SiC particle-reinforced AC4C based alloy (AC4C–Mg and AC4C–Cu) composites were fabricated by the melt stirring-gravity casting method in atmospheric, and the microstructures, strength and fatigue properties were studied. The tensile strength and the fatigue lives were substantially higher in the SiC particle-reinforced AC4C based alloy composites. Compared with the AC4C-based alloys, the fatigue lives of SiCp/AC4C–Mg and SiCp/AC4C–Cu composites at 250°C for 10⁷ cycles increased by 29% and 27%, respectively. It was found that the effect of SiC particle on the fatigue life was more remarkable for the SiC particle-reinforced AC4C-based alloy composite tested at elevated temperature. SEM observations of the fractured specimens of the SiC particle-reinforced AC4C based alloy composites showed that almost no particles were found from the regions of the crack propagation of the fatigue surfaces. The formation of the plastic zone in the matrix due to the thermal expansion mismatch between the SiC particle and the matrix resisted the generation and the propagation of the fatigue crack, and contributed to the improvement of fatigue life.

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The aging behavior of excess Mg type Al–Mg–Si alloy processed by high pressure torsion (HPT) and effect of Cu addition to this alloy have been investigated by hardness test and microstructure observation. The samples were solution heated, quenched, processed by HPT and aged. The hardness of HPT-processed samples increased about 100 HV rather than samples without HPT, and they showed hardening with peak hardness during aging. The level of hardness on alloys was increased by increasing with amount of Cu-addition. The mean grain size in HPT-processed samples was about 200–250 nm for each alloy. All HPT-processed alloys showed peak hardness during aging, and two Cu added alloys show positive value of age-hardening ability for the aging condition in this study. Precipitates in HPT-processed and aged samples existed on and near the grain boundaries, and a few small precipitates are found at some dislocations. According to the analysis of selected area diffraction (SAED) patterns, aluminum, Mg₂Si and Q-phase are found in the HPT-processed and aged Cu-added alloys. The hardening of HPT-processed samples is probably caused by precipitation at the homogeneous grain boundary networks of 200–250 nm in the sample which acts as heterogeneous nucleation sites for alloys.

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