High entropy design driven the strengthening and toughening of multi-scale gradient stainless steel joints

The development of alloys with both high strength and ductility remains a critical challenge. Here, a multiscale gradient-structured joint was fabricated by combining alloying with a two-pass laser welding strategy based on additive manufacturing principles. Using AlCoCrNiCu_0.5Ti_x (x = 0.5, 1) high-entropy alloys with 304L stainless steel, the joint exhibited columnar–equiaxed–columnar grains and compositional heterogeneity arising from varied heat input and dilution. Ti enrichment in the second pass promoted an FCC-to-BCC transition, establishing a phase gradient across the weld. This architecture enabled a remarkable strength–ductility synergy, achieving a yield strength of 368.8 MPa, ultimate tensile strength of 741.3 MPa, and elongation of 75 %, surpassing conventional 304L welds. Fracture occurred in the base metal, with dimpled morphology confirming ductile failure. The superior performance stems from multi-mechanism strengthening, particularly back-stress hardening from geometrically necessary dislocations, together with enhanced strain-hardening capacity imparted by the gradient structure. By utilizing the spatial design flexibility of additive manufacturing, this work presents a novel strategy for fabricating multiscale gradient structures, offering a promising pathway toward advanced structural materials with exceptional mechanical synergy.