Atomic structure of clusters and GP-zones in an Al-Mg-Si alloy
Calin D. Marioara, Sigmund J. Andersen, Christoph M. Hell, Jonas Frafjord, Jesper Friis, Ruben Bjørge, Inga G. Ringdalen, Olaf Engler, Randi Holmestad
Abstract
The structures of atomic clusters and GP-zones preceding precipitation in an Al-0.70Mg-0.85Si-0.15Fe-0.25Mn (wt.%) alloy have been investigated by annular dark field scanning transmission electron microscopy imaging and density functional theory calculations. One analysed condition consisted of one-month natural aging, and another was in a pre-baked state after 24 h aging at 90°C. To quantify the effect of clusters and GP-zones on microstructure development during a subsequent isothermal artificial aging at 185°C, hardness evolution was measured, and precipitate statistics was performed at the peak hardness corresponding to 5 h aging. Several types of clusters and GP-zones have been observed and based on structure and visual appearance named 'Disordered Frank-Kasper', 'Square', 'Binocular', and 1β", 2β", 3β", 4β". The first four are most common in the natural aged condition and are associated with a well-known influence on the initial hardness evolution. The last three are fractions of the hardening β" phase and refer to the number of structural units they contain. These GP-zones have an increased density in the pre-baked condition, leading to a rapid increase in hardness during artificial aging and to a finer microstructure of high-density small needle precipitates at peak hardness.