The influence of manganese content on the stacking fault and austenite/ε-martensite interfacial energies in Fe-Mn-(Al-Si) steels investigated by experiment and theory

The stacking fault and interfacial energies of three transformation- and twinning-induced plasticity steels (TRIP/TWIP) (Fe-22/25/28Mn-3Al-3Si wt.%) were determined by experimental and theoretical methods. Analysis of Shockley partial dislocation configurations in the three alloys using weak-beam dark-field transmission electron microscopy yielded stacking fault energy (SFE) values of 15 ± 3, 21 ± 3 and 39 ± 5 mJ m^-2 for alloys with 22, 25 and 28 wt.% Mn, respectively. The experimental SFE includes a coherency strain energy of ∼1-4 mJ m^-2, determined by X-ray diffraction, which arises from the contraction in volume of the stacking fault upon the face-centered cubic (fcc) to hexagonal close-packed (hcp) phase transformation. The ideal SFE, computed as the difference between the experimental SFE and the coherency strain energy, is equal to14 ± 3, 19 ± 3 and 35 ± 5 mJ m^-2, respectively. These SFE values were used in conjunction with a thermodynamic model developed in the present work to calculate the free energy difference of the fcc and hcp phases and to determine a probable range for the fcc/hcp interfacial energy in the three Fe-Mn-(Al-Si) steels investigated. In addition, the interfacial energies of three Fe-18Mn-0.6C-0/1.5(Al/Si) TWIP and five Fe-16/18/20/22/25Mn binary alloys were also determined from experimental data in the literature. The interfacial energy ranged from 8 to 12 mJ m^-2 in the TRIP/TWIP steels and from 15 to 33 mJ m^-2 in the binary Fe-Mn alloys. The interfacial energy exhibits a strong dependence on the difference in Gibbs energy of the individual fcc and hcp phases. Accordingly, an empirical description of this parameter is proposed to improve the accuracy of thermodynamic SFE calculations.