Improvement in the superelasticity of a Ti–35.5Ni–15Cu (at.%) alloy using Ti(Ni,Cu)2 phase

In this study, microstructures, martensitic transformation behavior and superelasticity of a (Ni,Cu)-rich Ti-35.5Ni–15Cu (at.%) alloy were investigated by means of scanning electron microscopy, differential scanning calorimetry, tensile tests and transmission electron microscopy. As-cast Ti-35.5Ni–15Cu alloy ingot fabricated by arc melting was hot-rolled without cracking into a sheet with a thickness of 1.2 mm, which was ascribed to the introduction of ductile Ti(Ni,Cu)2 phase beneficial for improving ductility of the alloy. Ti(Ni,Cu)2 phase remained after 1123 K solution treatment, while it dissolved into the matrix during 1273 K solution treatment. Dissolution of Ti(Ni,Cu)2 phase rendered matrix supersaturated with Ni and the subsequent annealing of the Ni-supersaturated matrix at 673 K induced precipitation of very fine C11b-type Ti(Ni,Cu)2 phase. The specimen annealed at 673 K showed transformation temperatures comparable to those of a Ti–35Ni–15Cu (at.%) alloy and almost complete superelasticity, which were ascribed to precipitation of C11b-type Ti(Cu,Ni)2 phase that made the (Ni + Cu) content of matrix close to 50 at.% and strengthened the matrix. The utilization of Ti(Ni,Cu)2 phase for improving workability and superelasticity without lowering transformation temperatures and increasing transformation hysteresis in this study was considered to be a good strategy for putting the Ti–Ni–Cu alloys with high Cu-content to practical use.