Non-monotonic strain-rate sensitivity in an ultra-coarse-grained high-Al and high-Mn low-density duplex steel: Competition between ferrite substructure hardening and strain-induced transformation

The room-temperature tensile deformation behavior of an ultra-coarse-grained duplex low-density Fe–30Mn–9Al–0.07C steel (wt.%) was investigated under quasi-static strain rates of 10 −1 -10 −5 s −1 . The alloy exhibited a non-monotonic strain-rate sensitivity: positive sensitivity was observed between 10 −1 and 10 −4 s −1 , whereas a reversal toward negative sensitivity occurred at 10 −5 s −1 . EBSD analyses reveal that high strain rates promote ferrite-dominated substructure development and a rotation toward <111>-type orientations, leading to enhanced hardening and ductility. At low strain rates, ferrite shows a weaker texture change and austenite become increasingly involved in strain accommodation, evidenced by enhanced <110>-type orientations and local γ to α transformation signatures. The results highlight a competition between ferrite substructure hardening at high strain rates and transformation-/load-transfer-assisted deformation at low strain rates, providing insight for microstructure design of lightweight steels under variable-rate loading.