Studies of composition, deformation temperature and pressure effects on structure formation in severely deformed TiNi-based alloys

Structure formation in TiNi-based shape memory alloys in dependence on deformation temperature (−196 to 400 °C) and pressure (4–8 GPa) under conditions of high-pressure torsion (HPT) was studied using transmission electron microscopy and X-ray diffraction methods. The tendency to form an amorphous structure depends on the relative positions of the deformation temperature and the martensite start temperature. Isothermal martensitic transformation is observed in the Ti–48.5 at.% Ni alloy as a result of 10-year keeping at room temperature after HPT. Increasing of pressure suppresses the tendency to form an amorphous structure. The upper deformation temperature limits for amorphous and nanocrystalline structures formation are determined. The thermomechanical conditions of the equal-channel angular pressing for obtaining actual nanocrystalline structure are recommended.