Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering

Abstract

High-entropy carbide ceramics (Ti0.2Hf0.2Nb0.2Ta0.2W0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the single-phase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of high-entropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.