Coupling between martensitic phase transformations and plasticity: A microstructure-based finite element model

A microstructural finite element (MFE) model is developed to capture the interaction between martensitic transformations and plasticity in NiTi shape memory alloys (SMAs). The interaction is modeled through the grain-to-grain redistribution of stress caused by both plasticity and phase transformation, so that each mechanism affects the driving force of the other. A unique feature is that both processes are modeled at a crystallographic level and are allowed to operate simultaneously. The model is calibrated to pseudoelastic data for select single crystals of Ti–50.9at.%Ni. For polycrystals, plasticity is predicted to enhance the overall martensite volume fraction at a given applied stress. Upon unloading, residual stress can induce remnant (retained) martensite. For thermal cycling under load bias, plasticity is observed to limit the net transformation strain/cycle and increase the hysteretic width. Deformation processing, via plastic pre-straining at elevated temperature, is shown to dramatically alter subsequent pseudoelastic response, as well as induce two-way shape memory behavior during no-load thermal cycling. Overall, the model is suitable at smaller imposed strains, where martensite detwinning is not expected to dominate.