Ageing behavior of as-cast SiCp/AZ91 Mg matrix composites

SiC particles exhibited necklace-type distribution in SiCp/AZ91 Mg matrix composites fabricated by stir casting, and thus, the SiC particles could be divided into segregated particles region and particle free region. The effects of this particle distribution on the discontinuous precipitation of Mg₁₇Al₁₂ phase during ageing were investigated. The high density dislocations were induced due to the sharp mismatch between the thermal expansion coefficients of SiC and AZ91 alloy. The overlapped mismatch among the segregated particles causes higher density dislocations. And as a result, the discontinuous precipitations as the ageing products started born mainly near particles, especially near segregated particles. However, few Mg₁₇A1₁₂ precipitated in the matrix within the particle segregation. The high-density dislocations and the numerous SiC/Mg interfaces made Mg₁₇A1₁₂ phases much easier and earlier to precipitate in the composite. Thus, the SiCp addition reduced the peak ageing time from 38 h (for AZ91 alloy) to 28 h (for the composite). The ultimate tensile strength of the composite was enhanced by 46% after peak-ageing. SiC particles could also improve the age hardening efficiency of the composite because the high-density dislocations, the fine grain sizes and the numerous SiC/Mg interfaces cause the finer and more uniform precipitation of Mg₁₇A1₁₂ Above all, the addition of SiC particles is effectively to improve the mechanical properties of magnesium matrix composite materials.