Strengthening effects of GP zone induced by SiC_p size variations in Al–Si–Mg matrix composites

In this work, SiC_p reinforced Al–Si–Mg matrix composites were prepared by stirring casting with different SiC_p sizes. The effect of SiC_p sizes on the microstructure and mechanical properties was investigated, and different strengthening effects were systematically calculated and analyzed. With the decrease of SiC_p sizes, the microstructure of composites is refined and the plasticity is improved significantly. The refined SiC_p also leads to a more intense grain refinement strengthening and thermal expansion mismatch strengthening. However, an abnormal decrease of yield strength is discovered with the refinement of SiC_p, which is due to the influence of SiC_p on GP zones strengthening in the matrix. As SiC_p sizes decreasing, the increased SiC_p/Al interfaces capture more Mg atoms, and then inhibits the formation of GP zones in the α-Al. Therefore, the refined SiC_p weaken the precipitation strengthening in composites. These acicular Mg–Si GP zones exhibit a preferentially growing direction along <100>_Al and present a same structure as α-Al with coherent interfaces which pin dislocations and provide precipitation strengthening. The high density of GP zones also increases nano-hardness and elastic modulus of α-Al, which improves the wear resistance of SiC_p/Al–Si–Mg composites. Therefore, it is suggested that the interaction between the SiC_p and precipitates in matrix should also be considered to obtain a suitable strengthening effect in the design of SiC_p/Al alloy composites.