Enhanced superelasticity of nanocrystalline NiTi/NiTiNbFe laminar composite

This study explores a new concept of the synergy between two constituent shape memory alloys (SMAs) in a composite to achieve a wide superelastic temperature window. In this work, a nanocrystalline NiTi/NiTiNbFe dual-SMA composite is designed, in which two constituent SMAs are both capable of stress-induced martensitic transformation, but within different temperature ranges. Based on the principle of lattice strain matching between the two SMAs, superelastic martensitic transformation in one induces superelastic transformation in the other at temperatures the latter will not when standing alone. Via this synergy the composite is able to achieve an ultra-wide superelastic temperature window of 336 °C, from −196 to 140 °C, with a ∼5% tensile strain recovery and high superelastic stresses in the range of 700–1400 MPa. Such properties appear superior to all NiTi-based shape memory alloys to date.