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

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