Effect of C on the phase state and microstructure of high‐entropy diborides produced via reactive spark plasma sintering

Abstract One‐step synthesis of high‐entropy diboride (HEB) samples from oxide‐based precursors is reported in this study. HEB samples (Hf _0.2 Nb _0.2 Ta _0.2 Ti _0.2 Zr _0.2 )B ₂ are prepared from metal oxide starting powders using reactive spark plasma sintering (R‐SPS) to conduct borocarbothermal reduction (BCTR) and sample densification in an individual high‐temperature processing unit. Excess precursor C black is added to the HEB precursor powders to aid in reducing residual oxygen in the final samples. The effect of precursor C black concentration on the phase state and microstructure of HEB samples is characterized. This study demonstrates that the conversion from metal oxide powder to the HEB through BCTR depends on the precursor C black concentration, which influences the secondary phase formation and microstructural features. The results reveal that 3.9 wt.% C black provides the optimal balance of oxide‐to‐HEB conversion, a more homogeneous microstructure, and intermediate grain sizes than HEB samples with both lower and higher amounts of precursor C black (0, 1.8, and 7.5 wt.%). We demonstrate the production of HEB samples from inexpensive oxide‐based precursor powders through BCTR and sample densification using an individual high‐temperature processing unit via R‐SPS.