Metastable phase formation in nickel-aluminum alloys during ion beam mixing

Phase transformations induced by ion beam mixing of nickel-aluminum alloys with 500 keV krypton ions have been investigated over a range of temperatures, compositions, and initial structures. 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 800K using the 2 MV ion accelerator at ANL. Phase transformations were observed during both in-situ irradiations in the High Voltage Electron Microscope at Argonne and also in subsequent electron diffraction analysis of an array of irradiated samples. Metastable phases formed include disordered crystalline structures, an amorphous structure, and a hexagonal close-packed structure. These phase structures were modeled using the embedded atom method to compute energies of transition from stable to metastable states. The effects of both thermodynamics and kinetics on metastable phase formation are evaluated