EXAFS of glassy metallic alloys: Amorphous and crystalline MoNi

Abstract

The Ni and Mo K-edge extended X-ray absorption fine structure (EXAFS) spectra of MoNi alloy in the amorphous and crystalline δ-phase have been measured in the transmission mode. The EXAFS spectrum of a Mo foil has also been measured for comparison. The EXAFS data were analyzed using both symmetrical (Gaussian) and asymmetrical (liquid-metal type) pair distribution functions. A newly developed fine adjustment technique based on model (FABM) was applied to the best fit results based on theoretical backscattering amplitude and phase functions. The following results were obtained. (1) There exist differences in distances between the crystalline (r c) and the amorphous ( r a) alloys, which follow the general trend: Ni Ni (r c − r a ≈ 0 A ̊ ⩽ Mo Ni(0.03 A ̊ ) < Ni Mo(0.10 A ̊ ) ⪡ Mo Mo(0.2 A ̊ ) . (2) The distances between unlike atoms obtained from the Ni K-edge spectra (NiMo) and from the Mo K-edge spectra (MoNi) are different for both amorphous and crystalline states. The differences, in terms of (NiMo)−(MoNi), amount to 0.06 Å in the amorphous and 0.13 Å in the crystalline states. (3) The coordination numbers obtained by EXAFS for the metallic alloys are abnormally low ( N = 1–4) in sharp contrast to those ( N = 5–8) expected from crystallography. These features are interpreted in terms of large local atomic disorder, asymmetry in the pair distribution functions, and phase cancellation between the nickel and the molybdenum waves (the backscattering phases are off by π radians). Fits with the asymmetric pair distribution function are, however, of similar or somewhat poorer quality than those with symmetric ones and follow the same trends. Parameter correlation characteristics reveal distinct differences among metal foils, crystalline metal alloys, and amorphous metal alloys. These differences can be attributed to the different pair distribution functions, indicating that neither the crystalline metal foils nor the crystalline metal alloy can be used as model for the amorphous metal alloy. Our results indicate that in the MoNi alloy the nearest neighbor interactions are to some extent preserved while the higher shell coordinations are increasingly diminished in going from the crystalline phase to amorphous phase.