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The strong covalent bonds and low self-diffusion coefficients of high-entropy diboride (HEBs) and diboride-carbide (HEB-HEC) ceramics render their densification difficult, which limits their applications. Here, we used an ultra-fast high-temperature sintering technique to synthesize and densify several HEBs and HEB-HEC (containing 5–8 cations) ceramics within an isothermal duration of 1 min and a total sintering period of 6 min. All ceramics formed a single-phase or dual-phase high-entropy solid solution, and showed high density, high hardness and good fracture toughness. The rapid densification mechanisms were determined, and the preferential occupancy of metal cations in HEB and HEC phases was explained using a simplified ideal solution model. A good fracture toughness in the range of 3.6–5.4 MPa·m1/2 resulted from crack deflection or blunting and bridging of the rod-like diboride or boron-rich phase. This paper presents a simple, economical, and efficient method for the synthesis and densification of high-entropy ceramics and other advanced materials. [ABSTRACT FROM AUTHOR]

High-entropy nitride and carbonitride ceramics have received wide attention for their excellent properties such as high hardness, high melting point, and high wear resistance, but the susceptibility of nitrides to decomposition at high sintering temperatures has rendered their densification challenging. In this work, four multicomponent nitrides and five multicomponent carbonitrides (containing five to seven cations) were prepared through ultrafast high-temperature sintering. While only (Cr 1/7 Zr 1/7 Nb 1/7 Hf 1/7 Ta 1/7 Ti 1/7 V 1/7)N formed a single phase among the nitrides, all carbonitride systems formed a single-phase high-entropy solid solution with a relative density exceeding 92%. The polyanionic structure of carbonitrides is responsible for their high configurational entropy, which in turn results in their high solid solubility. Although carbonitrides showed higher hardness and modulus than nitrides with the same cations, their fracture toughness was lower. Among carbonitrides with different C/N ratios, the system with a C/N ratio of 5:5 showed the highest solid solubility and best overall mechanical properties. [ABSTRACT FROM AUTHOR]

• Nearly fully dense HEPOs were prepared by RFS at a low furnace temperature in seconds. • The RFSed-HEPOs displayed compositional uniformity at the atomic level. • The roles of electric field, rapid heating and external pressure during RFS were discussed. • The RFSed-HEPOs exhibited superior modulus, hardness and fracture toughness. Pyrochlore-type high-entropy oxides (HEOs) are usually sintered at high temperatures for a long time to achieve full density. Herein, we synthesized pyrochlore-structured (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 2 O 7 HEOs with densities up to 99 % at a furnace temperature of 1200 °C in seconds via reactive flash sintering (RFS). The resultant HEOs achieved compositional uniformity at the atomic level and exhibited superior modulus, hardness and fracture toughness compared to the counterparts prepared by conventional solid-state sintering (at 1600 °C for 6 h). The underlying mechanisms for the ultrafast densification of the RFSed-HEOs were addressed in view of the roles of electric field, rapid heating, external pressure and internal reactions. [ABSTRACT FROM AUTHOR]