Microstructure and mechanical response of single-crystalline high-manganese austenitic steels under high-pressure torsion: The effect of stacking-fault energy

We investigate the kinetics of the structural deformation and hardening of single-crystalline austenitic Fe-13Mn-1.3C (Hadfield steel), Fe-13Mn-2.7Al-1.3C, and Fe-28Mn-2.7Al-1.3C (in wt%) steels with different stacking-fault energies after cold high-pressure torsion. Independently of the stacking-fault energy, mechanical twinning was found to be the basic deformation mechanism responsible for the rapid generation of an ultrafine-grained microstructure with a high volume fraction of twin boundaries. Under high-pressure torsion, the spacing between twin boundaries increases, and the dislocation density and microhardness decrease as the stacking-fault energy increases. The formation of a twin net from the beginning of plastic flow in Fe-13Mn-1.3C steel provides a homogeneous distribution of microhardness values across the discs independent of strain under torsion. Lower hardness values in the disk centers compared to the periphery were observed for the two other steels, Fe-13Mn-2.7Al-1.3C and Fe-28Mn-2.7Al-1.3C, with higher stacking-fault energies due to changes in the densities of the twin boundaries. An additional increase in the dislocation density for the Fe-13Mn-1.3C steel was detected compared with the Fe-13Mn-2.7Al-1.3C and Fe-28Mn-2.7Al-1.3C steels, which was a result of torsion in the temperature range of dynamic strain aging. The appearance of small fractions of ε and α′ phases in the structures of the Fe-13Mn-1.3C, Fe-13Mn-2.7Al-1.3C, and Fe-28Mn-2.7Al-1.3C steels is discussed.