Vacancies and substitutional defects in multicomponent metal diborides

Result code in IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F49777513%3A23520%2F22%3A43965856" target="_blank" >RIV/49777513:23520/22:43965856 - isvavai.cz</a>

Result on the web

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DOI - Digital Object Identifier

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Result language

angličtina

Original language name

Vacancies and substitutional defects in multicomponent metal diborides

Original language description

The effect of defects in hard and electrically conductive multicomponent metal diborides with high thermal stability on material characteristics is studied by ab-initio calculations. The defects include either vacancies or substitutional carbon atoms at boron sites (both relevant for numerous experiments including our own), and the composition Ti0.25Zr0.25Hf0.25Ta0.25B2 is used as a primary test case. Although both vacancies and carbon substitutions are found to be thermodynamically unfavorable, non-equilibrium material preparation techniques readily lead to their production. Various concentrations and distributions of such defects in the boron sublattice are investigated, and their effects on the formation energy and packing factor of the structures are identified. We show that the replacement of B with C is even more unfavorable than the formation of B vacancies. On the one hand, boron vacancies prefer to coalesce into a larger planar void, minimizing the number of broken B–B bonds. On the other hand, carbon substitutions at boron sites do not prefer coalescence and tend to occur far from each other, minimizing the number of formed C–C bonds. In both cases, however, the energetically preferred defect distribution minimizes the volume per atom. The predicted concentration of vacancies (stable at the moment of their formation at the melting temperature, metastable at the room temperature), obtained by minimizing the free energy (function of the configurational entropy) at the melting temperature, is around 1%. This is the lower bound of the actual concentration of vacancies resulting from using non-equilibrium preparation techniques under metal-rich conditions. We present a model which shows that once these distributed vacancies do not immediately heal but form a metastable planar void, the latter does not heal even though the corresponding formation energy is higher than that of a perfect crystal. Collectively, the achieved results explain experimental observations of planar voids in substoichiometric diborides and predict their functional properties. For example, the vacancy-containing Ti4Zr4Hf4Ta4B26 with the ratio of boron to metal equal to 1.625 exhibits higher metallicity and inferior mechanical properties (e.g., the predicted hardness is ≈ 40% lower) than the perfect defect-free Ti4Zr4Hf4Ta4B32.

Czech name

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Czech description

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Type

O - Miscellaneous

CEP classification

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OECD FORD branch

20506 - Coating and films

Project

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Continuities

S - Specificky vyzkum na vysokych skolach

Publication year

2022

Confidentiality

S - Úplné a pravdivé údaje o projektu nepodléhají ochraně podle zvláštních právních předpisů