HETEROGENEOUS NUCLEATION AND GRAIN-REFINEMENT IN CAST MG(AZ91) SICP METAL-MATRIX COMPOSITES/

Thermal analysis and microstructural investigation have been carried o ut to study the heterogeneous nucleation, crystal growth and grain ref inement in the solidification of magnesium alloy AZ91 and AZ91/SiCP co mposites with two different volume fractions of the reinforcement. The results show the evidence of heterogeneous nucleation of primary magn esium on SiC particles in the two composite samples, which is attribut ed to the small (4%) lattice disregistry between SiC and magnesium wit h an orientation relationship of (111)(SiC)//(0001)(Mg). Analysis of a nucleation/growth competition during the initial stage of grain growt h suggests that the particle/solidification front interaction during t he solidification of composites results in a reduced growth rate for t he primary phase, which is especially significant when the volume frac tion of reinforcement is high. Grain refinement in AZ91/SiCP composite s is the result of two separate and competing processes: nucleation an d growth. In the composite with 10 vol.% SiC, the significant grain ot her composite with 0.5 vol.% SiC, the moderate grain refinement (37%) is mainly attributed to a heterogeneous nucleation mechanism. Copyrigh t (C) 1996 Canadian Institute of Mining and Metallurgy.