Investigation on strain localization and hardening of dual-phase steel by hetero-deformation induced stress

Abstract

Strain hardening remains challenging for maintaining the plasticity of metals, especially those of high strength. The hetero-deformation induced (HDI) stress can effectively alleviate the problem, specifically with a large mechanical response mismatch. Accordingly, the strain localization and hardening in dual-phase steel consisting of ductile ferrite (F) and almost undeformed bainite (B) were investigated by calculating HDI stress (σh) based on the tensile load-unload-reload (LUR) hysteresis loop and modified equation. Of particular note is that strain localization first appears in the ferrite, and then transfers to the F/B interface during tensile LUR. The generated σh by the accumulation of geometrically necessary dislocations (GNDs) inhibits the mobile dislocation, thereby reducing the strain difference, which promotes the transfer of strain localization. The transformation of strain localization changes the hardening behavior from ferrite hardening to F/B interfacial hardening. The results state clearly that the σh enhances strain hardening while induced strain localization maintains tensile plasticity. Eventually, with the increase σh, the failure of dual-phase steel tends to transition from initial ductile fracture to brittle fracture.

Keywords

• Dual-phase steel
• HDI stress
• Strain localization
• Fracture
• Dislocation