Search

Pure nickel and lithium-implanted nickel were irradiated with 1 MeV electrons at the Argonne HVEM-Tandem facility. The lithium-implanted nickel was not damaged as readily as the pure nickel under 1 MeV electron irradiation in the temperature range 20-600°C. The decreased damage accumulation in the lithiated nickel could arise from both the precipitated lithium and that in solution. The latter could slow down the nickel interstitials produced during irradiation by association, thus enhancing their recombination with vacancies. The interfaces of the precipitated lithium could also act as unbiased sinks for the vacancies and interstitials. Above 600°C, under electron irradiation, a melting process which created small irregular shaped regions occurred in the lithium-implanted nickel. Electron diffraction analysis showed that the compound Li2NiO2 was formed inside these regions.

The growth of Ni3Si surface films on Ni-12.7at%Si alloys has been measured during lMeV electron irradiation. Stereoscopic techniques were used to determine film thickness from dark field images formed from Ni3Si superlattice reflections. Parabolic growth kinetics are observed at lower temperatures. However, at higher temperatures, deviations from parabolic kinetics are observed after short irradiation times. Such deviations have not been observed in bulk specimens during bombardment with energetic ions and, therefore, may be due to foil thickness effects.

The phase microstructure of alloys is frequently severely altered during irradiation. Sluggish precipitation reactions including precipitation coarsening are accelerated by irradiation-enhanced diffusion. Radiation-induced segregation redistributes existing precipitate phases within the microstructure, induces precipitation of nonequilibrium phases and affects the composition of phases in multicomponent alloys. The displacement process causes disordering of ordered alloys and frequently amorphization, especially in intermetallic compounds, at low temperature. Although a good qualitative understanding of the basic process involved, i.e., displacement mixing, radiation-enhanced diffusion and radiation-induced segregation exists, methods for detailed quantitative modeling of the evolution of the microstructure of alloys remain to be developed.

The radiation-induced growth of Ni3Si films on the surfaces of Ni(Si) alloys containing ≤ 13 at.% Si is well established. We have observed additional segregation of silicon during irradiation within the ordered Ni3Si phase. Post-irradiation depth profiling by Auger electron spectroscopy, as well as in situ analysis by high-resolution Rutherford backscattering spectrometry, reveals Si-enrichment at the surfaces of Ni(Si) alloys in excess of stoichiometric Ni3Si during irradiation. Thin, near-surface layers with silicon concentrations of 28-30 at.% are observed, and even higher Si enrichment is found in the first few atom layers. Transmission electron microscopy and selected area-electron diffraction were employed to characterize these Si-enriched layers. A complex, multiple-spot diffraction pattern is observed superposed on the diffraction pattern of ordered Ni3Si. The d-spacings obtained from the extra spots are consistent with those of the orthohexagonal intermetallic compound Ni5Si2.

A number of processes change the structure and the distribution of alloying elements in near-surface regions of alloys during ion bombardment. Displacements and displacement cascades lead to a randomization of atom locations and alloying elements: ordered alloys become disordered; concentration gradients are smoothed; crystalline materials become highly defective or even amorphous. Displacement processes dominate at low temperatures where thermal rearrangements of atoms are minimal. At higher temperatures, defects become mobile, and the resulting enhanced diffusion counteracts the randomization resulting from the displacement processes and aids in the approach to thermodynamic equilibrium. With increasing temperature and mobility of point defects, annihilation of defects at sinks, such as dislocations, grain boundaries and the surface, become dominant, causing persistent defect fluxes. Preferential coupling of certain alloying elements to defect fluxes may cause redistribution of alloying elements between regions around sinks and the matrix, a phenomenon termed radiation-induced segregation (RIS). The induced concentration changes frequently are large enough to precipitate a second phase in solid solution alloys or to dissolve precipitates in two-phase alloys. Complex redistribution of alloying elements and phases via RIS can occur throughout and well beyond the damage range. In addition, preferential loss of elements by sputtering affects the alloy composition in the near-surface region. Such loss can occur directly by preferential sputtering, or indirectly by Gibbsian surface adsorption or RIS which modify the composition at the surface, and hence, the composition of the sputtered material.

During ion bombardment, a number of processes can alter the compositional distribution and microstructure in near-surface regions of alloys. The relative importance of each process depends principally on the target composition, temperature, and ion characteristics. In addition to displacement mixing leading to a randomization of atomic locations, and preferential loss of alloying elements by sputtering, which are dominant at relatively low temperatures, several thermally-activated processes, including radiation-enhanced diffusion, radiation-induced segregation and Gibbsian adsorption, also play important roles. At elevated temperatures, nonequilibrium point defects induced by ion impacts become mobile and tend to anneal out by recombination and diffusion to extended sinks, such as dislocations, grain boundaries and free surfaces. The high defect concentrations, far exceeding the thermodynamic equilibrium values, can enhance diffusion-controlled processes, while persistent defect fluxes, originating from the spatial non-uniformity in defect production and annihilation, give rise to local redistribution of alloy constituents because of radiation-induced segregation. Moreover, when the alloy is maintained at high temperature, Gibbsian adsorption, driven by the reduction in free energy of the system, occurs even without irradiation; it involves a compositional perturbation in a few atom layers near the alloy surface. The combination of these processes leads to the complex development of a compositionally-modified layer in the subsurface region. Considerable progress has been made recently in identifying and understanding the relative contributions from the individual processes under various irradiation conditions. In the present paper, selected examples of these different phenomena and their synergistic effects on the evolution of the near-surface compositions of alloys during sputtering and ion implantation at elevated temperatures are discussed.

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+ ion bombardment as functions of time and temperature. The model predictions are in good qualitative agreement with recent experimental measurements.

An important aspect of displacement-induced radiation damage of ceramic materials that has received only occasional attention is the characterization of the microstructure by transmission-electron microscopy.1 The emphasis of the majority of the research has been on measurements of macroscopic properties such as density changes, mechanical strength, and electrical and thermal conductivity,2 also, some work has been performed on point defects, using such techniques as electron-spin resonance, electrical-resistivity measurements, and optical spectroscopy.2,3 Characterization of the radiation-induced defect microstructure should yield important complementary information for macroscopic as well as point-defect measurements.

A simple theory of the swelling of materials subjected to high energy particle irradiation is developed. Chemical reaction rate equations are used as a basis. Point defects, interstitials and vacancies, are assumed to be produced randomly throughout the solid. They move by random walk through the material until they cease to exist either by recombination with the opposite type of defect or by incorporation into the crystal at sinks such as dislocations, grain boundaries and voids. The rate equations for interstitials and for vacancies, which are coupled via the recombination term, are solved for steady state conditions under irradiation. Defect concentrations, supersaturations, recombination and total sink annihilation rates are obtained in terms of the production rate, sink annihilation probabilities, jump frequencies and thermal equilibrium concentrations of defects. The swelling rate is derived using sink annihilation probabilities at three principally different types of sinks, i.e. voids, sinks which have a bias with regard to the annihilation of interstitials and vacancies (such as dislocations), and sinks with no bias. The defect annihilation probabilities at void, precipitate, dislocation and grain boundary sinks are estimated by using a cellular model and solving the diffusion equation for geometries approximating that of the cells, e.g. a concentric sphere around a void. The relative effects of different types of sinks, i.e. the microstructure, on the swelling rate is discussed. The swelling rate is integrated to give swelling-time or swelling-dose relations, making some simplifying assumptions about the changes in the sink structure as the irradiation proceeds. It is shown that the relation obtained is rather sensitive to the type of assumptions made.

The effect of changes in strain rate ad temperature on the flow stress of Mg single crystals VIM determined m the range of 4·2° to 420°k. The present data, along with those obtained previously, indicate that the intersection of dislocations is the controlling mechanism. Cottrell's original model must, however, be modified to include an effect of stress on the activation volume. This effect appears to be due primarily to the fact that the force between two intersecting dislocations varies with their distance of separation rather than a change in the effective forest spacing.