The mechanisms of stress-induced transformation in ultimately fine-grained titanium nickelide, and critical grain size for this transformation

Abstract

The mechanisms of stress-induced transformation in ultimately fine-grained Ti-50.61 at.%Ni shape memory alloy subjected to severe cold rolling and short-term post-deformation annealing were studied using transmission electron microscopy and in situ X-ray diffractometry. An unambiguous evidence of a discrete mechanism of stress-induced B2→B19’ transformation in nanograined Ti–Ni alloys with an average grain size down to 2.3 ± 0.2 nm was obtained (this grain size is considered to be the smallest ever obtained for the severe plastic deformation processing route). No signs of continuous transformational lattice changes were found. It has been shown that the critical austenite grain size for the stress-induced transformation exists, and it amounts to ∼4.5 nm in the case of room-temperature tensile straining and further cold plastic rolling.