Dynamic hetero-deformation induced hardening through strength reversal between hetero-zones in austenite-ferrite duplex steel

The use of heterostructured materials has proven to be an effective strategy for achieving strength‒ductility balance, which is attributed to hetero-deformation induced (HDI) hardening between the hetero-zones. However, almost all heterostructured materials face an issue: the inherent contradiction between back stress hardening in soft zones and the damage caused by excessive localized strain in these zones. Using austenite-ferrite duplex steel as a model heterostructured material, we revealed a surprising strength reversal between the hetero-zones during room temperature uniaxial tension. Sample exhibiting this unique phenomenon achieved simultaneous enhancements in both strength and ductility, offering fresh perspectives for addressing this contradiction. Multi-scale in-situ experiments, combined with crystal plasticity simulations, revealed that the improvement in mechanical properties is attributed to strong strain/stress partitioning between the hetero-zones, as well as a sustainable, dynamically reinforcing HDI hardening mechanism triggered by strength reversal. Strength reversal allows hetero-zones to alternately bear strain, preventing premature damage from excessive strain in the initially soft zone. Importantly, the initially hard zone begins to provide back stress once the initially soft zone exceeds its strength. Alternate back stress between the hetero-zones ensures high sustained HDI hardening rather than exhaustion, thereby increasing strength. We also systematically discuss how the hardening ability of the initially soft zone and its content influence the occurrence of strength reversal and the critical engineering strain at which it occurs. Finally, we propose a novel, dynamic HDI hardening mechanism, which expands the conventional HDI theory and provides new insight for designing heterostructured materials with superior mechanical properties. © 2025 Acta Materialia Inc.