Recent Findings on the Effect of Aging Under Stress on Springback and Transformation Behavior of NiTi Shape Memory Alloys

Aging under stress is known to influence the precipitate formation in NiTi shape memory alloys (SMAs) and thus alter the transformation temperatures compared to the annealed state. In this work, we present some recent results related to aging under stress of Ni-rich and Ti-rich NiTi SMAs. In Ni-rich alloys we report on the dependance of springback on the aging time and temperature. In Ti-rich alloys, we report on a peculiar differential scanning calorimetry (DSC) response that is observed on stress-free aging, but which disappears on stress-aging. The response consists of an appearance multi-step transformation in stress-free aged Ti-rich samples, which reverts to single-step transformation in stress-aged samples. We propose a plausible mechanism to explain the occurrence of multi-step transformation as seen in the DSC pattern of Ti-rich NiTi SMAs.