MICROSTRUCTURAL ASPECTS OF THE STRENGTH AND HARDNESS OF CEMENTED TUNGSTEN CARBIDE

The balance of hardness and strength in sintered tungsten carbide- cobalt alloys is obtained by adjusting cobalt content and particle size of WC. The variations of hardness and strength with composition and particle size are discussed in terms of the mechanical interaction of the hard and ductile phases. The observed increase of hardness and yield strength with decreasing cobalt content and decreasing particle size is attributed to the plastic constraint of the cobalt layers in contact with the tungsten carbide. The plastic constraint also causes a transition between ductile and brittle modes of fracture, which, in turn, accounts for the observed maximum of fracture strength at small values of the carbide spacing. The theories of fracture pertaining to compositions of commercial alloys are based on the Griffith-Orowan equation, the parameters of which have been empirically related to the microstructure. Further progress in the design of WC-Co alloys can be achieved by control of the microstructure and the potential improvement of mechanical properties obtainable by the control of a number of variables is briefly mentioned.