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

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 degrees C and pressure (4-8 GPa) under conditions of high-pressure torsion and post-deformation annealing (200-400 degrees 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, is located about 300 degrees C. The upper limiting deformation temperature for actually nanocrystalline structure formation is located about 350 degrees C for non-aging Ti-Ni alloys and somewhat higher than 400 degrees 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. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.