Small-angle X-ray scattering from phase-separating amorphous metallic alloys undergoing nanocrystallization

We performed a combined small- and wide-angle X-ray scattering (SAXS/WAXS) study of nanocrystallization during annealing of binary Al92Sm8 and Al91Gd9 melt-spun glassy alloys. In course of the transformation an interference maximum develops in the SAXS region while WAXS spectra show formation of fcc-Al nanocrystals in the glassy phase. In order to determine the origin of the SAXS maximum the theoretical SAXS intensity and distance distribution function were calculated considering two structural models. The first model represented a single nanocrystal with a solute layer on its surface and the second one a phase-separated alloy with spatially correlated compositional fluctuations. The results of the calculations are compared with the experimental data. It is demonstrated that only the model representing correlated fluctuations reasonably reproduces the experimental results. We conclude that nano-scale glassy phase separation occurs in the investigated alloys and the nanocrystals form inside the Al-rich amorphous regions.