In-situ observation of elastic instability of stress-induced B19$^\prime$ martensite in thin NiTi wires

TL;DR

This study used laser-ultrasonic techniques to observe how the elastic properties of stress-induced martensite in NiTi wires change with stress and temperature, revealing phase softening and shape setting conditions.

Contribution

It provides in-situ measurements of elastic modulus evolution in NiTi martensite during stress and temperature variations, including shape setting, using a novel laser-ultrasonic approach.

Findings

Young's modulus ranges from 30 to 50 GPa in stress-induced martensite.

Martensite softens to nearly 10 GPa during shape setting at high temperatures.

Shape setting occurs within the same temperature interval for tension and torsion martensite.

Abstract

A laser-ultrasonic approach was used to measure elastic properties of a superelastic nickel-titanium wire with the aim to evaluate their evolution with stress and temperature in stress-induced martensite. It was observed that this evolution can be well described by a single smooth surface in the stress-temperature space, with the values of Young's modulus ranging from 30 to 50 GPa. The evolution of the modulus was then monitored in-situ with further heating under fixed strain, that is, during the shape setting. The results revealed that the martensite phase experienced further softening during this process, reaching Young's modulus of nearly 10 GPa at high temperatures and high stresses. In addition, the measurement enabled a direct detection of the initiation and termination temperatures of the shape setting from the elasticity data, which was used to show that it occurs in the same temperature interval for tension-induced and torsion-induced martensite.