Effect of Nb content on the phase composition, densification, microstructure, and mechanical properties of high-entropy boride ceramics

Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B-2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from similar to 100 % to similar to 99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 +/- 5.5 mu m for the composition without Nb additions to 5.2 +/- 2.0 mu m for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young's modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions.

Automated summary

Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B-2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction showed a single-phase with hexagonal structure in compositions without Nb, while compositions containing Nb had two phases with slightly different lattice parameters. The addition of Nb resulted in a Nb-rich second phase, increased as Nb content increased, leading to decreased grain size and increased mechanical properties.