Density functional theory study of solute cluster growth processes in Mg-Y-Zn LPSO alloys

Solute clusters in long period stacking order (LPSO) alloys play a key role in their idiosyncratic plastic behavior, for example kink formation and kink strengthening. Identifying atomistic details of cluster structures is a prerequisite for atomistic modeling of LPSO alloys and is crucial for improving their strength and ductility; however, there is much uncertainty regarding interstitial atoms in the cluster. Although density functional theory calculations have shown that the inclusion of Mg interstitial atoms is energetically most favorable in majority of LPSO alloys, solute elements have also been experimentally observed at interstitial sites. To predict the distributions of interstitial atoms in the cluster and to determine the kind of elements present, it is necessary to identify mechanisms by which interstitial atoms are created. In the present work, we use density functional theory calculations to investigate growth processes of solute clusters, specifically the Mg-Y-Zn LPSO alloy, in order to determine the precise atomistic structure of its solute clusters. We show that a pair of an interstitial atom and a vacancy are spontaneously created when a certain number of solute atoms are absorbed into the cluster, and that all full-grown clusters should include interstitial atoms. We also demonstrate that interstitial atoms are mostly Mg, while the rest are Y; interstitial Zn atoms are negligible. This knowledge greatly simplifies the atomistic modeling of solute clusters in Mg-Y-Zn alloys. Owing to the vacancies emitted from the cluster, vacancy density should be super-saturated in regions where solute clusters are growing, and increased vacancy density accelerates cluster growth.