Deformation-Induced Atomic Disordering and bcc → fcc Transformation in Heusler Alloy Ni54Mn21Ga25 Subjected to Megaplastic Deformation by High Pressure Torsion

Abstract

X-ray diffraction analysis, transmission electron microscopy, and scanning electron microscopy have been used for the first time to carry out in situ systematical studies of the structure and phase transformations in the Heusler alloy Ni54Mn21Ga25 with an L21 structure after megaplastic deformation by high pressure torsion. Deformation by high pressure torsion has been found to refine the polycrystalline structure of the alloy to a nanocrystalline and partially amorphous state. Total atomic disordering and stepwise structural-phase transformation according to a В2(bcc) → A2(bcc) → A1(fcc) scheme have been seen to occur with increasing the pressure (from 3 to 5 GPa) and the deformation degree (from two to five revolutions). Annealing at 573 K results in the devitrification of the amorphous phase. Annealing at 623 K and above has recovered the L21 structure. The size effect has been found to be responsible for the fact that cooling to 120 K suppresses martensite transformation in the nanostructured austenitic alloy with grain sizes less than 80 nm.