The effect of Cu on precipitation in Al–Mg–Si alloys

Abstract TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg + Si = 1. 3 at. %, 0. 127 at. % Cu, but with Mg/Si 0. 8 and 1. 25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as 100 Al planes. The Si grid is composed of nearly hexagonal sub-cells of a = b = 4. 05 Å, c = 4. 05 Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a = 10. 32 Å, b = 8. 1 Å, c = 4. 05 Å, γ = 101°. Acknowledgements This work was funded through the NorLight programme financed by industry and the Norwegian Research Council. Industrial sponsors are the following Norwegian companies: Hydro Aluminium AS, Raufoss ASA and Elkem Aluminium. More information is available online at: www. sintef. no/norlight/ProjectPortfolio/HeatTreatmentFundamentals/index. htm.