Computational investigation of interface structure and composition in cemented carbides at finite temperatures

Sammanfattning: 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. Atomic scale ab-initio calculations allow for accurately predicting interface energies for a given structure and composition. However, finding the ground-state interfacial structure and composition becomes a challenge 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) often becomes computationally unfeasible 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 the computational methods for calculating temperature dependent interfacial free energies are developed and applied to the WC-Co system. An interfacial phase diagram for cubic thin films in undoped WC-Co is constructed. Here, configurational degrees of freedom are treated using cluster expansion models and Monte Carlo sampling. Vibrations are treated in the harmonic approximation using force constant fitting to significantly reduce the number of DFT calculations. The temperature dependence of interface free energies for surfaces, grain boundaries and phase boundaries is using an analytic bond order potential. Here, multiple different free energy calculation methods are employed such as quasi-harmonic approximation, λ-integration​​, thermodynamic integration and surface tension calculation.