Convection-Induced Compositional Patterning at Grain Boundaries in Irradiated Alloys.

We consider the stability of precipitates formed at grain boundaries (GBs) by radiation-induced segregation in dilute alloys subjected to irradiation. The effects of grain size and misorientation of symmetric-tilt GBs are quantified using phase field modeling. A novel regime is identified where, at long times, GBs are decorated by precipitate patterns that resist coarsening. Maps of the chemical Péclet number indicate that arrested coarsening takes place when solute advection dominates over thermal diffusion right up to the precipitate-matrix interface, preventing interfacial local equilibrium and overriding capillary effects. This contrasts with liquid-solid mixtures where convection always accelerates coarsening.

Wear Resistance of Cu/Ag Multilayers: A Microscopic Study.

The microscopic wear behavior of copper-silver multilayer samples was studied by performing sliding wear tests using a tribo-indenter. Multilayers with an average composition of Cu90Ag10 and Ag layer thicknesses ranging from 2 to 20 nm were grown by magnetron sputtering. For reference, a homogeneous Cu90Ag10 solid solution film was similarly grown. The thin films were subjected to two-dimensional wear tests by rastering a cono-spherical diamond indenter under loads of 100-400 µN for 1-20 consecutive passes or cycles. The wear volumes were determined by atomic force microscopy. Characterization of the specimens employed nanoindentation, nanoscratch, and transmission electron microscopy (TEM). Wear rates were found to reach steady state after five cycles or less. The hardness values of the as-grown and worn samples both increased with decreasing thickness of the Cu and Ag layers, whereas the steady-state wear rates decreased. Notably, the wear resistance increased faster than the corresponding increase in indentation hardness, indicating a deviation from the Archard's law. An inverse relationship between the wear rate and hardness was, however, recovered when using scratch hardness, suggesting that scratch hardness is a better predictor of wear resistance. Characterization of subsurface wear microstructures by TEM revealed that forced chemical mixing and dissolution of layers occur to a depth of ≈40 to 50 nm, stabilizing a chemically homogeneous solid solution below the wear surface. Comparative wear tests on thicker multilayers revealed that Cu/Ag interfaces reduced the wear rate significantly, thus helping to rationalize the high wear resistance of thin multilayers.

Crossover from superdiffusive to diffusive mixing in plastically deformed solids.

We derive expressions for the effective diffusion coefficient of Richardson's pairs in plastically strained solids as a function of the pair separation distance R. We predict that a crossover from superdiffusive to diffusive mixing takes place when R becomes comparable to the coherence length of the shearing events underlying the plastic deformation. Molecular dynamics simulations on nanocrystalline and amorphous systems support this analysis, which thus provides new insight on deformation mechanisms in these systems. Superdiffusive mixing is experimentally observable by monitoring the rate of dissolution of precipitates as a function of their initial size.

Molecular-dynamics study of the density scaling of inert gas condensation.

The initial stages of vapor condensation of Ge in the presence of a cold Ar atmosphere were studied by molecular-dynamics simulations. The state variables of interest included the densities of condensing vapor and gas, the density of clusters, and the average cluster size, while the temperatures of the vapor and the clusters were separately monitored with time. Three condensation processes were explicitly identified: nucleation, monomeric growth, and cluster aggregation. Our principal finding is that both the average cluster size and the number of clusters scale with the linear dimension of the computation cell, L, and Ln, with the scaling parameter n approximately 4, corresponding to a reaction order of nu approximately 2.33. This small value of n is explained by an unexpected nucleation path involving the formation of Ge dimers via two-body collisions.

Forced chemical mixing in alloys driven by plastic deformation.

Molecular dynamics simulations of forced atomic mixing in crystalline binary alloys during plastic deformation at 100 K are performed. Nearly complete atomic mixing is observed in systems that have a large positive heat mixing and in systems with a large lattice mismatch. Only systems that contained a hard precipitate in a soft matrix do not mix. The amount of mixing is quantified by defining a mean square relative displacement of pairs of atoms, sigma(2)(R,t), that were initially separated by a distance R. Analysis of sigma(2)(R,t) and visual inspection of the displacement fields reveal that forced mixing results from dislocation glide, and that it resembles the forced mixing of a substance advected by a turbulent flow. Consideration of sigma(2)(R,t) also provides a rationalization of compositional self-organization during plastic deformation at higher temperatures.

Molecular dynamics simulations of cluster nucleation during inert gas condensation.

Molecular dynamics simulations of vapor-phase nucleation of germanium in an argon atmosphere were performed and a unexpected channel of nucleation was observed. This channel, vapor-induced cluster splitting, is important for more refractory materials since the critical nucleus size can fall below the size of a dimer. As opposed to conventional direct vapor nucleation of the dimer, which occurs by three-body collisions, cluster-splitting nucleation is a second-order reaction. The most important cluster-splitting reaction is the collision of a vapor atom and a trimer that leads to the formation of two dimers. The importance of the cluster-splitting nucleation channel relative to the direct vapor nucleation channel is observed to increase with decreasing vapor density and increasing ratio of vapor to carrier gas atoms.

Glassy vortex dynamics induced by a random array of magnetic particles above a superconductor.

The magnetic relaxation of a Nb film covered with a random array of permalloy particles has been studied using various procedures. When the sample undergoes a field-cooled process, the magnetic relaxation becomes logarithmic in time. The relaxation rate is nearly temperature independent at low temperature and characteristic glassy dynamics-aging and memory effects-are observed. These results are interpreted as the consequence of pinning by the statistical variation of the number of nanoparticles within the area of a vortex core.

Memory effects in an interacting magnetic nanoparticle system.

We have performed a series of measurements to study the low temperature dynamics of an interacting magnetic nanoparticle system. The results obtained demonstrate striking memory effects in the dc magnetization and magnetic relaxation that support the existence of a spin-glass-like phase in interacting magnetic nanoparticles. Moreover, we observe an asymmetric response with respect to temperature change that supports a hierarchical picture, rather than the droplet model discussed in other works on nanoparticle systems.

Mechanisms of radiation-induced viscous flow: role of point defects.

Mechanisms of radiation-induced flow in amorphous solids have been investigated using molecular dynamics computer simulations. It is shown for a model glass system, CuTi, that the radiation-induced flow is independent of recoil energy between 100 eV and 10 keV when compared on the basis of defect production and that there is a threshold energy for flow of approximately 10 eV. Injection of interstitial- and vacancylike defects induces the same amount of flow as the recoil events, indicating that point-defect-like entities mediate the flow process, even at 10 K. Comparisons of these results with experiments and thermal spike models are made.

Surface smoothing of rough amorphous films by irradiation-induced viscous flow.

Surface roughening and smoothing reactions on vapor codeposited glassy Zr65Al7.5Cu27.5 films by 1.8 MeV Kr+ ion beam irradiation is investigated. Irradiation causes significant smoothing of initially rough surfaces, and nearly atomically smooth films can be achieved. Smooth surfaces roughen at high doses and long wavelengths. By a Fourier analysis, radiation-induced viscous flow is identified as the dominant surface relaxation mechanism. Two noise terms are identified, which operate on different length scales: One is due to sputtering and the other to thermal spikes. The irradiation-induced viscosity is compared with radiation-enhanced diffusion.

Reactive epitaxy of Co nanoparticles on (111)Si.

The formation of epitaxial CoSi2 islands of nanoscopic dimensions is reported using the technique of reactive cluster deposition. Co clusters in the size range 5-50 nm were synthesized by sputtering of a high purity Co target inside a ultra high vacuum (UHV) sputtering chamber, using the technique of inert gas condensation. The clusters were then deposited on the reconstructed Si (111) surface. Upon annealing, the particles reacted with the Si substrate to form epitaxial CoSi2. Our observations were made using a JEOL 200CX transmission electron microscope modified for in situ sputtering and UHV conditions.

Antistructure and point defect response in the recovery of ion-irradiated Cu3Au.

The response of the point defect and antistructure systems to ion beam irradiation is investigated using methods of linear response on thin single crystals of ordered Cu3Au grown by molecular beam epitaxy. We demonstrate that antisite evolution, as measured by electrical resistance, quantitatively determines both the defect populations and diffusion in the irradiation field, and we explore new linear and nonlinear response processes as the antistructure system is driven from equilibrium.

Misclassification and under-reporting of acute myocardial infarction by elderly persons: implications for community-based observational studies and clinical trials.

We investigated the accuracy of self-report of hospitalization for acute myocardial infarction (MI) by elderly persons in a community-based prospective study. Among 3809 persons aged 65 years or older followed up for 6 years, self-reported hospitalization for MI was validated by review of primary records and Medicare diagnoses. Among 147 who self-reported MI and for whom hospital records were available, the diagnosis was confirmed in 79 (54%). Myocardial infarction was not a reason for hospitalization among the remaining 68 participants; misclassification with other cardiovascular diagnoses was common. Medicare diagnosis correlated well with primary hospital records. Using Medicare diagnoses as the standard, the diagnosis of MI was confirmed in 53% of self-reports; the sensitivity and specificity of self-report were 51% and 98%, respectively. False-negative reporting was common because only half of hospitalizations for MI were reported. Self-report of hospitalization for MI by elderly persons in the community may be unreliable for ascertaining trends in cardiovascular diseases.

Sintering and oxidation using a novel ultrahigh vacuum transmission electron microscope with in situ magnetron sputtering.

The synthesis and processing of materials is often highly sensitive to the presence of trace contaminants and a number of technologically important materials demand the clean conditions associated with an ultrahigh vacuum environment. With increasing interest in understanding materials phenomena occurring on smaller and smaller length scales, the transmission electron microscope is finding increasing application in the characterization of new materials and processes. The need for ex situ sample preparation prior to analysis can raise questions regarding the validity of the data, however, due to contamination and the introduction of microstructural artifacts. In this paper we discuss the application of the ultrahigh vacuum transmission electron microscope to in situ studies of materials synthesis. To illustrate the capabilities of the electron microscope in this context, we present two case studies: the synthesis and subsequent sintering of supported copper nanoparticles, and the initial stages of the growth of Cu2O on clean (001) Cu. We describe the novel aspects of the instrumentation used, the methods of sample preparation, and our application of the plan-view imaging technique to in situ investigations.