Plastic deformation modes in paramagnetic ?-Fe from longitudinal spin fluctuation theory

Using an efficient first-principles computational scheme, we calculate the intrinsic stacking fault energy (γisf) and the unstable stacking fault energy (γusf) of paramagnetic γ-Fe as a function of temperature. The formation energies are derived from free energies accounting for thermal longitudinal spin fluctuations (LSFs). LSFs are demonstrated to be important for the accurate description of the temperature-dependent magnetism, intrinsic and unstable stacking fault energies, and have a comparatively large effect on γif of γ-Fe. Dominated by the magneto-volume coupling at thermal excitations, γif of γ-Fe exhibits a positive correlation with temperature, while γuf declines with increasing temperature. The predicted stacking fault energy of γ-Fe is negative at static condition, crosses zero around 540 K, and reaches 71.0 mJ m−2 at 1373 K, which is in good agreement with the experimental value. According to the plasticity theory formulated in terms of the intrinsic and unstable stacking fault energies, twinning remains a possible deformation mode even at elevated temperatures. Both the large positive temperature slope of γif and the predicted high-temperature twinning are observed in the case of austenitic stainless steels.