Nanocrystallization of NiTi shape memory alloys made amorphous by high-pressure torsion

Almost complete amorphization is obtained in NiTi subjected to high-pressure torsion (HPT). Nanocrystallization occurs more than 100 °C lower than in amorphous sputtered and melt spun NiTi. This is caused by medium-range-ordered debris surviving HPT and triggering nanocrystallization. A high nucleation rate and low growth rate result from in situ heating transmission electron microscopy experiments. The kinetics of the heterogeneous nanocrystallization differs significantly from that leading to coarse grains. Therefore, HPT allows the nanostructure to be tailored to enhance shape memory properties.