Martensitic transformations and the evolution of the defect microstructure of metastable austenitic steel during severe plastic deformation by high-pressure torsion

Abstract

It has been shown that, in metastable austenitic Fe–18Cr–10Ni–Ti steel, under conditions of torsion under pressure, local reversible (forward plus reverse) (γ → α′ → γ) martensitic transformations can occur, which are one of the mechanisms of the formation of nanostructured states. An increase in the rotation rate, which leads to an increase in the deformation temperature, stimulates the reverse (α′ → γ) transformation. The evolution of the structural and phase states is represented as the following sequence: (1) mechanical twinning; (2) nucleation of martensitic plates in the microtwinned structure of the austenite with the formation of two-phase (γ + α′) structures, packet α′ martensite, and structural states with a high curvature of the crystal lattice; (3) reverse (α′ → γ)-transformations; and (4) the fragmentation of nanosized crystals via the formation of a nanotwinned structure in the austenite and of a nanoscale banded structure of the ε martensite in the α′ martensite.