PHASE-EQUILIBRIA EFFECTS ON THE ENHANCED LIQUID-PHASE SINTERING OF TUNGSTEN-COPPER

The sintering behavior and mechanical properties of W-Cu are improved by the addition of elements that have solubility for W, e.g., Co, Ni, Fe, and Pd. The degree of enhancement with small concentrations of add itive is dependent on specific phase diagram features, and the ranking of effectiveness does not follow the trend observed for the activated solid-state sintering of W. These observations are explained through a combination of liquid phase sintering and activated sintering theori es that considers the combined W, Cu, and activator phase equilibria e ffects. In small concentrations, Ni and Pd have little effect on densi fication because they go into solution with Cu, resulting in only a sl ight increase in the solubility of W in the liquid phase. In this case , the sintered density, strength, and hardness increase with increasin g additive concentration due to enhanced densification through solutio n-reprecipitation. Cobalt and Fe are the most effective activators due to their limited solubility in Cu and the formation of a stable inter metallic phase with W at the sintering temperature. This promotes the formation of a high-diffusivity interboundary layer which enhances sol id-state sintering of the tungsten grains at temperatures at which a l iquid phase is present. With Co and Fe additions, the sintered density , strength, and hardness peak with activator concentrations of 0.35 to 0.5 wt pct. An evaluation of models for activated solid-state sinteri ng and liquid phase sintering indicates a substantial solid-state cont ribution to densification when a high-diffusivity interboundary layer is present and the solubility of W in the liquid phase is small.