Mechanism of Incongruent Reactions Between Zr-Cu Melts and Solid Tungsten Carbide

The incongruent reactions between Zr-Cu melts and WC solid were studied. Mo was used as a tracer to track the movement of “heavy” W atoms during the incongruent reaction. In separate experiments, dense polycrystalline plates and porous preforms of W_0.9Mo_0.1C were reacted with Zr₂Cu and Zr₁₄Cu₅₁ melts at 1200 °C, 1300 °C and 1600 °C for 15 minutes. The microstructure of the reactive interfaces was studied using scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The change of microstructure and chemical composition at the reactive interface was observed as a function of temperature, Zr content in the melt and radius of the W_0.9Mo_0.1C solid. When the Zr-Cu melts contact the W_0.9Mo_0.1C solid, W, Mo and C all dissolve in the melt. The precipitation of W and W₂Zr is based on Zr activity in the Zr-Cu melt after a quick formation of ZrC_x. Both the dissolution rate and the amount of WC solid increase with decreasing radius of the WC particles, increasing Zr content in the Zr-Cu melts, and increasing reaction temperature. If the dissolution rate of WC is fast enough and the WC particles are in small sizes, the dissolution-precipitation is the main aspect. If a continuous ZrC layer quickly forms to separate the WC solid from Zr-Cu melts. The reaction is, therefore, controlled by the diffusion of carbon through the W and/or ZrC layers.