HAADF-STEM study of long-Period stacking-Ordered phases formed in the quaternary MgLiYZn alloys

The crystal structures of Long-Period Stacking-Ordered (LPS) Phases formed in Mg97¹ _xLi_xY₂Zn₁ (x = 3, 6, 10, 17 at%) alloys have been thoroughly investigated by means of High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy. All the quaternary alloys with different Li contents, especially when subjected to annealing at 500°C for 50 hrs (hereafter called ‘as-annealed alloy’), allow certain polytypes of LPS-phases to form with ¡-Mg solid solutions. The as-annealed alloy with x = 3 at%Li has 14H-type LPS-phase with disordered arrangements of Y and Zn in the solute-enriched layers, as in the case of the ternary counterparts without Li. It is evidenced that an increase of Li additions has substantial effects of changing the LPS structure, progressively inducing both the phase transformation from 14H-to 24R-type and the in-plane ordering of Y/Zn atoms in their enriched layers. The as-annealed alloys with x = 6 at%Li, in fact, contain LPS-crystal grains made up of the two polytypes of 14H and 24R, both of which have the long-range in-plane ordered structures. Among all, the as-annealed alloy with x = 10 at%Li has the 24R-type LPS-phase dominant over the LPS-grains combined with the in-plane ordering. The crystal structures of such highly-ordered LPS-polytypes are found to be well characterized by the concept of order-disorder (OD) intermetallic phases.