Formation of epsilon martensite by high-pressure torsion in a TRIP steel

An Fe-17% Mn-0.06% C-2% Si-3% Al-1% Ni steel exhibiting a phase transformation induced by room temperature deformation was processed by high-pressure torsion (HPT) using a pressure of 6.0 GPa and with the samples subjected to different amounts of torsional straining up to a maximum of 10 turns. A microstructural analysis revealed a phase transformation in the early stages of deformation and a gradual evolution towards a fully-deformed structure. Microhardness measurements showed two stages of hardening with eventual softening at large strains. From X-ray diffraction (XRD) analysis, there is evidence for a reverse martensitic transformation and the stabilization of an h.c.p. epsilon (ε) structure. The formation of an h.c.p. structure takes place in this steel at lower pressures than for pure iron but the results agree with earlier reports of the presence of an ε phase in stainless steel processed by HPT and with the expected reduction in the transition pressure due to the Mn addition.