[SY-E7] Structural Stability of Long-period Stacking Ordered Magnesium Alloys
Magnesium (Mg) alloys with long-period stacking ordered (LPSO) structures show excellent mechanical performances such as high yield strength and ductility. The LPSO structures consist of periodic arrangement of basal stacking faults (SFs) and enrichment of solute atoms in the vicinity of the SFs. HAADF-STEM measurements observed that the L12-type clusters of solute atoms are formed and aligned at each SF. The L12 clusters at each SF have a solute-enriched region of four close-packed planes. As SFs are periodically introduced, each type of the LPSO structures has the specifc close-packed layers between the quadrople solute-enriched layers. While a previous first-principles study showed that the binding energy of solute atoms to SF and the solute-solute pair interaction can describe the formation of the L12 clusters, the physical origin of the periodic arrangement of the quadrople solute-enriched layers with the L12 clusters remains controversial.
In order to elucidate the periodic arrangement of the quadrople solute-enriched layers, we investigate interactions mediated by electrons or phonons between the neiboring quadrople solute-enriched layers. The formation energy of the L12 cluster evaluted by first-principles calculations shows that the electron-mediated interaction is short-range repulsive with respect to the distance of the quadrople solute-enriched layers. On the other hand, we investigate effects of phonon on the inter-planer ordering of the solute-enriched layers using the 1-dimensional chain model with mass change. For heavy mass change, the ordering of the mass changes is stabilized by phonons and the energy gain increases with the concentration of the mass changes, i.e., the short LPSO period is favorable. Thus, a promising mechanism of the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople layers where heavy solute atoms are enriched as the L12 clusters at SFs.
In order to elucidate the periodic arrangement of the quadrople solute-enriched layers, we investigate interactions mediated by electrons or phonons between the neiboring quadrople solute-enriched layers. The formation energy of the L12 cluster evaluted by first-principles calculations shows that the electron-mediated interaction is short-range repulsive with respect to the distance of the quadrople solute-enriched layers. On the other hand, we investigate effects of phonon on the inter-planer ordering of the solute-enriched layers using the 1-dimensional chain model with mass change. For heavy mass change, the ordering of the mass changes is stabilized by phonons and the energy gain increases with the concentration of the mass changes, i.e., the short LPSO period is favorable. Thus, a promising mechanism of the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople layers where heavy solute atoms are enriched as the L12 clusters at SFs.