In Situ Synchrotron X-ray diffraction Measurement of Simple Bending of NiTi Shape Memory Alloy Wires

摘要

Shape memory alloys (SMAs) exhibit unique mechanical behavior due to the shape memory effect or pseudoelasticity depending upon whether the material is martensitic or austenitic, respectively. In the case of austenitic NiTi SMAs, a phase transformation from austenite to martensite allows for full recovery of up to 5-10 % strain. This phase transformation can be induced by either a stress or temperature change. While austenitic NiTi SMAs are used for many applications, one major concern is their fatigue life. In wire form, the fatigue life is often limited by the surface defect structure, such as microcracks which can initiate fracture in NiTi SMAs [1-3]. As austenitic NiTi SMA wires bend, the strain becomes localized at the top and bottom of the point of inflection, resulting in a stress-induced phase transformation from austenite to martensite. The goal of this study is to examine the effect of bending on localized strain and microcrack evolution. Austenitic pseudoelastic NiTi SMA wires were subjected to various degrees of bending and studied using scanning electron microscopy (SEM) and synchrotron X-ray diffraction, where the latter technique allows for internal measurement of phase microstrains.