Crystal Structure, Transformation Strain, and Superelastic Property of Ti–Nb–Zr and Ti–Nb–Ta Alloys
The composition dependences of transformation strain and shape memory, and superelastic properties were extensively investigated in Ti–Nb–Zr and Ti–Nb–Ta alloys in order to establish the guidelines for alloy design of biomedical superelastic alloys. The effects of composition on the crystal structure of the parent (β) phase and the martensite (α″) phase were also investigated. Results showed that not only transformation temperature but also transformation strain is tunable by alloy design, i.e., adjusting contents of Nb, Zr, and Ta. The lattice constant of the β phase increased linearly with increasing Zr content, while it was insensitive to Nb and Ta contents. On the other hand, the lattice constants of the α″ phase are mainly affected by Nb and Ta contents. The increase of Zr content exhibited a weaker impact on the transformation strain compared with Nb and Ta. The addition of Zr as a substitute of Nb with keeping superelasticity at room temperature significantly increased the transformation strain. On the other hand, the addition of Ta decreased the transformation strain at the compositions showing superelasticity. This study confirmed that the crystallography of martensitic transformation can be the main principal to guide the alloy design of biomedical superelastic alloys.
KeywordsMechanical behavior Superelasticity Stress-induced martensitic transformation
Over the past decade, Ti–Nb-base alloys have been extensively studied as promising candidates for Ni-free biomedical shape memory alloys, and many alloys have been reported to exhibit superelasticity at room temperature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. For the binary Ti–Nb alloys, superelastic recovery was observed when the Nb content is 26–27 at.% ; however, the recovery strain was as small as about 3 % even including elastic strain, which is quite smaller than those of practical Ti–Ni superelastic alloys . The small recovery strain in the Ti–(26–27)Nb alloys is due to the small lattice distortion strain upon stress-induced martensitic transformation from the parent (β) phase to the martensite (α″) phase as well as the low critical stress for slip . Extensive research has shown that the superelastic properties can be improved through microstructure control such as low temperature annealing and aging [4, 16, 17, 18].
Alloying is another effective way for improving superelastic properties. Among many alloying elements, Zr and Ta have attracted considerable attention due to their superior biocompatibility . Many kinds of ternary and multinary alloys including Zr and/or Ta have been developed up to date, e.g., Ti–Nb–Ta [20, 21, 22, 23, 24, 25], Ti–Nb–Zr [16, 18, 22, 24, 26, 27, 28, 29, 30, 31], Ti–Nb–Ta–Zr [17, 32, 33], Ti–Nb–Zr–Sn [34, 35, 36, 37, 38], Ti–Nb–Zr–Al , and Ti–Nb–Zr–Mo–Sn . Although substantial advances have been made in developing biomedical superelastic alloys, the understanding on the martensitic transformation behavior of Ti–Nb-base alloys is not sufficient yet. Particularly there is limited information regarding the effect of alloying element on the crystal structures of the β and α″ phases although they determine the transformation strain.
In this study, the effects of composition on the shape memory and superelastic properties of the Ti–Nb–Zr and Ti–Nb–Ta ternary alloys were extensively investigated. The composition dependence of lattice constants of the β and α″ phases were also investigated for Ti–Nb–Zr and Ti–Nb–Ta ternary alloys in order to clarify the effects of Zr and Ta on the transformation strain and to establish the design strategy for biomedical superelastic alloys.
Various compositions of Ti–Nb–Zr alloys and Ti–Nb–Ta alloys were prepared using the Ar-arc melting method. The ingots were melted at least six times and flipped over after each melting in order to maximize the homogeneity. The weight change during melting was less than 0.04 %. The ingots were sealed into quartz tubes under vacuum and homogenized at 1273 K for 7.2 ks, and then cold rolled with a final reduction ratio of about 98.5 %. Specimens for tensile tests and X-ray diffraction (XRD) measurements were cut using an electro-discharge machine. The slightly oxidized surface was removed using a solution containing H2O, HNO3, and HF (5:4:1) at room temperature. Then the specimens were encapsulated into quartz tubes in an Ar atmosphere and solution treated at 1173 K for 1.8 ks, and then quenched into water. Shape memory and superelastic properties were characterized by a tensile testing machine. Tensile tests were carried out at a nominal strain rate of 0.005 mm/s at room temperature. The dimensions of the tensile specimens were 40 mm in length and 1.5 mm in width. Both ends of test samples were gripped through chucks so that gage length was 20 mm. Phase constitutions and their lattice constants were determined by XRD with Cu Kα radiation at room temperature. It is noted that the superelastic properties and lattice constants of the β phase and α″ phase of Ti–Nb-base alloys do not change noticeably in the temperature range 293–310 K [17, 26, 41], and this allows the modeling of superelastic properties at body temperature using room temperature testing condition.
Results and Discussion
Shape Memory Effect and Superelasticity in Ti–Nb–Zr and Ti–Nb–Ta Alloys
Effect of Composition on Lattice Constants
Effect of Composition on Transformation Strain
Figure 11b reveals that the increase in the content of Nb or Ta reduces the transformation strain in the Ti–Nb–Ta alloys, but the impact of Nb is more significant than that of Ta. For instance, the transformation strain decreases by 0.35 % with 1 at.% increase of Nb content in Ti–Nb–20Ta alloys, while it decreases by 0.28 % with 1 at.% increase of Ta content in Ti–20Nb–Ta alloys. When comparing different composition alloys which reveal superelastic recovery at room temperature, it is clear that the increase of Ta content in Ti–Nb–Ta alloys decreases the transformation strain. For example, the transformation strain along the  direction is only 1.7 % in Ti–19Nb–10Ta and 1.2 % in Ti–13Nb–20Ta.
Superelastic Properties of Ti–Nb–Zr and Ti–Nb–Ta Alloys
As shown in Fig. 11, shape memory effect and superelasticity were observed in a wide composition range of Ti–Nb–Zr and Ti–Nb–Ta alloys; however, the properties were strongly dependent on the composition because not only transformation temperature but also transformation strain is changed by the addition of Zr or Ta in Ti–Nb alloys. Superelasticity is more applicable for biomedical applications than shape memory effect since most of medical applications of Ti–Ni alloys, e.g., orthodontic arch wires, self-expanding stents, and guide wires, use the superelasticity rather than shape memory effect. It is concluded that the addition of Zr is better than Ta in terms of superelastic properties owing to the increase in the transformation strain at the composition showing superelastic recovery at room temperature. This is due to the fact that Zr has a weak impact on the lattice constants of the α″ martensite phase, while it decreases the martensitic transformation temperature so as that the superelasticity occurs at compositions at lower Nb content. To verify the effect of Zr and Ta contents on superelastic recovery strain, loading–unloading cyclic tensile tests were carried out for various Ti–Nb–Zr and Ti–Nb–Ta alloys.
The shape memory effect and superelasticity were observed over a wide composition range in Ti–Nb–Zr and Ti–Nb–Ta ternary alloys.
The lattice constant of the β phase increased linearly with increasing Zr content, whereas the lattice constants of the α″ orthorhombic martensite phase are mainly affected by the Nb content in the Ti–Nb–Zr alloys.
The lattice constant of the β phase was insensitive to not only Nb content but also to Ta content in the Ti–Nb–Ta alloys. On the other hand, the lattice constants of the α″ orthorhombic martensite phase were mainly governed by the total amount of Nb and Ta in the Ti–Nb–Ta alloys.
Zr exhibited a weaker impact on the transformation strain compared with Nb and Ta. The addition of Zr as a substitute of Nb with keeping superelasticity at room temperature increased the transformation strain.
The superelastic recovery strain increased with increasing Zr content, while it decreased with increasing Ta content in both Ti–Nb–Zr and Ti–Nb–Ta alloys.
This work was partially supported by JSPS KAKENHI Grant Number 26249104 and MEXT KAKENHI Grant Number 25102704.
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