Effect of Low Temperature Aging on Superelastic Behavior in Biocompatible β TiNbSn Alloy

Abstract

Superelasticity of Ti-16 at%Nb-4.8 at%Sn alloy consisting of non-(or minimal-) cytotoxic elements was investigated for biomedical applications as functions of deformation temperature and aging heat treatment after quenching. As-quenched Ti-16Nb-4.8Sn having A f (the reverse martensitic transformation finish temperature) of 266 K exhibits superelasticity at human body temperature (310 K) with an elastic recovery strain of 3.2%. A f decreases with increasing aging temperature in the range of 353 to 423 K or with increasing aging time at 423 K. It is confirmed from TEM observation and Young's modulus measurements that ! transformation occurs on aging at 423 K for 500 h. Stress for inducing martensitic transformation, M , increases with aging. Stress-strain curves showing superelasticity are quite similar to each other when as-quenched or aged samples are deformed at such temperatures that they have the same M . It is concluded that maximum superelastic recovery strain can be obtained at intended temperature, e.g. human body temperature, in Ti-Nb-Sn alloys by aging at relatively low temperatures.

In-Situ Transmission Electron Microscopy Observation on the Phase Transformation of Ti-Nb-Sn Shape Memory Alloys

“…Therefore, we can propose that the b phase in the quenched alloy decomposes to isothermal x and more stable b phases when sufficient thermal energy is given, thereby resulting in the suppression of the martensitic a¢¢ transformation. This explanation is supported by the results obtained from our DSC analysis and in-situ TEM observation for the (Ti-35Nb)-0Sn alloy; the reversibility of the transformation between b and a¢¢ phases was not seen, once the martensite a¢¢ phase transformed to b phase by heating up to 523 K. This is explained by the precipitation of the isothermal x phase in the b phase of the Ti-35 wt pct Nb alloy at a temperature higher than 473 K. [12,13,34] The results obtained in the present study demonstrate that the reversible transformation between martensitic a¢¢ and austenitic b phases is due to the competition of martensitic a¢¢ and isothermal x phases from unstable b phase in the (Ti-35Nb)-Sn alloys. It is concluded, therefore, that appropriate amount of Sn addition to Ti-Nb alloys and suitable quenching rate are required for the reversible transformation caused by the unstable b phase without containing athermal and isothermal x precipitates, in order to enhance the shape memory effect in the ternary system.…”

“…4-M s and A f points of quenched (Ti-35Nb)-Sn alloys as a function of the Sn content, together with referred data. [8,13] reversible transformation between b and martensitic a¢¢ phase is confirmed for the (Ti-35Nb)-5Sn alloy.…”

“…The overall tendency indicated here is consistent with previous works. [8,13] C. Conventional TEM Observation Here, the DF images Figures 5(c) and (d) were taken using the spots marked by the dotted circles h1i and h2i in the SAD pattern, respectively. The SAD pattern of Figure 5(a) can be interpreted as a superposition of (1) strong spots arising from the b phase; (2) weak spots arising from the a¢¢ phase viewed along the [001] a¢¢ zone axis, with two variants seen simultaneously; and (3) the diffused streaks arising from the unstable b phase.…”

“…For example, it has been reported that the elastic modulus and shape memory properties of the alloys are sensitively influenced by ternary compositions, [3,[8][9][10] and by heat treatment procedure and manufacturing processes. [6,[11][12][13] These reports strongly suggest that the microstructure of the alloys plays a key role in giving rise to their mechanical and functional properties.…”

Martensitic Transformation and Superelastic Properties of Ti-Nb Base Alloys

“…2533) Particularly, there has been tremendous progress in developing Ti-Nb base shape memory alloys in the past decade and many Ti-Nb base alloys have been developed, e.g. Ti-Nb-Sn, 26,34) Ti-NbAl, 31,32,35) Ti-Nb-Ta, 36,37) Ti-Nb-Zr, 3844) Ti-Nb-Mo, 45,46) TiNb-Pd, 47) Ti-Nb-O, 48) Ti-Nb-N, 49,50) Ti-Nb-Pt, 51) Ti-Nb-TaZr, 5254) Ti-Nb-Zr-Sn, 5557) Ti-Nb-Mo-Sn, 5860) Ti-Zr-NbSn, 61) Ti-Nb-Zr-Al 62) and Ti-Nb-Zr-Mo-Sn. 63) This paper aims to provide an overview of the recent works on Ti-Nb base shape memory alloys.…”