Effect of nitrogen addition and annealing temperature on superelastic properties of Ti–Nb–Zr–Ta alloys

doi:10.1016/j.msea.2010.07.052

Materials Science and Engineering: A

Volume 527, Issue 26, 15 October 2010, Pages 6844-6852

https://doi.org/10.1016/j.msea.2010.07.052 Get rights and content

Abstract

The composition dependence of the mechanical properties and martensitic transformation behavior of Ti–Nb–4Zr–2Ta–N alloys is investigated. The effect of annealing temperature on the microstructural evolution and superelastic properties in the N-added and N-free alloys is compared. The addition of N decreases M_s of Ti–Nb–4Zr–2Ta alloys by about 200 K per 1 at.%N and improves the superelastic properties of Ti–Nb–4Zr–2Ta alloys. The dissolution of α phase increases the martensitic transformation start temperature and decreases the superelastic recovery strain for the N-free alloy, whereas it causes opposite effects for the N-added alloy. The different annealing temperature dependences of superelastic properties are discussed on the basis of microstructure observation.

Research highlights

In this study, the effects of composition and annealing temperature on microstructure, shape memory effect and superelastic properties were investigated in Ti–Nb–4Zr–2Ta–N alloys by measuring stress–strain curves at various temperatures and using transmission electron microscopy. Dissolution of α phase increases M_s and decreases the critical stress for slip for the Ti–22Nb–4Zr–2Ta alloy while it causes the decrease of M_s and the increase of the critical stress for slip for the Ti–20Nb–4Zr–2Ta–0.6N alloy. The different effect of dissolution of α phase can be attributed to the fact that N is absorbed in α phase.

Introduction

Recently, Ti–Nb base superelastic alloys which consist of only non-toxic elements have been studied actively due to their potential use as biomedical devices. Up to date, superelasticity has been confirmed in many Ti–Nb base alloys, such as Ti–Nb–Sn [1], Ti–Nb–Al [2], Ti–Nb–Ta [3], Ti–Nb–Zr [4], Ti–Nb–Ta–Zr [5], [6], Ti–Nb–Zr–Sn [7], Ti–Nb–O [8], Ti–Nb–N [9], Ti–Nb–Pt [10] alloys. However, the superelastic properties of Ti–Nb base alloys, such as recovery strain, functional stability and critical stress for slip, are relatively poor when compared with Ti–Ni superelastic alloys [11], [12], [13]. There have been many efforts to improve the superelastic properties through alloy design and microstructure control. The addition of Zr to Ti–Nb base alloys as a substitute of Nb is effective to increase the transformation strain with keeping the martensitic transformation start temperature (M_s) similar; however the decrease of Nb leads to acceleration of ω phase formation, which causes deleterious effects on mechanical properties [14]. On the other hand, it has been reported that the formation of ω phase is less in Ti–Ta alloys [15], when compared with Ti–Nb and Ti–Mo alloys having similar transformation temperatures. It has been also reported that Sn and Al are effective to improve the superelastic properties of Ti–Nb base alloys [1], [2] and to suppress the formation of ω phase [16], [17], [18]. However, these substitutional alloying elements exhibit little effect on the critical stress for slip deformation because of their relatively weak solid solution hardening effect. On the other hand, the addition of interstitial elements such as nitrogen (N) and oxygen (O) increases the critical stress for slip remarkably [8], [9], [19], [20], [21], [22], [23]. However, so far little has been known about the effect of interstitial elements on the martensitic transformation of Ti base alloys.

The superelasticity of Ti–Nb base alloys can be improved by intermediate temperature annealing and/or aging [24], [25]. Intermediate temperature annealing is effective in increasing the critical stress for slip and improving superelastic properties due to the formation of a fine subgrain structure and a high density of thermally rearranged dislocations. It has been also reported that fine ω precipitates formed during aging in the temperature range between 573 and 673 K are effective in increasing the critical stress for slip.

In this study, Ti–Nb–4Zr–2Ta–N alloys were designed in order to develop biomedical shape memory and superelastic alloys. The effect of Nb and N contents on the shape memory and superelastic properties was investigated by loading-unloading tensile tests and by tensile tests at various temperatures. The effects of annealing temperature on the microstructure and superelastic properties for the N-free and N-added alloys were compared and the different annealing temperature dependences of superelastic properties were discussed on the basis of microstructural observation.

Section snippets

Experimental

Ti–(18–24)Nb–4Zr–2Ta (at.%), Ti–(16–24)Nb–4Zr–2Ta–0.3N (at.%) and Ti–(14–24)Nb–4Zr–2Ta–0.6N (at.%) ingots were prepared by the Ar arc melting method. Hereafter, the composition of the alloys is represented in at.%. The N content of the alloys was controlled by the amount of TiN. The changes in weight due to arc melting were less than 0.02 wt.%, thus, it is considered that the composition did not change significantly by arc melting in this study. The ingots were homogenized at 1273 K for 7.2 ks in

Effect of Nb and N content on shape memory properties

Fig. 1 shows the stress–strain curves obtained at room temperature for Ti–(18–24)Nb–4Zr–2Ta, Ti–(16–24)Nb–4Zr–2Ta–0.3N and Ti–(14–24)Nb–4Zr–2Ta–0.6N specimens heat treated at 1173 K for 3.6 ks. The specimens were deformed to the strain of 2.5% and then the stress was removed. The dashed line with an arrow indicates the shape recovery by heating up to 500 K. For Ti–xNb–4Zr–2Ta alloys, shape memory effect and superelasticity were observed when the Nb content was 18–20 at.% and 21–23 at.%,

Conclusions

The effects of composition and annealing temperature on microstructure, shape memory effect and superelastic properties were investigated in Ti–Nb–4Zr–2Ta–N alloys. The obtained results are summarized as follows.

(1)
The addition of N decreases M_s of Ti–Nb–4Zr–2Ta alloys by about 200 K per 1 at.% N. Ti–(21–23)Nb–4Zr–2Ta, Ti–(20–23)Nb–4Zr–2Ta–0.3N and Ti–(18–22)Nb–4Zr–2Ta–0.6N alloys exhibit superelasticity at room temperature.
(2)
For the Ti–22Nb–4Zr–2Ta alloy, the maximum superelastic recovery strain of

Acknowledgments

This work was partially supported by the Grants-in-Aid for Fundamental Science Research (Wakate B(2006–2007), Kiban C(2008–2010)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and the Grant-in-Aid for JSPS Fellows from the Japan Society for the Promotion of Science. This work was also partially supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (grant

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Effect of nitrogen addition and annealing temperature on superelastic properties of Ti–Nb–Zr–Ta alloys

Abstract

Research highlights

Introduction

Section snippets

Experimental

Effect of Nb and N content on shape memory properties

Conclusions

Acknowledgments

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