Abstract
Transition metal diborides are ceramic materials with potential applications as hard protective thin films and electrical contact materials. We investigate the possibility to obtain age hardening through isostructural clustering, including spinodal decomposition, or ordering-induced precipitation in ternary diboride alloys. By means of first-principles mixing thermodynamics calculations, 45 ternary M11–xM2xB2 alloys comprising MiB2 (Mi = Mg, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta) with AlB2 type structure are studied. In particular Al1–xTixB2 is found to be of interest for coherent isostructural decomposition with a strong driving force for phase separation, while having almost concentration independent a and c lattice parameters. The results are explained by revealing the nature of the electronic structure in these alloys, and in particular, the origin of the pseudogap at EF in TiB2, ZrB2, and HfB2.
Highlights
As the field of thin film diborides for coatings applications is opening up, alloying is a natural step for property enhancement
Only few studies have investigated the electronic structure of ternary diborides 25 and there are outstanding questions for binary diborides appreciating considerable theoretical investigations 26–28
In this work we start by a first-principles scan of the mixing thermodynamics of all the 45 alloy systems formed by all M11−xM2xB2 combinations of the ten binary diborides MgB2, AlB2, ScB2, YB2, TiB2, ZrB2, HfB2, VB2, NbB2, and TaB2, all reported to crystalize in the AlB2 type structure
Summary
As an illustrative example of a temperature where hard coatings are used, the mixing trends at 1000 °C are derived for all considered 45 ternary systems. The resulting nanostructure in the lattice can decrease dislocation mobility and increase hardness For this reason, the alloys Al1–xTixB2, Al1–xVxB2, and Mg1–xHfxB2 deserves further attention as they are predicted to display clustering and a small volume misfit as can be seen in the Fig. 1 matrix. Al1–xTixB2 is an almost perfectly lattice matched system, as can be seen, but displays a strong driving force for phase separation in terms of a positive mixing enthalpy and mixing free energy at 1000 °C, in particular in the AlB2-rich compositions This is a very similar situation to the well studied Ti1–xAlxN alloys, where electronic structure effects have been found to be the origin of spinodal decomposition. Almost precisely two out of four Ti valence electrons occupy the
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