H. Wiedersich

Segregation of alloying elements to free surfaces during irradiation

Abstract Recent transmission electron microscopy examinations of a number of face-centered-cubic and body-centered-cubic metals and alloys irradiated by heavy ions or by high-energy electrons have shown thatdynamic interactions of displacement damage with impurities and alloying elements lead to segregation and/or to the formation of second phases at internal surfaces such as voids. To date, the phenomenon has been observed in an experimental 18Cr8Ni1Si stainless steel, in commercial 316L stainless steel, in vanadium and in nickel. In the electron irradiated Fe18Cr8Ni1Si alloy, analysis of the segregation-induced strain field around the voids indicates that during irradiation minor substitutional alloying elements with negative and positive size factors segregate towards and away from the void surface respectively. Preliminary Auger spectroscopy analysis indicates that a similar segregation phenomenon occurs at the external irradiated surface in nickel-ion bombarded 18Cr8Ni1Si stainless steel. These results suggest that undersized substitutional elements may tend to preferentially interchange positions with oversized solutes in interstitial sites, and that transport by interstitials may dominate segregation to defect sinks.

A theory of radiation-induced segregation in concentrated alloys☆

Abstract A new and simple theory for radiation-induced segregation in concentrated alloys is presented. The coupling between defect fluxes and atom fluxes is accounted for by the concept of preferential migration of vacancies and interstitials via A-atoms or B-atoms in a binary A-B alloy. Similarly, atom fluxes are partitioned into those occurring via vacancies and via interstitials. This approach permits expression of the defect fluxes and atom fluxes in terms of partial diffusivity coeffi- cients and concentration gradients of defects and alloy components. The time and space dependence of the defect concen- trations and composition of a binary alloy is described by a set of three coupled partial differential equations containing four partial diffusivity coefficients, i.e., those of A-atoms and B-atoms diffusing via vacancies and via interstitials. The set of differential equations has been integrated for some model binary alloys with complete miscibility, utilizing the geometry of a thin foil. The sample calculations are in good qualitative agreement with the general features of radiation-induced segregation as deduced from experiments. The temperature, dose and dose-rate dependencies of segregation in concentrated alloys are found to be similar to those predicted by the Johnson-Lam model for dilute alloys.

Effect of solute misfit and temperature on irradiation-induced segregation in binary Ni alloys☆

Abstract Four solid-solution, binary alloys of 1 at.% Al, Ti, Mo and Si in Ni were irradiated with 3.5-MeV Ni + ions at temperatures between 385 and 660° C. Auger analysis of the solute concentration as a function of depth shows that the oversize solutes, Al, Ti and Mo, are depleted near the irradiated surface, whereas the undersize solute, Si, is enriched. The magnitude of this irradiation-induced segregation in the Ni-1 at.% Si alloy is sufficient to cause precipitation of a surface layer of Ni 3 Si after a total dose of 5 dpa near 600° C; the segregation diminishes at both lower and higher temperatures. The observed temperature dependence is in qualitative agreement with a recently proposed theory of irradiation-induced solute segregation, but quantitative differences exist.

Nucleation theory without Maxwell Demons

The equations for steady-state nucleation are derived from the rates of growth and decay of clusters with emphasis on a clear distinction between thermodynamic quantities and inherently kinetic quantities. It is shown that the emission rates of molecules from embryos can be related to the equilibrium size distribution of clusters in a saturated vapor. It is therefore not necessary to invoke the existence of an embryo size distribution constrained be in equilibrium with a supersaturated vapor. The driving force for nucleation is shown to be a kinetic quantity called the condensation rate ratio, i.e., the ratio of the rates of acquisition of molecules by clusters in the supersaturated vapor to that in a saturated vapor at the same temperature, and not a thermodynamic quantity known as the supersaturation, i.e., the ratio of the actual pressure to the equilibrium vapor pressure.

Effect of mobile helium on void nucleation in materials during irradiation

Abstract The steady-state rate of void nucleation is calculated for irradiated materials containing mobile helium. At the low displacement rates typical of a fast-breeder reactor a concentration of less than 10 −10 atom fraction helium can cause a 10 20 increase in nucleation rate. The helium is less effective at the high displacement rates typical of accelerator experiments, but can increase the void-nucleation rate by 10 4 at a helium concentration of 10 −8 . The calculated void-nucleation rates for low displacement rates and without helium are too low to explain the void number densities observed in breeder-reactor irradiated materials. Therefore, void nucleation in reactor environments is helium-assisted. Accelerator experiments intended to simulate void nucleation under reactor conditions must be carefully designed to observe gas-assisted rather than homogeneous void nucleation.

Bombardment-induced segregation and redistribution

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.

Effects of solute segregation and precipitation on void swelling in irradiated alloys

Abstract A model of radiation-induced segregation of substitutional solutes has been used to study the effects of solute segregation and precipitation on void swelling behavior in metals and alloys. If the binding energies between point defects and solute atoms are sufficiently large, significant solute segregation takes place during irradiation, giving rise to a solute depletion or enrichment at defect sinks and a nonuniform distribution of defect-trapping centers in the matrix. The swelling reduction, which results from trapping of point defects by minor additions of alloying elements, is affected by radiationinduced solute redistribution in alloys. The effects of either dominant interstitial-solute interactions or dominant vacancy-solute interactions on segregation and swelling were studied. The present calculations are in good qualitative agreement with experimental results.

Kinetic processes during ion bombardment

Abstract 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.

Sputter-induced surface composition changes in alloys

Abstract Radiation-induced redistribution of alloying elements in the near-surface region of dilute binary alloys during low-energy Ar+ ion sputtering was calculated using a kinetic model that includes the effects of radiation-induced segregation and preferential sputtering. Changes in the alloy surface composition were calculated as functions of sputtering time, temperature, ion flux and initial alloy composition for Ni-based model alloys. In the temperature range 200–850°C, radiation-induced segregation is dominant initially and the surface is enriched with or depleted of solutes whose fluxes are coupled predominantly to interstitial or vacancy fluxes, respectively. As the bombardment time increases, the effects of preferential sputtering become dominant and the surface composition approaches a steady-state value determined by the sputtering coefficients of the alloy components. Below 200 and above 850°C, the surface composition is altered by preferential sputtering because radiation-induced segregation is insignificant. The time required to achieve steady state increases with increasing temperature and decreasing ion flux. The present calculations may be of importance in the areas of sputter depth-profiling, sputter etching, and plasma contamination in fusion reactors.

Modifications of subsurface alloy composition during high-temperature sputtering

Abstract 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+ 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.