A thermochemical model was developed for the stacking-fault energies in a Fe-Mn-C system with a  few percent of added Cu, Cr, Al and Si. It was found that aluminium strongly increased the stacking-fault energy, contrary to chromium, while the effect of silicon was more complex. Copper also increased the stacking-fault energy, but strongly decreased the Néel temperature. The stacking-fault energy was the relevant parameter that controlled the mechanical twinning which was known to be at the origin of the excellent mechanical properties. Using this model, copper-containing Fe-Mn-C grades were prepared having stacking-fault energies below 18mJ/m2, in the range where ε-martensite platelets formed instead of micro-twins during plastic deformation. This change in deformation modes, confirmed by X-ray diffraction, did not significantly damage the mechanical properties, as long as the stacking-fault energy was greater than 12mJ/m2; which avoided the formation of α′-martensite.

Influence of Addition Elements on the Stacking-Fault Energy and Mechanical Properties of an Austenitic Fe-Mn-C Steel. A.Dumay, J.P.Chateau, S.Allain, S.Migot, O.Bouaziz: Materials Science and Engineering A, 2008, 483-484[1-2], 184-7