The pressure infiltration of densely packed alumina particle preforms with molten aluminium produces defect-free composites containing roughly equal proportions of metal and ceramic. The process affords considerable latitude in microstructural design of the resulting composites. In particular, the matrix can be kept microstructurally simple, and the size of the reinforcement can be varied roughly between 3 and 100 μm.
Despite their high ceramic loadings, these composites display appreciable plasticity, often deforming in tension to elongations of several percent before breaking. Beyond yield, their flow stress is largely governed by that of the matrix; this matrix, in turn, is a metal deforming between hard inclusions only a few micrometres apart. As is known from the composite literature, the high constraint imposed by the reinforcement on matrix deformation is manifest in a “size-effect”, whereby the matrix flow stress, and hence that of the composite, depend on the microstructural scale of the composite, increasing as the reinforcement size decreases.
We present recent results on this phenomenon from an investigation of pressure-infiltrated alumina reinforced aluminium composites. The composites are model materials in that (i) their matrix (high-purity aluminium, or binary aluminium-copper alloys) is kept metallurgically simple, (ii) the matrix/reinforcement interface is mechanically strong and free of interfacial reaction products, and (iii) the reinforcement is monosized and uniformly distributed throughout the composite.
We describe a method of data analysis of this effect that takes into account both the complexity of the underlying mechanical problem and the influence of internal damage, which often develops extensively in such materials. Recent results for the non-linear deformation of power-law composites are used in conjunction with measurements of the rate of internal damage in these materials to derive in-situ flow curves of the matrix within such composites. Results show that there is a strong and systematic size effect for yield and plastic flow of highly reinforced pure aluminium; data also show an influence of the particle shape in additional to particle size. With binary Al-Cu alloys, the data show that there is a size effect on yield of the composite, but not on the flow stress upon subsequent deformation.