Precipitation sequence in Al-Sc-Zr alloys revisited

Sc and Zr additions to Al provide significant strengthening through the formation of Al3(Sc,Zr) L1 2 dispersoids. Fe impurities are always present in commercial Al alloys and the effect of these impurities on precipitation mechanisms in Al-Sc-Zr alloys remain poorly understood. This is particularly relevant when considering recycled aluminium as recycling results in the build-up of impurities. We used a combination of resistivity and thermo-electric power (TEP) measurements to monitor the evolution of solid solution during ageing of an Al-Sc, Al-Zr and Al-Sc-Zr alloy. Sc first comes out of solution to form Al3Sc dispersoids, followed by Zr to form Al3Zr. A range of Al3Sc-core/Al3Zr-shell faceted morphologies were observed with high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). The presence of a new class of Fe-rich plate-like precipitates in the {100}Al was revealed via HAADF-STEM. The ordered positions of the precipitate and matrix reflections in the selected area diffraction pattern and in high-resolution TEM images reveal that these precipitates display a multi-layered structure. The centre of the plates is pseudo-icosahedral with apparent five-fold symmetry, and is rich in Fe. The outside layer has a L1 2 structure and is rich in Sc and Zr. The layered structure allows complete coherency with the aluminium matrix. These precipitates are highly anisotropic with a thickness of 5-10 nm and a length up to similar to 1000 nm. This is the first report of the formation of this type of coherent pseudo-icosahedral precipitates in isothermal conditions which is named the D-phase.