n this dissertation, a quantitative, parameter free model to predict the flow stress as a function of temperature and strain rate of such alloys is presented. The model builds on analytic concepts developed by Labusch but introduces key innovations rectifying shortcomings of previous models. To accurately describe the solute/dislocation interaction energies in and around the dislocation core, density functional theory and a flexible-boundary-condition method are used. The model then predicts the zero temperature flow stress, the energy barrier for dislocation motion, and thus the finite temperature flow stresses. The model is first used to predict the flow stresses of various Al alloys. Excellent results are obtained for Al-Mg and Al-Mn. Al-Fe with ppm levels of Fe is not predicted well but, using experimental results for Fe, results for the quasi-binary Al-Cr-(Fe) and Al-Cu-(Fe) alloys agree well with experiments. The model is also consistent with the "stress equivalency” postulate of Basinski. The model is then applied to Mg-Al alloys undergoing basal slip. Due to the wide partial separation of the Mg basal edge dislocation, a smaller roughening is required to decorrelate the solute fluctuations in the partials compared to that required to decorrelate the fluctuations in the "far field”. As a consequence, the dislocation has two stable configurations. When these two configurations are taken into account, the model predictions are in good agreement with experiments over a wide range of solute concentrations and temperature. The model also explains the origins of the "plateau stress” observed in experiments. Finally, the model in conjunction with the standard Friedel strengthening model is used to study the transtion between weak-pinning and strong-pinning at finite temperatures. The transition concentration is found to be a strong function of the dislocation core structure and can be significantly different from the zero-temperature transition concentration. It is also found that, for the case of Al-X and basal Mg-Al alloys, the weak-pinning mechanism is always dominant at temperatures and concentrations relevant to engineering applications.
Leyson, Gerard Paul M.,
"Solute strengthening from first-principles and applications to Al and Mg alloys"
(2012).
Mechanics of Solids Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z047485S
