Influence of chemical disorder on atomic structure in high-entropy diborides

Conference Dates

September 17-20, 2017


Density functional theory (DFT) calculations were performed on a set of high-entropy metal diborides composed of five equimolar transition metals in the layered hexagonal AlB2 structure. Atomic structure data was explored and related to that of experimentally synthesized bulk samples of this new class of ultra-high temperature ceramics. Charge disorder and lattice distortions of the relaxed structures were measured and compared between compositions. Interactions between near-neighbor atom pairs were analyzed to explore the effects of constituent elements on the local atomic structure. The high-entropy compositions allow for the incorporation of Mo into the AlB2 structure where it is typically not stable, as well as allowing for Cr concentrations well above the low solubility limit in conventional early transition metal diborides. The presence of these group six elements in certain compositions creates large lattice distortions within a stable single phase structure.

Atom pair interactions were further explored by the introduction of vacancies in the structure. Vacancy formation energies were calculated by DFT methods for lattice sites with varying chemical coordination. Preferential vacancy configurations were examined as well as possible effects of atom pair interactions on short-range ordering of elements. Unexpected diffusion behavior observed in high temperature oxidation experiments was explored as it relates to vacancy configurations and vacancy mediated self-diffusion in high-entropy diborides.

This work is supported by the U.S. Office of Naval Research MURI program (grant No. N00014-15- 1-2863).

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