Predicting the onset strain for strain-induced martensitic transformation: A thermodynamics-informed calculation and experimental validation

The prediction of the onset of strain-induced martensitic transformation has traditionally relied on empirical parameters, lacking a robust physical basis. This study presents a method for the physical calculation of the critical plastic strain, ε_p^cr, based on the classical thermodynamic energy balance framework by incorporating dislocation energy from the plastic deformation process. This approach comprehensively considers the balance between the chemical driving force, strain energy, grain boundary energy, mechanical driving force, and dislocation energy, wherein the chemical composition and initial austenite grain size are key variables determining the phase transformation energy barrier. The calculated ε_p^cr for QP980 steel is 0.00577, which shows good agreement with experimental observations. Substituting this theoretical value as a fixed parameter, ε₀, into Shin model maintains high predictive accuracy while reducing the number of free parameters (RMSE ≈ 0.032%, adjusted R² = 0.99967). Application to Fe-0.1C-5Mn steel further supports its transferability with comparable accuracy and fewer empirical fitting parameters. Using this computational framework, the influence mechanism of alloying elements is quantitatively revealed: C and Si significantly reduce ε_p^cr by promoting strain localization; conversely, by improving the thermodynamic stability of austenite, ε_p^cr increases monotonically with increasing Mn content. The influence of Al is comparatively weak. Grain refinement substantially increases ε_p^cr by inhibiting the propagation of shear bands. Based on the thermodynamic criterion, this work achieves the quantitative calculation of the critical plastic strain, providing a theoretical foundation for predicting the activation conditions of the TRIP effect.