Metastable phase formation by ion beam mixing

There are essentially four basic types of metastable alloys which may be formed through heavy ion irradiation of crystalline structures: amorphous phases with no long range order; crystalline phases with structures different from that of stable intermetallic alloy; disordered crystalline phases with structures based on the same lattice as that of the stable intermetallic; and a quasicrystalline structure. With the exception of the quasicrystalline structure, all of these metastable structures are produced by ion beam mixing of nickel-aluminum alloys with 500 keV krypton ions. Ion beam mixing was performed on samples formed by alternate evaporation of layers of nickel and aluminum as well as on the intermetallic compounds at both 80 and 300 K. The structure resulting from ion beam mixing depended strongly on composition, and hence its formation was governed primarily by thermodynamic considerations. The thermodynamically favored state was determined analytically using the embedded atom method, and the model results are in qualitative agreement with observations of metastable phase formation. However, kinetic considerations are needed to explain the dependence of the final structures on initial structure and temperature. (orig.)