo, G., Caram, R., Guastaldi, A.C., 2007. Development of Ti–Mo alloys for biomedical applications: microstructure and electrochemical characterization. Mater. Sci. Eng. A 452–453, 727–731. Prima, F., Vermaut, P., Texier, G., Ansel, D., Gloriant, T., 2006. Evidence of α-nanophase heterogeneous nucleation from ω particles in a β-metastable Ti-based alloy by high-resolution electron microscopy. Scripta Mater. 54, 645–648. Settefrati, A., Aeby-Gautier, E., Dehmas, M., Geandler, G., Appolaire, B., Audion, S., Delfosse, J., 2011. Precipitation in a near β titanium alloy on ageing: influence of heating rate and chemical composition of the β-metastable phase. Solid State Phenom. 172–174, 760–765. Sukedai, E., Hashimoto, H., Hida, M., Mabuchi, H., 1992. Formation of ω phase in Ti–Mo alloys after aging and deforming. Mater. Sci. Technol. 8, 3–9. Sukedai, E., Yukihiro, T., Miyaji, D., Matsumoto, H., Nishizawa, H., Hashimoto, H., 2003. β to ω phase transformation due to aging in Ti–Mo alloy deformed in impact compression. Mater. Sci. Eng. A 350, 133–138. Sun, F., Prima, F., Gloriant, T., 2010. High-strength nanostructured Ti–12Mo alloy from ductile metastable beta state precursor. Mater. Sci. Eng. A 527, 4262–4269. Terauchi, H., Sakaue, K., Hida, M., 1981. Study of the ω-phase in Ti–Mo alloys by diffuse X-ray scattering. J. Phys. Soc. Jpn. 50, 3932–3936. Wood, R.M., 1963. Martensitic alpha and omega phases as deformation products in a titanium–molybdenum alloy. Acta Metall. Mater. 11, 907–914. Zhou, Y.-L., Luo, D.-M., 2011. Microstructure and mechanical properties of Ti–Mo alloys cold-rolled and heat treated. Mater. Charact. 62, 931–937.
Available online at www.sciencedirect.com
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Research Paper
Ti–Mo alloys employed as biomaterials: Effects of composition and aging heat treatment on microstructure and mechanical behavior Flavia F. Cardosoa, Peterson L. Ferrandinib, Eder S.N. Lopesa, Alessandra Cremascoa, Rubens Carama,n a
University of Campinas, School of Mechanical Engineering, Campinas, SP 13083-860, Brazil Sao Paulo State University, School of Engineering, Guaratingueta, SP, Brazil
b
art i cle i nfo
ab st rac t
Article history:
The correlation between the composition, aging heat treatments, microstructural features
Received 19 June 2013
and mechanical properties of β Ti alloys is of primary significance because it is the
Received in revised form
foundation for developing and improving new Ti alloys for orthopedic biomaterials.
17 October 2013
However, in the case of Ti–Mo alloys, this correlation is not fully described in the literature.
Accepted 26 November 2013
Therefore, the purpose of this study was to experimentally investigate the effect of
Available online 3 December 2013
composition and aging heat treatments on the microstructure, Vickers hardness and
Keywords:
elastic modulus of Ti–Mo alloys. These alloys were solution heat-treated and water-
Titanium alloys
quenched, after which their response to aging heat treatments was investigated. Their
Aging heat treatment
microstructure, Vickers hardness and elastic modulus were evaluated, and the results
Vickers hardness
allow us to conclude that stabilization of the β phase is achieved with nearly 10% Mo when
Elastic modulus
a very high cooling rate is applied. Young's modulus was found to be more sensitive to phase variations than hardness. In all of the compositions, the highest hardness values were achieved by aging at 723 K, which was attributed to the precipitation of α and ω phases. All of the compositions aged at 573 K, 623 K and 723 K showed overaging within 80 h. & 2014 Published by Elsevier Ltd.
1.
Introduction
Ti alloys are successfully employed in the area of orthopedic biomaterials due to their enhanced biocompatibility, high biocorrosion resistance and high strength-to-weight ratio (Lütjering and Williams, 2003; Niinomi, 2003). In recent years, research into metallic biomaterials has focused on the β Ti alloys produced from nontoxic elements, particularly using Mo as the alloying element (Zhou and Luo, 2011; Oliveira n
et al., 2007). In addition to superior mechanical strength, orthopedic biomaterials employed to replace hard tissues must present low elastic modulus values to avoid the stress-shielding phenomenon (Geetha et al., 2009). This phenomenon is caused by elastic modulus mismatch and leads to the insufficient loading of bone adjacent to an implant (Ahn and Grodzinsky, 2009). The effect deserves attention because it can result in bone mass loss, osteoporosis and sometimes bone failure (Carter et al., 1989). To
Correspondence to: Rua Mendeleiev, 200, Campinas 13083-860, SP, Brazil. Tel.: þ55 19 35213314; fax: þ55 19 32893722. E-mail addresses: [email protected], [email protected] (R. Caram).
1751-6161/$ - see front matter & 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jmbbm.2013.11.021
32
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
prevent the occurrence of stress-shielding, low-elasticmodulus implants are desirable. The microstructural features and hence the mechanical properties of β Ti–Mo alloys depend directly on alloy composition and the processing routes to which the alloys are subjected (Flower et al., 1974; Davis et al., 1979a). It is well known that solution heat-treating β Ti alloys at high temperatures and then cooling them rapidly may lead to microstructures composed of martensite and β phase (Ferrandini et al., 2007; Mantani et al., 2004). The two metastable phases, i.e., orthorhombic martensite (α″) and the bcc phase, display low elastic modulus values and mechanical strength (Lee et al., 2002). The effect of Mo content on the structure and properties of Ti–Mo alloys cast in graphite molds was investigated by Ho et al. (1999), who found that the crystal structure and phase morphology were composition-sensitive. Samples containing 6 wt% Mo resulted in acicular orthorhombic martensite. As the Mo content was increased, β phase became the only dominant phase. However, no details were provided about the cooling rates applied to the samples during the casting processes. A strengthening effect can be achieved in β Ti alloys by applying suitable heat treatment processes, causing the transformation of metastable phases into α phase (Mantani and Tajima, 2006). The precipitation of α phase, which is tougher and more rigid than orthorhombic martensite and β phase, can be controlled by submitting rapidly quenched β Ti alloys to aging heat treatment. However, the aging of β Ti alloys may also lead to the precipitation of the undesirable ω phase (Terauchi et al., 1981; Sukedai et al., 1992, 2003; Ho, 2008). Although ω phase precipitation significantly increases an alloy's mechanical strength, it also causes severe brittleness. Moreover, the presence of ω phase in a β phase matrix may play an important role in acting as a nucleation substrate for the formation of α phase, producing a fine and uniform distribution of α phase in the β phase matrix (Prima et al., 2006). The phase transformation of orthorhombic martensite by isothermal aging in Ti–8Mo (wt%) alloy was investigated by Mantani et al. (2004). The researchers' results indicated that orthorhombic martensite remained after aging at 723 K for 9.0 ks and was fully decomposed into α and β phases after aging at 923 K for 9.0 ks. In another interesting study, the sequence of phase transformation of Ti–12Mo (wt%) alloy was investigated by electrical resistivity and dilatometric measurements (Sun et al., 2010). It was found that under heating, samples of Ti–12Mo alloy showed the transformation of metastable β into ω and α precipitates, leading to very high tensile strength. The correlation between composition, aging heat treatments, microstructural features and mechanical properties of β Ti alloys is of primary significance because it is the foundation for developing and improving new Ti alloys for orthopedic biomaterials. However, in the case of Ti–Mo alloys, this correlation is not fully described in the literature. Therefore, the purpose of this study was to experimentally investigate the effect of composition and aging heat treatments on the microstructure, Vickers hardness and elastic modulus of Ti–Mo alloys. These alloys were solution heat-treated and
water-quenched, after which their response to aging heat treatments was investigated.
2.
Experimental procedure
Ingots of different compositions of the Ti–Mo system were arc melted using a water-cooled copper hearth and nonconsumable tungsten electrode in an argon atmosphere. The ingots were flipped and re-melted 5 times to ensure their chemical homogeneity. They were then homogenized at 1273 K for 24 h and hot rolled (initially heated to 1273 K) to obtain 2 mm thick plates. The nominal compositions of the 2 mm plates (wt%) were 3, 5, 6, 7.5, 8, 8.5, 9, 9.5, 10 and 15 wt% Mo (samples 3, 5, 6 and so on). The plates were solution heattreated for 1 h at 1273 K under inert atmosphere, waterquenched, aged at different temperatures and then air cooled to room temperature. The measured cooling rate provided by water quenching (WQ) was 160 K/s. In order to carry out the aging heat treatment, the samples were sealed in quartz ampoules in an argon atmosphere and inserted in a furnace heated at proper temperatures. Samples were conventionally prepared for metallography and etched in Kroll solution (3 vol% HF, 6 vol% HNO3 and 91 vol% H2O). The samples were analyzed by optical microscopy (OM) (Olympus BX60M). Vickers hardness was determined by applying a load of 1000 gf for 15 s, and the results presented herein are the averages of five measurements. Young's modulus was determined dynamically by the standard through-transmission technique, using coupled longitudinal and shear transducers with an active diameter of 6.35 mm. X-ray diffraction (XRD) measurements were taken with a Rigaku DMAX 2200 diffractometer and were performed at room temperature, using Cu-Kα radiation and operating at 40 kV/30 mA.
3.
Results and discussion
3.1.
Water-quenched samples
Initially, the aim of the experiments was to evaluate the effect of the Mo content on the phase formation in water-quenched samples, particularly the formation of hexagonal (α′) and orthorhombic (α″) martensites as well as the suppression of α″ and stabilization of β phase at room temperature. Samples containing 3 to 15 wt% Mo were solution heattreated and water-quenched, and their hardness, elastic modulus and microstructure were investigated. Fig. 1 presents the Vickers hardness values of water-quenched samples as a function of composition. In principle, the smaller strains involved in the formation of α′ and α″ martensites lead to low-hardness structures. Sample 3 presented a hardness value of HV 292, and samples 5 and 6 presented similar values. Davis et al. (1979b) reported that water-quenched Ti–4Mo (wt%) samples contained a thin layer of β phase between individual martensite plates due to some diffusion that occurred during quenching. An increase in hardness was observed when the Mo content reached 8 wt%. This high hardness is attributed to ω phase precipitation, which was
33
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
enabled by the amount of β phase that formed during quenching. For high-Mo-content alloys, the stabilization of β phase resulted in a decrease in hardness. Fig. 2 depicts optical microscopy images of waterquenched samples 3, 5, 6 and 7.5, and Fig. 3 presents their X-ray diffraction patterns. According to Davis et al. (1979b), Ti–4Mo (wt%) alloy presents α′ martensite, whereas Ti–6Mo (wt%) presents α″ martensite (orthorhombic martensite). Ho et al. (1999) reported the coexistence of α′ and α″ martensites in Ti–6Mo (wt%) alloy. Similar results were obtained for a different metallic system, Ti–Nb. Lee et al. (2002) reported the presence of both α′ and α″ martensites in Ti–15Nb (wt%) alloy. It should be emphasized that samples 3 and 5 presented only α′ martensite and sample 6 presented both α′ and α″ martensites. When the Mo content was increased to 7.5%, the microstructure presented a small volume fraction of β phase,
which should lead to some reduction in hardness, as opposed to what was observed for alloy 6. The results of X-ray diffraction analyses do not confirm the occurrence of β phase in sample 7.5 due to its small volume fraction. However, sample 7.5 presented higher hardness, which is explained by ω phase precipitation. Micrographs of water-quenched samples 8, 9.5, 10 and 15 are shown in Figs. 4 and 5 presents their X-ray diffraction patterns. The sample with Mo content of 8% exhibited α″, β and ω phases, while the one with Mo content of 9.5% exhibited α″ and β phases and possibly the ω phase. The amount of α″ phase decreased with increasing Mo content, and the hardness values revealed the strong influence of the ω phase. The microstructure of water-quenched sample 10 was also evaluated. According to the literature, the " "
" " "
650
120
600
80
500 Vickers Hardness Elastic Modulus
60
450 400
40 350 20
' "
'
Intensity (a.u.)
550
Vickers Hardness (HV)
Young´s Modulus (GPa)
100
"
"
"
" "
" " "
Ti-7.5Mo
" " "
Ti-6Mo
'
Ti-5Mo
" " "
"
"
" '
' " " "
' '
'
' '
'
300
'
'
'
'
'
'
'
Ti-3Mo
250
0 2
4
6
8
10
12
14
30
16
Fig. 1 – Hardness and Young's modulus of water-quenched Ti–Mo alloys.
40
50
60
70
80
90
Angle, 2
Mo content (wt. %)
Fig. 3 – X-ray diffraction patterns of water-cooled Ti–3Mo, Ti–5Mo, Ti–6Mo and Ti–7.5Mo (wt%).
Ti-3Mo
Ti-5Mo
Ti-6Mo
Ti-7.5Mo
Fig. 2 – Optical microscopy images of water-cooled Ti–3Mo, Ti–5Mo, Ti–6Mo and Ti–7.5Mo (wt%) samples.
34
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Ti-8Mo
Ti-9.5Mo
Ti-10Mo
Ti-15Mo
Fig. 4 – Optical microscopy images of water-cooled Ti–8Mo, Ti–9Mo, Ti–10Mo and Ti–15Mo (wt%) samples.
Intensity (a.u.)
Ti-15Mo
"
Ti-10Mo
"
Ti-9.5Mo
" " " " 30
" 40
" "
" " 50
60
70
Ti-8Mo 80
90
Angle, 2
volume fraction of α′ phase decreased in response to higher Mo contents; according to Lee et al. (2002), α′ martensite is stiffer than α″ martensite. Sample 3, which showed α′ martensite, presented an elastic modulus of 91 GPa, whereas sample 5 showed an elastic modulus of 80 GPa and sample 6 (α′ and α″ martensite) presented an elastic modulus of 75 GPa. As mentioned previously, samples 7.5 (82 GPa) and 8 (89 GPa) exhibited small amounts of β phase, which allowed for ω precipitation. Again, examination of the values of the elastic modulus reveals that they show a much clearer trend than the Vickers hardness values. Samples 8.5 to 10 presented high elastic modulus values, as expected because they displayed the presence of ω phase, and sample 15 presented an elastic modulus of 75 GPa, which is consistent with the β phase structure.
Fig. 5 – X-ray diffraction patterns of water-cooled Ti–8Mo, Ti–9.5Mo, Ti–10Mo and Ti–15Mo (wt%).
3.2.
addition of 10 wt% Mo suffices to retain β at room temperature (Ho et al., 1999). In this work, water-quenched sample 10 presented a structure consisting of β phase with ω precipitation. However, a small volume fraction of α″ martensite was detected by X-ray diffraction. The Vickers hardness values observed for the water-quenched samples are consistent with the phases observed. Sample 15 presented an entirely β phase structure and the lowest hardness value (HV 284). The Young's modulus of the water-quenched samples was determined, and the variation thereof is also illustrated in Fig. 1. Note that the elastic modulus is more effective than hardness in indicating changes in phase equilibrium. The
Alloy samples 5, 7.5, 10 and 15 were aged at 523 K, 573 K, 623 K and 723 K for periods of up to 100 h. Vickers hardness and X-ray diffraction analyses were performed to identify the constituent phases at the end of the heat treatments. Fig. 6 shows that sample 5 presented hardness plateaus despite the increase in aging temperature. When aged at 523 K, sample 5 showed a negligible increase in hardness after 40 min, which then leveled off, and finally, after 16 h, the increase in hardness was slightly more pronounced. Fig. 7 shows the X-ray diffraction pattern of sample 5 aged at 523 K. The figures shows the presence of α/α′ phase only. The X-ray diffraction patterns of α phase and α′ hexagonal martensite are very similar, and therefore, it is difficult to distinguish
Aging heat treatment
35
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
450
400
350
300
0.01
0.1
1
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100
Time (h)
Fig. 6 – Vickers hardness values of Ti–5Mo alloy as a function of aging time.
Intensity (a.u.)
Ti-5Mo
"
"
"
"
723 K
"
623 K
"
573 K
"
"
' ' 30
' ' 40
50
'
' 60
70
'
' 80
'
523 K ' 90
Angle, 2
Fig. 7 – X-ray diffraction patterns of Ti–5Mo alloy aged at different temperatures.
between these phases. Eventually, aging at such a temperature may have initiated the precipitation of α phase after a long aging period, causing the hardness to increase. No evidence of the presence of β and ω phases was observed in the diffraction pattern. The sample aged at 573 K showed similar behavior, but the first increase in hardness occurred after 10 min of aging, with a more pronounced increase occurring after 2 h. A decrease in hardness was observed after 40 h of aging. As expected, the hardness values obtained were always higher and the periods of time involved shorter than those obtained with aging at 523 K. The X-ray diffraction results show that at the end of the heat treatment, sample 5 presented α and ω phases and evidence of the precipitation of an orthorhombic symmetry phase, with an X-ray diffraction pattern similar to that of the orthorhombic martensite phase, α″. This result is somewhat surprising, considering the fact that orthorhombic martensite is not expected to be formed in low-Mo-content Ti alloys. Orthorhombic martensite, α″, is formed when β phase is rapidly cooled to low temperatures from high temperatures
or during the deformation of β phase at room temperature (Wood, 1963). In spite of the fact that this precipitation is very intriguing, a number of studies have also reported the formation of an orthorhombic symmetry phase during aging heat treatments of Ti alloys. Malinov and co-authors applied high-resolution synchrotron X-ray diffraction to investigate phase transformation in Ti–6Al–4V alloy and found that samples that were water quenched from high temperature and aged showed evidence of the precipitation of an orthorhombic symmetry phase (Malinov et al., 2002). Ivasishin et al. (2005) investigated the aging of some Ti alloys and observed that in the early stages of β decomposition in VT22 and TIMETAL-LCB alloys, an orthorhombic symmetry phase was formed. Settefrati et al. (2011) investigated phase transformation in Ti-5553 alloy using high-energy X-ray diffraction. The researchers found that an orthorhombic symmetry phase is formed during aging and decomposition of the β metastable phase. Finally, in a recent study, Aeby-Gautier et al. (2013) investigated the isothermal aging of Ti17 and Ti-5553 alloys and found that an orthorhombic symmetry phase forms at the very beginning of β phase decomposition. In sample 5, the presence of an orthorhombic symmetry phase is consistent with the pronounced decrease in hardness at the end of the aging treatment. With regard to the aging heat treatments at 623 K and 723 K, the hardness vs. aging time curves confirm that hardness begins to increase in less time, which is consistent with the expected C-curve behavior. In addition, the highest hardness values were reached in less time at higher aging temperatures. According to the X-ray diffraction patterns, the sample aged at 623 K presented α, the orthorhombic symmetry phase and ω phases and possibly a small volume fraction of β phase, whereas the sample aged at 723 K presented α and β phases and a reduced amount of the orthorhombic symmetry phase. The orthorhombic phase appears to be partially decomposed at 723 K, and no evidence of ω phase was observed at this temperature. The hardening effect observed at 723 K was stronger than that observed at 623 K.
500
Vickers Hardness (kgf/mm2)
Vickers Hardness (kgf/mm2)
Ti-5Mo
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450
523 K 573 K 623 K 723 K
Ti-7.5Mo
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Fig. 8 – Vickers hardness values of Ti–7.5Mo alloy as a function of aging time.
36
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Fig. 8 shows the hardness vs. aging time curves for sample 7.5. An increase in hardness after 8 h of aging at 523 K is visible, albeit not pronounced, as expected. It is likely that a small amount β phase was precipitated, followed by the isothermal precipitation of ω phase. Neither phase was identified by X-ray diffraction due to the phases' low volume fraction. As presented in Fig. 9, the X-ray diffraction results show the presence of α″ phase, which is consistent with the quenched condition of the alloy. When aged at 573 K, the alloy presented a plateau between 20 min and 1 h, steadily increasing thereafter up to HV 371 at an aging time of 40 h and followed by overaging. The amount of ω phase precipitated during aging at 573 K was sufficient to be detected by X-ray diffraction. Thus, α″, β and ω phases were identified. When aged at 623 K, sample 7.5 presented the same plateau behavior, albeit more pronounced because higher hardness values were obtained. X-ray diffraction showed evidence of the presence of α phase, as well as α″, β and ω phases. The hardness vs. aging time curve obtained for sample 7.5 aged at 723 K indicates that the higher aging temperature enabled more intense precipitation, resulting in considerably higher hardness values. Unlike those for sample 5, which presented α and β phases when aged at 723 K, the X-ray diffraction results for sample 7.5 showed α, α″, β and ω phases, Ti-7.5Mo
723 K
Intensity (a.u.)
"
"
623 K
"
"
" "
"
" "
"
"
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" "
"
"
"
" " 30
40
50
60
" " "
"" 70
" " 80
573 K
"
523 K 90
Angle, 2
Fig. 9 – X-ray diffraction patterns of Ti–7.5Mo alloy aged at different temperatures.
indicating that the ω phase solvus temperature for sample 7.5 is higher than 723 K. Fig. 10 shows the hardness vs. aging time curves obtained for sample 10. The hardening that occurred in response to 523 K aging was unexpectedly high and is attributed to ω phase precipitation. After 100 h of aging, the X-ray diffraction results shown in Fig. 11 revealed the presence of ω and β phases but not α phase, indicating that the nucleation and growth of α phase from ω phase particles requires more time. The same degree of hardness was attained with aging at 573 K. However, after 80 h of aging, X-ray diffraction showed the presence of α and β phases but not ω phase. A lower and more coherent hardness was attained after aging at 623 K, which according to the X-ray diffraction results, was due to the precipitation of α phase. Although hardness values should be analyzed carefully, the hardness vs. aging time curve indicates that the hardness began to increase after 40 min of aging at 623 K and that overaging occurred after 30 h. On the other hand, after 1 h of aging at 573 K, the hardness began to increase, with overaging occurring after 40 h. Again, this is evidence of C-curve behavior. The more intense precipitation that occurred in response to aging at 723 K led to the highest hardness value, with overaging occurring after 4 h. X-ray diffraction analysis revealed the presence of α and β phases. In Fig. 12, the hardness vs. aging time curves of sample 15 aged at 523 K and 573 K show very similar values. However, the latter temperature exhibits a clear tendency for overaging after 40 h, whereas the former presents more constant values between 16 h and 56 h and a slight decrease in hardness after 80 h of aging. The X-ray diffraction results show the presence of β and ω phases after 80 h of aging, as well as minor evidence of α phase precipitation, as indicated in Fig. 13. The same behavior was observed in response to aging at 623 K. The incubation period was approximately the same as that observed during aging at 523 K and 573 K. However, the more intense precipitation that occurred at a higher temperature (623 K) led to higher hardness values and overaging after 16 h of heat treatment. After 40 h of aging, the X-ray diffraction results indicated the presence of β and ω phases and a small amount of α phase. The rapid hardening observed Ti-10Mo
Ti-10Mo
723 K Intensity (a.u.)
Vickers Hardness (kgf/mm2)
450
523 K 573 K 623 K 723 K
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Fig. 10 – Vickers hardness values of Ti–10Mo alloy as a function of aging time.
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Fig. 11 – X-ray diffraction patterns of Ti–10Mo alloy aged at different temperatures.
journal of the mechanical behavior of biomedical materials 32 (2014) 31 –38
Vickers Hardness (kgf/mm2)
Ti-15Mo
523 K 573 K 623 K 723 K
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450
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strong peaks of α phase, whereas the hardness values revealed pronounced overaging. These findings lead to the conclusion that hardening was due mostly to ω phase precipitation, followed by α phase precipitation, which started to coarsen after 12 h of aging. Precipitates of ω phase acted as nucleation sites for α phase and were completely depleted after 12 h of heat treatment, as indicated in Fig. 14. In addition, a clear decrease in hardness was observed after 12 h of aging.
4.
300
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Fig. 12 – Vickers hardness values of Ti–15Mo alloy as a function of aging time.
37
Conclusions
Samples of Ti–Mo alloys were solution heat-treated and water-quenched, after which their response to aging heat treatments was investigated. Their microstructure, Vickers hardness and elastic modulus were evaluated, and the results allow us to conclude that:
Ti-15Mo
Intensity (a.u.)
723 K
623 K
573 K
523 K 30
40
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70
80
90
Angle, 2
Fig. 13 – X-ray diffraction patterns of Ti–15Mo alloy aged at different temperatures. Ti-15Mo - aging at 723 K
Intensity (a.u.)
12 h
(a) Martensitic transformation in dilute Ti–Mo alloys results in the formation of hexagonal martensite, α′. As the solute content reached 6% Mo, martensite transformation led to the formation of orthorhombic martensite α″. The coexistence of α′ and α″ martensites in the microstructure of the rapidly cooled Ti–6Mo (wt%) alloy was observed. (b) As the Mo content was increased, rapidly cooling from temperatures in the β phase region produces microstructures with α′; α′þα″; α″þβþω and βþω phases. The precipitation of ω phase, which was enabled by the retained β phase, resulted in higher hardness and elastic modulus values, which was followed by a decrease in those values due to an increase in the β volume fraction. Stabilization of the β phase was achieved at a composition near 10% Mo when high cooling rates were applied; (c) Young's modulus was found to be more sensitive than hardness to phase variations; (d) The aging of solution heat-treated and water-quenched samples of alloy 5 resulted in the precipitation of an unexpected orthorhombic symmetry phase; (e) In all of the compositions, the highest hardness values were achieved with aging at 723 K, which was attributed to the precipitation of α and ω phases. All of the compositions aged at 573 K, 623 K and 723 K showed overaging within 80 h.
4h 5 min
Acknowledgements WQ 78.0
78.5
79.0
79.5
80.0
80.5
81.0
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82.0
Angle, 2
Fig. 14 – X-ray diffraction patterns (low scanning speed) of water-cooled Ti–15Mo sample aged at 723 K for 5 min, 4 h and 12 h.
The authors gratefully acknowledge the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development) for their financial support of this work.
r e f e r e nc e s in response to aging at 723 K revealed that ω phase precipitation was intense during the first 5 min of aging. After 28 h of aging, the X-ray diffraction patterns indicated the presence of
Aeby-Gautier, E., Settefrati, A., Bruneseaux, F., Appolaire, B., Denand, B., Dehmas, M., Geandier, G., Boulet, P., 2013.
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Isothermal α´´ formation in β metastable titanium alloys. J. Alloy Compd. 577, S439–S443. Ahn, A.C., Grodzinsky, A.J., 2009. Relevance of collagen piezoelectricity to “Wolff’s Law”: a critical review. Med. Eng. Phys. 31, 733–741. Carter, D.R., Orr, T.E., Fyhrie, D.P., 1989. Relationship between loading history and femoral cancellous bone architecture. J. Biomech. 22, 231–244. Davis, R., Flower, H.M., West, D.R.F., 1979a. Martensitic transformations in Ti–Mo alloys. J. Mater. Sci. 14, 712–722. Davis, R., Flower, H.M., West, D.R., 1979b. The decomposition of Ti–Mo alloy martensites by nucleation and growth and spinodal mechanisms. Acta Metall. Mater. 27, 1041–1052. Ferrandini, P.L., Cardoso, F.F., Souza, S.A., Afonso, C.R., Caram, R., 2007. Aging response of the Ti–35Nb–7Zr–5Ta and Ti–35Nb–7Ta alloys. J. Alloy Compd. 433, 207–210. Flower, H.M., Henry, S.D., West, D.R.F., 1974. The β⇄α transformation in dilute Ti–Mo alloys. J. Mater. Sci. 9, 57–64. Geetha, M., Singh, A.K., Asokamani, R., Gogia, A.K., 2009. Ti based biomaterials, the ultimate choice for orthopaedic implants—a review. Prog. Mater. Sci. 54, 397–425. Ho, W.-F., Ju, C.P., Chern Lin, J.H., 1999. Structure and properties of cast binary Ti–Mo alloys. Biomaterials 20, 2115–2122. Ho, W.-F., 2008. Effect of omega phase on mechanical properties of Ti–Mo alloys for biomedical applications. J. Med. Biol. Eng. 28, 47–51. Ivasishin, O.M., Markovsky, P.E., Semiatin, S.L., Ward, C.H., 2005. Aging response of coarse- and fine-grained β titanium alloys. Mater. Sci. Eng. A 405, 296–305. Lee, C.M., Ju, C.P., Chern Lin, J.H., 2002. Structure–property relationship of cast Ti–Nb alloys. J. Oral Rehabil. 29, 314–322. Lu¨tjering, G., Williams, J.C., 2003. Titanium. Springer, Germany, p. 28. Malinov, S., Sha, W., Guo, Z., Tang, C.C., Long, A.E., 2002. Synchrotron X-ray diffraction study of the phase transformations in titanium alloys. Mater. Charact. 48, 279–295. Mantani, Y., Takemoto, Y., Hida, M., Sakakibara, A., Tajima, M., 2004. Phase transformation of α´´ martensite structure by aging in Ti-8 mass % Mo alloy. Mater. T. JIM 45, 1629–1634.
Mantani, Y., Tajima, M., 2006. Phase transformation of quenched α” martensite by aging in Ti–Nb alloys. Mater. Sci. Eng. A 438– 440, 315–319. Niinomi, M., 2003. Fatigue performance and cyto-toxicity of low rigidity titanium alloy, Ti–29Nb–13Ta–4.6Zr. Biomaterials 24, 2673–2683. Oliveira, N.T.C., Aleixo, G., Caram, R., Guastaldi, A.C., 2007. Development of Ti–Mo alloys for biomedical applications: microstructure and electrochemical characterization. Mater. Sci. Eng. A 452–453, 727–731. Prima, F., Vermaut, P., Texier, G., Ansel, D., Gloriant, T., 2006. Evidence of α-nanophase heterogeneous nucleation from ω particles in a β-metastable Ti-based alloy by high-resolution electron microscopy. Scripta Mater. 54, 645–648. Settefrati, A., Aeby-Gautier, E., Dehmas, M., Geandler, G., Appolaire, B., Audion, S., Delfosse, J., 2011. Precipitation in a near β titanium alloy on ageing: influence of heating rate and chemical composition of the β-metastable phase. Solid State Phenom. 172–174, 760–765. Sukedai, E., Hashimoto, H., Hida, M., Mabuchi, H., 1992. Formation of ω phase in Ti–Mo alloys after aging and deforming. Mater. Sci. Technol. 8, 3–9. Sukedai, E., Yukihiro, T., Miyaji, D., Matsumoto, H., Nishizawa, H., Hashimoto, H., 2003. β to ω phase transformation due to aging in Ti–Mo alloy deformed in impact compression. Mater. Sci. Eng. A 350, 133–138. Sun, F., Prima, F., Gloriant, T., 2010. High-strength nanostructured Ti–12Mo alloy from ductile metastable beta state precursor. Mater. Sci. Eng. A 527, 4262–4269. Terauchi, H., Sakaue, K., Hida, M., 1981. Study of the ω-phase in Ti–Mo alloys by diffuse X-ray scattering. J. Phys. Soc. Jpn. 50, 3932–3936. Wood, R.M., 1963. Martensitic alpha and omega phases as deformation products in a titanium–molybdenum alloy. Acta Metall. Mater. 11, 907–914. Zhou, Y.-L., Luo, D.-M., 2011. Microstructure and mechanical properties of Ti–Mo alloys cold-rolled and heat treated. Mater. Charact. 62, 931–937.
