Structure and properties of Ti–Ni-based alloys after equal-channel angular pressing and high-pressure torsion

Abstract

Structure formation and functional properties of Ti–48.5, 50.0, 50.6 and 50.7 at.% Ni and Ti–47 at.% Ni–3 at.% Fe shape memory alloys under conditions of high-pressure torsion (HPT) and equal-channel angular pressing (ECAP) in dependence on deformation temperature and post-deformation annealing were studied using electron microscopy and mechanical testing methods. The upper limiting deformation temperature for nanocrystalline structure formation under continuous severe deformation in HPT were determined for aging (somewhat higher than 400 °C) and non-aging (about 300–350 °C) alloys. As a result of ECAP of Ti–Ni and Ti–Ni–Fe alloys at 350–500 °C in six to eight passes, a submicrocrystalline structure with the grain size of 0.1–0.2 μm (at 350 °C), 0.2–0.4 μm (at 450 °C) and 0.3–0.5 μm (at 500 °C) was obtained. The highest functional properties of Ti–50.6% Ni alloy which exceed the best results provided by traditional thermomechanical treatment, correspond to the finest submicrocrystalline structure formation after ECAP at 350 °C. The low-temperature annealing after ECAP does not deteriorate the functional properties as it does not increase the austenite grain size. For obtaining actual nanocrystalline structure in bulk samples under ECAP conditions, the ECAP temperature should be below 350 °C.