Nanocrystalline NiTi shape memory alloys with huge superelasticity and high mechanical damping

Abstract

Superelasticity and mechanical damping are important functional properties of nickel–titanium (NiTi) shape memory alloys (SMAs). Owing to the generation and accumulation of dislocation during loading, the recovery strain of commercial NiTi SMAs is usually smaller than 10%, which limits their ability to dissipate energy. In this paper, the superelasticity and mechanical damping of a nanocrystalline NiTi SMA was studied. The results show that the nanocrystalline NiTi SMA alloy possessed a large recovery strain of about 14%, greater than that of the commercial NiTi SMAs, and a high level of absorbed energy, or toughness, of 111 MJ m−3, which is higher than the highest value (about 81 MJ m−3) of all SMAs reported so far. The transmission electron microscopy (TEM) studies suggest that few full dislocations were generated in the nanocrystalline NiTi alloy during loading. Instead, the dominant deformation modes after stress induced martensitic transformation were elastic deformation and detwinning. The detwinning process decreased the twin boundary energy, which stabilised the martensitic phase.