Optimal concentration of SiC in SiC/Al composites: experiment and percolation theory prediction of lower and upper bounds

We have measured the mechanical strength of composites made by pressing mixed powders of deformable aluminum (Al) and non-deformable silicon carbide (SiC) as a function of the volume fraction of SiC and porosity. The experimental results show a peak in green strength at low SiC concentration. We explain these results as a competition between strengthening by clusters of interlocking Al-SiC sub-units and weakening by clusters of SiC particles. This clustering behavior of the composites is modeled by a random binary mixture model and described in terms of percolation theory. From extensive simulations of the model in three different lattice structures, we find the critical concentrations of SiC required to form two different types of percolating clusters, thus providing lower and upper bounds for the optimal concentration. Quantitative comparison between the simulations and experimental results shows good agreement.