Structural changes of precipitates by aging of an Mg-4at%Dy solid solution studied by atomic-scaled transmission electron microscopy

Phase transformation of solid solution decomposition occurring in a 96 at%Mg-4 at%Dy alloy, which was solutiontreated 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 Dysegregated regions and that the short-range ordered state allows full of the nuclei of β phase associated with an Mg₇Dytype 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∼100 nm and a diameter of 200∼400 nm. With an advance of over-aging effect, the β precipitates are gradually reduced in volumes and replaced by b precipitates of cubic structure.