Microstructural-Mechanical Property Relationships in WC-Co composites

While empirical relationships between the mechanical properties of WC-Cocomposites and the carbide grain size and carbide volume fraction are qualitatively well-known, the influence of other microstructural features on the macroscopic properties hasbeen less clear. In this thesis, a comprehensive study of the effect of the interfacecharacter distribution (grain shape and misorientation distribution of WC crystals), grainsize, size distribution, contiguity, and carbide volume fraction on the mechanicalproperties (fracture strength) of WC-Co composites is described. To do this, methodswere developed to accurately measure microstructural features, characterize interfaces,and predict mechanical properties.Atomic force microscopy (AFM) and orientation imaging microscopy (OIM)have been used to determine the WC/WC boundary and WC/Co interface positions inWC-Co composites with Co volume fractions from 10 % to 30 % and mean carbide grainsizes from 1 to 7 microns. Direct measurements of the carbide and cobalt area fractionsshow that the log-normal distributions of carbide grain size and binder mean free pathdistributions are Gaussian and that there is a very strong linear correlation between thevolume fraction of the binder and the contiguity of carbide phase. The number of verticesper carbide grain is nearly constant and it is weakly influenced by the Co volume fraction.Empirical expressions involving the grain size and carbide contiguity fit well to measuredhardness and fracture toughness data.Stereological analysis of the carbide grain shapes shows that WC particles inthese materials have similar distributions of habit planes, with {101 0} prism facets and the {0001} basal planes in contact with Co. The orientation imaging microscopy (OIM)measurements demonstrate an absence of orientation texture and that the carbide grainboundaries have similar misorientation distributions, with a high population grainboundaries that have a 90° twist misorientation about the [101 0] axis and 30° twist andasymmetric misorientations about the [0001] axis.Using a two-dimensional finite element analysis (FEM) of the stress-strain field inthese materials, a brittle fracture model for the prediction of fracture strength has beendeveloped. The model assumes that a crack initiates either along carbide/carbideboundaries or in carbide grains. When the fracture energy is ~49 J/m, the calculatedfracture strength under a combined (thermal and mechanical) load shows good agreementwith the experimentally measured data. The calibrated model was then applied to thehypothetical microstructures of WC-Co composites. The degree of carbide connectivity(contiguity), orientation and/or misorientation texture appear to be the most importantparameter for improving the fracture strength of these materials.