Variations in martensitic transformation parameters due to grains evolution during post-deformation heating of Ti-50.2 at.% Ni alloy amorphized by HPT

Abstract

The structure and martensitic transformations in Ti-50.2at.% Ni alloy subjected to high-pressure torsion (HPT) and post-deformation heating were studied. It was shown that HPT under 8GPa for 3.5 turns resulted in an amorphization of the alloy structure but the crystalline debris remained. Further heating of the sample led to alloy crystallization and the bimodal grain size structure formed. The first group of grains underwent a B2↔R transition when the average grain size was varied from 20 to 50nm, and a B2→R→B19′I transformations when the average grain size was 80nm or more. The second group of grains underwent a B2↔B19′II transformation. An increase in temperature of amorphous sample heating led to a rise in grain size and as a result, the temperatures of the martensitic transformations changed. The temperatures of the B19′I→B2 transformation and the B2↔R transition increased when the heating temperature changed from 359 to 550°C whereas, the temperatures of the B2↔B19′II transformation increased if the amorphous sample was subjected to heating from 415 to 550°C when the recrystallisation took place. It was assumed that the bimodal grain distribution was caused by the grain appeared during crystallization in two ways: from the amorphous phase and from the debris remained in the amorphous phase after HPT.