A thermodynamic and kinetic basis for understanding metastable phase formation during ion-beam mixing of nickel-aluminum alloys

Abstract

A quantitative thermodynamic explanation for the formation of metastable phases in the nickel-aluminum alloy system through heavy-ion irradiation is presented. The role of kinetics in the transformation to a metastable state is also investigated. Experiments involved the irradiation of both layered nickel-aluminum samples and ordered intermetallic compounds with 500 keV krypton ions over a range of temperatures and compositions. Samples were formed by alternate evaporation of layers of nickel and aluminum. A portion of these samples was subsequently annealed to form intermetallic compounds. Irradiations were performed at both room temperature and 80 K using the 2 MV ion accelerator at Argonne National Laboratory. Phase transformations were observed during both in situ irradiations in the high-voltage electron microscope at Argonne and also in subsequent electron diffraction analyses of an array of irradiated samples. Metastable phases formed included disordered crystalline structures, an amorphous structure, and a hexagonal-close-packed structure. These phase structures were modeled using the embedded atom method to compute heats of transformation ΔHs–ms from stable to metastablestates. It was found that metastable states that have moderate heats of transformation, ΔHs–ms ≍ 15%–20% of the heat of formation of the stable phase, form under irradiation. Metastable states with high heats of transformation, ΔHs–ms ≍ 50% of the heat of formation of the stable phase, do not form under irradiation. Kinetics also play an important role in determining the effect of temperature and initial structure on the formation of metastable phases.