The formation of twins during deformation in fully austenitic high-Mn steels (HMnS) with 12-30 wt.% Mn and stacking fault energies (SFE) in the range of 18-50 mJ/m² leads to a gradually progressing grain refinement. This so-called dynamic Hall-Petch effect is widely considered to be a significant contributing factor to the high work hardening rate and high dislocation density of Twinning-Induced Plasticity (TWIP) steels that results in combinations of strength and ductility upwards of 50000 MPa%. While the uniform elongation in tensile tests of more than 40% speaks to good deformation behaviour and formability, a dynamic strain aging effect already occurs at ambient temperatures. This effect manifests itself in form of a serrated flow curve resulting from heavily localized deformation zones, which are undesirable since they introduce a variety of complex problems during sheet forming processes, as is also visible in the significant drop in formability in the plane strain and biaxial strain state of Forming Limit Diagrams calculated from Nakajima tests. In austenitic steels, diffusion rates are not high enough to allow for long-range diffusion of interstitial atoms like carbon or nitrogen that could result in a dislocation pinning effect. Several theories for the occurrence of the PLC-like effect have been offered in the past, most of them targeting two phenomena: (i) Short-range jumps of carbon atoms during deformation into preferential octahedral lattice positions or (ii) the activation of additional twin systems, leading to localized hardening phenomena.
Based on a previously published model for the calculation of strength increases by short-range ordering effects, several high-Mn TWIP steels, with and without aluminium additions, have been characterized via quasistatic tensile tests with regards to the occurrence of the serrated flow phenomena. The onset of deformation bands is investigated in connection to the level of strength increases that is predicted by the model.
To investigate the impact of deformation twinning on the occurrence of serrated flow in high-Mn TWIP steels, EBSD measurements have been performed, targeting the onset of deformation bands. Subsequent analysis of grain orientations was carried out to detect possible twin systems with high probability of activation during tensile tests. Additionally, the impact of aluminium on the suppression of serrated flow and its effect on sheet formability has been studied.