The nature of the scattering tail in Cu-Ni-Fe and Invar alloys investigated by anomalous small-angle X-ray scattering

A large rapidly decreasing intensity called the `scattering tail' is generally observed at the smallest recorded angles during small-angle measurements of metallic alloys. Since this tail was interpreted as caused by a bimodal phase separation in Cu-Ni-Fe alloys and by long-wavelength concentration fluctuations in Invar alloys, these two systems were re-examined with anomalous X-ray scattering. The variation of the alloying atomic contrasts allows a discrimination between the different types of particles or defects. In neither of the two systems can the tails be interpreted as caused by large-scale concentration fluctuations. In Cu-Ni-Fe alloys, the tail is due to some kind of superficial defect (surface roughness etc.). In Invar alloys, the tail is probably due to residual impurity particles.