Effects of size and interstitial element of L12-type clusters on formation of long-period stacking ordered structures in 10H-type Mg–Al–Y alloy

We investigated the effects of size and interstitial elements of the L12-type Al6Y8 cluster on the formation of the long-period stacking ordered (LPSO) structure in the 10H-type Mg–Al–Y alloy, Mg92Al12Y16-i2, i=interstitial elements, Mg, Al, Y or vacancy (no interstitial atom), by using first-principles calculations. We found that there exist two local minima of total energy (two stable states) in a function of the Al–Al distance of the vacancy-centered L12 cluster. The A1-type stacking is the most stable at the 1st energy minimum with the smaller Al–Al distance whereas the A3-type stacking is the most stable for the 2nd minimum with the greater Al–Al distance. Meanwhile, the Al–Y distances are identical at both minima. We also found that the sizes of the solute atom-centered L12 clusters in Mg92Al12Y16-i2(i=Mg, Al, Y) are similar to that for the 2nd minimum with the larger cluster size. The Al–Y distances are again identical for all the solute atom-centered clusters. We show that these behaviors in the Al–Al and Al–Y distances can be explained by the softness of the Al–Al bonds and the strong Al–Y bonds predicted from the distance dependence of the pair interaction energies between the solute atoms in the Mg-matrix. These results suggest that the Al–Y bond is a key factor to determine the structure in the solute-enriched layer in the Mg–Al–Y LPSO alloy, especially in the L12 cluster.