Thermomechanical model for NiTi-based shape memory alloys including R-phase and material anisotropy under multi-axial loadings

This work presents a thermomechanical model for polycrystalline NiTi-based shape memory alloys developed within the framework of continuum thermodynamics of irreversible processes. The model is capable of realistic simulations of several physical phenomena, involving transformation between austenite, R-phase and martensite and martensite reorientation, which may simultaneously occur under general thermomechanical loading. This is due to three key features of the model: a novel form of the dissipation function coupling martensite transformation and reorientation processes, inclusion of the material responses associated with the transformation between austenite and R-phase and implementation of the influence of material anisotropy. Based on a mathematically consistent formulation, the model was implemented into finite elements providing a numerical tool particularly useful for analysis of NiTi-based highly-textured components, which are of great industrial importance. To explore and demonstrate further features of the proposed model several numerical simulations were performed and compared with experimental results.