The phase composition, microstructure, and mechanical properties of the WB2–B4C composites fabricated by a combination of boro/carbothermal reduction and spark plasma sintering (SPS) method with WO3, B4C, and graphite as raw materials were investigated in this study. The experimental results showed that the relative density of the as‐sintered WB2–B4C composites was ∼93.1% and ∼99.5%, respectively, after being SPS sintered at 1600°C under the applied load of 30 MPa for 10 min. Scanning electron microscope analysis showed that a network structure with WB2 grains surrounded by B4C grains was observed after sintering. Analyses of high‐resolution TEM showed semi‐coherent interface and lattice distortion transition region between WB2 and B4C grains. The Vickers hardness of WB2–B4C composite increased to 22.3 ± 0.9 GPa at 9.8 N owing to the fully dense, solid solution of C, and three‐dimensional network structure. Moreover, the fracture toughness and flexural strength of WB2–B4C composite reach 6.04 ± 0.81 MPa m1/2 and 750 ± 80 MPa, respectively, which could be attributed to the semi‐coherent interface between WB2 and B4C grains.
The thermal stability of metastable ReB2‐type transition metal diborides (TMB2), which are considered as new type of superhard material, is of vital importance to obtain bulk samples. In the present work, thermal stability of four kinds of ReB2‐type TMB2 powders, ReB2, OsB2, Os1–xRexB2, and Os1‐xWxB2, were synthesized with varied transition metal (TM)‐to‐B molar ratio by mechanochemical methods and the subsequent annealing was compared. The as‐synthesized powders were then consolidated using a pressureless sintering technique. The results showed that the B content required to obtain the pure hexagonal ReB2‐type Os1–x(TM)xB2 phase varied, which indicated different thermal stabilities, such as OsB2 < Os0.1W0.1B2 < Os0.9Re0.1B2 < Os0.8W0.2B2 < ReB2 < Os0.6W0.4B2 and Os0.5W0.5B2. Among them, Os0.6W0.4B2 and Os0.5W0.5B2 were found to be relatively thermally stable and could be synthesized with a stoichiometric molar ratio of (Os + W):B = 1:2. It was also found that the thermal stability of TMB2 with a hexagonal ReB2 structure could be mainly governed by the length of lattice constant c. The results have guiding significance for the design and preparation of new type of TM borides. In addition, the hardness of TMB2 can be increased by tailoring the B content in the raw materials more precisely.
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