Automatic iterative fitting of Rutherford backscattering spectra from multielement samples

A computer code (BASF) has been constructed to perform automatic iterative fitting of Rutherford backscattering spectra using only the experimental spectrum and the parameter set defining the experiment. The code may be used to analyze samples containing anywhere from two to five elements. The code output consists of the total amount of each element present and a composition versus depth profile.The code's performance was verified on both computer generated and experimental backscattering spectra. Samples consisting of nickel substrates onto which layers of pure nickel and pure aluminum have been alternately evaporated in thicknesses of 130 and 100 A, respectively, were used to produce backscattering spectra. These spectra, when analyzed, demonstrated that the code was able to determine the total aluminum content to within 3% and the ratio of aluminum to nickel to within 1% of the thickness monitor readings taken during evaporation. The code has shown the ability to recognize sharp interfaces in well resolved spectra. The code performs equally well on slowly varying concentration profiles which are created during the annealing of layered samples. Limitations on the code and its use include the precise knowledge of the relevant experimental parameters used as input, and complete specification of all elements in the sample. The ultimate limits on the code's accuracy are the resolution of the spectrum and the accuracy of the computed stopping powers.This code provides a significant advantage over other spectrum fitting codes in that the process is fully automated and does not require constant user interaction. Further, it provides the capability of accurately determining concentration profiles in layered samples where the layer thickness is of the order 100 A.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25485/1/0000025.pd

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 the 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26736/1/0000287.pd

Grain growth and phase morphology in ion beam mixed, two phase Ni---Al and Ni---Cr---Al alloys

Multilayers of Ni---21Al and Ni---20Cr---10Al were subjected to ion beam mixing using 350 keV Ni+ ions and/or thermal annealing at 440[deg]C to study the development of grain growth and phase morphology. Two film thicknesses of Ni---21 Al (60 and 120 nm) were investigated. Both thermal annealing and irradiation resulted in grain growth in the Ni---21Al samples. Grain size increased by a factor of 7 after irradiation and only 3-4 after annealing. Annealing produced a two phase [gamma] + [gamma]' structure and nonuniform grain sizes while the irradiation produced a supersaturated solid solution with more uniform grain size. Annealing subsequent to irradiation produced a structure consisting of [gamma] + [gamma]' and an HCP phase. There was no difference in grain growth behavior as a function of film thickness. The Ni---Cr---Al film exhibited no grain growth during annealing and only a factor of 6 increase during irradiation. Irradiation alone or post-irradiation annealing produced nearly identical structures of [gamma] and the HCP phase. The [gamma]' phase was never observed in the Ni---Cr---Al film. All irradiated samples showed a more uniform grain size compared to that following annealing. Considerable texture was observed in all irradiated samples in which the gamma grains in the film were aligned with the Ni grains in the substrate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28023/1/0000459.pd

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

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.Ph.D.Nuclear engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161237/1/8702725.pd

Dual phase formation in multilayered Ni‐Al by ion beam mixing

Ion beam mixing is used to homogenize multilayered thin films of nickel and aluminum, vapor deposited onto pure nickel substrates. Doses of 500‐keV Kr+ at fluences of 2×1016 ions/cm2 produced a supersaturated solid solution of between 16 and 23 at. % aluminum in nickel at room temperature. Subsequent thermal treatment at 425 °C for 1 h resulted in the formation of a dual phase structure of γ and γ′ with grain sizes in the range 640 to 710 Å. Although the dual phase structure was obtained either by ion beam mixing followed by thermal annealing or by thermal annealing alone, only the former process resulted in a surface film with a texture. The films are stable against thermally induced grain growth at temperatures up to 700 °C for 10 h.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70280/2/JAPIAU-62-5-2145-1.pd