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

Abstract

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/m²) estimated according to x-ray diffraction line profile analyses. The Al-free steel manifested maximum strain hardening rate ~ 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: $(\gamma \to \varepsilon )$ transformation $\to$ deformation twinning $\to$ 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 $\varepsilon$-martensite in the austenite microstructure. The incidences of $\varepsilon$-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.