Influence of microalloying elements on the precipitation behavior of Al-Mg-Si alloys

This work deals with a phenomenon called the negative effect of natural pre-aging on artificial aging, shown by many industrial important Al-Mg-Si alloys. An extensive literature research including the precipitation sequence in Al-Mg-Si alloys, fundamentals of vacancy assisted diffusion and theories treating the negative effect, served as a basis for the interpretation of experimental results. The approach of the work is based on a theory regarding interactions between the main alloying elements Si and Mg with quenched-in vacancies as the main reason for the negative effect of natural pre-aging on artificial aging. In the experimental part selected microalloying elements (Li, Ca, Sr, Ba, Ag, Cd, In, Ge, Sn, Sb, Pb and Bi) were added to the industrial important alloy AA6061. Some of these microalloying elements are believed to interact with vacancies in the aluminum matrix and are able to prohibit the negative effect of natural pre-aging on artificial aging. Using calculations from literature-based data of element-vacancy interactions in pure aluminum facilitated not only to verify the aforementioned theory, but also to interpret the effect which some tested microalloying elements exhibited on the overall precipitation kinetics of AA6061.