Processing and microstructure development of reactive sintered (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy ceramics

Abstract

A novel dual-phase (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy, compositionally complex composite was synthesized by reactive spark plasma sintering using commercial ZrB2, NbB2, HfB2, TaB2 and TiC powders as starting materials. The composite with high density, consists predominantly of boride (~39.7 vol.%) and carbide (~56.4 vol.%) phases and with residual amount of (Hf-Zr)O2. The microstructure is homogenous and relatively fine due to the reaction and solid solution coupling effect with average grain sizes of 3.4 μm and 2.2 μm of the boride and carbide phases, respectively. Large-area SEM analyses confirmed no microcracks or micropore formation and no presence of large grains or clusters. Investigation from micro to nano and atomic level, focused on local chemical composition and spatial distribution of constituent elements, revealed that Ti preferentially dissolves into the boride and all other elements to the carbide phase creating a (Ti0.82Zr0.04Nb0.08Hf0.03Ta0.03)B2 + (Ti0.49Zr0.12Nb0.13Hf0.11Ta0.15)C system. The analysis of grain and phase boundary interfaces revealed a continuous, <1.5nm wide, segregation of impurity atoms of Fe and Co. Continuous (>20nm) and atomically flat basal interfacial termination planes (001) decorated with monoatomic Zr enriched layer in the boride phase were observed. Triple points at grain and phase boundaries exhibit typical chemical composition rich in Si (40at.%), Co (35at.%), Fe (20at.%), and Ni (5at.%).