In situ observation on temperature dependence of martensitic transformation and plastic deformation in superelastic NiTi shape memory alloy

In situ digital image correlation (DIC) and in situ X-ray diffraction (XRD) are applied to investigate the effect of temperature on martensitic transformation and plastic deformation in superelastic NiTi shape memory alloy. Via in situ DIC, two well-known deformation modes of NiTi are identified at various temperatures: (A) localized forward and reverse transformations with little residual strain (< 1%); (B) localized forward transformation and homogenous reverse transformation with considerable residual strain (> 1%). As temperature increases from 25 degrees C to 120 degrees C, the mechanical response of NiTi gradually transits from Type A to Type B. We verify that plastic strain accumulates concurrently as the traverse of the front of localized deformation band. Via in situ XRD observation, we conclude that it is material plasticity rather than retained martensite that plays a dominant role in the irreversibility of NiTi. The experimental results provide both macroscopic and lattice level scenarios to understand the temperature dependence of complicated thermomechanical coupling and plasticity in superelastic NiTi. (C) 2017 Elsevier Ltd. All rights reserved.