An investigation on particle weakening in T6-treated SiC/Al-Zn-Mg-Cu composites

This study found weakening effect of SiC particles on strength of a T6-treated SiC/Al-Zn-Mg-Cu composite. The mechanisms for the weakening were investigated in detail for the first time. The 15 vol% SiC/7085Al composite and 7085Al alloy were fabricated by powder metallurgy (PM) method and subjected to microstruttural examination and mechanical properties evaluation. It was indicated that the as-extruded composite had 92 MPa enhancement in the ultimate tensile strength (UTS) compared to the as-extruded unreinforced alloy. However, After T6 treatment (solution treatment and artificial ageing), the UTS of the composite was 34 MPa lower than that of the unreinforced alloy. Microstructural examination disclosed that the content of eta'-MgZn2 was reduced in the T6-treated composite due to interface reaction between Mg and oxide on the surface of SiC particles. Meanwhile, compared to the unreinforced alloy, the composite had coarser precipitates with relative lower density under the T6 condition. Furthermore, it was found that SiC particles accelerated ageing process, leading to formation of coarse precipitates and precipitate free zone in the vicinity of SiC-Al interface. According to the theoretical calculation, the weakened precipitation strengthening value was not compensated by mismatch dislocation strengthening and load transfer strengthening. By adding additional Mg (1.0 wt%) into the composite, the precipitation strengthening effect could be basically recovered, resulting in significantly enhanced strength compared to the 7085Al alloy.