Anomalous small-angle X-ray scattering study of the early stages of decomposition in Cu-15wt%Ni-8wt%Sn

The decomposition process of the supersaturated solid solution Cu-15wt%Ni-8wt%Sn prepared by powder metallurgy has been studied by anomalous small-angle scattering of X-rays. The ageing has been followed at 623 K. The analysis of the small-angle scattering intensities shows that the spinodal decomposition model accounts neither for the measured contrast of the electronic densities nor for the anomalous effects observed near the absorption edges of the elements contained in the sample. The measured intensity variation is large and of opposite sign when the Cu and Ni edges are approached from the lower energy side: a decrease of 30% for an energy variation from 292 to 8 eV below the Ni K-absorption edge and a sharp increase of 140% from 300 to 7 eV below the Cu K edge. A comparison with previous studies on the same system indicates that the decomposition process investigated in our samples corresponds more to the nucleation and growth regime than to the spinodal one. We found that the assumption of (Cu_0.52Ni_0.48)₃Sn precipitates having D0₂₂ structure is in agreement with our experimental results.