Friction stir welding of CoCrNi medium-entropy alloy: Recrystallization behaviour and strengthening mechanism

Dynamic recrystallization (DRX), deformation-induced mechanical twins, and strengthening mechanism of CoCrNi equi-atomic medium-entropy alloy (MEA), welded by friction stir welding (FSW) at different welding speeds, were systematically investigated in this study. The results indicated that FSW led to grain refinement in the stir zones (SZs) with diameters fluctuating in the range of 2.1–9.6 μm. The hardness was improved from ∼154HV (base material, BM) to 238–263HV (SZ) by FSW. The yield strength and ultimate tensile strength of the optimal SZs were 601 MPa and 844 MPa, respectively, which were 260% and 134% higher than those of the as-received material, respectively. Analyses of DRX behaviour indicated that discontinuous DRX (DDRX), continuous DRX (CDRX), and geometric DRX (GDRX) were successively activated with increasing strain during FSW to facilitate grain refinement. The larger the grain size, the higher the fraction of mechanical twins. However, the thickness of the twins had a negative correlation with the grain size. Although a few HCP structures were formed on the twin boundaries, grain refinement and dislocation hardening mechanisms remained dominant in enhancing the mechanical properties of the SZs. The strength-ductility synergy by FSW, under low heat input conditions, was caused by the formation of thin mechanical twins in fine equiaxed grains. Thinner twins were more effective in transferring and homogenising plastic deformation, thereby contributing to the postponing of plastic instability and further promoting an excellent combination of strength and ductility. These findings confirm that FSW is a promising approach for strengthening the MEA for superior performance.