Properties of Al-Mg Alloys and Vacancies in Al: a First-Principles Investigation

Abstract: Phase transformations such as precipitation are used to control the strength of many technologically important aluminum alloys. In precipitation hardening, the material is heat treated and aged to produce precipitates, small particles of a different phase, in the matrix. The age-hardened alloy is stronger because dislocation motion is impeded by the precipitates. In the binary Al-Mg alloys, the first stages of the precipitation sequence remain partly unclear. Also, they are interesting as model systems for the development of new methods for simulation and analysis of precipitation processes.

This thesis is a study of atomic scale structures and energetics in Al and Al-Mg alloys. In the first part, vacancies in aluminum have been studied with density functional theory (DFT). The formation energies for mono- and divacancies were calculated, as well as the corresponding vibrational formation entropies. The temperature dependence of the formation energies and entropies was calculated with an embedded atom model (EAM) potential fitted to data from DFT calculations. We show that the divacancy is thermodynamically unstable, and that anharmonic atomic vibrations explain the non-Arrhenius temperature dependence of the vacancy concentration found experimentally.

In the second part, the properties of coherent precipitates in Al-Mg alloys, especially the β" phase, are analyzed. The performance of the orbital-free density functional theory (OF-DFT) method was tested by calculation of the lattice parameters, bulk moduli, and energies of the fcc Al, hcp and bcc Mg, and β" phases. The results confirm the transferability of the recently developed kinetic energy functional with a density-dependent kernel.

Furthermore, the properties of the first stages of the nucleation of the β" phase have been investigated with both experiments and simulations. With the mixed space cluster expansion (MSCE) method, the configurations for different temperatures, aging times, and alloy compositions have been calculated and the results are analyzed with the same methods as for the experimental data. The conclusion is that clustering of Mg atoms occur relatively quickly in the material, while the transformation to ordered β" requires longer aging times. Also, the energy of precipitation of coherent clusters in Al-Mg alloys has been investigated with both the MSCE and EAM potentials. The results are within the range of experimental data derived from calorimetric measurements.