Microstructures and mechanical behavior of Ti-(38+x)Ni-12Cu (at%) (x=0∼3) alloys

In this study, various aging conditions were done on (Ni,Cu)-rich Ti-(38+x)Ni-12Cu (at %) (x=0∼3) alloy sheets to improve the superelasticity and it is found that precipitation hardening is available on some alloys at proper aging conditions, leading to the realization of perfect superelasticity with small stress hysteresis. For the solution-treated specimens, their microstructures and mechanical properties are dependent on the Ni content and solution treatment temperature. In 39 at %Ni and 40 at %Ni alloys, Ti(Ni,Cu)2 particles were embedded in the matrix of as-rolled specimens and disappeared after solution treatment at 1373 K, while they remained in the 41 at %Ni alloy. The 40 at %Ni alloy containing Ti(Ni,Cu)2 particles showed a fracture strain of 30.0 % which was large compared with the alloy without the particles (∼10.2 %). Aging treatment on the specimen solution treated at 1373 K increased micro Vickers hardness and the maximum hardness of the aged 39 at %Ni and 40 at %Ni alloys were Hv 274 and Hv 377, respectively, which was ascribed to the precipitation of the C11b-type precipitates. After optimizing aging conditions, 39 at %Ni and 40 at %Ni alloys showed perfect superelasticity, which was due to the precipitation hardening effect by the C11b-type precipitates. More importantly, the properly precipitation-hardened 40 at %Ni alloy specimen aged at 723 K for 2.5 h following the solution treatment at 1373 K for 1 h showed perfect superelasticity with a small stress hysteresis of 36 MPa associated with the B2-B19 transformation, which was ascribed to the short mean free path between the C11b-type precipitates. With raising the aging temperature to 873 K, the stress hysteresis increased up to 112 MPa, which was attributed to the increase in the mean free path between the C11b-type precipitates.