Precipitation in Mg-Gd-Y-Zr Alloy: Atomic-scale insights into structures and transformations

This paper reports on a comprehensive investigation, using atomic-scaled C-s corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), of the precipitation in Mg-9.28Gd-2.93Y-0.35Zr, including structures of the precipitates and several structural transformation processes. The precipitation sequence is identified as: Super saturated solid solution -> clusters -> nucleation beta' (major)/beta(H) (minor) -> precipitate beta' (major)/beta(M),beta(T)' (minor)->beta(1)->beta(Equilibrium). Under peak-age condition, the strengthening structure is independent defect-free beta' with little interaction among each other. Under over-age condition, the beta' precipitates start to interact with each other, during which coarsening and orientation transformation of beta' precipitates take place. The coarsening and orientation transformation, in which some RE atoms in the precipitates need to change their positions, are achieved by beta(M) and beta(T)' as transitional structures. (C) 2016 Elsevier Inc. All rights reserved.