Stoichiometric precipitates owe their fixed composition to an ordered crystal structure. Deviations from that nominal value, however, are encountered at times. Here we investigate composition, structure and diffusion phenomena of ordered precipitates that form during heat treatment in an industrially cast Al–Mg–Sc–Zr alloy system. Experimental investigations based on aberration-corrected scanning transmission electron microscopy and analytical tomography reveal the temporal evolution of precipitate ordering and formation of non-equilibrium structures with unprecedented spatial resolution, supported by thermodynamic calculations and diffusion simulations. This detailed view reveals atomic-scale spinodal decomposition to majorly define the ongoing diffusion process. It is illustrated that even small deviations in composition and ordering can have a considerable impact on a system’s evolution, due to the interplay of Gibbs energies, atomic jump activation energies and phase ordering, which may play an important role for multicomponent alloys.

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The authors thank the Austrian Cooperative Research Facility, the Austrian Ministry for Transport, Innovation and Technology (project GZ BMVIT- 612.011/0001-III/I1/2015) and the Austrian Research Promotion Agency FFG (TAKE OFF project 839002) for funding. We would like to express our gratitude to F. Hofer for supporting the project and to W. Sprengel for advice concerning the manuscript. Furthermore, we would like to thank L. Allen and his group for support with µSTEM and M. Weyland, J. Etheridge and S. Findlay for support concerning quantitative STEM.

Author information


  1. Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz, Austria

    • Angelina Orthacker
    • , Georg Haberfehlner
    •  & Gerald Kothleitner
  2. Graz Centre for Electron Microscopy, Graz, Austria

    • Angelina Orthacker
    • , Georg Haberfehlner
    •  & Gerald Kothleitner
  3. Institute of Materials Science, Joining and Forming , Graz University of Technology, Graz, Austria

    • Johannes Taendl
    • , Maria C. Poletti
    •  & Bernhard Sonderegger


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A.O. performed all STEM investigations including sample preparation for tomography and STEM HAADF simulations, interpreted the results and wrote the bigger part of the manuscript. G.H. provided the software for tomographic alignment and reconstruction and supported its application. J.T. and M.C.P. provided the samples and information and performed the re-solidification and ageing process. B.S. coded the Gibbs energy calculation and the 3D atomistic diffusion simulation and wrote the parts of the manuscript treating them. G.K. supervised the project and the writing of the manuscript.

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The authors declare no competing interests.

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Correspondence to Angelina Orthacker or Gerald Kothleitner.

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