Effects of short time heat treatment on superelastic properties of a Ti–Nb–Al biomedical shape memory alloy

Abstract

The effect of short time heat treatment on microstructure, texture formation and anisotropy on superelastic properties was investigated for a Ti–24Nb–3Al (at.%) alloy, which is a Ni-free and β-titanium based biomedical shape memory alloy. An ingot was made by Ar-arc melting, homogenized at 1273 K for 7.2 ks, and then cold rolled with 99% reduction in thickness. Then, the alloy was heat-treated at 973, 1123 and 1273 K for 60, 120, 180 and 300 s, etc. X-ray diffraction analysis, X-ray pole figure measurement and scanning electron microscopy (SEM) combined with the recording of electron backscattering patterns (EBSP), were made for alloy characterization. Cyclic loading–unloading tensile test was performed to evaluate superelasticity for the alloy heat-treated at 1123 K for 120 s. It was found that 〈1 1 0〉 β{0 0 1} β-type deformation texture, which is developed by cold rolling, still appears after the heat treatment at 973 K for 300 s. On the other hand, it was also found that {1 1 2} β〈1 1 0〉 β-type recrystallization texture appears when heat-treated at 1123 K for 180 s and 1273 K at 60 s. Only when heat-treated at 1123 K for 120 s, SEM–EBSP revealed that the alloy contains both deformation texture with a grain size of 0.5 μm in diameter and recrystallization texture with a grain size of 20 μm. By tensile tests, the alloy with such bimodal texture exhibits more isotropic superelastic property of a 4% in all tensile directions than fully recrystallized and cold rolled alloys.