Surface exafs study of the crystallization of a metallic glass

The crystallization of an amorphous solid on high-temperature annealing may occur progressively from the surface into the bulk or uniformly throughout the whole sample. We have studied crystallization by means of surface EXAFS in the metallic glass Fe40Ni32Cr8P12B8. Samples were annealed at various temperatures and times ranging from 240°C for 10 min to 400°C for 42 h; X-ray diffraction showed the gradual formation of crystalline phases. EXAFS measurements were made from the surface to about 4000 Å deep, with a depth resolution of a few hundred Å. The EXAFS results were quite sensitive to the crystallization, an effect quantified by model fitting in which the Debye-Waller factor σ2 was varied as a parameterization of the local static disorder. We find that for partly crystallized samples σ2 increases at the surface compared to deeper inside the sample. Since σ2 is a direct measure of local static disorder (thermal vibrations are not important here as all measurements were made at room temperature), in this sytem these results imply that crystallization begins at the surface before it occurs in the bulk.