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

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.