The structure of long period stacking/order Mg–Zn–RE phases with extended non-stoichiometry ranges

Abstract

We propose structural models of the unique long period stacking/order (LPSO) phases formed in Mg–Zn–RE alloys, based on Z-contrast scanning transmission electron microscopy observations and first principles calculations. The LPSO structures are long period stacking derivatives of the hcp Mg structure, and the Zn/RE distributions are restricted at the four close-packed atomic layers forming local fcc stacking (i.e. a local ABCA stacking). Chemical order is well developed for the LPSO phases formed in Mg 97Zn 1Er 2 (14H type) and Mg 85Zn 6Y 9 (18R type) alloys with pronounced superlattice reflections, and the relevant Zn/RE distributions clearly emerge in the Z-contrast atomic images. Initial ternary ordered models were constructed by placing all the atoms at the ideal honeycomb sites, leading to plausible space groups of P6 3/ mcm for the 14H type and C2/ m, P3 112 or P3 212 for the 18R type. The characteristic ordered features are well represented by local Zn 6RE 8 clusters, which are embedded in the fcc stacking layers in accordance with the L1 2 type short-range order. Energy favored structural relaxations of the initial model cause significant displacement of the Zn/RE positions, implying that strong Zn–RE interactions may play a critical role in phase stability. The LPSO phases seem to tolerate a considerable degree of disorder at the Zn and RE sites with statistical co-occupations by Mg, extending the non-stoichiometric phase region bounded along the Zn/RE equiatomic line from ∼Mg 94.0Zn 2.0Y 4.0 to ∼Mg 83.3Zn 8.3Y 8.3.