First-principles prediction, fabrication and characterization of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 high-entropy borides

The microstructure and mechanical properties of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B4C on microstructure and mechanical properties were evaluated. Due to the addition of B4C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B4C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m1/2. Nonetheless, 10 wt% excess of B4C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B4C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics.