Microstructure of laser-melted zirconium carbide ceramics

Zirconium carbide (ZrC) is a candidate material for advanced nuclear fuels for high-temperature gas-cooled reactors, but its durability and performance under irradiation is still under investigation. In particular, the effect of carbon vacancies on thermodynamic, mechanical, and heat transport properties of ZrC must be better understood. As part of a novel thermal analysis technique, melting of ceramics in the Zr-C system was investigated via pulsed laser heating. Ceramics were prepared by spark plasma sintering of commercial powders or powders derived from carbothermal reduction of zirconia. Laser pulses (400-3600 ms) heated samples above their liquidus temperatures (3200-3700 K) up to 4000 K. Dendritic microstructure observed on laser-heated surfaces confirmed the formation of liquid. The laser-modified microstructure consisted of single-phase ZrC and lamellar ZrC+C eutectic, in proportions consistent with the equilibrium phase diagram. Non-equilibrium distribution of phases prevailed in subsurface regions of samples undergoing non-isothermal solidification. Porosity, generated by impurities and coalesced by material flow, existed throughout the heat-affected region.