WEB Stable and metastable structures of atomic defects in transition metal carbidesWednesday (01.01.2020) 00:30 - 00:45 Part of:
Transition-metal (TM) carbides combine ceramic-like properties and metal-like properties, which is desirable for many existing and emerging applications of these compounds. Ab inito calculations give us the possibility to compute the energies of various atomic defects and defect arrangements, as well as to simulate their dynamics and migration through the lattice [1-3]. This information is essential for modeling the structure and property evolution of carbide materials during their manufacturing and service. With the goal to revisit the diffusion mechanisms, we study the structure and dynamics of various vacancy-based point defects in Group 4 and 5 TM carbides. We find a new (symmetry-broken) configuration of a metal vacancy in TiC that is twice as low in energy as the usually considered symmetric configuration. We explore the configurational space of a metal vacancies in TM carbides using molecular statics simulations to locate a number of other metastable structures of carbon atoms inside the vacancies. All these structures are stabilized by short C–C bonds that are absent in the perfect carbide structure. Ab initio molecular dynamics simulations reveal that these carbon-bonded structures inside a metal vacancy in TiC become highly dynamic at high temperature, giving rise to a high vacancy formation entropy, which is essential for describing Ti self-diffusion in TiC by the monovacancy mechanism. Results of similar simulations for other TM carbides are also reported.
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2. W. Sun, H. Ehteshami, and P. Korzhavyi, Phys. Rev. B 91, 134111 (2015).
3. W. Sun, H. Ehteshami, P. R. C. Kent, and P. Korzhavyi, Acta Mater. 165, 381-387 (2019).