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.
Mixed ground state in Fe-Ni Invar alloys
