AbstractThe formation of OD (order-disorder) phases of the LPSO (long-period stacking ordered)-type in Mg-Al-RE (RE (rare earth) = Y, La, Ce, Nd, Sm, Dy, Ho, Er and Yb) ternary systems has been investigated for both as-solidified and annealed conditions. The OD phase is found to form in those systems with RE = Y, Nd, Sm, Dy, Ho and Er. The Mg-Al-RE OD phase formed is of the 18R-LPSO-type consisting of 6-layer structural blocks with the RE enrichment occurring in the four consecutive atomic layers in the structural block in the form of the Al₆RE₈ L1₂-type atomic clusters. The Mg-Al-RE OD phases are found to be stabilized by the inclusion of any atoms (either Mg, Al or RE) in the central site of the Al₆RE₈ L1₂-type atomic cluster. The occupancy ratio of the central site among Mg, Al and RE atoms varies with the RE element, so that the occupancy ratio of RE atoms increases with the increase in the atomic number of the RE element in particular for the late rare-earth elements. Based on the results obtained, a criterion based on the volume of the Al₆RE₈ atomic cluster is proposed to predict the formation of the Mg-Al-RE OD phases.