The Effect of Trace Elements on Precipitation in Al-Mg-Si alloys - A Transmission Electron Microscopy Study

Abstract

Atomic scale studies of aluminum alloys

The thesis focuses on structural analysis of particles (precipitates) in the important aluminum alloys – Al-Mg-Si alloys, studied by transmission electron microscopy (TEM) including the state of art modern aberration corrected scanning transmission electron microscopy (STEM) techniques.

Aluminum recycling has recently become important in industry. Recycled aluminum will contain some undesired elements, and it is often difficult to estimate the amounts and effects of these elements. They may induce changes in structures of precipitates and consequently mechanical properties of the aluminum alloys. The thesis covers investigations of the effects of copper and zinc as trace elements on the structure of precipitates in and mechanical properties of Al-Mg-Si alloys. The investigations lead to development of recycled aluminum alloys as well as understanding of fundamental mechanisms of formation of precipitates in aluminum alloys.

The results show that copper leads to significant effects on mechanical properties. This is due to change in the structure of the precipitates affected by a strong interaction with other solute elements. Moreover, specific local structures around Cu atoms are observed on the atomic scale. Zn, on the other hand, has weaker effects on mechanical properties and the structure of precipitates. However, Zn atoms segregate along grain boundary, which leads to higher corrosion susceptibility.

Analysis of images taken by modern aberration corrected STEM techniques has huge potential, but is complicated because of complex electron scattering mechanisms. The thesis combines these studies with theoretical calculations and simulations. This combination opens for further insight from atomic investigations of trace elements in aluminum alloys.