Atomic-scale investigation of interfacial structures in WC-Co at finite temperatures

Abstract: WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in high-speed machining of steels and in wear resistance tools. These excellent mechanical properties are to a large extent dependent on the microstructure and thus the interfacial properties of the material. Hence, being able to predict and understand interfacial properties in this material can allow for, e.g., optimizing the manufacturing process in order to improve mechanical properties further. Electronic structure calculations allow for accurately predicting interface energies for a given structure and composition. However, finding the ground-state interfacial structure and composition is challenging as the search space is very large when considering all degrees of freedom. Furthermore, direct sampling of interfacial properties at finite temperature using density functional theory (DFT) is usually computationally impractical as hundreds, thousands or even millions of calculations may be required. Therefore, employing atomic-scale models based on DFT calculations is advantageous and allows for investigation of the interface structure, composition and free energy at finite temperatures. In this thesis computational methods for calculating temperature-dependent interfacial free energies are developed and applied to the WC-Co system. The emphasis is on understanding under which conditions cubic interfacial structures (complexions) can form on the WC basal plane in contact with Co. Configurational degrees of freedom are treated with cluster expansions and Monte Carlo simulations. Vibrational properties are mainly treated in the harmonic approximation using a regression approach to extract the harmonic force constants, which significantly reduces the number of DFT calculations. Interfacial phase diagrams are obtained for both the undoped WC-Co system and the Ti-doped system. Detailed information pertaining to structure and composition of the interfacial phases are obtained and show good agreement with experimental observations.