L. Funk

In situ ion irradiation /implantation studies in the HVEM-tandem facility at argonne national laboratory

Abstract The HVEM-Tandem User Facility at Argonne National Laboratory interfaces two ion accelerators, a 2 MV tandem accelerator and a 650 kV ion implanter, to a 1.2 MV high-voltage electron microscope. This combination allows experiments involving simultaneous ion irradiation/ion implantation, electron irradiation and electron microscopy/electron diffraction to be performed. In addition the availability of a variety of microscope sample holders permits these as well as other types of in situ experiments to be performed at temperatures ranging from 10 to 1300 K, with the sample in a stressed state or with simultaneous determination of electrical resistivity of the specimen. This article summarizes the details of the Facility which are relevant to simultaneous ion beam material modification and electron microscopy, presents several current applications and briefly describes the straightforward mechanism for potential users to access this US Department of Energy-supported facility.

Shock-wave production of nanoparticles during high-energy ion sputtering

Abstract Several previous studies have shown that the size distributions of smaller nanoparticles (n⩽40 where n is the number of atoms in a given cluster) generated by ion sputtering obey an inverse power law, with an exponent varying between −8 and −4, dependent upon the total sputtering yield. Such large negative exponents have not been explained by any simple physical mechanism. We reported electron microscopy studies of the size distributions of the larger nanoparticles (n>500) that are sputtered from the surface by high-energy ion impacts. These measurements also yielded an inverse power law, but one with an exponent of −2, and one that is independent of total sputtering yield. This inverse-square dependence indicates that the clusters are produced when shock waves, generated by sub-surface displacement cascades, impact and ablate the surface. Many smaller clusters can result from fragmentation of these larger ones, which helps explain the large negative exponents that have been reported previously. In this paper, we briefly review the previous results. In addition, we present new results demonstrating that the same inverse-square size distribution is generated in both transmission and reflection sputtering geometries. An important corollary from these results is that the sputtered nanoparticles consist of simple fragments of the original surface, that is particles which have not experienced any large thermal excursions. Hence high-energy ion sputtering should provide a convenient method for synthesizing a broad distribution of nanoparticles of a wide variety of alloy phases.

Intercascade annihilation of freely migrating defects

To characterize putative interactions between freely migrating defects (FMD) and remnants of energetic displacement cascades, radiation‐induced segregation (RIS) in Cu‐1 at. % Au was measured by Rutherford backscattering during separate and simultaneous irradiation at 400 °C with 1.5 MeV He and 800 keV Cu ions. The strong RIS observed during only He irradiation was greatly reduced under simultaneous Cu irradiation at approximately the same displacements per atom rate; increasing the Cu flux by a factor of 5 suppressed the RIS from the He beam almost completely. The suppression of RIS at 400 °C disappeared quickly when the Cu irradiation ceased. These results demonstrate that a transient population of interstitial and/or vacancy clusters from the Cu irradiation greatly reduces the survival rate of FMD produced by the He.

Annihilation of freely migrating defects by cascade remnants in Cu-1% Au alloys

Abstract The effects of cascade remnants on freely migrating defects (FMD) were studied by measuring radiation-induced segregation in Cu-1%Au at 400°C during simultaneous irradiation with 1.5 MeV He and (400–800) keV heavy ions (Ne, Ar or Cu). Radiation induced segregation during single-ion irradiation with He, Ne, Ar and Cu, and the effects of pre-irradiation with the same ions, were also investigated. The large segregation observed during 1.5 MeV He-only irradiation was dramatically suppressed under simultaneous Cu irradiation. This suppression disappeared immediately after the Cu irradiation ceased, indicating that it was caused by a transient population of cascade remnants, i.e., vacancy and/or interstitial clusters. For simultaneous inert-gas (He and Ne or Ar) irradiation, a similar suppression of the radiation induced segregation was observed. However, in contrast to the Cu-irradiation results, the suppression persisted after the Ne or Ar beam was turned off. The present results demonstrate that the energetic displacement cascades created by heavy ions introduce additional point-defect annihilation sites, which reduce the steady-state concentration of FMD. This finding implies that recombination dominates defect annihilation under the present irradiation conditions, which is indeed what is observed experimentally. As the cascade remnants produced by the Cu ions are thermally unstable at 400°C, the suppression of radiation induced segregation occurs only during simultaneous irradiation. During Ne and Ar irradiation, the inert gas atoms which accumulate in the specimen apparently stabilize the cascade remnants, allowing the suppression to persist during subsequent He-only irradiation.

Reduction of freely migrating defect concentrations by cascade remnants in NiSi alloys

Abstract Interactions of cascade remanants with freely migrating defects in Ni-12.7% Si were investigated using in-situ Rutherford backscattering spectrometry during simultaneous irradiation with 1.5 MeV He and 400 keV Ne ions at elevated temperatures. Radiation induced segregation of Si atoms toward the specimen surface caused γ′-Ni 3 Si layers to grow on the surface during irradiation. The γ′-Ni 3 Si growth rate during He bombardment was strongly suppressed by simultaneous irradiation with Ne ions, even when the calculated defect production rate for the Ne ions was only a few percent of that for the He ions. This results shows that the cascade remnants generated by the Ne ions act as additional recombination sites for freely migrating defects, reducing their steady state concentration. Despite a large difference in defect properties and in the kinetics of radiation induced segregation between the two alloys, the effects of cascade remnants on freely migrating defects in Ni-12.7% Si are remarkably similar to those found previously in Cu-1% Au.

Heavy-ion cascade effects on radiation-induced segregation kinetics in Cu-1%Au alloys

Abstract Various dual ion irradiations were conducted to investigate the effect of heavy-ion cascades on the fluxes of freely migrating defects which drive radiation-induced segregation (RIS) in Cu–1at.%Au alloys. In situ Rutherford backscattering spectroscopy (RBS) was used to measure the RIS suppression effect of heavy-ion bombardment (with 300-keV Al + , 800-keV Cu + , and 1.2-MeV Ag + ) on 1.5-MeV He + -RIS of Au in the near-surface region of the alloy during concurrent He + and heavy-ion irradiations at 400°C. Results demonstrated that the suppression of He + -RIS correlated well with the cascade volume produced by concurrent Al + , Cu + , and Ag + irradiation per second and was independent of the weighted average primary recoil energy. Model calculations of the kinetics of RIS during dual beam irradiation were also performed and compared with the measurements. Information regarding the energetics of freely migrating point defects and their relative production efficiencies was obtained from systematic fitting. Using the values previously reported for the energies of formation and migration for vacancies and interstitials in Cu, the binding and migration energies of Au-interstitial and Au-vacancy complexes in the alloy were found to be −0.14, 0.15, and 0.03 and 0.76 eV, respectively. The respective derived efficiencies of freely migrating defect production by energetic He + , Al + , Cu + , and Ag + ions were 0.25, 0.12, 0.09, and 0.08.

Effects of He implantation on radiation induced segregation in Cu–Au and Ni–Si alloys

Effects of He implantation on radiation induced segregation (RIS) in Cu-Au and Ni-Si alloys were investigated using in-situ Rutherford backscattering spectrometry during simultaneous irradiation with 1.5-MeV He and low-energy (100 or 400-keV) He ions at elevated temperatures. RIS during single He ion irradiation, and the effects of pre-implantation with low-energy He ions, were also studied. RIS near the specimen surface, which was pronounced during 1.5-MeV He single-ion irradiation, and during simultaneous irradiation with 1.5-MeV He and low-energy He ions. A similar RIS reduction was also observed in the specimens pre-implanted with low-energy He ions. The experimental results indicate that the accumulated He atoms cause the formation of small bubbles, which provide additional recombination sites for freely migrating defects.

Reduction of Defect Fluxes Using Dual-Ion-Beam Processing

Radiation-induced segregation (RIS) in Ni-12.7% Si and Cu-1% Au alloys was studied using Rutherford backscattering spectroscopy during He and Ne irradiation at elevated temperatures. During single ion-beam irradiation with 1.5 MeV He, strong RIS of Si toward the surface was observed in Ni-12.7% Si. Simultaneous irradiation with 400 keV Ne and 1.5 MeV He almost completely suppressed the Si segregation, even when the calculated damage production rate by Ne was only a few percent of that by He ions. A similar effect of dual-beam irradiation was observed in the Cu-1% Au alloy, i.e., the rate of near surface Au depletion was strongly reduced under simultaneous irradiation. The present result shows that dual-beam irradiation can be applied to control RIS and RED (Radiation Enhanced Diffusion) during ion beam processing.

Elevated-temperature implantations of Au into Zr

Abstract Polycrystalline Zr samples were implanted with 200 keV Au ions at several temperatures between 295 and 790 K to a dose of 5 × 10 15 /cm 2 . The implanted Au distributions were analyzed with RBS. A strong dependence of the Au distribution on implantation temperature was found for temperatures ⩾ 450 K, in good agreement with previous studies of interdiffusion and ion-beam mixing in Au/Zr multilayer specimens. For implantation temperatures between 500 and 600 K, a narrowing of the implanted distribution and an increase in the peak concentration were observed. At implantation temperatures ⩾ 650 K, a substantial fraction of the Au migrated toward the implanted surface. Systematic shifts of the peak in the implanted distribution were also found. The effects observed at the higher temperatures can be understood qualitatively in terms of radiation-induced segregation (RIS) and radiationenhanced diffusion (RED) becoming dominant. The combined effects of sputtering, RIS and RED substantially reduce the fraction of Au retained at elevated temperature.

Shock-wave synthesis of nanoparticles during ion sputtering.

We report electron microscopy studies of nanoparticles ( 500 ≤ n ≤ 10, where n is the number of atoms in a given cluster) that are sputtered from the surface by high-energy ion impacts. Measurements of the sizes of these clusters yielded an inverse power-law distribution with an exponent of –2 that is independent of irradiating ion species and total sputtering yield. This inverse-square dependence indicates that these nanoclusters are produced when shock waves, generated by sub-surface displacement cascades, impact and ablate the surface. Such nanoparticles consist of simple fragments of the original surface, i.e., ones that have not undergone any large thermal excursion. As discussed below, this “ion ablation” technique should therefore be useful for synthesizing nanoparticles of a wide variety of alloy compositions and phases.