Nanocrystallization and amorphization of NiTi shape memory alloy under severe plastic deformation based on local canning compression

Abstract

As a new method for severe plastic deformation (SPD), local canning compression was applied in order to fabricate amorphous and nanocrystalline nickel–titanium shape memory alloys (NiTi SMA) at room temperature. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were used for understanding microstructural evolution as well as deformation mechanism of NiTi SMA under local canning compression. Deformation mechanism of NiTi alloy under local canning compression involves stress-induced martensite transformation, deformation twinning, dislocation slip, nanocrystallization and amorphization. The (001) martensite compound twins, the stacking faults, primary deformation bands, secondary deformation bands and amorphous bands are observed in the NiTi sample subjected to SPD. The (001) martensite compound twins belong to the deformation twins and thus lead to mechanical stabilization of stress-induced martensite. Deformation twinning and dislocation slip are of great importance in amorphization and nanocrystallization of NiTi SMA subjected to SPD based on local canning compression.