Reinforcement distributions play an important role in various aspects of the processing and final mechanical behaviour of particulate metal matrix composites (PMMCs). Methods for quantifying spatial distribution in such materials are, however, poorly developed, particularly in relation to the range of particle size, shape and orientation that may be present in any one system. The present work investigates via computer simulations the influences of particle morphology, homogeneity and inhomogeneity on spatial distribution measurements obtained by finite-body tessellation. Distribution inhomogeneity was simulated both by the segregation of particles away from specified regions within a microstructure and by generating point density peaks at random locations within a microstructure. Both isotropic and anisotropic inhomogeneous distributions were considered to simulate distribution patterns in PMMCs before and after mechanical working. It was found that the coefficient of variation of the mean near-neighbour distance (COV(d(mean))), derived from particle interfaces using finite-body tessellation, was essentially independent of particle shape, size distribution, orientation and area fraction in homogeneous (random) distributions, but showed great sensitivity to inhomogeneity. Increased values of COV(d(mean)) were seen for both forms of inhomogeneous distributions considered here, with little influence of particle morphology. The COV(d(mean)) was also seen to be sensitive to anisotropic clustering, the presence of which was identified via nearest-neighbour angles and cell orientations. Although generally formulated for PMMCs, the present results may be generalized to other systems containing low aspect ratio finite bodies of low to moderate area fraction.

Various reports in the literature have highlighted the effects of particle distribution on the fatigue behaviour of particulate reinforced metal matrix composites (PMMCs), although few attempts have been made at modelling such effects. A micromechanical understanding of the effects of clustering on short crack growth behaviour in Al–SiCp composites has been achieved via finite element modelling. Comparison of preliminary models with the literature has shown that shielding/anti-shielding effects were significantly affected by the relative sizes of the particle and the overall model such that, when edge effects were removed, a crack was predicted to be accelerated rather than decelerated as it propagated through closely spaced pairs of particles. Consistent differences were identified between models with homogeneous versus clustered particle arrangements in terms of crack path morphologies and local crack–tip stress intensity fluctuations. Furthermore, predicted influences of clustering on growth rates in the numerical models were found to be consistent with previous experimental results (i.e. growth rates rose with increased clustering), demonstrating that load transfer effects associated with changes in particle distribution may play a direct role in controlling the growth of short cracks in these materials.

A series of finite-size particle distributions mere simulated to investigate the effects of particle size, shape, orientation, and area fraction on the quantification of homogeneity in structural particulate metal matrix composites (MMCs). It is found that, for nominally random distributions, the values of conventional centre-to-centre nearest-neighbour spacing parameters are influenced by particle morphology, and, as such, are unsuitable for characterising distributions of finite-size particles. However, the coefficient of variation of the mean near-neighbour distance COV(d(mean)), derived from particle interfaces using finite-body tessellation, appears independent of particle shape, size distribution, orientation, and area fraction, while showing great sensitivity to particle clustering. In the range of particle morphological characteristics studied, the random distributions mere found to exhibit a consistent value of COV(d(mean)) equal to 0.36 +/- 0.02. The degree of inhomogeneity of any given distribution may then be evaluated by simply comparing the measured COV(d(mean)) with this value. MST/4568. (C) 2000 IoM Communications Ltd.

It is well established that the clustering of failure initiating particles may limit fatigue performance, particularly where extensive multiple crack formation and coalescence occurs. compromising conventional 'dominant crack' lifing approaches. To address this, detailed tessellation approaches have been developed, which extend the applicability of Dirichlet tessellation to systems containing any shape/size range of secondary phases. Austempered ductile iron (ADI), represents an important 'inverse composite' system containing mechanically void-like graphite: particles, which is now being used in camshaft manufacture. Although contact fatigue is the dominant service problem, an assessment of short crack initiation and growth provides a ranking for optimisation of a fatigue resistant microstructure. Fatigue evaluation of high hardness austempered ductile iron (ADI) has established the role of graphite nodule clustering and size in crack initiation.

The concept of a Dirichlet tessellation has been extended to that of a 'finite body' tessellation to provide a more meaningful description of the spatial distribution of non-spherical secondary phase bodies on two-dimensional sections. A finite body tessellation consists of a network of cells constructed from the interfaces of each individual secondary phase body such that every point within a cell is closer to the corresponding body than to any other. Spatial distribution related cell characteristics derived from Dirichlet tessellations have been extended to finite body tessellations. Quantitative comparisons between the two methods indicate that finite body tessellation measurements are more physically representative as well as more sensitive to local distribution characteristics of secondary phases. To reflect the potential application of finite body tessellations, a methodology is described for analysing the effects of particle distribution and morphology on short crack behaviour in particulate reinforced metal matrix composites.

Ceramic reinforcements are known to have a marked effect on fatigue in Al-based materials. Critical physical processes include localized load transfer mechanics and associated stress/strain concentrations, the presence of matrix/reinforcement differential thermal contraction stresses and the modification of matrix microstructure by the presence of ceramic reinforcements (e.g. dislocation densities and precipitation characteristics). The extent to which reinforcements may affect crack growth will depend on a range of materials parameters, including component phase elastic moduli, thermal expansivities and strength levels, interfacial strength, reinforcement morphology (size, aspect ratios, volume fraction), and the spatial distribution of the reinforcement. The identification of Dirichlet cells around individual secondary phase bodies, such that every point within the cell is closer to the centroid of the associated body than any other, has been previously identified as a uniquely powerful analysis tool in describing distribution characteristics on a particle-by-particle basis. In a separate publication by the authors, such a tessellation process has been extended to finite bodies to provide a more physically meaningful description of non-spherical secondary phase bodies exhibiting a wide size range. A finite body tessellation consists of a network of cells such that every point within a cell is closer to the interface of themore » corresponding body than to any other. In an initial investigation of the effects of particle distribution on fatigue failure processes of MMCs, crack propagation associated with the short crack regime in an aluminum-based metal matrix composite has been analyzed quantitatively using finite body tessellations. The main aim of this paper is to illustrate the value of tessellation analysis for the study of crack tip interactions in particle reinforced materials.« less