Dissecting functional degradation in NiTi shape-memory-alloys containing amorphous regions via atomistic simulations

TL;DR

This study uses atomistic simulations to understand how amorphous regions affect the functional degradation of NiTi shape-memory alloys under cyclic loading, revealing atomic-scale mechanisms and proposing mitigation strategies.

Contribution

It provides detailed atomic-scale insights into degradation mechanisms caused by amorphous regions in NiTi alloys and suggests new methods to prevent this degradation for improved durability.

Findings

Amorphous surface regions influence the mechanical response under cyclic compression.

Plastic deformation and retained martensite cause superelasticity degradation.

A new method is proposed to prevent amorphous-crystalline synergy, enhancing alloy longevity.

Abstract

Molecular dynamics simulations are performed to provide a detailed understanding of the functional degradation of shape memory alloys at small scale. The origin of the experimentally reported accumulation of plastic deformation and the anomalous sudden increase of the residual strain under cyclic mechanical loading are explained by detailed insights into the relevant atomic scale processes. Our work reveals that the mechanical response of shape-memory-alloy pillars under cyclic compression is significantly influenced by the presence of an amorphous-like surface region as experimentally induced by focused ion beam milling. The main factor responsible for the observed degradation of superelasticity under cyclic loading is the accumulated plastic deformation and the resultant retained martensite originating from a synergetic contribution of the amorphous and crystalline shape-memory-alloy regions. We show that the reported sudden diminishment of the stress plateaus and hysteresis under cyclic loading is caused by the increased stability of the martensite phase due to the presence of the amorphous phase. Based on the identified mechanism responsible for the degradation, we validate reported methods of recovering the superelasticity and propose a new method to prohibit the synergetic contribution of the amorphous and crystalline regions, such as to achieve a sustainable operation of shape memory alloys at small scale.