Constant-strain tensile cyclic behavior and microstructure of Ni-rich NiTi (-Sc) alloys

The superelastic behavior of Ni52Ti48, Ni55Ti45, and Ni55Ti44.9Sc0.1 alloys after solution treatment was in-vestigated by cyclic tensile tests, electron probe microanalyzer (EPMA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Ni-rich NiTi(-Sc) alloys exhibited excellent su-perelastic properties and underwent stress-induced martensite transformation during tensile loading. The residual strain accumulated in the Ni52Ti48 alloy was approximately 0.05% after loading/unloading tests. Coarse martensite laths were observed, indicating incomplete recovery after unloading. In contrast, no residual martensite was found in Ni55Ti45 alloy and Ni55Ti44.9Sc0.1 alloy after unloading. This was due to the presence of a large amount of Ni4Ti3 phases, which cut off the continuity of the matrix and confined the martensite transformation in a narrow matrix, thus inhibiting the formation of large-size martensite. The recovery ratio of Ni55Ti44.9Sc0.1 alloy was maintained at 96.7% after 5 cycles at 2.5% tensile strain. Moreover, the tensile strength of Ni55Ti45 and Ni55Ti44.9Sc0.1 were 1567.8 and 1667.9 MPa, respectively, which re-presented an apparent improvement compared to Ni52Ti48 (1223.5 MPa). This was attributed to the in-creased Ni content, resulting in an increase in the volume fraction of the strengthening phases including the Ni4Ti3 phase and the Ni3Ti2 phase. (c) 2023 Published by Elsevier B.V.