Dynamic carbon diffusion induced sustainable strain-hardening at quasi-static strain rates in high-C Al-added austenitic steels

Abstract

The design of strain-hardening behavior in steel typically involves controlling the activation of deformation mechanisms and the evolution of microstructure during deformation. This research proposes a novel strategy to promote sustained hardening by leveraging dynamic strain aging (DSA) through a high-C design to pin dislocations, thus enhancing tensile strength and ductility at quasi-static strain rates, independent of microstructure tailoring. This study reveals that lower strain rates are more conducive to achieving greater strain-hardening capacity in the new alloys within the thermally-activated regime (strain rates of 10^–3 to 10^–1 s^–1). Intriguingly, heavily deformed microstructures show reduced substructure density at lower strain rates, yet exhibit enhanced flow stress. This discrepancy indicates that the observed changes in the alloy’s hardening deviate from conventional substructure evolution law. Transmission electron microscopy and electron backscatter diffraction analyses show that low strain rates inhibit the formation of additional twin systems and promote a predominantly cellular structure dominated by cross-slip. Theoretical calculations of dislocation dynamics and carbon diffusion rates confirm that DSA dominates strain rate sensitivity. Tensile tests at elevated temperatures demonstrate notable improvements in both ultimate tensile strength and ductility. This observation, combined with cyclic aging-reloading tests, underscores the critical role of DSA in the hardening of C-enriched alloys. By demonstrating the substantial impact of dynamic interstitial diffusion on strain-hardening and strain rate response, this study confirms that DSA induces substantial hardening at ambient temperature. This results in the alloy at a strain rate of 10^–3 s^–1 exhibiting a strain-hardening capacity exceeding 100 MPa higher than that at 10^–1 s^–1, while also achieving improved resistance to instability and an elongation increase of nearly 10 %. Despite limited twinning and dislocation density, the alloy achieves superior mechanical properties through solute-dislocation interactions, surpassing predictions of conventional hardening models that over-rely on substructure evolution. This study offers a promising avenue for designing future alloys with superior strength and ductility at quasi-static strain rates, potentially overcoming the traditional strength-ductility trade-off via solute-dislocation interactions.