The stacking fault and interfacial energies of three transformation- and twinning-induced plasticity steels (TRIP/TWIP) (Fe-22/25/28Mn-3Al-3wt%Si were determined by using 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 values of 15, 21 and 39mJ/m2 for alloys with 22, 25 and 28wt%Mn, respectively. The experimental stacking-fault energy included a coherency strain energy of some 1 to 4mJ/m2, determined by X-ray diffraction, which arose from the contraction in volume of the stacking fault during the face-centered cubic to hexagonal close-packed phase transformation. The ideal stacking-fault energy, computed as the difference between the experimental stacking-fault energy and the coherency strain energy, was equal to 14, 19 and 35mJ/m2, respectively. These stacking-fault energy values were used, in conjunction with a thermodynamic model developed here, 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. 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 published experimental data. The interfacial energy ranged from 8 to 12mJ/m2 in the TRIP/TWIP steels and from 15 to 33mJ/m2 in the binary Fe-Mn alloys. The interfacial energy exhibited a strong dependence upon the difference in Gibbs energy of the individual fcc and hcp phases.

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. D.T.Pierce, J.A.Jiménez, J.Bentley, D.Raabe, C.Oskay, J.E.Wittig: Acta Materialia, 2014, 68, 238-53