Anomalous stress-strain behavior of NiTi shape memory alloy close to the border of superelastic window

In this work, we report an anomalous phenomenon on superelastic cycling of NiTi shape memory alloys when deforming at the temperature close to the border of superelastic window. New unexpected effects are found-(i) critical stress for inducing martensite transformation during the second loading cycle is higher than that of the first cycle; ( ii ) the plateau stress of the second cycle decreases to the original level when the strain overcomes the limit of the first cycle; ( iii ) transition from good superelasticity in the first cycle to fully irreversible strain in the second. We propose that defects generated during the first superelastic cycle close to the border of superelastic window impede following stress-induced martensitic transformations, leading to the increase of critical stress beyond yield stress of the B2 matrix, and thus functional fatigue of NiTi alloys. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This work reports anomalous phenomena in superelastic cycling of NiTi shape memory alloys near the temperature border, including higher critical stress, plateau stress changes, and transition from good superelasticity to fully irreversible strain, which may be caused by defects generated during the first superelastic cycle near the border impeding stress-induced martensitic transformations.