Atomic-scale characterization of the precipitates in a Mg-Gd-Y-Zn-Mn alloy using scanning transmission electron microscopy

Precipitates are critical in strengthening Mg-Gd-Y-Zn-Mn alloys. However, lack of sufficient atomic-scale char-acterization restricts further understanding of the precipitates. Therefore, in the present study, precipitates in the as-cast, solution-annealed, and isothermally aged (200 degrees C) Mg-Gd-Y-Zn-Mn alloy have been comprehensively examined on the atomic scale by aberration-corrected (Cs) high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) combined with energy dispersive X-ray spectrometry (EDS). When observed in the [0001]alpha direction in the early-aged alloy for 6 min, uneven local rare earth solute clusters in the form of beta H and beta Z phases suggest a transition from beta H to beta Z. In the peak-aged alloy for 16 h, Mn atoms are found to segregate into the gamma '/long-period stacking ordered (LPSO) phase and beta ' phase, participating in their formation. In the over-aged alloys for 200 h, gamma '' phase is detected with enrichment of Mn, coexisting with the gamma ' and beta ' phases. Furthermore, beta ''-like structures are observed, which may be defective beta M or beta ' T as a result of their overlapping, thus providing a possible explanation for the controversy regarding the existence of the beta '' phase. Our findings serve as evidence that deepens the understanding of the precipitates in Mg-Gd-Y-Zn-Mn alloys at the atomic level.