AN ELECTRON CRYSTALLOGRAPHY AND ATOM PROBE INVESTIGATION OF ZONES AND PRECIPITATE PHASES IN INDUSTRIAL Al-Zn-Mg ALLOYS

X.Z. Li1, V. Hansen1, K. Stiller2, G. Waterloo3, R. Wallenberg4, and J. Gjonnes1

1 Centre for Materials Science, University of Oslo, N-0317 Oslo, Norway.
2 Chalmers University of Technology, S-412 96 Göteborg, Sweden.
3 Hydro Raufoss Automotive Research Centre, 2830 Raufoss, Norway.
4 Nat. Center for HREM, Inorganic Chem. 2 Chemical Center, Box 124, Lund Sweden.

Keywords: GP zones, precipitate phase, TEM, APFIM, Al-Zn-Mg alloys

The artificial ageing treatment in industrial Al-Zn-Mg-based alloys relies on precipitation sequences that involve several clusters, zones and precipitate phases [1]. Apart from the equilibrium phases h -MgZn2 and T-(Al,Zn)49Mg32, structures and compositions are not established. Our electron microscope and atom probe (APFIM) investigations aim at clarifying structures in relation to an industrial hardening sequence: solution treatment and quenching to room temperature followed by a two-step artificial ageing, at 100oC and 150oC. Electron microscopy was performed in a JEM2000FX and a JEM4000EX microscope; APFIM analyses in the Chalmers atom probe instrument [2].

During the earlier 100oC-stage two different zones GP(I) and GP(II) appear, with characteristic electron diffraction patterns. GP(I) is described as an ordered zone formed on {100}-planes with a periodic antiphase structure, related to early proposals [3] based on X-ray diffraction. GP(II) is characterized by strong diffuse <111>-streaks through positions outside (422)/3 and is proposed as a precursor for the metastable precipitate phase h ' which is formed after the subsequent, 150o heat treatment stage. Diffraction patterns and high resolution images from h '-particles confirm the 14A c-axis in a hexagonal unit cell [4]; but HREM images taken along this axis indicate lower symmetry and disorder. Atom probe analyses of the precipitates formed at this stage showed the Zn/Mg-ratio to depend on particle size. Crystallographic relations between h ' and other phases in the system will be discussed.

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