Determining Phase Stability in the Co-Al-W System using First Principles Methods
Co-based superalloys are a promising new material for high strength, high temperature applications due to their gamma-gammaprime microstructure. Within the Co-Al-W ternary system, the gammaprime phase manifests as an L1-2 structure in a small temperature range. Determining the thermodynamic stability of the L1-2 phase relative to other phases in this system is important for guiding the alloy development process. Density functional theory allows the direct calculation of free energy for these phases. The configurational energy component of the energy for the L1-2 structure was calculated via a cluster expansion, and the vibrational energy component for all structures was calculated in the quasiharmonic approximation. The results show that the L1-2 structures become stable at 600K and increase stability at higher temperatures, which has not been shown in previous work.
The purpose of this thesis is both to show the encouraging research results in the Co-Al-W system, and also to be a useful guide for performing first principles calculations and connecting theory, VASP usage, and the utilization of the CASM code.