Evolution of elastic constants of the NiTi shape memory alloy during a stress-induced martensitic transformation

We carried out an in situ ultrasonic characterization of a NiTi shape memory alloy polycrystal subjected to pseudoplastic straining. The measurement leads to a full elastic tensor of the transversely isotropic polycrystal during the stress-induced austenite -> R-phase -> B19' martensite transformation. The results reveal a strong anisotropy of the R-phase already in the low-strain state, which is then retained in character throughout the R-phase -> martensite transition and even in the stress-free oriented martensite after the unloading. By using a micromechanical model based on homogenization approaches, we show that this strong anisotropy is probably related to twin boundaries both in the R-phase and in martensite rather than to the anisotropy of the unit cell and the crystallographic texture. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.