Grain coalescence in (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C during spark plasma sintering

High-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C ceramics (HEC) are fabricated via spark plasma sintering using different die configurations, including the conductive and insulating dies. Compared to the conductive die, the grain sizes of samples sintered in the insulating die are significantly larger, which is attributed to the higher local temperature as a result of the higher current density in the sample. Furthermore, the microstructure evolution and grain growth mechanism of HEC are investigated for the first time. We find that at moderate temperatures (similar to 1600 degrees C), the grain growth of HEC can occur by a grain coalescence mechanism, forming numerous irregular grains in the porous sample. Three factors are crucial to induce grain coalescence, including the formation of partial melting layers on particle surfaces, nanograin rearrangement via rotation and sliding, and the formation of low-angle grain boundaries. During the final sintering stage, the irregular grains will change into polyhedral shapes by grain boundary migration. These findings are of assistance to better understand and control the microstructure evolution of HEC and other ultrahigh-temperature carbide ceramics.