Modelling on grain size dependent thermomechanical response of superelastic NiTi shape memory alloy

Abstract

To describe the grain size dependence and rate dependence of superelastic NiTi, a constitutive model involving intrinsic material instability and thermomechanical coupling is proposed and implemented implicitly into finite element software. The material is regarded as a mixing of transformable phase and non-transformable phase. The thermodynamic driving force for martensitic transformation and the heat equilibrium equation are deduced in the framework of irreversible thermodynamics. The global thermomechanical response, i.e. stress–strain curve, and local thermomechanical response, i.e. strain and temperature profiles, are simulated. If the grain size is above 68 nm, NiTi is characterized with Lüders like deformation associated with strong thermomechanical coupling. If the grain size is 42 nm and 27 nm, Lüders like deformation does not take place because Considere criterion is not fulfilled. Strain localization is visible owing to the collective effect of thermomechanical coupling and temperature inhomogeneity. If the grain size is 10 nm, thermomechanical coupling is suppressed and NiTi deforms homogeneously.