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

In this work, SiCp reinforced Al–Si–Mg matrix composites were prepared by stirring casting with different SiCp sizes. The effect of SiCp sizes on the microstructure and mechanical properties was investigated, and different strengthening effects were systematically calculated and analyzed. With the decrease of SiCp sizes, the microstructure of composites is refined and the plasticity is improved significantly. The refined SiCp 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 SiCp, which is due to the influence of SiCp on GP zones strengthening in the matrix. As SiCp sizes decreasing, the increased SiCp/Al interfaces capture more Mg atoms, and then inhibits the formation of GP zones in the α-Al. Therefore, the refined SiCp 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 SiCp/Al–Si–Mg composites. Therefore, it is suggested that the interaction between the SiCp and precipitates in matrix should also be considered to obtain a suitable strengthening effect in the design of SiCp/Al alloy composites.