CONCENTRATION-DEPENDENCE OF THE LOCAL ORDER IN AMORPHOUS NI-NB ALLOYS

The atomic structure of amorphous Ni-Nb alloys has been studied via molecular-dynamic simulations on the basis of effective interatomic potentials calculated using the hybridized nearly-free-electron-tight-binding-bond technique. The partial static structure factors and pair correlation functions at four different compositions are compared with results from neutron diffraction with isotopic substitution. Theory and experiment agree in finding a certain chemical order (similar to that in the crystalline Ni-Nb compounds) and strong Ni-Nb bonds that are contracted compared with the average Ni-Ni and Nb-Nb bond lengths. The chemical short-range order depends only weakly on composition. In addition to the laboratory experiment, the computer simulation also yields information on the partial triplet correlation functions. The bond-angle distributions indicate that the local atomic arrangement is essentially of a polytetrahedral type, with broad peaks in the total bond-angle distribution close to the icosahedral bond-angles. In the partial bond-angle distributions, the peaks are slightly shifted as a consequence of size differences and chemical order. This shift is again similar to the bond angles in the coordination polyhedra of the crystalline Frank-Kasper phase. Compared with Ni-Zr and Ni-Y and Ni-metalloid glasses, Ni-Nb grasses are found to be more random and show no sign of the trigonal prismatic order characteristic of these systems. The physical origin of these structural trends is discussed.