First-principles investigations of solid solution strengthening in Al alloys

Any material properties, in principle, can be reproduced or predicted by performing firstprinciples calculations. Nowadays, however, we are dealing with complex alloy compositions and processes. The complexities cannot be fully described by first-principles, because of the limited computational power. The primary objective of this study is to investigate an important engineering problem, solid solution strengthening, in a simplified manner. The simplified scheme should allow fast and reliable prediction of the solid solution strengthening. It is simplified by capturing the key features of the solid solution strengthening, namely the volume and modulus mismatch. Those two quantities are calculated by density functional theory (DFT) and inserted into the linear elasticity models. Seven Al binary solid solutions were investigated (Al-X, X=Ca, Sr, Ir, Cu, Mg, Li and Zn). The calculated strengthening effects of the solute elements by this simplified approach are in very reasonable agreement with what is observed in the experiments and another direct DFT calculation. It is also observed that the decisive parameters of the solid solution strengthening follow a trend on the periodic table, so does the strengthening effect. The volume mismatch parameters are correlated with the solubility enthalpies of the studied Al binary solid solutions. This correlation allows to estimate the optimal strengthening parameter to achieve the maximum strengthening at the solubility limit. Another objective is to better understand the solid solution strengthening from the electronic origin. This was tentatively investigated by observing the perturbation caused by the solute elements. It is observed that the perturbation can be correlated with the strengthening effect and the supercell size dependence. All the calculated materials properties in this thesis entered Ashby’s materials property and design charts. The performance of the studied Al binary solid solutions indicated by those charts reasonably coincides with their current applications. Although Ca, Sr and Ir cause larger strengthening effect in Al than other studied solute elements, they do not always help Al perform better in some tasks