Atomic Scale Investigation of Defects in High-Performance Materials

Abstract

Transition metal carbides of groups 4 and 5 (TiC, ZrC, HfC, VC, NbC, TaC) with the rocksalt (B1) structure are critical refractory materials for extreme temperature applications due to their exceptional hardness, high melting points, and thermal stability. This high-temperature behavior governed by point defects and diffusion has long been preplexing, with experimental metal self-diffusion activation energies ( 7.5 eV in TiC and ZrC) and anomalously high prefactors (entropy 10–14.5 𝑘𝐵 in TiC)conflicting with traditional ab initio predictions assuming unreconstructed vacancies.

This thesis focuses on these discrepancies through systematic density functional theory (DFT) investigations, revealing that metal vacancies in group 4 and certain group 5 carbides spontaneously reconstruct by displacing neighboring carbon atoms to form strong C–C bonds. A combinatorial enumeration in TiC identified a rich landscape of reconstructed configurations, with the ground-state structure featuring a planar graphene-like C dimer lowering the Ti vacancy formation energy by 3.5 eV relative to the unreconstructed state. This reconstruction dramatically reduces Schottky defect formation energies from 7–8 eV (unreconstructed) to 3.98 eV (TiC), 6.08 eV (ZrC), 7.14 eV (HfC), and 1.97 eV (VC), while NbC and TaC retain unreconstructed vacancies ( 2.7 eV). Trends across the MeX (X = C, N, O) series correlate with valence electron count and bond covalency. Ab initio molecular dynamics (AIMD) at 1500–3000 K demonstrate that the C-dimer in the 2G structure undergoes thermally activated rotation above 1500 K, periodically opening the vacancy site and enabling Ti jumps into metastable open configurations with migration barriers of 3.5–4.0 eV. The resulting activation energy of 7.5 eV in agreement with experimental values. The anomalously high diffusion entropy arises from the large configurational and vibrational entropy of the reconstructed vacancy ensemble, particularly the dimer’s rotational degree of freedom (rotational diffusion coefficient 1.5 × 10¹² s⁻¹ at 2500 K) and numerous low-energy C-bonded metastable states. Reconstruction also induces strong short range repulsion between vacancies,preventing clustering and restoring the classical dissociated Schottky picture contrary to earlier cluster-based models. These findings establish a monovacancy mediated diffusion mechanism driven by dynamic carbon reconstruction as the dominant metal transport pathway in group 4 carbides. The insights are extended to technologically vital WC–Co cemented carbides, where vacancy-reconstruction-mediated processes at the surface of WC particles and WC/Co interfaces control Ostwald ripening,abnormal grain growth, and phase stability during liquid-phase sintering. The reconstructed vacancy framework provides a new atomic-scale foundation for defect engineering of refractory carbides, enabling predictive modeling of creep, sintering,and microstructural evolution in ultra-high-temperature ceramics and cemented carbides for aerospace, nuclear, and cutting-tool applications.

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