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Now showing items 1 - 16 of 25

  • Irradiation-induced patterning in dilute Cu–Fe alloys

    B. Stumphy   S.W. Chee   N.Q. Vo   R.S. Averback   P. Bellon   M. Ghafari  

    Abstract Compositional patterning in dilute Cu 1− x Fex ( x ≈ 12%) induced by 1.8 MeV Kr + irradiation was studied as a function of temperature using atom probe tomography. Irradiation near room temperature led to homogenization of the sample, whereas irradiation at 300 °C and above led to precipitation and macroscopic coarsening. Between these two temperatures the irradiated alloys formed steady state patterns of composition where precipitates grew to a fixed size. The size in this regime increased somewhat with temperature. It was also observed that the steady state concentrations of Fe in Cu matrix and Cu in the Fe precipitates both greatly exceeded their equilibrium solubilities, with the degree of supersaturation in each phase decreasing with increasing temperature. In the macroscopic coarsening regime, the Fe-rich precipitates showed indications of a “cherry-pit” structure, with Cu precipitates forming within the Fe precipitates. In the patterning regime, interfaces between Fe-rich precipitates and the Cu-rich matrix were irregular and diffuse.
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  • Fundamental aspects of defect production in solids

    R.S. Averback   Mai Ghaly  

    Irradiation of solids with energetic particles leads to the production of lattice defects in the form of Frenkel pairs, anti-site defects, defect clusters, dislocation loops and amorphous zones in the bulk and to sputtered atoms, adatoms, and craters near the surface. Predicting which types of damage prevail in specific situations has been a difficult chore; however, recent developments in computer simulations have greatly facilitated this task. This review highlights the results of these simulations and provides a fundamental understanding of the damage process in a variety of pure metals, intermetallic compounds, and pure Si. The special role of surfaces on damage production is a central focus of this work. In addition, recent experimental investigations of alloy disordering in Cu 3Au during 1 MeV He bombardment and STM imaging of single ion impacts on Pt will be reviewed to illustrate the level of accuracy that has now been achieved by these simulations
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  • Interfacial effects during ion beam processing of metals

    R.S. Averback   Mai Ghaly   P. Bellon  

    Microstructural changes of surfaces and interfaces were investigated at the atomic level by computer simulations. Molecular dynamics simulations were employed to examine the effects of self-atom bombardment of Au and Pt in the energy range 10-20 keV. It was observed that significantly more damage and atomic mixing were produced near surfaces than in corresponding recoil events in the bulk. Most damage is created by viscous flow of liquid metal onto the surface, although in some cases, micro-explosions were observed. Interfacial mixing and roughening during ion irradiation was simulated by Monte Carlo methods for multilayered structures of immiscible metals. It was observed that only few ballistic jumps relative to diffusion jumps are needed to roughen the interface, but that many more ballistic than diffusive jumps are needed to mix the layers
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  • Atomic displacement processes in irradiated metals

    R.S. Averback  

    Irradiation of solids with energetic particles such as electrons, heavy ions and neutrons, results in the displacement of atoms from their lattice sites. In some cases the displaced atoms find metastable interstitial sites in the lattice, leaving vacant sites behind. In other cases, the atoms undergo switching processes with neighbors. No Frenkel defects are produced but atomic disorder is created. In this paper, the fundamental theory underlying these processes is presented. Topics include primary recoil spectra, spatial distributions of damage energy, defect production, and ion beam mixing. In addition to the traditional analytical theories of atomic collisions in solids, selected results of computer simulation are presented to elucidate the discussion and to introduce the latest developments in this field
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  • Processing of nano-grained materials

    R.S. Averback   H.J. H?fler   R. Tao  

    Sintering and deformation were studied in nano-grained (n-)TiO 2 and n-TiAl as part of a program to develop materials for near-net shaping and superplasticity applications. An important concern for processing nano-grained materials is the control of grain growth during both densification and deformation. In this study, the effectiveness of doping n-TiO 2 with yttrium for controlling grain growth during isothermal annealing was examined. In addition, an empirical constitutive law for the densification of n-TiO 2, was determined. comparison of sinter-forging in n-TiO 2 with larger grained oxide ceramics shows many similar features. The studies on TiAl examined the hardness as a function of grain size, indentation time and temperature. At large grain sizes, the hardness obeys the Hall-Petch relation, but below a critical grain size, approximately 30 nm, the hardness decreases with decreasing grain size. Finally, the potential for synthesizing metallic glasses with nanoscale amorphous particles is discussed
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  • MD studies of the interactions of low energy particles and clusters with surfaces

    R.S. Averback   Mai Ghaly  

    The interaction of low-energy particles and clusters with metal surfaces has been investigated using molecular dynamics computer simulations. For the particle interactions, a new mechanism for producing damage at surfaces was revealed in simulations of 10 and 20 keV Au bombardment of Au surfaces. Macroscopic modeling of this phenomenon illustrates the important parameters. For the cluster interactions, many different clusters and substrates were employed: the energies were varied from less than one eV (soft landings) to over 3 keV; the size of the clusters was varied from 87 to 1000, the angle of incidence was varied from near glancing to near normal, and both solid and liquid clusters were examined. Embedded atom method potentials were employed to represent the different metals and alloys. A broad range of behavior was observed in the simulations: splatting of the cluster over the surface, the formation of globs, the penetration of the clusters deep into the substrate. General rules are suggested for predicting which type of behavior prevails
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  • Molecular dynamics studies of displacement cascades

    R.S. Averback   H. Hsieh   T.Diaz de la Rubia   R. Benedek  

    Molecular-dynamics simulations of cascades in Cu and Ni with primary-knock-on energies up to 5 keV and lattice temperatures in the range 0-700 K are described. Interatomic forces were represented by either the Gibson II (Cu) or Johnson-Erginsoy (Ni) potentials in most of the work, although some simulations using `embedded atom method' potentials, e.g. for threshold events in Ni 3Al, are also presented. The results indicate that the primary state of damage produced by displacement cascades is controlled by two phenomena, replacement collision sequences during the collisional phase of the cascade and local melting during the thermal spike. As expected, the collisional phase is rather similar in Cu and Ni, however, the thermal spike is of longer duration and has a more pronounced influence in Cu then Ni. When the ambient temperature of the lattice is increased, the melt zones are observed to both increase in size and cool more slowly. This has the effect of reducing defect production and enhancing atomic mixing and disordering. The implications of these results for defect production, cascade collapse, atomic disordering are discussed
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  • Interactions of energetic particles and clusters with solids

    R.S. Averback   T.Diaz de la Rubia   Horngming Hsieh   R. Benedek  

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  • Dynamics and structure of energetic displacement cascades

    R.S. Averback   T.Diaz De La Rubia   R. Benedek  

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  • Ion beam mixing and radiation-enhanced diffusion in metallic glasses

    R.S. Averback   H. Hahn   Fu-Rong Ding  

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  • Fundamental aspects of ion beam mixing

    R.S. Averback  

    Irradiation of solids with energetic particles results in the reorganization of constituent target atoms, i.e. ion beam mixing (IM). At low temperatures, IM is characterized by prompt (10 -10 s) diffusion processes which are localized in the vicinity of the displacement cascade. Mixing at low temperatures can cause the system to depart far from the equilibrium state. At elevated temperatures, the diffusion of radiation-induced defects extends the mixing to longer times and greater distances. These delayed IM processes tend to return the system toward equilibrium. Recent experimental progress has led to a qualitative understanding of the fundamental aspects of IM in both temperature regimes. This has been achieved through systematic measurements of the influence of temperature, dose, dose-rate, cascade energy density and chemical interactions on IM. The results of these experiments are reviewed and compared to IM models based on collisional, thermal spike, and radiation-enhanced diffusion processes. The relation of IM to other fundamental radiation damage effects will also be discussed
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  • Ion beam mixing and superconductivity in Cu-Bi

    R.S. Averback   P.R. Okamoto   A.C. Baily   B. Stritzker  

    Ion beam mixing in Cu-Bi bilayered samples was studied using measurements of the superconducting transition temperature, Tc, backscattering analysis and electron microscopy. All measurements could be performed in situ, without warming above the irradiation temperature. Irradiation of the bilayered samples with energetic Kr + at 10K induces an amorphous phase which is also superconducting below ~5.2. On annealing, complete recrystallization did not occur until room temperature. The backscattering results revealed that irradiation at 10K causes significantly interdiffusion of Cu and Bi but that at 295K, there is essentially no mixing
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  • Defect production in ion-irradiated aluminum

    R.S. Averback   R. Benedek   K.L. Merkle   J. Sprinkle   L.J. Thompson  

    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
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  • Ion-irradiation studies of cascade damage in metals

    R.S. Averback  

    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.
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  • Self-organized, size-selection of precipitates during severe plastic deformation of dilute Cu-Nb alloys at low temperatures

    J.A. Beach   M. Wang   P. Bellon   S. Dillon   Yu. Ivanisenko   T. Boll   R.S. Averback  

    Abstract High pressure torsion (HPT) experiments have been carried out at very low temperatures (−78 °C) on dilute Cu-Nb alloys. The samples were prepared by magnetron sputtering to create solid solutions containing nominally between 1 at.% and 10 at.% Nb. Some samples were pre-annealed to 700 °C to form large Nb precipitates, ∼75 nm in radius, prior to HPT. For the unannealed alloys, HPT at low temperatures led to alloy decomposition for alloys with concentrations greater than ∼2 at.% Nb, with the steady state concentration of Nb increasing from ∼1.5 to ∼3 at.% as the initial concentration of Nb was increased. It was also observed that BCC Nb precipitates formed during low-temperature HPT, with a steady state precipitate size ∼10 nm in radius, which was insensitive to alloy concentration. HPT of pre-annealed Cu-10 at.% alloy led to the same steady state microstructure as the unannealed sample, indicating that the steady state during the low temperature HPT process is independent of the initial state for these Cu-Nb alloys. A model for phase evolution in strongly immiscible alloys during severe, low-temperature deformation is offered and implemented through kinetic Monte Carlo simulations. Good agreement with experiment is obtained. Graphical abstract The solubility vs strain of niobium in copper and the size distribution of niobium precipitates found after a strain of ∼2000. It can be seen that HPT of pre-annealed Cu- 10 at.% alloy led to the same steady state microstructure and solubility as the unannealed sample, indicating the low temperature HPT process is ergodic for these Cu-Nb alloys. Image 1
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  • In situ Rutherford backscattering analysis of radiation-induced segregation

    R.S. Averback   L.E. Rehn   W. Wagner   P.R. Okamoto   H. Wiedersich  

    Radiation-induced segregation of Ni-12.7 at.% Si during 2.0 MeV He and 3.25 MeV Kr irradiation has been investigated. Using high resolution Rutherford backscattering spectrometry, the rate of segregation of Si at the irradiated surface was determined. During the He irradiation, the amount of Si segregation was found to be proportional to the square root of the dose. From the dependence of segregation on temperature, for both He and Kr irradiations, as apparent activation enthalpy of 0.3 eV was deduced for the segregation process. A comparison of the segregation rates during the He and Kr irradiations revealed that on the basis of calculated dpa rates, He irradiation is significantly (~6times) more effective than Kr irradiation for inducing segregation
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