ELECTRONIC ORIGINS OF ORDERING IN MULTICOMPONENT METALLIC ALLOYS - APPLICATION TO THE CU-NI-ZN SYSTEM

We investigate the ordering tendencies of the fee Cu-Ni-Zn system usin g a recently developed first-principles, density-functional-based theo ry of atomic short-range order (ASRO) in disordered substitutional all oys of an arbitrary number of components. We find for the binary alloy s a variety of effects which should lead to competition in the ternari es: commensurate ordering (Ni-Zn), long-period ordering (Cu-rich Cu-Zn ), and clustering (Cu-Ni), in agreement with experiment. We calculate the ASRO of various disordered ternary alloys (as described by the War ren-Cowley pair-correlation function) and discuss its relationship to the electronic structure of the binary and ternary disordered alloys. We find [100]-type ASRO over an extensive composition range for the te rnary alloys, even though all of the ordering tendencies for binaries and ternaries have a Fermi-surface-driven component. We discuss how al loying and disorder broadening lead to these ASRO properties. For Cu50 Ni25Zn25, the agreement for our calculated ASRO and its indication of two ordered states at low temperature are in good agreement with exper iments.