Several Issues in the Development of Ti–Nb-Based Shape Memory Alloys
Ni-free Ti-based shape memory alloys, particularly Ti–Nb-based alloys, have attracted increasing attraction since the early 2000s due to their wide application potentials in biomedical fields. Recently, there has been significant progress in understanding the martensitic transformation behavior of Ti–Nb-based alloys and many novel superelastic alloys have been developed. The superelastic properties of Ti–Nb-based alloys have been remarkably improved through the optimization of alloying elements and microstructure control. In this paper, in order to explore and establish the alloy design strategy, several important issues in the development of Ti–Nb-based shape memory alloys are reviewed. Particularly, the effects of alloying elements on the martensitic transformation temperature and the transformation strain are analyzed. The effects of omega phase and texture on the superelastic properties are also discussed.
KeywordsMartensite Mechanical behavior Shape memory alloys Superelasticity Transformation strain Beta Ti alloy Biomaterial
Over the past decade, there has been significant progress not only in understanding the martensitic transformation behavior of Ti-based alloys but also in developing novel biocompatible shape memory alloys [1, 2, 3]. In binary Ti–Nb alloys, superelasticity has been reported to occur when the Ni content is in the range of 26–27 at.% [4, 5]. However, the superelastic properties of Ti–(26, 27)Nb alloys are not good enough, particularly in terms of recovery strain, when compared with practical Ti–Ni superelastic alloys . The superelasticity of Ti–Nb alloys is associated with the stress-induced martensitic transformation from the parent (β) phase to the martensite (α″) phase and its revision. It has been concluded that the small recovery strain is mainly due to the intrinsic small transformation strain of the β–α″ transformation. [3, 5]. Another drawback of Ti–Nb alloys to be mentioned is low critical stress for slip. These drawbacks have prompted researchers to develop new alloys. Many kinds of Ti–Nb-based alloys have been developed to date, e.g., Ti–Nb–Sn [6, 7, 8, 9], Ti–Nb–Al [10, 11, 12], Ti–Nb–O [13, 14], Ti–Nb–N [14, 15], Ti–Nb–Mo , Ti–Nb–Pt , Ti–Nb–Pd , Ti–Nb–Ta [19, 20, 21, 22, 23], Ti–Nb–Zr [24, 25, 26, 27, 28, 29, 30, 31], Ti–Nb–Ta–Zr [32, 33, 34], Ti–Nb–Mo–Sn [35, 36, 37, 38], Ti–Nb–Zr–Sn [39, 40, 41, 42, 43], Ti–Nb–Zr–Al , and Ti–Nb–Zr–Mo–Sn . Through these efforts, superelastic properties have been significantly improved over the last 10 years. However, the influence of alloying elements on the martensitic transformation characteristics has not been fully elucidated. There have been only limited quantitative studies to assess the effect of alloying elements on the crystal structure of martensite phase, transformation temperature, and transformation strain. In this paper, several important issues in the development of Ti–Nb-based shape memory alloys are reviewed in order to establish an alloy design strategy for biomedical superelastic alloys. All the compositions mentioned in the paper are given in at.%.
Transformation Strain and Transformation Temperature
Effects of alloying elements on Ms and transformation strain along the \(\) direction in Ti–Nb binary and Ti–Nb–X ternary alloys
Effect in changing Ms temperature (K/at.%)
Effect in changing transformation strain (%/at.%)
Alloy composition showing superelasticity at RT
Ti–24Nb–1Mo, Ti–21Nb–2Mo, Ti–18Nb–3Mo
−160 to 200
Interstitial Alloying Elements
Recent researches have provided important insight into the understanding of the effect of alloying elements on the microstructure, martensitic transformation behavior, and superelastic properties of Ti–Nb alloys. It has been well documented that the superelastic properties of Ti–Nb alloys can be markedly improved by the addition of alloying elements. Zr is the most effective alloying element in increasing the superelastic recovery strain because the addition of Zr in replacement of Nb decreases the transformation temperature but has a relatively weak impact on the transformation strain. Mo is also effective to increase the transformation strain if it is added while maintaining the same transformation temperature. On the other hand, the decrease in the amount of β phase stabilizing elements facilitates the formation of ω phase causing detrimental effects on the superelastic properties. The addition of Sn is effective for suppressing the formation of ω phase. As a result, it is proposed that Ti–Zr–Nb–Sn and Ti–Zr–Mo–Sn alloys are promising candidates for practical biomedical superelastic alloys. For example, it has been reported to exhibit a large recovery strain of about 7% in Ti–24Zr–10Nb–2Sn and Ti–Zr–1.5Mo–3Sn alloys. Texture is one of the important factors to determine the superelastic properties. Not only the deformation texture but also the recrystallization texture is changed by the alloying element. However, much remains unclear about the effect of alloying element on the texture development of Ti alloys. Interstitial alloying elements such as oxygen and nitrogen have attracted particular attention because of many unique properties such as larger elastic strain, invar-like behavior, and nonlinear elastic behavior. In addition, the interstitial alloying element is very effective in increasing the critical stress for plastic deformation and improving the superelastic properties. Although there still remains a lack of quantitative research, Ti-based superelastic alloys are expected to expand the application fields of shape memory alloys.
This work was partially supported by JSPS KAKENHI Grant Number 26249104 and MEXT KAKENHI Grant Number 25102704.
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