Analysis of amorphous regions in severely marformed NiTi shape memory alloy

In this work Ni-rich NiTi alloy subjected to 35 % cold-rolling in the martensitic state followed by annealing at 450 °C for 15 min has been studied by transmission electron microscopy observations. It has been found that the material after thermo-mechanical treatment has a non-uniform nanostructure with canal-like shaped amorphous areas containing crystallites of R and B19′ phases. Zones surrounding these regions contain grains elongated in different directions in different areas. They are identified to be a mixture of B2 parent, R rhombohedral and B19′ martensitic phases. Selected area electron diffraction as well as transmission electron microscope orientation maps taken from these zones exhibit differences in their crystallographic orientations. It is proposed that the amorphous areas form along grain boundaries during severe plastic deformation.