Influence of cold work and texture on the high-strain-rate response of Nitinol

The high-strain-rate (1200/s) tensile response of Ti–50.8Ni in cold-worked and annealed conditions has been studied using a split-Hopkinson bar between −196°C and 400°C. The dynamic stress–strain behavior was compared with the corresponding behavior at quasi-static strain-rates (∼10−3/s). To study the influence of cold work, the stress–strain properties of cold-rolled sheet were contrasted with the annealed sheet (800°C). High-strain-rate behaviors of annealed rolled sheet and annealed drawn rod products were compared to evaluate the influence of crystallographic texture. Additionally, texture influence was also investigated by studying the dynamic behavior of cold-rolled and annealed sheet along longitudinal and transverse direction. Cold working affected the overall stress–strain behavior and mechanical properties such as critical stress and stress plateau characteristics to a greater degree than texture, especially at higher temperatures. While twinning/detwinning was predominant in the cold-worked material followed by martensite plasticity at 10−3/s, the dynamic stress–strain curves exhibited non-flat plateau regions with increasing slope followed by parabolic hardening, indicative of deformation predominantly by slip. The deformation in the annealed material was similarly dominated by plastic slip with some twinning/detwinning due to the ease of slip in the annealed matrix. Fractography observations showed quasi-cleavage type failure in the martensite at low temperatures (−196°C to −50°C), ductile void growth in austenite at high temperatures (200–400°C) and mixed-mode failure consisting of micro-void and cleavage in the superelastic austenite (0–100°C) at high-strain-rates.