Effect of Transition Metal Elements (TM = Zn, Cu, Ni) on Phase Stability of LPSO Phases and Experimental Verification of Occupancy Positions of TM Atoms in Mg–Co–Y Based Alloys

Phase stability and microstructure of long-period stacking ordered (LPSO) phases formed in Mg–Co–TM (TM: transition metal, Zn, Cu, Ni)–Y and Mg–Au–Y heat-treated alloys were investigated. In the Mg–Co–TM–Y alloys, the LPSO phases were stable at 773 K, regardless of the type or amount of TM elements. The LPSO phase contained Mg, Co, TM, and Y, with the TM concentration higher than that of Co. The area fraction of the LPSO phase increased with increasing TM content. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) observations suggested that the solute elements were enriched through four consecutive atomic layers, forming L12-type clusters within the LPSO phase. In the Mg80Au4Y16 alloy, the experimentally obtained HAADF-STEM images of the L12 clusters were consistent with the simulated results, indicating that Au and Y atoms occupied the atomic positions predicted by theoretical calculations. In addition, no Au or Y atoms have been observed at any interstitial sites, suggesting that a Mg atom could be occupied at these sites. These results suggest that in the Mg–Au–Y alloy, the Au atom occupies face-centered sites. These results also indicate that in Mg–Co–TM–Y alloys, Co and TM atoms occupy the face-centered sites as substitute atoms to form L12 clusters, playing a key role in stabilizing the LPSO phase.