Modeling the microstructural evolution of metallic polycrystalline materials under localization conditions

In general, the shear localization process involves initiation and growth. Initiation is expected to be a stochastic process in material space where anisotropy in the elastic-plastic behavior of single crystals and inter-crystalline interactions serve to form natural perturbations to the material's local stability. A hat-shaped sample geometry was used to study shear localization growth. It is an axi-symmetric sample with an upper “hat” portion and a lower “brim” portion with the shear zone located between the hat and brim. The shear zone length was 870-890 μm with deformation imposed through a Split-Hopkinson Pressure Bar system at maximum top-to-bottom velocity in the range of 8-25 m/s. The deformation behavior of tantalum tophat samples is modeled through direct polycrystal simulations. An embedded Voronoï-tessellated two-dimensional microstructure is used to represent the material within the shear zone of the sample. A thermo-mechanically coupled elasto-viscoplastic single crystal model is presented and used to represent the response of the grains within the aggregate shear zone. In the shoulder regions away from the shear zone where strain levels remain on the order of 0.05, the material is represented by an isotropic J₂ flow theory based upon the elasto-viscoplastic Mechanical Threshold Stress (MTS) model for flow strength. The top surface stress versus displacement results were compared to those of the experiments and over-all the simulated stress magnitude is over-predicted. It is believed that the reason for this is that the simulations are two-dimensional. A region within the numerical shear zone was isolated for statistical examination. The vonMises stress state within this isolated shear zone region suggests an approximate normal distribution with a factor of two difference between the minimum and maximum points in the distribution. The equivalent plastic strain distribution within this same region has values ranging between 0.4 and 1.5 and is not symmetric. Other material state distributions are also given. The crystallographic texture within this isolated shear zone is also compared to the experimental texture and found to match reasonably well considering the simulations are two-dimensional.     

Systems of oblate molecules. Monte Carlo study

Monte Carlo (MC) simulations were performed for systems of hard oblate spherocylinders with breadth-to-height ratios φ = 0.5–3.5 and packing fractions y = 0.25–0.45 and for Kihara oblate molecule systems of φ = 1 at reduced temperatures T* = 0.75 and 1.0 and y = 0.05–0.45. The compression factors and the dependence of the average correlation functions on the shortest surface-to-surface distance were determined for the case of hard oblate spherocylinders and the compression factors, residual internal energies and average correlation functions for the case of the generalized Kihara molecule systems. In addition, values of the third virial coefficient of the hard oblate spherocylinders were evaluated in the range of φ = 1–3. Results of the MC simulations for the hard oblate spherocylinders compare well with the available data in the literature and theoretical values; thermodynamic functions of the Kihara molecule systems were determined from the second-order perturbation theory. They agree well with our MC values at lower densities and higher reduced temperatures.     

Surface States in a Simple Crystal Model

A very simple crystal model of a semi-infinite heteroatomic planar lattice like boronitride is investigated for the energy, wave functions and existence conditions of surface states, using the one-electron Green function method. σ and π electrons are treated separately. For π electrons, we find two surface state bands, both for the electropositive and the electronegative surface. The consequence is the existence of Shockley “subsurface” states. For σ electrons, similar results were found as for the sphalerite-type lattice. The investigated model can also be used to draw qualitative conclusions about the effect of electronic correlation on surface states of planar graphite. The possibility of finding a certain type of “antiferromagnetism” with π electrons localized on the planar graphite boundary is suggested. On étudie un modèle très simple d'un cristal avec un réseau sémi-infini, héteroatomique, comme le boronitride. L'énergie, les fonctions d'onde et des conditions d'existence d'états de surface ont été obtenus par la méthode de la fonction de Green à un électron. Les électrons σ et π ont été traités séparément. Pour les électrons π on trouve deux bandes d'états de surface pour la surface électropositive ainsi que pour la surface électronégative, ce qui implique l'existence d'états de “sous-surface” de Shockley. Pour les électrons σ on trouve des résultats semblables aux ceux qu'on a obtenu pour le réseau de type sphalérite. Le modèle étudié ici peut aussi être employé pour tirer des conclusions qualitatives sur l'effet de la corrélation électronique sur les états de surface du graphite plan. On propose qu'il serait possible de trouver un certain type de “antiferromagnétisme” avec les électrons π localisés sur le bord du graphite plan. Es wurde ein einfaches Modell für ein halb-unendliches, heteroatomares Kristallgitter, wie Boronitrid untersucht. Die Energie, die Wellenfunktionen und Existenzbedingungen für Oberflächenzustände wurden mit der Einelektron-Green-Funktionsmethode erhalten. Die σ- und π-Electronen wurden getrennt behandelt. Für die π-Elektronen finden wir zwei Bände für Oberflächenzustände, sowohl für die elektropositive als für die elektronegative Oberfläche. Als Folgerung erhalten wir die Shockleysche unteroberflächenzustände. Für die σ-Elektronen wurden ähnliche Resultate als für das Sphaleritgitter gefunden. Das untersuchte Modell kann auch dafür angewendet werden, urn den Effekt der elektronischen Korrelation auf die Oberflächenzustände planares Graphits in qualitativer Weise zu diskutieren. Es wurde vorgeschlagen, dass es möglich wäre, eine gewisse Art von “Antiferromagnetismus” der π-Elektronen die auf der Grenze planares Graphits lokalisiert sind, zu finden.     

Dimensional quantification of embedded voids or objects in three dimensions using X-ray tomography

Scientific digital imaging in three dimensions such as when using X-ray computed tomography offers a variety of ways to obtain, filter, and quantify data that can produce vastly different results. These opportunities, performed during image acquisition or during the data processing, can include filtering, cropping, and setting thresholds. Quantifying features in these images can be greatly affected by how the above operations are performed. For example, during binarization, setting the threshold too low or too high can change the number of objects as well as their measured diameter. Here, two facets of three-dimensional quantification are explored. The first will focus on investigating the question of how many voxels are needed within an object to have accurate geometric statistics that are due to the properties of the object and not an artifact of too few voxels. These statistics include but are not limited to percent of total volume, volume of the individual object, Feret shape, and surface area. Using simple cylinders as a starting point, various techniques for smoothing, filtering, and other processing steps can be investigated to aid in determining if they are appropriate for a specific desired statistic for a real dataset. The second area of investigation is the influence of post-processing, particularly segmentation, on measuring the damage statistics in high purity Cu. The most important parts of the pathways of processing are highlighted.     

Antibacterial Activity and Synergistic Effect of Biosynthesized AgNPs with Antibiotics Against Multidrug-Resistant Biofilm-Forming Coagulase-Negative Staphylococci Isolated from Clinical Samples

Silver nanoparticles form promising template for designing antimicrobial agents against drug resistant pathogenic microorganisms. Thus, the development of a reliable green approach for the synthesis of nanoparticles is an important aspect of current nanotechnology research. In the present investigation, silver nanoparticles synthesized by a soil Bacillus sp. were characterized using UV–vis spectroscopy, FTIR, SEM, and EDS. The antibacterial potential of biosynthesized silver nanoparticles, standard antibiotics, and their conjugates were evaluated against multidrug-resistant biofilm-forming coagulase-negative S. epidermidis strains, S. aureus, Salmonella Typhi, Salmonella Paratyphi, and V. cholerae. Interestingly, silver nanoparticles (AgNPs) showed remarkable antibacterial activity against all the test strains with the highest activity against S. epidermidis strains 145 and 152. In addition, the highest synergistic effect of AgNPs was observed with chloramphenicol against Salmonella typhi. The results of the study clearly indicate the promising biomedical applications of biosynthesized AgNPs.     

Effects of texture,temperature and strain on the deformation modes of zirconium

Clock-rolled, high-purity, textured polycrystalline zirconium exhibits significant plastic anisotropy for compression along the through-thickness and in-plane directions and strong temperature dependence of flow stress for both orientations. Orientation imaging microscopy in a scanning electron microscope and defect analysis via transmission electron microscopy are used to characterize the defect microstructures as a function of initial texture, deformation temperature and plastic strain. The observed deformation mechanisms are correlated with the measured mechanical response.     

RELATIONSHIP BETWEEN SURVIVAL AND THYMINE-DIMER EXCISION IN ULTRAVIOLET-IRRADIATED ESCHERICHIA COLI

Abstract— The influence of amino acid prestarvation on both the resistance to u.v. light and excision of thymine dimers of bacterial strains E. coli B/r hcr ⁺ thy- trp ^-, E. coli B/r hcr ^-thy^- trp ^-, and E, coli 15 T^- 555–7 thy ^- meth ^- trp ^- arg ^- has been studied.
The prestarvation increased the resistance of all the strains but reasonably inhibited excision of thymine dimers. Thus the enhancement of u.v. resistance after amino acid prestarvation was not due to more complete excision of thymine dimers.     

Comparing Nonsmooth Nonconvex Bundle Methods in Solving Hemivariational Inequalities

Hemivariational inequalities can be considered as a generalization of variational inequalities. Their origin is in nonsmooth mechanics of solid, especially in nonmonotone contact problems. The solution of a hemivariational inequality proves to be a substationary point of some functional, and thus can be found by the nonsmooth and nonconvex optimization methods. We consider two type of bundle methods in order to solve hemivariational inequalities numerically: proximal bundle and bundle-Newton methods. Proximal bundle method is based on first order polyhedral approximation of the locally Lipschitz continuous objective function. To obtain better convergence rate bundle-Newton method contains also some second order information of the objective function in the form of approximate Hessian. Since the optimization problem arising in the hemivariational inequalities has a dominated quadratic part the second order method should be a good choice. The main question in the functioning of the methods is how remarkable is the advantage of the possible better convergence rate of bundle-Newton method when compared to the increased calculation demand.