Unveiling the microstructure evolution and mechanical properties of submerged friction stir welded joint of fine-grained NiCoCr medium entropy alloy

Architecting fine-grained (FG) structure in NiCoCr medium-entropy alloy (MEA) is one of the few effective strategies to attain enhanced strength-ductility synergy, thereby improving its engineering practicality and applicability. However, obtaining welded joint of FG NiCoCr MEA without losing the mechanical properties dominated by FG structure is of great challenge. Herein, we demonstrate the feasibility and effectiveness of submerged friction stir welding (SFSW) in welding of FG NiCoCr MEA without losing its original superior strength-ductility combination. During FSW of FG NiCoCr MEA, its low stacking fault energy makes dynamic recovery be greatly suppressed, so that discontinuous dynamic recrystallization and grain growth dominate the microstructure evolution, leading to position-dependent grain refinement across the whole nugget zone (NZ) while introducing high-density dislocations into the recrystallized grains. Water cooling seems to act mainly after the material is deposited, and exerts an inhibitory effect on the thermally activated grain growth, resulting in finer grain sizes and higher dislocation densities across the NZ. The resultant microstructures strengthen the entire NZ and no softening occurs around the NZ. As compared to the base material, this strategy enables the joint to exhibit significantly enhanced yield strength, comparable ultimate tensile strength and only a slightly decreased elongation. This work provides a feasible and effective way to obtain high-quality FG NiCoCr MEA joint with superior strength-ductility combination. We also anticipate that the SFSW would be applicable to the welding of many other FCC-based MEAs/HEAs systems for obtaining high-performance joints.