Variant selection map of external load during Ni₄Ti₃ precipitation in nitinol: a theoretical and phase field study

The morphology of Ni₄Ti₃ precipitates is important in tuning the martensitic transformation (MT) behavior and mechanical properties of nitinol. Constrained ageing is effective in engineering the morphology of Ni₄Ti₃ precipitates due to the variant selection effect of external load which is still lacking. In this work, maps of variant selection effect of external load applied along all crystallographic directions are obtained by using a combination of theoretical analyses and phase field simulations. It is found that maps produced by uniaxial tension and uniaxial compression are quite different. The number and types of Ni₄Ti₃ variants preferred by external load vary as the loading direction changes. Moreover, factors influencing the strength of variant selection effect are discovered. This work provides insights on understanding the Ni₄Ti₃ precipitation process and sheds light on the engineering of morphology of Ni₄Ti₃ precipitates for desired mechanical and functional properties.