Laser Melting of Zirconium Carbide: Determination of Phase Transitions in Refractory Ceramic Systems

Pulsed laser heating and optical pyrometry were used to generate time-temperature thermogram data suitable for the determination of extremely high-temperature (>43000 K) solidus, liquidus, and eutectic transitions for ceramics in the Zr-C system. Transition temperatures correlated well with phase boundaries and individual measurements published previously. Microstructural and diffraction analysis of melted specimens confirmed that ZrC existed in the liquid phase and resolidified to ZrC or a ZrC+graphite eutectic. Transition temperatures were insensitive to laser pulse timescale and repeated melting, and microstructures of melted surfaces were consistent with the phase diagram, indicating the local attainment of thermodynamic equilibrium. Subsurface nonequilibrium microstructures were attributed to thermal gradients with depth and solute partitioning during freezing. The present work indicates that pulsed laser heating is a viable technique for producing equilibrium microstructures in ZrC as a prerequisite for precision measurement of phase transition temperatures. The main source of uncertainty in reported temperatures was the estimation of ZrC emittance. A consistently observed discontinuous temperature decrease upon the solid-liquid phase transition indicated a decrease in the emittance of liquid ZrC with respect to solid ZrC. Based on an estimated emittance of solid ZrC of 0.6, emittance of liquid ZrC was estimated at 0.44-0.58.