Microstructural evolution and mechanical properties of ultrafine-grained pure α-iron and Fe-0.02%C steel processed by high-pressure torsion: Influence of second-phase particles

A Fe-0.02 wt%C, containing cementite particles, and a pure α-iron are subjected to unconstrained high-pressure torsion and their microstructural refinement with strain are examined by electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction EBSD (TKD-EBSD), based on which the influence of second-phase particles on grain refinement mechanisms is investigated. Both materials are refined rapidly by formation of subgrain boundaries and grain boundaries at low and medium strains. The single-phase iron generates a higher density of geometrically necessary dislocations and forms small grains in the deformation inhomogeneity regions. However, at a higher strain of ~12.3, the cementite particles facilitate to overcome the saturation microstructure that occurrs in the pure iron and promote further grain refinement. Continuous dynamic recrystallization (CDRX) by transforming subgrain boundaries to grain boundaries is the major grain refinement mechanism before evm~12–13. Geometric dynamic recrystallization (GDRX) is also operating during ultrahigh strains (evm~12–30), particularly prevalent in the cementite-containing specimen. Mechanical properties of the HPT-processed microstructures are examined by nanoindentation and micropillar compression.