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

Abstract

Equimolar quinary diboride powders, with nominal composition of (Ti 0.2 Hf 0.2 Zr 0.2 Nb 0.2 Ta 0.2)B 2 , were synthesized by boro/carbothermal reduction (BCTR) of oxide mixtures (MO x , M = Ti, Hf, Zr, Nb and Ta) using B 4 C as source of B and C in vacuum. By adjusting the B 4 C/MO x ratios, diboride mixtures without detectable MO x were obtained at 1600°C, while high-entropy diboride (HEB) powders with particle size of < 1 μm was obtained at 1800°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°C were in a single-phase AlB 2 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°C. (Ti 0.2 Hf 0.2 Zr 0.2 Nb 0.2 Ta 0.2)B 2 ceramics with a relative density of 97.9% were obtained after spark plasma sintering the HEB powders at 2050°C under 50 MPa. Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050°C, and the averaged grain size increased from 6.67 to 41.2 μm. HEB ceramics sintered at 2000°C had a Vickers hardness of 22.44 ± 0.56 GPa (under a load of 1 kg), a Young's modulus of 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.

A new class of high-entropy M3B4 borides

“…In 2016, Gild et al [8] first reported the synthesis of single-phase high-entropy diborides in AlB 2 -prototyped hexagonal structure, which represents the first highentropy boride (HEB) fabricated. Follow-up studies synthesized the high-entropy diborides from boro/ carbothermal reduction of metal oxides [19][20][21] or elemental precursors [22][23][24]. Only a limited number of studies explored the HEBs with other metal-toboron stoichiometric ratios in monoboride (MB) or hexaboride (MB 6 ) structures [9,10,25].…”

High‐temperature oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics in air

“…It should be noted that the formed oxides may be the solid solutions of five metal elements but there are a small amount of some of metal elements in the formed oxides and they can be ignored. 20 In this case, the composition of the irregular particle phase and the molten-like phase (BSE) cross-sectional images ( Figure 4A,C,E), it can be found that the size and density of the holes in the generated oxide layers first increase moderately from 1573 to 1673 K and then increase sharply from 1673 to 1773 K. In other words, the generated oxide layer is dense at 1573 K but involves a large number of holes at 1773 K. It should also be noted that the formed oxide layer exhibits the interesting "dense-porous-dense" structure along the crosssectional direction at 1673 K. Meanwhile, no microcracks can be found in all the formed oxide layers. In addition, the thickness of the formed oxide layers at 1573 and 1673 K is summarized in Table 1.…”

Optimal Preparation of High-entropy Boride-silicon Carbide Ceramics

“…In recent years, high-entropy ultra-high temperature boride (HEB) ceramics have been extensively studied by combining the concepts of ultra-high temperature boride ceramic (UHTC) and high-entropy materials [1][2][3]. HEB ceramics were found to exhibit high hardness and superior oxidation resistance [4][5][6].…”