Segregation of rare earth atoms in Mg-Gd-Y-Zr alloy after a 6-year natural ageing at room temperature: Atomic-scale direct imaging

Abstract

The segregation of rare earth (RE) atoms in solution-treated Mg-Gd-Y-Zr alloy after a 6-year natural ageing at room temperature has been comprehensively investigated by atomic-scale high angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) technique. When the aged specimen is viewed from [0001]Mg, RE enriched regions without a definite RE atom arrangement as well as three types of segregation with exact RE atom arrangements, dots, zigzag lines and hexagons, are characterized. The single dots are formed by columns of RE atoms occupying trials of Mg positions along [0001]Mg. The number of atoms in one trial is usually no more than three. The zigzag lines and hexagons are the predecessors of β′ and βM precipitate structures, respectively, which are common in artificially-aged alloys. Moreover, nano-sized zirconium-rich cores in the alloy are observed to facilitate the natural ageing because it provides both elements sources and nucleation sites for the precipitation, particularly under this low-temperature condition which determines the kinetic feasibility to be the crucial factor.