Medium-ranged interactions of transition-metal (3d and 4d) impurity pairs in Al and atomic structures of Al-rich Al-transition-metal alloys

We give systematic ab initio calculations for the interaction energies (from first to eighth neighbors) of impurity pairs X-X (X = Sc-Zn, Y-Cd) in Al and discuss the micromechanism of the structural stability of Al-rich AlX alloys. The calculations are based on the generalized gradient approximation in the density-functional theory and employ the all-electron full-potential Korringa-Kohn-Rostoker Green's function method for point defects, which guarantees the correct embedding of the cluster of impurities and vacancies in an otherwise perfect crystal. We show: (1) the fundamental features of phase diagrams of these alloys, such as ordering and segregation, are understood by the sign of first-neighboring X-X interaction energies; (2) the structural stability of Al-rich AlX alloys such as L12 (Al3Sc and Al3Y), DO22 (Al3V and Al3Nb), and Mackay icosahedron in AlMn quasicrystal, are understood qualitatively by using the medium-ranged and oscillating X-X interactions, due to the strong sp-d (Al-X) hybridization; (3) the strength and oscillating behavior of the medium-ranged X-X interaction energies are specified very well by the d-electron numbers of X impurities