Investigating the twining transformation in the Hadfield single crystal using VPSC and phase field modeling

The plastic deformation mechanisms of single crystal Hadfield steel were investigated by examining the interplay between dislocation slip and mechanical twinning under uniaxial tension and compression along the , , and crystallographic directions. Deformation mechanisms were analyzed using viscoplastic modeling and thermodynamically informed phase field simulations, and the predictions were systematically compared with experimental observations. Viscoplastic results indicate that system activation is governed primarily by Schmid factors and critical shear stresses, leading to the prediction of multiple active twinning variants within a single grain. However, the model consistently overestimates twinning activity relative to experiments. In particular, loading along is predicted to activate several twin variants, whereas experimental studies report either no twinning in tension or activation of at most two variants in compression. The discrepancy is further amplified by the VPSC assumption of abrupt grain reorientation once the twin volume fraction exceeds specific amount, which significantly alters the simulated deformation response. To resolve these inconsistencies, phase field simulations incorporating thermodynamic aspects were performed. The results reveal that competition among twin variants critically governs twin growth. When multiple variants possess similar conditions such as similar driving forces, strong competition suppresses stable twin formation, limiting the number of active variants. These findings demonstrate that purely mechanical viscoplastic approaches are insufficient to accurately predict twinning behavior in Hadfield steel. Incorporating thermodynamic effects and variant competition mechanisms is essential for physically consistent modeling of deformation twinning in single crystals.