A phase-field study of the martensitic detwinning in NiTi shape memory alloys under tension or compression

The degree of shape recovery in shape memory alloys is highly associated with their deformation due to martensitic detwinning. In this work, we studied the microstructural evolution of the martensitic variants in NiTi shape memory alloys under applied tension or compression. We first carried out a phase-field simulation based on the Landau-type energy function to investigate the nucleation and growth of the B19^′ variants that form a polytwinned structure right after the material is cooled below the martensitic transformation temperature. Then, the detwinning of the polytwinned martensites was studied under a uniaxially applied tensile or compressive strain. The simulation was run in a 2-variant system to confirm the applicability of the approach, and then, it was performed in a 6-variant system to highlight the microstructure-based mechanism for the overall mechanical response with a higher degree of microstructure complexity. The detwinning simulation confirms many features found by micrograph observation. What was not experimentally reported is the continuing martensitic reorientation in the early stage of the subsequent unloading, which caused the nonlinear stress–strain behavior. Although dislocation was not considered in our model, we still found tension–compression asymmetry in the calculated stress–strain behavior. It is due to the initial configuration of the unevenly distributed polytwinned martensitic structure and the separate evolution paths under tension or compression. Finally, we carried out the sensitivity study of the numerical results to the applied strain rate and the kinetic coefficient and we found that the two effects are interrelated in the framework of the current model.