Effects of Size and Interstitial Element of L1 <sub>2</sub>-Type Clusters on Formation of Long-Period Stacking Ordered Structures in 10H-Type Mg-Al-Y Alloy

Abstract

We investigate effects of size and interstitial element of L12-type Al6Y8 cluster on formation of long-period stacking ordered (LPSO) structures in 10H-type Mg-Al-Y alloy, Mg92Al12Y16-i2, i=interstitial elements, Mg, Al, Y or vacancy ( no interstitial atom), by using first-principles calculation based on density functional theory. We find that there exist two local minima of total energy in a function of the cluster size for vacancy-centered cluster, Al6Y8. Between the two local minima, the lower energy depends upon the stacking polytypes A1-A4; the lowest (the second lowest) is for the A1 with the smaller clusters (for the A3 with the larger clusters). For the clusters with the center atom of Al or Mg, the cluster sizes are similar to the larger size for the vacancy-centered cluster, and the stacking polytype of the lowest state is also A3 in accordance with the experimental observation. Furthermore, we find that, a spot of Mg-centered cluster among the vacancy-centered clusters can change the size of the vacancy-centered clusters from the smaller to the larger one, according as the lowest stacking polytype changes from A1 to A3. These results explain the structure of Mg75Al10Y15 alloy in which the regularly ordered A3 stacking polytype has been experimentally observed although the constituent atoms or vacancies randomly occupy the cluster centers. We also show a distance dependence of pair interaction energies(PIEs) between Al-Al/Al-Y/Y-Y atoms in Mg matrices and explain the structural features of the L12-type clusters in Mg-Al-Y alloys, that is, the intra- and inter-cluster structures in the impurity enriched layer. The reason that there exist two local energy minima in the Al6Y8 cluster is that both the Al-Al PIEs of the 1st and 2nd nearest neighboring pair are negative (energetically favorable).