Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

Equimolar quinary diboride powders, with nominal composition of (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B-2, were synthesized by boro/carbothermal reduction (BCTR) of oxide mixtures (MOx, M = Ti, Hf, Zr, Nb and Ta) using B4C as source of B and C in vacuum. By adjusting the B4C/MOx ratios, diboride mixtures without detectable MO, were obtained at 1600 degrees C, while high-entropy diboride (HEB) powders with particle size of < 1 mu m was obtained at 1800 degrees C. The phase, morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated. Although X-ray diffraction pattern indicated the powders synthesized at 1800 degrees C were in a single-phase AlB2 structure, elemental mappings showed that (Ta, Ti)-rich and (Zr, Nb)-rich solid solution coexisted in the HEB powders. The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000 degrees C. (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B-2 ceramics with a relative density of 97.9% were obtained after spark plasma sintering the HEB powders at 2050 degrees C under 50 MPa. Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050 degrees C, and the averaged grain size increased from 6.67 to 41.2 mu m. HEB ceramics sintered at 2000 degrees C had a Vickers hardness of 22.44 +/- 0.56 GPa (under a load of 1 kg), a Young's modulus of similar to 500 GPa and a fracture toughness of 2.83 +/- 0.15 MPa m(1/2). This is the first report for obtaining high density HEB ceramics without residual oxide phase, benefiting from the high quality HEB powders obtained.