Thermodynamic stability of in situ W–ZrC and W–Zr(CN) composites

Powders of W–ZrC and W–Zr(CN) were carbothermally synthesized in situ from milled mixtures of graphite, WO3 and ZrO2. The thermal stability of Zr(CN) in a W matrix was simulated and compared with that of ZrC in W in terms of free energy change and carbide coarsening. Carbon and nitrogen had high mutual affinity in Zr(CN) of B1 crystal structure, which led their activity curves to exhibit strong negative deviation from ideal mixing behavior. Zr(CN) was more stable than ZrC up to 2075 K; however, a microstructural study showed that it became less stable than ZrC at around 1975 K. This result is attributed to the decreasing thermodynamic stability of ZrN with increasing temperature. Other transition metal carbonitrides containing group 4–6 elements are expected to show similar coarsening behaviors at high temperatures. [Display omitted] •The Zr(CN) phase formed due to the high affinity between C and N in ZrC.•A complete reversal of the slope is found in the formation energy curves.•The growth of the carbonitride is due to the nitrogen, reducing the stability.•Solid solutions containing group 4 elements would show similar growth behavior.