Tensile behaviour of discontinuous two-phase materials : influence of matrix flow stress and internal damage

Particle reinforced metal matrix composites are manufactured by high-pressure infiltration. These composites contain roughly 50% by volume of discrete alumina inclusions dispersed homogeneously in a continuous aluminium-based matrix. Three matrices, namely pure Al, Al-2%wtCu and Al-4.5%wtCu are compared. Two reinforcement types, namely angular (or irregular) and polygonal (or equiaxed) Al2O3 particles of different sizes (5 to 60 µm for angular, and 5 to 25 µm for polygonal alumina), are used. These materials are tested under uniaxial tensile loading, recording stress and strain. The evolution of Young's modulus using periodic unload/reload cycles, and several mechanical properties are measured. The development of internal damage is studied qualitatively using metallography, and quantified using the measured evolution of the Young's modulus as a function of tensile strain. In general, composites with polygonal particles have higher mechanical properties and accumulate internal damage more slowly than composites with angular particles, in which there is a significantly greater propensity for particle cracking under stress. Mean-field models are used to approximate the elastic behaviour of these two-phase materials. The Mori-Tanaka method is found to agree, within error, with the experimental data. Ponte Castañeda's variational estimate, shown by Suquet to be equivalent to a modified secant modulus approach, is used to effect the passage from linear to non-linear composite deformation. The three damage mechanisms present in composite materials of this class, namely particle cracking, matrix voiding, and interphase debonding, are finally incorporated in these models, again building on recent progress in the micromechanics of two-phase materials such as these. It is shown, for a pure aluminium matrix reinforced with angular 35 µm alumina, that the proposed analytical method gives solutions very close to experimental in-situ matrix flow stress found with neutron diffraction. On this basis, the in-situ matrix stress-strain curve is calculated for each composite and the effect of the intrinsic characteristics of the components is studied. A strong particle size effect is found with a pure aluminium matrix, while it practically disappears in alloyed matrix composites; this latter observation is explained by the role of (much finer) intermetallic precipitates in the matrix. Analysis of matrix flow stress data using the Taylor relation leads to conclude that the size effect is also dependent on particle shape. The presence of sharp angular ridges, existing in angular but not in polygonal particles, is probably the main reason. Finally, the mean-field approximations and the variational estimate are extended to composites of metal and pores, i.e., to microcellular metallic materials. Comparison with data for open-pore aluminium foam data leads to the conclusion that features of the phase distribution exert a very strong influence on both the elastic modulus a