Melting and phase transformation of hardmetal powders

Hardmetals are excellent tool materials that are made of WC–Co powder mixture by liquid phase sintering. The sintering occurs above 1320°C, the melting point of the eutectics formed by the components, and the melt acts as the binder phase for the WC grains. Efforts are made to reduce grain coarsening during heat treatment because fine-grained materials have considerably better mechanical properties. One possible way might be the use of powders with grain size ranging from 10 to 100 nm. When experiments were carried out with such powders, it was found that they have no well-defined melting point. Individual grains melt at different temperature values, sometimes much below the eutectic temperature given in the equilibrium phase diagram. Solidification also may occur below the equilibrium melting point resulting in different compositional variations. A possible explanation of this phenomenon is given based on the high specific surface of the ultrafine powders.