Particle reinforcement distributions and solidification structures in directionally solidified metal matrix composites

The solidification of a metal alloy in the presence of a more dense, chemically inert solid particulate is studied using the aluminum-silicon alloy system reinforced with silicon carbide particulate.It was found that the well-known Stokes' settling rule, without correction for hindering, may be applied to model the observed settling rates.Melt composition has little effect on the rate of settling of the particulate, however, the presence of large, detached and buoyant Si-rich primaries in solidifying hyper-eutectic AlSi alloys leads to compositional banding in the growth direction as the particles impinge upon physical obstructions in the mould. In the reinforced regions of the hyper-eutectic based composites the Si-rich primary phase is refined by the SiC particulate.Bridgman-grown samples of A356 (predominantly Al7wt%Si) and A356 containing 15 volume percent SiC particulate were grown over the range of 0.1 to 50 micrometres/second. At rates at which a very coarse cellular structure occurred in the alloy, a somewhat finer cellular structure persisted. The inter-cellular regions were comprised of SiC particulate and eutectic phase. Only a minute degree of particle entrapment within the aluminum-rich cell occurred even at the lowest rates of growth.Samples, processed using the DuralFor certain dilute metal matrix composites the rates of sedimentation are so large that mould-filling conditions and solidification times will play a significant role in the ultimate distribution of the reinforcement. The presence of the SiC in the hyper-eutectic AlSi alloy introduces a degree of primary-Si refinement. (Abstract shortened by UMI.)...