Making metals linear super-elastic with ultralow modulus and nearly zero hysteresis

Abstract

We demonstrate a novel materials design approach to achieve unprecedented properties by utilizing nanoscale chemo-mechanical coupling. In particular, by using computer simulations we demonstrate how to engineer ultralow modulus (12 GPa), nearly hysteresis-free, and linear super-elastic metals with a giant elastic strain limit (2.7%) by creating appropriate concentration modulations (CMs) at the nanoscale in the parent phase and by pre-straining to regulate the stress-induced martensitic transformation (MT). The nanoscale CMs created via spinodal decomposition produce corresponding phase stability modulations, suppress autocatalysis in nucleation, impose nano-confinements on growth, and hinder long-range ordering of transformation strain during the MT, which changes the otherwise sharp first-order transition into a smeared, macroscopically continuous transition over a large stress range. The pre-straining generates retained martensitic particles that are stable at the test temperature after unloading and act as operational nuclei in subsequent load cycles, eliminating the stress-strain hysteresis and offering an ultralow apparent Young's modulus. Materials with a high strength and an ultralow apparent Young's modulus have great potential for application in orthopaedic implants.