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 demon- strate 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 conti- nuous 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 ultra- low apparent Young’s modulus. Materials with a high strength and an ultralow apparent Young’s modulus have great potential for applica- tion in orthopaedic implants.