Heterogeneous process of disordering and structural refinement in Ni ₃Al during severe plastic deformation by high-pressure torsion

The process of grain refinement in Ni₃Al by high-pressure torsion (HPT) was investigated up to 100 turns. The entire diametric sections of the deformed samples were analyzed by optical microscopy and image processing methods in order to evaluate both the spatial distribution and the volume fractions of the nanocrystalline and coarse grains. A thick band of nanocrystalline phase was formed in the middle section of the samples, and a structure containing mainly coarser ordered fragments was present in the vicinity of the top and bottom surfaces. Pseudotwinning along (111) was observed at the boundaries of coarse-fragmented grains as well as the inside of the fragments and is put forward as a possible mechanism for disordering and nanocrystalline structure formation in Ni₃Al.