Response of NiTi shape memory alloy at high strain rate: A systematic investigation of temperature effects on tension-compression asymmetry

A systematic experimental investigation into the effects of temperature on the stress-strain response of NiTi (55.6 wt.% Ni) shape memory alloy at high strain rate was conducted and compared with the behavior at quasi-static strain rate. Dynamic compression and tension tests, to total strains of 16-24%, were performed at high strain rate (similar to 1200/s) and under quasi-static conditions at temperatures from -196 to 400 degrees C. Since the superelastic range for the NiTi alloy used in this work is A(f) (2 degrees C) < T < M-d (150 degrees C), it was possible to study the high strain rate deformation of both the austenite phase (where stress-induced martensite transformation occurs) at temperatures > 0 degrees C and the (thermally induced) martensite phase at lower temperatures. The results indicate differences in the stress-strain response of thermally induced martensite and stress-induced martensite, in terms of plateau stress characteristics and critical stress (as determined by 0.2% strain offset). The observations illustrate a complex interplay of test temperature, stress state (compression and tension) and martensite type (thermally induced vs. stress induced) that lead to the asymmetry in compression vs. tension response of the shape memory alloy in both quasi-static and dynamic loading conditions. This asymmetry was captured in the variation of critical stress with temperature that exhibited a three-stage character, with the critical stress being higher in compression than in tension. These findings have significant implications for the understanding and exploitation of the underlining functional characteristics of shape memory alloys, especially at high strain rates. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.