Phase evolution and mechanical properties of an intercritically-annealed Fe-10Ni-7Mn (wt. %) martensitic steel severely deformed by high-pressure torsion

Experiments were conducted on an Fe-10Ni-7Mn (wt. %) martensitic steel processed by high-pressure torsion (HPT) after intercritical annealing in the dual phase (α+γ) region at 600°C for 2 hours. Microstructural examination and phase analysis using electron backscatter diffraction (EBSD) confirmed that post-HPT processing led to a phase transformation of austenite to α-martensite (γ→α and γ→ε→α) such that after 20 turns of HPT the microstructure included multi-phases of α-martensite, austenite and ε-martensite. The HPT processing also produced a significant reduction in the α-martensite and austenite grain sizes from initial values of 3.8 and 3.9 μm to ~225 and ~60 nm after 20 HPT turns, respectively, while the ε-martensite final grain size was ~44 nm. Micro-hardness measurements and tensile testing showed a significant increase in hardness and in the ultimate tensile strength to about 680 Hv and 2235 MPa after 20 turns, respectively. Finally, the fracture surfaces of samples after HPT processing showed a combination of dimples, cracks and shear bands which confirmed a transition in fracture behavior from ductile to a mixture of a ductile and brittle mode