Metastable Phase Formation in Ion Irradiated Nickel-Aluminum Alloys (Amorphous).

Abstract

Phase transformations induced by ion beam mixing of nickel-aluminum alloys with 500 keV krypton ions have been investigated over a range of temperatures (80 K to 300 K), composition (NiAl(,3), NiAl, Ni(,1)Al), initial structures (both nickel-aluminum layers and ordered intermetallic compounds), and doses (ranging from 2 x 10('14)cm('-2) to 5 x 10('16)cm('-2)). Samples were formed by alternate evaporation of layers of nickel and aluminum in high vacuum onto copper grids. These samples were checked for purity with energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, and Rutherford backscattering spectrometry. A portion of these samples was annealed to form the intermetallic compounds appropriate to the given composition. Irradiations were performed at both room temperature (300 K) 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 analysis of an array of samples irradiated in a target chamber. Metastable phases formed include disordered crystalline structures at compositions of 25% and 50% aluminum, an amorphous structure at 75% aluminum, and a hexagonal close-packed structure formed at 25% aluminum. These metastable states were all converted to the stable intermetallic compounds through annealing treatments. The thermodynamic heats of formation of the stable and metastable phases were computed using the embedded atom model. The results indicate that metastable phases with small heats of transformation (15-20% of the heat of formation of the stable intermetallic) are likely to form under irradiation, while phases with high heats of transformation (50% of the heat of formation) will not form. In addition, the effect of temperature and initial structure on the formation of the metastable states indicates that the kinetics of the transformation from the stable to the metastable state is important in determining when a metastable state will form. Previous work in this field is evaluated in light of the experimental results and the connection between several empirical rules of metastable phase formation and the thermodynamic and kinetic arguments proposed here is examined.