Nanostructuring and hardness evolution in a medium Mn steel processed by high-pressure torsion technique.

Abstract

Severe plastic deformation (SPD) is performed on a newly developed medium Mn steel with the composition of Fe-7.66Mn-2Ni-1Si-0.23C-0.05Nb (wt.%) to achieve a nanocrystalline microstructure. The SPD process utilizes the high-pressure torsion (HPT) technique, resulting in a nominal shear strain of approximately 36,000% after processing the disk for 10 turns. X- ray diffraction line profile analysis reveals an increase in dislocation density to around 230 × 1014 m-2 . In addition, under high strains, a face-centered cubic (fcc) secondary phase emerges within the body-centered cubic (bcc) matrix. Analytical transmission electron microscopy using energy dispersive X-ray spectroscopy indicates that the secondary phase particles are enriched in Al, Mn, and Si. As the strain imposed during HPT increases, the simultaneous rise in dislocation density and nanostructuring lead to material hardening. However, the partial phase transformation from bcc to fcc contributes to material softening. As a result of these two opposite effects, the hardness exhibits a non-monotonic variation with the shear strain, displaying, for 10 turns of HPT, a lower hardness compared to fewer turns, despite the continuous increase in dislocation density and decrease in crystallite size.