Favourable conditions for strain induced transformations in unstable high-manganese steel

The present research is part of a project that includes several grades of high-manganese steels with different chemical compositions and various pre-treatment parameters. The main objective of this work was to determine favourable conditions for the occurrence of strain-induced phase transitions in unstable Fe-20Mn-3Al-3Si steel. After preliminary treatment the steel showed a two-phase microstructure consisting of austenite matrix and 29% of ferrite volume fraction. The deformation process of the examined steel was strongly influenced by the low stacking fault energy value of austenite matrix. Initially, the slip of dissociated dislocations led to the formation of numerous bands of stacking faults. Then, the phase transformations induced by plastic deformation took place, i.e., gamma (fcc) -> e (hcp) and subsequently e (hcp) -> alpha' (bcc). The beginning and further progress of these phase transitions depended primarily on the crystallographic orientations of the austenite grains. The more favourable the orientation of the grains with respect to external stresses, the more effective was the activation of dislocation slip and the greater number of deformation bands was formed. These processes directly resulted in a locally induced phase transition gamma -> e. Simultaneously, in grains showing the formation of e-bands in at least two intersecting systems, the transformation e -> alpha' gradually occurred.