Precipitation Sequence in the Mg–Gd–Y System Investigated by HRTEM and HAADF-STEM

Yuuki Matsuoka; Kenji Matsuda; Katsumi Watanabe; Junya Nakamura; Williams Lefebvre; Daisuke Nakagawa; Seiji Saikawa; Susumu Ikeno

doi:10.2320/matertrans.L-M2014824

Yuuki Matsuoka, Kenji Matsuda, Katsumi Watanabe, Junya Nakamura, Williams Lefebvre, Daisuke Nakagawa, Seiji Saikawa, Susumu Ikeno

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Keywords: magnesium–gadolinium–yttrium alloys, age-hardening, precipitation, high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM)

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2014 Volume 55 Issue 7 Pages 1051-1057

DOI https://doi.org/10.2320/matertrans.L-M2014824

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Abstract

In this work, the early stage of precipitation have been investigated in a Mg–2.9 at% Gd–0.8 at% Y by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and high resolution transmission electron microscopy (HRTEM).
At the underaged condition, precipitates observed by HRTEM were classified as follows; mono-layer, a part of β″, β′. By HAADF-STEM, zig-zag structure, small hexagonal network, and β′ could be recognized. The small “super hexagons” are the first precipitates in this alloy. This structure, referred to as the pre β″-phase, displays a short range similar to the one present in the D0₁₉ structure. This phase is formed during quenching and prevails over the zig-zag structure at the beginning of aging. The zig-zag contrast is one of the variations of this small hexagonal network or heterogeneous nucleation during aging. Finally, we concluded that the proposed precipitation sequence is as follows: SSSS → pre β″-phase having D0₁₉-SRO → β″-phase → β′-phase. The comparison of HRTEM, HAADF-STEM and simulated images have helped to clarify the meaning of HRTEM features.

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