Evolution of Mechanical Properties and Microstructures with Equivalent Strain in Pure Fe Processed by High Pressure Torsion

Abstract

Pure Fe (99.96%) was processed by high pressure torsion (HPT) using disc and ring samples. When the microhardness and tensile properties are plotted against the equivalent strain, the individual properties fall well on unique single curves, level off at the equivalent strain of ∼40. At the saturated level, the tensile strength of 1050 MPa and the elongation to failure of 2% are attained. Transmission electron microscopy showed that a subgrain structure containing dislocations develops at an initial stage of straining. More dislocations form within the grains and the subgrain size decreases with further straining. At the saturation stage, the average grain size reaches ∼200 nm, the misorientation angle increases and some grains which are free from dislocations appear. It is suggested that at the saturation stage, a steady state condition should be established through a balance between hardening by dislocation generation and softening by recrystallization.