Reply to comments on “Microstructural features of a type 304L stainless steel deformed at 1473 K in the strain rate interval 10⁻³ s⁻¹ to 10²s⁻¹”

Using an efficient first-principles computational scheme, we calculate the intrinsic stacking fault energy (${\gamma }_{isf}$) and the unstable stacking fault energy (${\gamma }_{usf}$) 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 ${\gamma }_{isf}$ of γ-Fe. Dominated by the magneto-volume coupling at thermal excitations, ${\gamma }_{isf}$ of γ-Fe exhibits a positive correlation with temperature, while ${\gamma }_{usf}$ 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 ${\gamma }_{isf}$ and the predicted high-temperature twinning are observed in the case of austenitic stainless steels.

The reaction layers of friction stir welded joints made from austenitic stainless steel and Al alloy consisted of mixed layers of elongated and ultra-fine grains and the intermetallic compound layer. The intermetallic compound layer was identified as Al₄Fe with a hexagonal close-packed structure and a thickness of approximately 250 nm. The elongated grains were shown to consist of a ferrite phase because of their ferromagnetism and a body-centered cubic crystal structure.

The influence of strain rate and state-of-stress on the formation of ferrite in stainless steel type AISI 304L, 304 and 304 as-cast, during hot working has been studied. Compression and torsion tests were conducted in the temperature range 1100 to 1250 °C and strain rate range 0.001 to 100 s⁻¹ on these materials. Ferrite formation occurs during deformation at temperatures above 1150 °C and strain rates above 10 s⁻¹, in stainless steel type AISI 304L and 304. The tendency for the formation of ferrite is more in as-cast 304 than in wrought 304. In as-cast 304 the ferrite forms at lower temperatures and strain rates. The tendency for the ferrite formation is more in torsion than in compression.