Annealing and strain rate effects on the mechanical behavior of ultrafine-grained iron produced by SPD

Thermal stability and strain rate sensitivity of ultrafine-grained (UFG) Fe produced by severe plastic deformation (SPD) were investigated. The UFG Fe was processed by equal-channel angular pressing (ECAP) via route Bc. After 6 passes, the grain size of UFG Fe reaches 600 nm, as confirmed by means of electron back scatter diffraction (EBSD). Examination of micro-hardness and grain size of UFG Fe as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization. The critical transition temperature is approximately 500 °C, and the material has a bimodal structure after annealing at this temperature. Deformation behaviors of ECAP Fe and ECAP + annealing Fe were studied under both quasi-static and dynamic compressive loadings. The UFG iron shows increased strength and reduced strain rate sensitivity compared with its coarse-grained counterparts. The appropriate post-ECAP annealing can increase strain hardening ability and cancel out thermal softening effect with only a small loss of strength under dynamic loading.