The microstructure of cemented carbides produced by liquid-phase sintering.

A detailed study has been made of the microstructure of a number of cemented carbides, prepared by liquid-phase sintering. The alloys investigated were of TiC, ZrC, HfC., VC, NbC, TaC, Mo2C, WC, and certain combinations of these, all with cobalt as the binder phase metal. The dependence of the microstructures on sintering conditions and on physico-chemical properties of the alloy systems, has been studied. For this purpose, solubilities of certain carbides in liquid cobalt, and the interfacial energies between certain carbides and cobalt, have been measured. It is concluded that the carbide grain growth, which occurs during sintering, occurs by solution-reprecipitation processes rather than coalescence. Possible mechanisms of growth have been analysed theoretically. It appears that, in alloys of VC-Co and Mo2C-Co, the growth is controlled by diffusion through the liquid. For the other carbides, other rate-controlling mechanisms are involved. The growth behaviour of WG-Co alloys is untypical in relation to the other carbides. A second carbide, added to an alloy, usually inhibits the growth, especially if it is present as a separate phase. The cubic monocarbides develop a cubic grain shape during sintering. The degree of rounding of this shape is explained quantitatively in terras of the anisotropy of the interfacial energy. In all the alloys there is contact between the carbide grains. It is, shown that this develops during sintering, towards a state dependent on the relative values of the interfacial and carbide-boundary energies. With the exception of VC, all the carbides form a continuous skeleton in alloys of 20 vol% cobalt. It is shown that this is not necessarily inconsistent with densification processes. The hardness and relative toughness of the alloys have been measured. The relative effect of the microstructural properties on the hardness, has been evaluated empirically