Reprints on TWIP Steels

High-manganese steels have received much interest in recent years due to their outstanding mechanical properties combining high strength and ductility. This property profile is attributed to their high strain hardening capacity.

High-manganese steels are typically austenitic steels, i.e. they are face-centered cubic (fcc) alloys, with a high Mn content (above 20% wt.%) and additions of elements such as carbon (<1 wt.%), silicon (<3 wt.%) and aluminum (<10 wt.%).

This steel grade exhibits different hardening mechanisms, such as transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP) or microband-induced plasticity (MBIP). The activation of these mechanisms is strongly dependent on the stacking fault energy. TRIP is observed in very low stacking fault steels (below 20 mJ m-2) and is associated with the transformation of austenite (fcc phase) into e-martensite (hexagonal close-packed phase), which in turn further acts as nucleus of a'-martensite (body-centered cubic or tetragonal phase). TWIP is observed in medium stacking fault energy steels (20–40 mJ m-2) and is characterized by the formation of deformation twins with nanometer thickness. MBIP has been recently reported in steel grades with high stacking fault energy (ca. 90 mJ m-2) and is attributed to the formation of microbands, which are in-grain shear zones that are confined by geometrically necessary boundaries or conventional grain boundaries. These microstructure features (e-martensite plates, deformation twins and microbands) lead to a remarkable variety of strain hardening phenomena as they all act as effective obstacles for dislocation glide. High-manganese TWIP steels are characterized by a hierarchical microstructure refinement that includes complex dislocation and twin substructures, and their interactions. Although there are some previous studies on the strain hardening behavior in TWIP steels, the details of the underlying kinetics of the substructure evolution and its correspondence to the stress–strain and strain hardening evolution is not yet fully understood.