Enthalpy driving force and chemical bond weakening: The solid-solution formation mechanism and densification behavior of high-entropy diborides (Hf1−x/4Zr1−x/4Nb1−x/4Ta1−x/4Scx)B2

Abstract

(Hf0.2Zr0.2Nb0.2Ta0.2Sc0.2)B2 was designed to improve the densification and solid-solution formation of high-entropy transition metal diborides, and its phase stability was predicted using the energy distribution of the local mixing enthalpy of all possible configurations. It was found that (Hf0.2Zr0.2Nb0.2Ta0.2Sc0.2)B2 are enthalpy-stabilized materials. The two-component metal diborides formed by transition metal diborides (HfB2, ZrB2, TaB2 and NbB2) with ScB2 are thermodynamically favorable, based on the mixing enthalpy. Therefore, the introduction of ScB2 in high-entropy metal diborides is beneficial to reduce the mixing Gibbs free energy during the boro/carbothermal reduction process, which enables the formation of single-phase solid solution at low temperatures. Even high-entropy metal diboride powders with large particle sizes, 25–57 µm, can achieve sintered density up to ~97% due to the introduction of ScB2 in high-entropy metal diborides, owing to its weakening action on the TM d - B p and the TM dd bonding.