Alloy composition, deformation temperature, pressure and post-deformation annealing effects in severely deformed Ti–Ni based shape memory alloys

Abstract

Structure formation in Ti-48.5, Ti-50.0, Ti-50.7 at.% Ni and Ti-47 at.% Ni-3 at.% Fe shape memory alloys depending on deformation temperature (−196 to 400 °C) and pressure (4–8 GPa) under conditions of high-pressure torsion and post-deformation annealing (200–400 °C) was studied using transmission electron microscopy and X-ray diffraction methods. The tendency to form an amorphous structure depends on the relative values of the deformation temperature and martensite start ( M s) temperature as follows: it is strongest in initially martensitic alloy, intermediate in a premartensitic austenite, and the weakest in initially thermally stable austenitic alloy. Lowering of the deformation temperature in the range below the martensite finish ( M f) temperature facilitates amorphization. Raising of the deformation temperature in the austenitic temperature range suppresses amorphization. The upper limiting deformation temperature for partial amorphization of the alloy having the highest M s is located about 300 °C. The upper limiting deformation temperature for actually nanocrystalline structure formation is located about 350 °C for non-aging Ti–Ni alloys and somewhat higher than 400 °C for aged Ti–Ni alloy. The thermomechanical conditions of the equal-channel angular pressing for obtaining actually nanocrystalline structure are recommended. Isothermal martensitic transformation is observed in the Ti-48.5 at.% Ni alloy as a result of keeping for 10 year at room temperature after high-temperature severe plastic deformation. Increasing the pressure suppresses the tendency to form an amorphous structure. The nanocrystalline structure formed under post-deformation annealing from the amorphous structure remains finer than the nanostructure formed as a result of severe plastic deformation through the annealing temperature range covering a nano-grain size scale.