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.

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 CuAg and Ag layer thicknesses ranging from 2 to 20 nm were grown by magnetron sputtering. For reference, a homogeneous CuAg solid solution film was similarly grown.

Improving atomic displacement and replacement calculations with physically realistic damage models.

Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations.

Mechanical Properties of Molybdenum Disulfide and the Effect of Doping: An in Situ TEM Study.

Direct observations on nanopillars composed of molybdenum disulfide (MoS2) and chromium-doped MoS2 and their response to compressive stress have been made. Time-resolved transmission electron microscopy (TEM) during compression of the submicrometer diameter pillars of MoS2- and Cr-doped MoS2 (Cr: 0, 10, and 50 at %) allow the deformation process of the material to be observed and can be directly correlated with mechanical response to applied load. The addition of chromium to the MoS2 changed the failure mode from plastic deformation to catastrophic brittle fracture, an effect that was more pronounced as chromium content increased.

Irradiation induced grain boundary flow--a new creep mechanism at the nanoscale.

A new mechanism of irradiation enhanced creep is proposed for nanocrystalline materials. It derives from local relaxations within the grain boundaries as they absorb point defects produced by irradiation. The process is studied by inserting point defects into the grain boundaries and following the materials response by molecular dynamics.

Solidification velocities in deeply undercooled silver.

We measure the solidification velocity of pure Ag as a function of undercooling temperature from the melting point (Tm=1235 K) to 0.6Tm using ultrafast, pump-probe laser experiments. The thickness of the liquid layer, while it solidifies, is measured using optical third harmonic generation.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Techniques for Studying Nanoparticle Sintering by Plan-View In Situ Transmission Electron Microscopy.

: We discuss various techniques for the characterization of supported nanoparticles by in situ plan-view transmission electron microscopy. In particular, we discuss here mechanisms of image contrast formation by particles undergoing reorientation on the surface of a single crystal substrate. We consider reorientation by a variety of mechanisms including rotation, sintering and grain growth, and surface diffusion.

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.