Stacking Fault Energy of Austenite Phase in Medium Manganese Steel

The stacking fault energy (SFE) of the austenite phase in a medium manganese steel (Fe-4.75Mn-0.18C-0.8Si-0.4Al wt pct) has been determined by X-ray diffraction (XRD) using the modified Reed-Schramm formalism. SFE determination involved XRD line broadening analysis of mean square strain due to dislocations and calculation of stacking fault probability (SFP) from the analysis of diffraction peak shift due to both stacking faults and residual stress in the deformed austenite phase. Determination of SFP revealed a significant change in the separation between two neighboring peak reflections (111 and 200) due to compressive residual stress as compared to the corresponding change in peak separation due to stacking faults. Obtained SFE values were found to vary from 9 to 20 mJ m(-2) for the deformed specimens, annealed at an intercritical temperature for different durations for austenite stabilization prior to deformation. SFP determination neglecting residual stress led to a significant decrease in the SFE value of the austenite phase. Dislocations in deformed austenite phase were predominantly < 110 >{111} edge type, and dislocation density was of the order of 10 15 m(-2). Both XRD and transmission electron microscopy observations suggested twinning-induced plasticity as the prevalent mode of deformation of austenite phase having an SFE value of similar to 20 mJ m(-2), whereas transformation-induced plasticity was found to be the major deformation mode for the specimen having an SFE value of similar to 9 mJ m(-2)