Superelastic Behavior of Biomedical Metallic Alloys

In this this work, superelastic NiTi and Ni-free Ti-23Hf-3Mo-4Sn biomedical alloys were investigated by tensile tests in relationship with their microstructures. To follow the stress-induced martensitic transformations occurring in these alloys, in situ tensile tests under synchrotron beam were conducted. In NiTi, an intermediate trigonal R phase, which is first stress-induced before the B19′ martensitic phase, was identified. However, the Ti-23Hf-3Mo-4Sn alloy does not present a transitional phase, and a direct β into α″ reversible stress-induced martensitic transformation was observed. With NiTi, all the applied strain is recovered after unloading, and no residual plastic deformation occurs. However, the strain is not completely recovered with the Ti-23Hf-3Mo-4Sn alloy, and residual plastic strain was observed to prevent a complete recovery, thus explaining why the strain recovery is lower for Ti-23Hf-3Mo-4Sn compared with NiTi. We also showed that the maximum strain recovery depends on the texture in the Ti-23Hf-3Mo-4Sn alloy. The favorable texture leading to the highest strain recovery (4.6 pct) is the {111}〈110〉_β texture, which can be obtained by a short-time solution treatment (0.3 ks) at 1073 K with this alloy.