Lüders-band-mediated spatial partitioning of TRIP and TWIP in a hot-rolled Fe66(CoNi)29Cr5 ferrous medium-entropy alloy

초록

Designing ferrous medium-entropy alloys with a cryogenic strength-ductility synergy requires a mechanistic understanding of how strain localization redistributes deformation pathways in metastable dual-phase microstructures. Here, a hot-rolled Fe-6(6)(CoNi)(2)Cr-9(5) ferrous medium-entropy alloy was investigated using interrupted tensile testing at 77 K combined with electron backscatter diffraction and electron channeling contrast imaging. The as-hot-rolled alloy exhibited an FCC/BCC dual-phase microstructure with a BCC fraction of 19.1%, a high dislocation density, and pre-existing deformation twins in the FCC phase. During L & uuml;ders elongation, the propagation of the L & uuml;ders band markedly accelerated the FCC -> BCC martensitic transformation, increasing the BCC fraction to similar to 74.3% by the end of the L & uuml;ders regime. In contrast, retained FCC regions remained TWIP-active, as evidenced by reduced twin spacing after deformation. This band-redistributed plasticity, together with the hot-rolling-induced defect structure, produced a high yield strength of similar to 1280 MPa at 77 K while retaining substantial tensile ductility. These results establish a processing-structure-property linkage between hot-rolling-induced metastability, L & uuml;ders-band propagation, phase-transformation kinetics, and cryogenic deformation, and provide guidance for designing Fe-rich ferrous medium-entropy alloys with balanced strength and ductility at low temperature.

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