Phase transformation of solid solution decomposition occurring in a 96 at %Mg-4 at % Dy alloy, which was solution-treated at 540°C and subsequently aged at 250°C for various lengths of time, has been investigated by conventional transmission electron microscopy (TEM) in combination with high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The atomic-scaled observations based on both techniques provide the evidence that the first appreciable change in microstructure caused by aging is the occurrence of a short-range ordered state in Dy-segregated regions and that the short-range ordered state allows full of the nuclei of β′ phase associated with an Mg7Dy-type structure to occur in the domains, just as in cases of Mg-Gd and Mg-Y systems. With an increase of age-hardening effect, the β′ precipitates become larger and increasingly anisotropic in morphology, accompanying three orientation variants in coherent with the Mg-matrix. When reaching at the stage of hardness maximum (as-aged at 250°C for 100 h), the β′ precipitates, which have an orthorhombic structure with lattice parameters of a = 0.659 nm, b = 2.231 nm, c = 0.523 nm, take the form of a thin disk-shape with a thickness of 20∼100nm and a diameter of 200∼400nm. With an advance of over-aging effect, the β′ precipitates are gradually reduced in volumes and replaced by β precipitates of cubic structure.
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