First-principles interatomic potentials for transition-metal aluminides. III. Extension to ternary phase diagrams

Modeling structural and mechanical properties of intermetallic compounds an d alloys requires detailed knowledge of their interatomic interactions. The first two papers of this series [Phys. Rev. B 56, 7905 (1997); 58, 8967 (1 998)] derived first-principles interatomic potentials for transition-metal (TM) aluminides using generalized pseudopotential theory (GPT). Those paper s focused on binary alloys of aluminum with first-row transition metals and assessed the ability of GPT potentials to reproduce and elucidate the allo y phase diagrams of Al-Co and Al-Ni. This paper addresses the phase diagram s of the binary alloy Al-Cu and the ternary systems Al-Co-Cu and Al-Co-Ni, using GPT pair potentials calculated in the limit of vanishing transition-m etal concentration. Despite this highly simplifying approximation, we find rough agreement with the known low-temperature phase diagrams, up to 50% to tal TM concentration provided the Co fraction is below 25%. Full compositio n-dependent potentials and many-body interactions would be required to corr ect deficiencies at higher Co concentration. Outside this troublesome regio n, the experimentally determined stable and metastable phases all lie on or near the convex hull of a scatter plot of energy versus composition. We ve rify, qualitatively, reported solubility ranges extending binary alloys int o the ternary diagram in both Al-Co-Cu and Al-Co-Ni. Finally, we reproduce previously conjectured transition-metal positions in the decagonal quasicry stal phase.