New polytypes of LPSO structures in an Mg-Co-Y alloy

The magnesium alloys containing long-period stacking ordered (LPSO) structures exhibit excellent mechanical properties. Each LPSO structure is known to contain either ABCA or ABC building block and feature its own stacking sequences. By atomic-scale high-angle annular dark field scanning transmission electron microscopy, we find the co-existence of ABCA and ABC building block in a single LPSO structure of the as-cast Mg92Co2Y6 (at.%) alloy, leading to the formation of six new polytypes of the LPSO structures determined as 29H, 51R, 60H, 72R, 102R and 192R. The lattice parameter of each LPSO structure is derived as a(LPSO) =a(Mg) = 0.321 nm and c(LPSO) = 1/2n x c(Mg) (n presents the number of basal layers in a unit cell). The stacking sequences and the space groups of these newly identified LPSO structures are proposed based on the electron diffraction and atomic-scale aberration-corrected high-resolution images. A random distribution of Co/Y elements in the basal planes of ABCA and ABC structural units is also observed and discussed.