Site-occupation tendencies for ternary additions (Fe, Co, Ni) in β-phase transition-metal aluminides

We have investigated, using extended x-ray-absorption fine-structure spectroscopy and x-ray-absorption near-edge spectroscopy, the local environment of Co dopants in β-phase transition-metal aluminides. In contrast to our previous findings that Fe atoms in FexNi50Al50-x and FexCo50Al50-x occupy the Al sublattice exclusively, we now find that in CoxNi50Al50-x, Co atoms predominantly occupy the transition-metal sublattice. Further, in this transition-metal-rich system the available sites on the Al sublattice are occupied by both Co and Ni, with greater preference for such occupation by Co relative to Ni. These findings are discussed using a simple model based on heats of formation of the binary transition-metal aluminides, as well as by referring to studies by others that relate the effect of microalloying, using various dopants, to alloy-ductility enhancement.