High-performance welds with superior cryogenic strength-ductility in ferrous medium-entropy alloys by compositional heterogeneity-driven metastability engineering

This study investigated the cryogenic tensile behavior of ferrous medium-entropy alloy (FeMEA) welds produced by gas tungsten arc welding, focusing on their microstructural deformation behavior and strain-hardening mechanisms. The FeMEA welds exhibited a 300% increase in strength and a 201% improvement in elongation as the temperature decreased from 298 to 77 K. Furthermore, the welds achieved 102% of the strength and 105% of the elongation compared to the base metal (BM) at 77 K, outperforming traditional alloys. This unprecedented performance originates from compositional heterogeneity induced by elemental segregation during welding, which beneficially affects the deformation behavior of the FeMEA welds. Fe-enriched regions further destabilize the FCC phase, thereby accelerating deformation-induced martensitic transformation with substantial strain-hardening ability. This effect was supported by the decreased ΔG^FCC→BCC in the Fe-enriched weld metal (WM) compared to the BM using third-generation thermodynamic calculations applicable to cryogenic conditions. Moreover, interstitial-free BCC martensite accommodated plastic strain more effectively through dynamic stress-strain partitioning with the FCC matrix during plastic deformation. These synergistic mechanisms enable the FeMEA welds to achieve superior strength without sacrificing ductility at cryogenic temperatures. This study demonstrates that FeMEA welds are promising candidates for cryogenic structural materials, offering a viable solution to the long-standing challenge of the mechanical inferiority of the WM under cryogenic conditions.