Occurrence of Special Boundaries in hot deformed NiTi Shape Memory Alloy

NiTi Shape memory alloys (SMA) unlike other intermetallics, exhibit anomalous ductility in wide temperature range. NiTi in austenitic phase has only three independent slip system with burgers vector <100> on {011} plane. So, deformation twinning is essential to undergo large plastic strain. Earlier investigations have shown the formation of deformation twin on {114} plane, at lower processing temperatures. Since, these reports were solely based on diffraction patterns from transmission electron microscope, they lag from good statistics. So, in the present work, we demonstrate the occurrence of various special boundaries with twin relationship in addition to {114} twin at very high temperature rolling, using electron back scattered diffraction (EBSD) technique covering large number of grains.

Ni52Ti48 alloy was produced by vacuum arc melting technique. The as-cast material was homogenized at 1000°C for 24 hrs, to dissolve any second phase. Hot rolling was carried out at 700°C up to a strain of 2. The rolled sample was further characterized by EBSD in a scanning electron microscope (SEM). The special boundaries that are observed in the present investigation include twinning on {114}, {320}, {331}, {651}, {541} and {441} planes.

Twinned regions formed by deformation have undergone larger plastic strain compared to the surrounding matrix and they exhibit large intra grain misorientation. So, they act as potential nucleation sites for dynamically recrystallized grains.  Evidences have been presented for both the cases. In addition to the special boundaries, evolution of crystallographic texture with strain will also be discussed.