Formation of long-period stacking-ordered (LPSO) structures and microhardness of as-cast Mg-4.5Zn-6Y alloy

Long-period stacking-ordered (LPSO) structures formed in both the a-Mg component and the interdendritic component in as-cast Mg-4.5Zn-6Y-0.5Zr (wt.%) alloy were studied from micro to atomic scales. Inside the alpha-Mg component, stacking-ordered structure took three forms: single unit, scattered cluster, and a single layer of short range 18R structures. Chemical ordering of clusters of Zn and Y formed in the short range scale but not in the long range. Inside the interdendritic component LPSO-layered structure took two forms: 18R and 14H LPSO. Long-range 18R formed at locations with high concentrations of Zn and Y. 14H LPSO phase formed at the edge of any layer of 18R LPSO phase, sandwiched between 18R and alpha-Mg band. We concluded that the formation of LPSO had occurred in two distinct steps. First, Zn and Y atoms were segregated into two adjacent atomic layers of the original hexagonal close-packed matrix. Then, the nucleation of stacking fault occurred, followed by the growth of a stacking fault, which attracted more Zn and Y atoms into four constitutive close-packed layers, forming a new segment of an LPSO structural unit.