R. S. Averback

Ion beam mixing at nickel-silicon interfaces

Ion‐beam mixing at Ni‐Si interfaces was studied as a function of dose, temperature, and dose rate. In the temperature range 10–443 K, the thickness of the mixed Ni‐Si layer grew proportionally to the square root of dose for 250‐keV Ar+ and 280‐keV Kr+ irradiations. The apparent diffusion coefficient for mixing during irradiation comprises temperature‐dependent and independent contributions. The activation enthalpy for the temperature‐dependent part is ∼0.09 eV. Varying the dose rate by a factor of ∼20 had little effect upon mixing at either 80 or 368 K. Analysis of these results using chemical rate theory showed that the low activation enthalpy for diffusion was not associated with point‐defect motion. More likely the temperature dependence of mixing in Ni‐Si is associated with the effect of temperature on compound formation. The results for mixing at 10 K in Ni‐Si were compared with those in Ni‐Al, at 10 K, to study the effect of crystal structure on displacement mixing.

Solute redistribution processes in ion bombarded alloys

Abstract An overview of radiation-induced solute redistribution process in ion bombarded alloys is presented and some applications to damage correlation studies are described. The type and amount of solute redistribution which occurs and its temporal and spatial dependence are affected not only by the target material and irradiation conditions, but also by the mass and energy of the incident ion. Because of the sensitivity of radiation damage effects in alloys to changes in composition, an understanding and an appreciation of the magnitude of these redistribution effects are essential for a proper interpretation and correlation of damage microstructures produced in alloys by different bombarding ions. Furthermore, once understood, these redistribution effects can be used to quantify differences in the calculated atomic displacement rates and the net production rates of freely migrating defects for different ions. Such information is a prerequisite for quantitative damage correlation, and requires a quantitative understanding of the dose-rate dependence of solute redistribution effects. Recent experimental and theoretical studies which provide the basis for such applications of solute redistribution effects are discussed.

Correlations between ion and neutron irradiations: Defect production and stage I recovery

Abstract A comparison is made between results of recent neutron- and ion-irradiation experiments on metals. Specifically, lowtemperature defect production and stage-I annealing are discussed. Defect production efficiencies observed for neutron irradiations are shown to be consistent with results for ion irradiations. A phenomenological model describing both defect production and stage-I annealing in terms of thermal-spike induced recombinations is introduced. The stage-I recovery fraction for ion and neutron irradiations is found to correlate with cascade-energy density as represented by an appropriate nuclear stopping power.

Fundamental aspects of ion beam surface modification: defect production and migration processes

Abstract Ion beam modification of metals is generating increasing scientific interest not only because it has exciting technological potential but also because it has raised fundamental questions concerning radiation-induced diffusion processes. In addition to the implanted species, several defect production and migration mechanisms contribute to changes in the near-surface composition of an alloy during ion bombardment, e.g. atoms exchange positions via displacements and replacement sequences, preferential sputtering effects arise, and radiation-enhanced diffusion and radiation-induced segregation occur. The last two defect migration mechanisms are of particular significance since they can alter the composition to depths which are much greater than the implanted ion range. By altering various parameters such as irradiation temperature, ion mass, energy and current density and by changing the initial alloying distributions, a rich variety of near-surface composition profiles can be created. We have utilized changes in ion mass and energy and irradiation temperature to distinguish defect production from defect migration effects. Experimental results are presented which provide a guide to the relative efficiencies of different mechanisms under various irradiation conditions.

Efficiency of defect production in cascades

In copper and silver, the defect production by energetic ions has been investigated for projectile masses between 1 and 209 in the energy range between 10/sup 4/ and 10/sup 6/ eV. Thin-film residual-resistivity measurements were used to determine the defect production below 10K. Calculations of the number of Frenkel pairs produced by various projectiles as a function of energy were performed, based on Lindhards cross sections and a modified Kinchin-Pease expression. A comparison of measured and calculated damage rates shows that the defect-production efficiency decreases with ion mass for light projectiles, whereas at masses >20 the cascade efficiency remains rather constant and assumes a value of xi approximately 0.3 for both Ag and Cu. The decrease in defect efficiency indicates a transition from isolated Frenkel-pair formation to the production of energetic displacement cascades.

Defect production in ion-irradiated aluminum☆

Abstract Residual-resistivity measurements on ion-irradiated thin films are used to study defect production in aluminum. The energy and mass dependences of damage rates indicate that defect production efficiency (relative to the modified Kinchin-Pease relation) strongly decreases at recoil energies between 5 and 10 keV. At high energy the efficiency approaches an asymptotic value of 0.5. These results are similar to those observed in copper and silver. Binary-collision simulations were performed for copper, silver and aluminum to investigate the relationship of cascade structure to defect production efficiency. The mechanism primarily responsible for the reduced defect production efficiency in cascades in aluminum is thought to be the subthreshold recombination of Frenkel pairs during the thermal-spike phase of the cascade.

Radiation‐enhanced diffusion in Ni/Zr diffusion couples

Radiation‐enhanced diffusion and ion‐beam mixing in Ni/Zr diffusion couples were investigated. The Kirkendall geometry was employed for this experiment to determine the radiation‐enhanced diffusivities of both Ni and Zr in the amorphous (a‐) NiZr interdiffusion zone. W, Au, and Hf were employed as markers. The activation enthalpy for radiation‐enhanced interdiffusion was determined to be ≊0.4 eV, and Ni was found to be the faster moving species. The dose‐rate dependence of the interdiffusion coefficient in the a‐NiZr was typical of that of crystalline materials and suggests a point‐defect‐like diffusion mechanism in irradiated metallic glasses.

Ion-irradiation studies of cascade damage in metals

Abstract Ion-irradiation studies of the fundamental aspects of cascade damage in metals, which have been performed at Argonne National Laboratory, are reviewed. The emphasis of these studies has been the determination of the primary state of damage (i.e. the arrangement of atoms in the cascade region prior to thermal migration of defects). Progress has been made towards understanding the damage function (i.e. the number of Frenkel pairs produced as a function of primary recoil atom energy), the spatial configuration of vacancies and interstitials in the cascade and the cascade-induced mixing of atoms. It is concluded from these studies that the agitation of the lattice in the vicinity of energetic displacement cascades stimulates defect motion and that such thermal-spike motion induces recombination and clustering of Frenkel defects.

Magnetic phase transitions in nanocrystalline erbium

Nanocrystalline samples of 99.9% pure erbium with particle sizes between 10 and 70 nm have been produced by the inert gas condensation technique. Magnetic susceptibility measurements on samples compacted to between 50% and 75% of bulk density indicate a modification in the high‐temperature phases and superparamagnetic behavior at low temperatures. ESR signals near g=10 are probably due to Er compounds which may form during the production process.

Defect clustering in copper, silver and aluminum during heavy-ion irradiations at low temperatures

Abstract Point defect clustering during heavy-ion irradiation at low temperature was investigated in copper, silver, and aluminum using electrical-resistivity measurements. Clustering effects were studied by determining the average recombination volume per Frenkel defect, V, as a function of defect concentration. Within single unoverlapped displacement cascades, the clustering increased with increased energy density in the cascades. For Ar-irradiated aluminum no clustering was observed in individual cascades, whereas for Kr-irradiated silver V was deduced to be V 0.2 Vo, where Vo is the value of V for unclustered defects. Clustering was observed for all ion-target combinations at sufficiently high dose. For copper and silver at high doses, a regime was reached in which additional irradiation increased the Frenkel defect number N without increasing the total recombination volume N V. Defect clustering in copper and silver during isochronal annealing of damage produced by 150-keV proton irradiation was inve...