Effect of annealing temperature on shape memory characteristics in Ti–20Nb–6Zr–0.5O(at%) biomedical alloy

Abstract

► The martensitic transformation start temperature of Ti–20Nb–6Zr–0.5O(at%) alloy is hardly affected by annealing temperature. ► Ti–20Nb–6Zr–0.5O(at%) alloy annealed at 923 K exhibited the maximum total recovery strain higher than 4.2%. ► The critical stress for slip of Ti–20Nb–6Zr–0.5O(at%) alloy annealed at 923 K is higher than that of Ti–22Nb–6Zr(at%) by over 200 MPa. ► With increasing annealing temperature, accumulated plastic strain decreases, recovery strain by superelasticity decreases and stress hysteresis decreases during cyclic deformation. Effect of annealing on shape memory characteristics of Ti–20Nb–6Zr–0.5O(at%) biomedical alloys was investigated by using tensile test and optical microscopy (OM). The ingots were cold-rolled with a reduction up to 95% in thickness. After severe cold-rolling, the plate was annealed at 923–1173 K. The martensitic transformation start temperature ( M s ) is hardly affected by annealing temperature. The superior shape memory properties and cyclic deformation behavior were observed at room temperature in all specimens. The specimen annealing at 923 K exhibits the most stabilized superelasticity. The maximum shape recovery strain of 4.2% and the critical stress for slip of 780 MPa were obtained in the specimen annealed at 923 K. Maximum recovery strain and critical stress for slip decrease with increasing annealing temperature. According to OM observation the grain size increases from 10 to 47 μm with increasing annealing temperature. It is considered that the critical stress for slip and the stability of superelasticity was considerably affected by grain size. This indicates that grain refinement is important method to improve shape memory characteristics.