52 research outputs found
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A unified description of crystalline-to-amorphous transitions
Amorphous metallic alloys can now be synthesized by a variety of solid-state processes demonstrating the need for a more general approach to crystalline-to-amorphous (c-a) transitions. By focusing on static atomic displacements as a measure of chemical and topological disorder, we show that a unified description of c-a transformations can be based on a generalization of the phenomenological melting criterion proposed by Lindemann. The generalized version assumes that melting of a defective crystal occurs whenever the sum of thermal and static mean-square displacements exceeds a critical value identical to that for melting of the defect-free crystal. This implies that chemical or topological disorder measured by static displacements is thermodynamically equivalent to heating, and therefore that the melting temperature of the defective crystal will decrease with increasing amount of disorder. This in turn implies the existence of a critical state of disorder where the melting temperature becomes equal to a glass-transition temperature below which the metastable crystal melts to a glass. The generalized Lindemann melting criterion leads naturally to an interpretation of c-a transformations as defect-induced, low-temperature melting of critically disordered crystals. Confirmation of this criterion is provided by molecular-dynamics simulations of heat-induced melting and of defect-induced amorphization of intermetallic compounds caused either by the production of Frenkel pairs or anti-site defects. The thermodynamic equivalence between static atomic disorder and heating is reflected in the identical softening effects which they have on elastic properties and also in the diffraction analysis of diffuse scattering from disordered crystals, where the effect of static displacements appears as an artificially-enlarged thermal Debye-Waller factor. Predictions of this new, unified approach to melting and amorphization are compared with available experimental information
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In-situ HVEM studies of radiation-induced segregation in Ni-Al alloys during simultaneous irradiation with electrons and ions
The effects of 75-keV Ne{sup +} and 300-keV Ni{sup +} bombardment on electron radiation-induced segregation (RIS) in a Ni-9at.% Al alloy were investigated in-situ using the HVEM (high voltage electron microscope) / Tandem accelerator facility at Argonne National Laboratory. The radial component of defect fluxes generated by a highly-focused 900-keV electron beam was used to induce segregation of Al atoms towards the center of the electron irradiated area via the inverse Kirkendall effect. The radial segregation rate was monitored by measuring the increase in the diameter of the Al enriched zone within which {gamma}{sup `}-Ni{sub 3}Al precipitates form during irradiation. Both dual electron-ion and pre-implanted ion- electron irradiations were performed in an attempt to separate the contributions of energetic displacement cascades and implanted ions acting as defect trapping sites to RIS suppression. It was found that 75-keV Ne{sup 3} implantation has a retarding effect on RIS
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Effects of ion implantation and temperature on radiation-induced segregation in Ni-9Al alloys
Effects of Ne and Sc implantation on radiation-induced segregation (RIS) in Ni-9at.%Al were studied in-situ using the high-voltage electron microscope/Tandem accelerator at ANL. A highly-focused 900- keV electron beam generated radial defect fluxes which, in turn, induced transport of Al atoms toward the center of the electron- irradiated area via the inverse Kirkendall effect. Radial segregation rate of Al atoms was monitored by measuring the diameter of the {gamma}{prime}-Ni{sub 3}Al zone which formed in the Al-enriched area during irradiation. Ne and Sc implantation effects on RIS were investigated at 550 C; Ne effects were also examined at 625 C to determine effect of temperature on ability of Ne to act as defect trapping sites, causing RIS suppression. It was found that the RIS suppression effect of Ne increased with irradiation temperature and that Sc had a small RIS suppression effect which increased with Sc implantation dose. Ne bubbles which formed during implantation are believed to be responsible for its strong suppression effect. 6 figs, 12 ref
Effects of Ion Implantation and Temperature on Radiation-Induced Segregation In Ni-9Al Alloys
Effects of Ne and Sc implantation on radiation-induced segregation (RIS) in Ni-9at.%Al were studied in-situ using the high-voltage electron microscope/Tandem accelerator at ANL. A highly-focused 900- keV electron beam generated radial defect fluxes which, in turn, induced transport of Al atoms toward the center of the electron- irradiated area via the inverse Kirkendall effect. Radial segregation rate of Al atoms was monitored by measuring the diameter of the {gamma}{prime}-Ni{sub 3}Al zone which formed in the Al-enriched area during irradiation. Ne and Sc implantation effects on RIS were investigated at 550 C; Ne effects were also examined at 625 C to determine effect of temperature on ability of Ne to act as defect trapping sites, causing RIS suppression. It was found that the RIS suppression effect of Ne increased with irradiation temperature and that Sc had a small RIS suppression effect which increased with Sc implantation dose. Ne bubbles which formed during implantation are believed to be responsible for its strong suppression effect. 6 figs, 12 ref
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Ion implantation at elevated temperatures
A kinetic model has been developed to investigate the synergistic effects of radiation-enhanced diffusion, radiation-induced segregation and preferential sputtering on the spatial redistribution of implanted solutes during implantation at elevated temperatures. Sample calculations were performed for Al and Si ions implanted into Ni. With the present model, the influence of various implantation parameters on the evolution of implant concentration profiles could be examined in detail
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Modifications of subsurface alloy composition during high-temperature sputtering
Changes in the subsurface composition of concentrated binary alloys during high-temperature sputtering were studied using a kinetic model that includes Gibbsian adsorption, preferential sputtering, displacement mixing, radiation-enhanced diffusion, and radiation-induced segregation. Numerical solutions were obtained for Cu-Ni alloys under 5-keV Ar/sup +/ ion bombardment as functions of sputtering time and temperature. The effects of these various phenomena were examined in detail. The present calculations may be of importance in the areas of plasma contamination in fusion reactors, sputter depth-profiling, and elevated-temperature ion implantation
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Damage-rate gradient effects on radiation-induced segregation and phase stability in irradiated alloys
Recent studies have shown that significant compositional redistribution in irradiated alloys can be induced by the gradients in the atomic displacement rates resulting from nonuniform defect production, in addition to the commonly-observed solute segregation at defect sinks. This process gives rise to complex local phase transformations during light-ion bombardment or irradiation with focused electron beams in the high-voltage electron microscope. Results of our theoretical and experimental investigations of this phenomenon in Ni-Al and Ni-Si are discussed. The implications of the observed effect in a number of areas of materials science are assessed
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Radiation-induced compositional redistribution and local phase transformation in irradiated alloys
In order for radioinduced segregation to occur, certain alloy elements need to be coupled to radioinduced defect fluxes. Some observations of local phase changes in Ni-based alloys during ion bombardment and high-voltage electron-microscopic irradiation are reported in this paper. (DLC
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Ion-bombardment-induced subsurface composition modifications in alloys at elevated temperatures. [Cu - 40 at. % Ni]
Modifications of subsurface alloy composition during high-temperature sputtering were studied using a comprehensive kinetic model that includes Gibbsian adsorption, preferential sputtering, displacement mixing, radiation-enhanced diffusion, and radiation-induced segregation. Numerical solutions were obtained for a Cu-40 at. % Ni alloy under 5-keV Ar/sup +/ ion bombardment as functions of time and temperature. The model predictions are in good qualitative agreement with recent experimental measurements
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Nonequilibrium segregation and phase instability in alloy films during elevated-temperature irradiation in a high-voltage electron microscope
The effects of defect-production rate gradients, caused by the radial nonuniformity in the electron flux distribution, on solute segregation and phase stability in alloy films undergoing high-voltage electron-microscope (HVEM) irradiation at high temperatures are assessed. Two-dimensional (axially symmetric) compositional redistributions were calculated, taking into account both axial and transverse radial defect fluxes. It was found that when highly focused beams were employed radiation-induced segregation consisted of two stages: dominant axial segregation at the film surfaces at short irradiation times and competitive radial segregation at longer times. The average alloy composition within the irradiated region could differ greatly from that irradiated with a uniform beam, because of the additional atom transport from or to the region surrounding the irradiated zone under the influence of radial fluxes. As a result, damage-rate gradient effects must be taken into account when interpreting in-situ HVEM observations of segregation-induced phase instabilities. The theoretical predictions are compared with experimental observations of the temporal and spatial dependence of segregation-induced precipitation in thin films of Ni-Al, Ni-Ge and Ni-Si solid solutions
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