First-Principles Investigation of Bulk and Interfacial Properties of Cu-Co Binary System

Abstract: Due to the complex nature of phase interfaces, acquiring precise interfacial energies is usually a big challenge for both experimental measurements and computational modelings. In this thesis, we put forward an efficient route for assessing the temperature dependence of the interfacial energy using density functional theory (DFT). For our investigations, we select the Cu-Co binary system as a model with large miscibility gap. Most of the first-principles calculations presented here are carried out using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA), but other alternative DFT methods are also included in the various stages of the project.The first step is to acquire an accurate thermodynamical description of the Cu-Co binary system. We assess the quality of the predicted thermodynamic properties by an effort to reproduce the phase diagram for the entire range of composition using first-principles calculations and alloy theory. The calculations are performed for the random Cu-Co alloys with face-centered cubic (fcc) structure at both ferromagnetic (FM) and paramagnetic (PM) states, depending on the composition. We demonstrate that the equilibrium volumes and magnetic states are crucial for the proper description of the magnetic entropy of the Cu-Co system at elevated temperatures. More specifically, the contribution of magnetic entropy to the free energy in the Cu-rich region obtained for the PM state turns out to be critical. Furthermore, the adopted equilibrium volumes strongly affect the contribution of the vibrational entropy to the free energy. When all effects are properly accounted for, we find that the ab initio phase diagram of the Cu-Co system agrees well with the Thermo-Calc phase diagram and the experimental observations.The Cu-Co system has a large miscibility gap. The interface between the decomposed Cu-rich and Co-rich phases plays critical roles in the precipitation nucleation and growth, therefore having huge effects on the physical and mechanical properties of the alloys. Therefore, adopting the thermodynamical properties of the bulk Cu-Co alloys successfully obtained by our ab initio calculations, we go further and investigate the interfacial properties of the Cu-Co alloys using a coherent interface model. The chemical, magnetic, and strain energy contributions to the formation energy of the interfaces are analyzed separately. We find that the chemical interfacial energies generally decrease with increasing concentrations, namely when the compositions accross the interface become more homogenous. We identify a sizable contribution to the interfacial energies from the magnetic effects. The temperature dependence of the interfacial energy is estimated, to the first-order approximation, through considering how the equilibrium compositions of the two phases vary at different temperatures. Our results show that the temperature dependence of the interfacial energy originates primarily from the temperature-induced increase of the mutual solubility of the alloy constituents and the loss of the magnetic long range order near the Curie temperature. Our ab initio results are compared with the experimental data as well as with those extracted from Thermo-Calc modeling. The present thesis provides an atomic-level description of the bulk and interfacial properties of the Cu-Co binary system using quantum mechanics simulations. This approach is believed to be useful for a complete thermodynamical description of other similar immiscible alloy systems as well from first-principles.

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