First-principles demonstration of band filling-induced significant improvement in thermodynamic stability and mechanical properties of Sc1-xTaXB2 solid solutions

Mixtures of different metal diborides in the form of solid solutions are promising materials for hard-coating applications. Herein, we study the mixing thermodynamics and the mechanical properties of AlB2-structured Sc1-xTaXB2 solid solutions using the first-principles method, based on the density functional theory, and the cluster-expansion formalism. Our thermodynamic investigation reveals that the two diborides readily mix with one another to form a continuous series of stable solid solutions in the pseudo-binary TaB2 -ScB2 system even at absolute zero. Interestingly, the elastic moduli as well as the hardness of the solid solutions show significant positive deviations from the linear Vegard's rule evaluated between those of ScB2 and TaB2. In case of Sc1-xTaXB2, the degrees of deviation from such linear trends can be as large as 25, 20, and 40% for the shear modulus, the Young's modulus, and the hardness, respectively. The improvement in the stability as well as the mechanical properties of Sc1-xTaXB2 solid solutions relative to their constituent compounds is found to be related to the effect of electronic band filling, induced upon mixing TaB2 with ScB2. These findings not only demonstrate the prominent role of band filling in enhancing the stability and the mechanical properties of Sc1-xTaXB2, but also it can potentially open up a possibility for designing stable/metastable metal diboride-based solid solutions with superior and widely tunable mechanical properties for hard-coating applications.

Automated summary

In this study, the mixing thermodynamics and mechanical properties of AlB2-structured Sc1-xTaXB2 solid solutions were investigated using first-principles calculations. The results showed that TaB2 and ScB2 readily mix to form a continuous series of stable solid solutions, with significant positive deviations in the elastic moduli and hardness compared to the linear Vegard's rule. The improved stability and mechanical properties of Sc1-xTaXB2 solid solutions were attributed to the effect of electronic band filling induced by mixing TaB2 with ScB2.