Microstructure evolution and mechanical properties of SiC nanoparticles reinforced magnesium matrix composite processed by cyclic closed-die forging

Magnesium composites containing different amounts of SiC nanoparticles were processed by a two-step cyclic closed-die forging (CCDF), which was carried out at 400°C for 3 passes (1st-CCDF) and further at 300°C for 2 passes (2nd-CCDF). Microstructure evolution and mechanical properties of the composites were investigated. After processing, the average grain size is significantly refined to ~2.5μm, and the morphology exhibits a flow-lined feature. The β-Mg17Al12 phases in the as-cast alloy dissolve completely after 1st-CCDF, and then precipitate out with an average size of ~650nm during 2nd-CCDF. After CCDF, the as-cast SiC clusters are uniformly dispersed as separate particles, and the yield strength and ultimate strength of the composites reach 258MPa and 365MPa, respectively. The ductility of the composites is enhanced after 1st-CCDF but decreased after 2nd-CCDF, which is in accordance with the observed fracture surfaces. From the perspective of strength and ductility, the optimal content of SiC is 0.5wt%.