INVESTIGATION OF PHASE-FORMATION SEQUENCE IN THE IRON-ALUMINUM PHASE-DIAGRAM USING SUPERLATTICE COMPOSITES AS REACTANTS

The low-temperature reaction of iron-aluminum superlattice composites as a function of composition and layer thicknesses was explored. Sampl es were largely amorphous but with embedded nanocrystalline FeAl or al pha-Fe in the as-deposited state. It was not generally possible to for m a homogeneous amorphous alloy via a solid-state amorphization reacti on. In most of the composites investigated, FeAl was the first interfa cial compound observed to form regardless of layer thicknesses or over all composition. The second phase formed was Fe2AI5. Several exception s to this general behavior were seen, however, illustrating the kineti c nature of the nucleation process. It is suggested that FeAl generall y formed first for two reasons. The first is the extremely wide combin ed stability field of disordered bcc alpha-Fe and ordered bcc FeAl. Be cause the starting multilayer had a large number of interfaces, and be cause intermixing at these interfaces appears to have been relatively high, a significant fraction of the starting multilayer must have had a composition falling within this broad stability field, making it eas y for crystallites of the bec material to form. The second reason is t hat FeAl has a small unit cell, containing only two atoms. All other b inary Fe/Al phases have larger unit cells.