Investigation into post constrained recovery properties of nickel titanium shape memory alloys

Abstract

Experimental investigations have found that nickel titanium (NiTi) shape memory alloys (SMAs) exhibit a phenomenon wherein the material can be recoverably deformed in its low temperature martensitic state, thermally actuated to its austenite state while constrained from returning to its parent geometry, and upon ceasing actuation and return to the low temperature martensite state it will continue to generate post constrained recovery residual stresses (PCRRS). The ability to generate a PCRRS creates a new way for SMAs to be used but the stability of the PCRRS, response to subsequent loading, and underlying microstructural mechanisms that generate the PCRRS are poorly understood. This paper presents an experimental investigation into the thermomechanical response of NiTi beginning from a PCRRS state. Multiple formulations of NiTi were exposed to training and sequences of cyclic deformation and thermal actuation in this study. It was found that repeated application of 0.5% strain reduced the residual stress with each application and thermal actuation would restore the original PCRRS. Training and mechanically working the material stabilized the thermomechanical response and did not significantly degrade the magnitude of the PCRRS. Understanding of PCRRS supports a new way for materials to be used as actuators, storing potential energy in material structures that may be beneficial to self-healing material, fatigue crack prevention and other applications.