Microstructure and Properties of Metal Matrix Composites

Research project P 14333 Microstrukture and Properties of Metal Matrix Composites Otmar KOLEDNIK 06.03.2000 The aim of the project is to improve the basic physical understanding of particle reinforced metal matrix composites (MMCs) and to find general relations between the architecture of the composites and their mechanical properties. It is important that both the architectural parameters and the mechanical properties will be considered locally. The most important architectural parameters are the particle volume fraction, the scaling relations between the particles and the matrix, the spatial distribution of particles in the matrix, and the defects and inhomogeneities in the composite structure. The basic material for the investigation will be a powder metallurgy AMC with Al 6061 as the matrix material and SiC particles with a great variety in size and volume fraction; Al 6061 reinforced with A1203 particles, and an ultra fine grained MMC will be also considered. A set of special experimental techniques, part of which developed recently, enables us to investigate the local mechanical behavior of NMCs in a way that has not been possible before: The local deformation behavior will be investigated applying m-situ experiments in the scanning electron microscope, automatic local deformation analyses, and orientation image microscopy. By in-situ neutron difliraction experiments, the mean elastic residual stresses in the particles and in the matrix in the undeformed condition will be measured, as well as the changes of the stress distribution during the plastic deformation. Transmission electron microscopy will be applied to measure the local lattice strains and to study the dislocation structure at the particle-matrix interfaces, and to detect the first occurrence of interface or matrix failure . Automatic fracture surface analyses will enable us to determine the local fracture toughness in regions with different particle configurations. From these experiments, general criteria will be deduced how the local composite architecture influences the local deformation behavior and the local damage evolution, such as the appearence of inhomogeneous plastic deformation, the onset of micro-damage (particle cracking, interfacial debonding, or void formation within the matrix), crack initiation, and crack growth.

The main attractiveness of the application of metal matrix composites (MMCs) in engineering are the weight reduction and the associated environmental and economic advantages, especially in the transportation industry. The primary obstacles against the widespread use of MMCs are their low ductility and fracture toughness. The aim of the project was, therefore, to improve the basic physical understanding of the deformation and fracture behavior of particle reinforced MMCs. The novelty was to consider both the architectural parameters of the material and the deformation and fracture properties locally. The local fracture properties were deduced from the shape of corresponding fracture surface regions of broken specimens which were analyzed by a system for automatic fracture surface analysis. A new procedure was developed where the results of the fracture surface analysis were combined with analytical computations to estimate the maximum stress at the moment of particle fracture for individual particles in front of the crack tip. The local deformation behavior was investigated by in-situ experiments in a scanning electron microscope. At different stages of loading, images were taken and processed by a computer program in order to generate the local in-plane strain and rotation fields at the different loading stages. This procedure was used to study in detail the very onset of plastic deformation, the formation of shear bands, and the evolution of damage in MMCs with different particle volume content, particle sizes, and heat treatments. The pure matrix material was also investigated. It was found that the global ductility of the materials decrease with increasing inhomogeneity of the local deformation. For a high ductility, the formation of far-reaching damage induced shear bands should be avoided which have a very high local strain. Such bands are induced by the fracture of large particles, but also by clusters of much smaller particles. The investigations have also lead to a new possibility for the improvement of the homogeneity of MMCs with small particles, which suffer especially from the presence of particle clusters, by severe plastic deformation.