The ductile fracture process in materials containing a high volume fraction of brittle particles has been studied by means of the quantitative characterization of damage in an Al-Si-Mg model system. It has been found that the damage process in these materials consists of initiation by cracking of the Si particles and growth of the damage in the form of intraparticle voids, which was terminated by the linkage of the damage through local impingement or shear processes, depending on microstructure. In form, the damage appeared similar to that observed in materials with lower particle volume fractions, although the development of damage was appreciably accelerated.

The microstructural effects of particle volume fraction, particle size, and matrix characteristics on the damage process in uniaxial tension were examined. Particle volume fraction and the matrix characteristics as controlled by thermal treatment had the dominant effect on fracture ductility, while particle size in the somewhat limited range studied was less important. A higher rate of particle cracking with strain was seen with increasing particle volume fraction, as measured both by a damage parameter based on modulus loss as well as by quantitative metallographic evaluation. This was interpreted in light of the role of particle-particle interactions at the higher particle volume fractions.

The particle cracking process was found to be primarily stress-controlled, although there are effects of matrix characteristics on the stress dependence of particle cracking that could not be explained on this basis. The growth of the damage in the form of intraparticle voids proceeded by opening but not transverse growth in a strain-controlled manner until a critical strain value. At this critical strain, which was only slightly less than the fracture strain, the rate of damage increased rapidly, possibly through a transition from growth of single damage features to local linking and growth of damage in aggregate form. Final fracture occurred by linking these regions in the form of a macrocrack.