Strength-ductility synergy enhancement of as-cast Mg-Gd-Sm-Zr alloy via friction stir processing

This study selected three groups of different processing parameters to perform FSP on the as-cast Mg-7Gd-3Sm-0.5Zr alloy, and measured and recorded the thermal history generated during the processing. The influence of tool rotation speed and traverse speed on the microstructural evolution, DRX mechanism, texture modification, and mechanical properties of the alloy were systematically investigated. The results indicated that FSP mainly achieves rapid grain refinement through the CDRX mechanism, and the grain sizes refined by 93.7%, 93.3%, and 91.7% under the three processing parameters, respectively. Under intense mechanical stirring, the coarse eutectic phases at grain boundaries were broken down into nanoscale particles, primarily consisting of elliptical Mg 5 Gd phases and a small amount of elongated Mg 41 Sm 5 phases. As the ω/ν ratio increases, the peak temperature within the material rises, enabling more solute atoms to form solid solutions within the matrix and strengthening it. The texture in SZ gradually changed from <0001> α ∥TD to <0001> α ∥PD, and promoted the activation of the pyramidal <a> slip and pyramidal <c+a> slip. Compared to the as-cast alloy, the strength and ductility of the alloy were both enhanced synergistically after FSP. Under the P2 parameter, the alloy exhibited the most outstanding mechanical properties, with ultimate tensile strength and elongation increased by 38.1% and 143.7%, respectively, and the strength-ductility product reaching 4.5 GPa•%. This is the result of the combined effects of grain refinement, the fragmentation and dissolution of coarse eutectic structures, the weakening of strong basal texture, and the presence of high-density dislocations.