High temperature superelasticity realized in equiatomic Ti-Ni conventional shape memory alloy by severe cold rolling

Abstract

Great interests on high temperature shape memory alloys (HTSMAs) are recently raised, driven primarily by the aerospace and automotive industries, for their potential to serve as solid state actuators at high temperatures over 373 K. Near equiatomic Ti-Ni conventional SMAs, as the most applied SMA system, were considered to possess appreciable shape memory effect and superelasticity only below 373 K, which owes to low martensitic transformation temperatures. Here we utilized the martensite stabilization effect and successfully expanded the viable zone of B19′ (monoclinic) martensite to 616 K in a 35% cold-rolled Ti-50Ni (CR35) SMA. After a training cycle, it exhibited quasi-linear superelasticity of narrow hysteresis, ~3% superelastic strain and high strength over 1.2 GPa in a wide temperature range up to 483 K. It is the first exploration of high temperature superelasticity associated with the stabilized martensite. Using transmission electron microscopy and in-situ heating/tensile synchrotron X-ray diffraction, we reveal the origin of martensite stabilization and the underlying mechanisms of unique superelasticity in the CR35. Our work provides an attractively facile approach to realize high temperature superelasticity in Ti-Ni conventional SMAs, as well as in other kinds of SMAs, for precise actuation within an expanded range of working temperature and stress.