Phase structure, mechanical properties and thermal properties of high-entropy diboride (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂

A new high-entropy diboride (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂ was designed to investigate the effect of introducing rare-earth metal diboride ScB₂ into high-entropy diborides on its structure and properties. The local mixing enthalpy predicts that (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂ has high enthalpy driving force, which more easily allows the formation of single-phase AlB₂-type structures between components. The experiments further demonstrate that (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂ possesses excellent phase stability, lattice integrity and nanoscale chemical homogeneity. (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂ showed relatively high hardness (30.7 GPa), elastic modulus (E, G, and B of 522, 231 and 233 GPa, respectively), bending strength (454 MPa), and low thermal conductivity (13.9 W·m⁻¹·K⁻¹). The thermal expansion of (Hf_0.25Zr_0.25Ta_0.25Sc_0.25)B₂ is higher than that of ZrB₂ and HfB₂ due to weakened bonding (M d - B p and M dd bonding) and enhanced anharmonic effects. Thus, incorporating Sc into high-entropy diborides can tailor the properties associated with the bonding, which further expands the compositional space of high-entropy diborides.