Combined First Principles Electronic Structure Calculations and Thermodynamic Study of Binary Alloys
In the past decade, density functional theory (DFT), combined with the highly precise computational methods and the increasing computer power, has become a most successful tool to study the physical properties of atoms, molecules, solids, surfaces and disordered systems. In this dissertation, we present a common framework, based on the density functional theory, to study the electronic structure, structural stability and the phase equilibria of both ordered compounds and solid solution of the binary alloys which usually have very small energy differences. As an illustrative example, we have made a systematic study on the Al-Li alloys which have become promising low density, high strength aerospace materials. The Al-Li ordered compounds are calculated by the all electron self-consistent, full potential linearized augmented plane wave (FLAPW) method within the local density approximation. All the stable and metastable phases are correctly predicted due to the high precision of the method. The phase stability in Al-Li alloys can be understood by our assumption that the Li atoms basically transfer their valence electrons in between the Al bonds and the resultant strengthened bonds stabilize the Al-Li compounds. The unusually high elastic modulus of the Al-Li alloys is due to the increased anisotropic Al bonding (decrease of the Poisson's ratio) with increasing Li content. Very good agreement with experiment is obtained. To utilize the existing highly precise band calculation method, we describe the Al-Li solid solution by a supercell method based on the "theory of locality". The relatively small size of a supercell is shown to give a very good description of Al-rich Al-Li solid solution. A thermodynamic model is proposed, as a first step, to calculate the phase diagrams of the binary alloys. The grand partition function, constructed from volume-dependent internal energies obtained from local-density total-energy supercell calculations, permits the determination of the entropy and thus, in principle, all thermodynamic quantities. Illustrative results for the Al-Li alloys show: (i) structural properties versus concentration in very good agreement with experiment and (ii) features on the Al-rich side of the phase diagram of the fcc solid solution which are important for alloy formation. This thermodynamic model shows promise for studying alloy phase diagrams entirely from first principles.
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- Physics: Condensed Matter; Engineering: Materials Science