Influence of Al Variation on the Mechanical Properties and Critical Shear Stress of Twinning in Fe-Mn-Al-C Steels

The deformation behavior of three tensile deformed high-Mn steels with varying Al contents (0, 1 and 3 wt. %) is discriminated on the basis of their respective critical shear stress of twinning, derived from their respective stacking fault energies (18.1-41.6 mJ/m(2)) estimated according to x-ray diffraction line profile analyses. The Al-free steel manifested maximum strain hardening rate similar to 3 GPa that gradually dropped with addition of Al. The critical twinning stress, tau(c), of the steels was estimated as: 246, 348 and 561 MPa. Increasing stacking fault energies lead to an adaptation in deformation mechanism from: (gamma -> epsilon) transformation -> deformation twinning -> dislocation plasticity-as was noted from x-ray diffraction and transmission electron microscopy studies. The highest strain hardening observed in Al-free steel was attributed to the presence of epsilon-martensite in the austenite microstructure. The incidences of epsilon-martensite and/ or twinning were found to be directly related to the equilibrium width of the overlapping stacking faults. The critical shear stress-based calculations could better explain the adaptation of plasticity mechanisms in Fe-Mn-Al-C steels than the prediction based on the stacking fault energy alone.