The predominant role of Zn₆Y₉ cluster in the long period stacking order structures of Mg-Zn-Y alloys: A first-principles study

The strengthening mechanisms underlying the long period stacking order (LPSO) phases in Mg-Zn-Y alloys have largely remained unsolved due to unclear understanding of the local arrangements of Zn and Y atoms in the LPSO phases. The local arrangements of Zn and Y atoms in the LPSO structures are theoretically refined with first-principles method in this study. The calculations clearly demonstrate that Zn and Y atoms prefer clustering in the form of Zn₆Y₉ rather than arranging in random or ordered arrangements, or in the form of Zn₆Y₈ cluster, as proposed in previous experiments. The Zn₆Y₉ cluster, which leads to the formation of ABCA-type building block of the LPSO structures, can be particularly regarded as the ideal stoichiometric component of the LPSO structures. Various non-stoichiometric Zn _m Y _n (Mg) clusters derived from the Zn₆Y₉ cluster can also easily exist in the LPSO structures. The non-stoichiometric Zn _m Y _n (Mg) clusters may correspond to the arrangements of Zn and Y atoms in the LPSO phases with different Zn/Y ratios. This study indicates that the novel structures of the LPSO phases consisting of ABCA-type building blocks stem from the formation of the Zn₆Y₉ cluster and its derivative clusters, Zn _m Y _n (Mg).