Effect of rotary swaging on the microstructure and mechanical properties of Mg-Zn-Ca/SiCp composites

Due to the poor plasticity and deformability, magnesium matrix composites are difficult to strengthen using traditional severe plastic deformation (SPD) processes. In this study, we applied a large deformation of 0.36 to SiC particles reinforced Mg-Zn-Ca (ZX50/SiCp) composites at room temperature via rotary swaging (RS). The RS process significantly increased the yield strength of the composite from 174.1 MPa after extrusion to 346.5 MPa after RS with ten passes, representing a 98% enhancement. The remarkable strengthening effect was primarily attributed to grain boundaries strengthening, dislocation strengthening and precipitation strengthening effects. The results indicated that the average grain size of the composite refined to 16 µm after ten passes, and a substantial number of deformation twins were developed in the host grains. The interactions between twin boundaries and precipitates as well as reinforcement particles blocked the twin growth, resulting in dense twin networks. With increasing strain, multiple twins developed to further refine the twin lamella. Furthermore, the activation of numerous dislocations developed dislocation arrays and the geometrically necessary dislocations (GNDs) density increased from 2.8 × 10¹⁴ to 7.2 × 10¹⁴ m^-2. In addition, dynamic precipitation occurred during the RS process, resulting in the formation of substantial nano-scale Mg-Zn second phases (average diameter ~70 nm). The severe shear strain during the RS process promoted the uniformly dispersion of reinforcement particles. These findings provide valuable insights into the fabrication and strengthening of magnesium matrix composites through the proposed RS process.