Origins of Strength, Strain Hardening, and Fracture in B2 Tailored Fe–0.8C–15Mn–10Al–5Ni Wt Pct Austenitic Low Density Steel

Abstract

Low density steel is a future hope for automotive industries with high strength–ductility, concerning passenger safety. To investigate the microstructural contributions, an as-cast Fe–0.8C–15Mn–10Al–5Ni wt pct alloy is fabricated by hot rolling, cold rolling, and thereafter annealing at 900 °C. The phase constituents are austenite (γ) as the matrix, B2 as the second phase, and B2 stringer bands originating from heat-treated deformed δ-ferrite in this high-Al-alloyed steel. The work analyzes the effective contribution of intra-granular B2 to yield strength (YS) through precipitation strengthening. The strain-hardening mechanism is rather complex. One group believes that strain incompatibility between softer γ and hard B2 localizes geometrically necessary dislocations at the phase interface for high-strain hardening in this alloy by back stress; whereas, the other considers planar glide with a restricted cross-slip behind the strain-hardening mechanism. The present work successfully resolves the debate by stating that strain hardening is a matrix’s characteristic, dominated by lattice friction stress and dislocation strengthening. The lattice friction of solute-enriched γ-matrix reduces the width of dislocation core merely to an interplanar spacing. Thereby, a difficulty in dislocation movement during tensile deformation results in extraordinary strain hardening for this steel through a constrained planar glide. The static recrystallization and polygonization of heat-treated cold-rolled B2 stringer band induces intergranular micro-cracks, and thereafter its annihilation as a stress concentration site for tensile failure without necking.