Structure-property relations and homogeneity of tungsten carbide-cobalt composites

Bulk mechanical properties of particulate reinforced composites are greatly influenced by their microstructure. Besides the grain size, volume fraction, and contiguity of the reinforcing brittle particulate, the crack path in the composite is of critical importance. To explain the dependence of mechanical properties on the microstructure, the above parameters need to be treated in combination, since no feature is capable of explaining the relation individually.; This thesis is focused on developing relations between bulk mechanical properties such as hardness and fracture toughness of particulate reinforced composites, and their microstructure. By isolating the micromechanism of failure in tungsten carbide-cobalt composite, it is shown that the plastic deformation in the cobalt matrix and microcracks in the WC grains influence the failure of the material. The mechanical properties have been explained in terms of a single structural parameter S_c, which combines the effect of the grain size, volume fraction, intergrain distance, contiguity of WC, and crack path in the composite. The model predicts that the fracture toughness increases with WC grain size and cobalt volume fraction, while the hardness decreases. The unique dependence between the mechanical properties and S_c has been exploited to create optimization charts, which provides guidelines for suitable selection of grain size and volume fraction of the carbide grains to achieve required mechanical properties.; Carbide clusters are potential sites for crack initiation. A microstructural-based reliability model is developed based on the cluster size distribution. This model allows the reliability of the composite to be microstructurally determined. This is a potent tool as it is independent of the testing conditions, which significantly influences the measured properties of a composite. This model predicts a higher reliability for WC-Co composite made of powders, which were initially milled for 24 hour than that milled for 12, 6, and 2 hours. This result is consistent with experimentally determined fracture toughness values for the composite which ranged from 16.9 to 19.9 MPa m1/2.