The structure of beta(1)' precipitates in Mg-Zn-Y alloys

It is shown here by high-resolution electron microscopy that the structure of rod-like beta(1)' precipitates perpendicular to the basal planes of the magnesium matrix in Mg-Zn-Y alloys consist of complex domains at nanoscale. These domains can be recognized to be those of monoclinic Mg4Zn7 phase and hexagonal Laves phase MgZn2 with axial orientations [1(2)over-bar10](MgZn2)parallel to[010](Mg4Zn7)parallel to[0001](Mg). Due to disorder, often complete unit cells of the Mg4Zn7 structure cannot be recognized. Inside a domain of Mg4Zn7, the structure may locally transform to that of MgZn2. The structure of both the phases are composed of similar units of icosahedral coordinations. Maintaining the above axial orientation, the Mg4Zn7 can exhibit two orientation relationships with the matrix, (802)(Mg4Zn7)parallel to{10(1)over-bar0}(Mg) or (603)(Mg4Zn7)parallel to{10(1)over-bar0}(Mg). The hexagonal MgZn2 forms in the monoclinic Mg4Zn7 in two different orientations, related to rhombic units (with angle similar to 72 degrees) in the Mg4Zn7 unit cell. With the common axial relationship given above, these two orientation relationships can be given as (0001)(MgZn2)parallel to(120(1)over-bar)(Mg4Zn7) or (0001)(MgZn2)parallel to(206)(Mg4Zn7). One of these two variants forms a known orientation relationship [11(2)over-bar0](MgZn2)parallel to[0001](Mg), (0001)(MgZn2)parallel to{11(2)over-bar0}(Mg) with the matrix. The structure of the MgZn2 was found to be modified by the ordering of zinc layers perpendicular to the hexagonal axis.