Heterogeneous nucleation and grain refinement in cast Mg(AZ91)/SiCp metal matrix composites

Thermal analysis and microstructural investigation have been carried out to study the heterogeneous nucleation, crystal growth and grain refinement in the solidification of magnesium alloy AZ91 and AZ91/SiCP composites with two different volume fractions of the reinforcement. The results show the evidence of heterogeneous nucleation of primary magnesium on SiC particles in the two composite samples, which is attributed to the small (4%) lattice disregistry between SiC and magnesium with an orientation relationship of (111)(SiC)//(0001)(Mg). Analysis of a nucleation/growth competition during the initial stage of grain growth suggests that the particle/solidification front interaction during the solidification of composites results in a reduced growth rate for the primary phase, which is especially significant when the volume fraction of reinforcement is high. Grain refinement in AZ91/SiCP composites is the result of two separate and competing processes: nucleation and growth. In the composite with 10 vol.% SiC, the significant grain other composite with 0.5 vol.% SiC, the moderate grain refinement (37%) is mainly attributed to a heterogeneous nucleation mechanism. Copyright (C) 1996 Canadian Institute of Mining and Metallurgy.