Precipitation in Al-Au Dilute Alloys

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  • Sumida Naoto
    Department of Materials Science and Engineering, Faculty of Engineering, Osaka University
  • Okano Tetsuyuki
    Department of Materials Science and Engineering, Faculty of Engineering, Osaka University
  • Okada Tatsuaki
    Department of Materials Science and Engineering, Faculty of Engineering, Osaka University

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  • Al-Au希薄合金中の金の析出挙動
  • Al Au キハク ゴウキンチュウ ノ キン ノ セキシュツ キョドウ

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Abstract

Precipitates in Al-Au dilute alloys are η-phase (Al2Au), which appear in the form of plate on the matrix cube planes. However, it is likely that Au-rich zones (GP-zone) are formed at the beginning stage of precipitation. Consideration of the atomic radii of Au and Al leads us to expect the spherical shape. In this paper, electron microscope observation of the precipitates in an Al-0.2 mass%Au alloy aged under various conditions was made to study the precipitation process in detail.<BR>Small platelets about 1-4 nm in size appear on the {100} planes at the beginning stage of the precipitation. The platelets thicken and lengthen with aging and finally form the η-phase. It follows from the diffraction patterns and the high resolution images of the various forms of precipitates that the small platelets are not the η-phase.<BR>High resolution images of the η-phase precipitates show that the interface between the precipitates and the matrix is not steep in atomic scale and a transition phase exists around the interface. The transition phase can be considered as an intermediate state of the transformation from the Al to the η-phase.<BR>It is concluded that the precipitation process of Au in Al is as follows. The Au atoms aggregate to form the small platelets at the beginning stage of precipitation and the platelets grow up to be the η-phase forming the transition phase around them.

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