Small-Angle Neutron Scattering Analyses of Phase Separation Processes in Amorphous Alloys

During thermal annealing, amorphous Fe₄₀Ni₄₀P₂₀ becomes brittle via a two-step process at 220 and 300°C. The first step results from a loss of excess free volume. This embrittlement is reversible: During subsequent neutron irradiation, a swelling of the alloy is observed, which corresponds to an increase in excess volume and a complete restoration of the ductility. Small-angle neutron scattering reveals that the second step of embrittlement, during which the specimen remains fully amorphous, is induced by phase separation into regions enriched and depleted in phosphorus. If amorphous Fe₄₀Ni₄₀P₂₀ is exposed to neutron irradiation prior to the heat treatment, a similar phase separation into amorphous phosphorus-enriched and phosphorus-depleted regions occurs. While the radius of the phosphorus-rich regions is about the same regardless of whether or not the specimen has been irradiated, the onset of phase separation occurs at lower temperatures for preirradiated samples; under identical annealing conditions, the volume fraction of phosphorus-rich clusters is much larger in preirradiated FeNiP than in unirradiated material. The faster phase separation kinetics are a consequence of the irradiation-induced excess volume that allows for an increased mobility of individual atoms.