Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4-7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to similar to 13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (similar to 4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress-strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.