Strong high-entropy diboride ceramics with oxide impurities at 1800°C

In this study, a type of high-entropy boride (HEB) ceramics ((Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)B-2) with similar to 2 vol% oxides and similar to 1 vol% porosity was successfully consolidated by spark plasma sintering at 2000 degrees C under a uniaxial load of 50 MPa for 10 min, using self-synthesized high entropy diboride powders from a boro/carbothermal reduction approach. The residual oxides were determined to be m(Hf,Zr)O-2, which were incorporated with minor amounts of boron and carbon. Elastic modulus, Vickers hardness and fracture toughness of HEBs at room temperature were 508.5 GPa, 17.7 +/- 0.4 GPa and 4.2 +/- 0.2 MPa m(1/2), respectively. The as-obtained ceramics possessed excellent flexural strength, particularly at high temperatures. The four-point flexural strengths of HEBs at room temperature, 1600, and 1800 degrees C are 400.4 +/- 47.0, 695.9 +/- 55.9, and 751.6 +/- 23.2 MPa, respectively. Postmortem analysis was conducted on HEB fractured at 1800 degrees C, in the region near their tensile and fracture surfaces, and microstructure observations show that limited dislocation lines were present adjacent to the crack front and edge of the pore, without noticing any trace for dislocation motions. It is the first report on the high-temperature strength of high-entropy diboride ceramics and the strengthening mechanism at high temperatures was also discussed.