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. Samples were largely amorphous but with embedded nanocrystalline FeAl or alpha-Fe in the as-deposited state. It was not generally possible to form a homogeneous amorphous alloy via a solid-state amorphization reaction. In most of the composites investigated, FeAl was the first interfacial compound observed to form regardless of layer thicknesses or overall composition. The second phase formed was Fe2AI5. Several exceptions to this general behavior were seen, however, illustrating the kinetic nature of the nucleation process. It is suggested that FeAl generally formed first for two reasons. The first is the extremely wide combined stability field of disordered bcc alpha-Fe and ordered bcc FeAl. Because the starting multilayer had a large number of interfaces, and because 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 easy for crystallites of the bec material to form. The second reason is that FeAl has a small unit cell, containing only two atoms. All other binary Fe/Al phases have larger unit cells.