First-principles study of crystallographic slip modes in ω-Zr.

We use first-principles density functional theory to study the preferred modes of slip in the high-pressure ω phase of Zr. The generalized stacking fault energy surfaces associated with shearing on nine distinct crystallographic slip modes in the hexagonal ω-Zr crystal are calculated, from which characteristics such as ideal shear stress, the dislocation Burgers vector, and possible accompanying atomic shuffles, are extracted. Comparison of energy barriers and ideal shear stresses suggests that the favorable modes are prismatic 〈c〉, prismatic-II [Formula: see text] and pyramidal-II 〈c + a〉, which are distinct from the ground state hexagonal close packed α phase of Zr. Operation of these three modes can accommodate any deformation state. The relative preferences among the identified slip modes are examined using a mean-field crystal plasticity model and comparing the calculated deformation texture with the measurement. Knowledge of the basic crystallographic modes of slip is critical to understanding and analyzing the plastic deformation behavior of ω-Zr or mixed α-ω phase-Zr

Deformation mechanism map of Cu/Nb nanoscale metallic multilayers as a function of temperature and layer thickness

The mechanical properties and deformation mechanisms of Cu/Nb nanoscale metallic multilayers (NMMs) manufactured by accumulative roll bonding (ARB) are studied at 25C and 400C. Cu/Nb NMMs with individual layer thicknesses between 7 and 63 nm were tested by in-situ micropillar compression inside a scanning electron microscope Yield strength, strain-rate sensitivities and activation volumes were obtained from the pillar compression tests. The deformed micropillars were examined under scanning and transmission electron microscopy in order to examine the deformation mechanisms active for different layer thicknesses and temperatures. The analysis suggests that room temperature deformation was determined by dislocation glide at larger layer thicknesses and interface-related mechanisms at the thinner layer thicknesses. The high temperature compression tests, in contrast, revealed superior thermo-mechanical stability and strength retention for the NMMs with larger layer thicknesses with deformation controlled by dislocation glide. A remarkable transition in deformation mechanism occurred as the layer thickness decreased, to a deformation response controlled by diffusion processes along the interfaces, which resulted in temperature-induced softening. A deformation mechanism map, in terms of layer thickness and temperature, is proposed from the results obtained in this investigation

Knowledge transfer and management consulting: A look at “The firm”

In the authors 'view, a firm's competitive advantage comes from the value it can develop for its customers. Most successful firms today can be considered „intelligent enterprises” because they transform intellectual assets into product and service outputs. It follows that knowledge transfer is especially critical for the functioning of a management consulting firm, because knowledge is the cornerstone of the services such a firm offers its clients

Sub-Pixel Response Measurement of Near-Infrared Sensors

Wide-field survey instruments are used to efficiently observe large regions of the sky. To achieve the necessary field of view, and to provide a higher signal-to-noise ratio for faint sources, many modern instruments are undersampled. However, precision photometry with undersampled imagers requires a detailed understanding of the sensitivity variations on a scale much smaller than a pixel. To address this, a near-infrared spot projection system has been developed to precisely characterize near-infrared focal plane arrays and to study the effect of sub-pixel non uniformity on precision photometry. Measurements of large format near-infrared detectors demonstrate the power of this system for understanding sub-pixel response.Comment: 9 pages, 13 figures, submitted to PAS

Comment on "Giant Plasticity of a Quantum Crystal"

In their Letter, Haziot et al. [Phys. Rev. Lett. 110 (2013) 035301] report a novel phenomenon of giant plasticity for hcp Helium-4 quantum crystals. They assert that Helium-4 exhibits mechanical properties not found in classical plasticity theory. Specifically, they examine high-quality crystals as a function of temperature and applied strain, where the shear modulus reaches a plateau and dissipation becomes close to zero; both quantities are reported to be independent of stress and strain, implying a reversible dissipation process and quantum tunneling. In this Comment, we show that these signatures can be explained with a classical model of thermally activated dislocation glide without the need to invoke quantum tunneling or dissipationless motion. Recently, we proposed a dislocation glide model in solid Helium-4 containing the dissipation contribution in the presence of other dislocations with qualitatively similar behavior [Zhou et al., Philos. Mag. Lett. 92 (2012) 608].Comment: 1 page, 1 figure, comment; minor revision

Structural Phase Transition in the Superconducting Pyrochlore Oxide Cd2Re2O7

We report a structural phase transition found at Ts = 200 K in a pyrochlore oxide Cd2Re2O7 which shows superconductivity at Tc = 1.0 K. X-ray diffractionexperiments indicate that the phase transition is of the second order, from a high-temperature phase with the ideal cubic pyrochlore structure (space group Fd-3m) to a low-temperature phase with another cubic structure (space group F-43m). It is accompanied by a dramatic change in the resistivity and magnetic susceptibility and thus must induce a significant change in the electronic structure of Cd2Re2O7.Comment: 4 pages, 4figures, proceeding for ISSP

In vivo brain connectivity: optimization of manganese enhanced MRI for neuronal tract tracing

One of the main problems in systems biology is to obtain information on signal processing between interconnected groups of neurons in highly distributed networks. The recently introduced technique of manganese (Mn2+) enhanced MRI (MEMRI) to study neuronal connectivity in vivo opens the possibility to these studies. However, several drawbacks exist that challenge its applicability. High Mn2+ concentrations produce cytotoxic effects that can perturb the circuits under study. In the other hand, the MR signal is proportional to the Mn2+ concentration in tissue and thus, significant amounts of Mn2+ are required to produce detectable contrast and reliable connectivity maps. Here we attempt to optimize the MEMRI technique by preventing toxicity and improving the quality and extension of the obtained connectivity maps. The somatosensory cortex of male SD rats was stereotaxically injected with different Mn2+-containing solutions. Total amount of injected Mn2+ ranged between 1 and 16 nmol and the injected volumes between 10 and 80 nL. Osmolarity and pH effects were investigated injecting pH buffered solutions of Mn2+ (pH 7.3 in Tris-HCl buffer vs. 5.5 in H2O) at different concentration (0.05, 0.1 and 0.8 M MnCl2). Same amounts of Mn2+ (8nmol) delivered to the tissue at different infusion rates were also compared. Following the injection, T1-weighted MR imaging (250 mm isotropic resolution) was performed in a 7T scanner at different time points. Fifteen days after the injection animals were sacrificed and brains processed for histology. Nissl staining as well as GFAP and NeuN immunohistochemistry (selective staining for astrocytes and neurons, respectively) were performed in the brain sections to examine cellular toxicity. All injections produced connectivity maps consistent with the known anterograde projections of SI cortex based on classical neuronal tract-tracing techniques. Our results show that pH buffered solution improve the effectiveness of MEMRI, increasing T1 contrast in the projection sites. In addition, injections of pH buffered and isotonic solutions of 50 and 100 mM MnCl2 yielded more extensive connectivity maps, in particular, ipsiand contra-lateral corticocortical connections were evident in all animal injected with those solutions but not with the more usual MEMRI protocol (0.8M MnCl2 in H2O). Hypertonic and non-buffered solutions containing 8nmol Mn2+ resulted in neuronal death and astrogliosis in extensive areas around the injection point. In sharp contrast, no neuronal toxicity was observed with injections containing up to 8nmol of Mn2+ in isotonic solutions of up to 100 mM MnCl2 and pH 7.3. Slow infusion rates demonstrated also to be advantageous and permitted application of larger amounts of Mn2+ without toxic effects, resulting in better T1 contrast in the low density projection fields. Any sign of toxicity was observed in any condition in the projection fields. We conclude that refined protocols for MEMRI improve the quality and extension of connectivity maps and preserves tissue viability, assuring the application of this technique in longitudinal experiments

Strength and ductility with 10 11 10 12 double twinning in a magnesium alloy

Based on their high specific strength and stiffness, magnesium alloys are attractive for lightweight applications in aerospace and transportation, where weight saving is crucial for the reduction of carbon dioxide emissions. Unfortunately, the ductility of magnesium alloys is usually limited. It is thought that one reason for the lack of ductility is that the development of double twins DTW cause premature failure of magnesium alloys. Here we show with a magnesium alloy containing 4 amp; 8201;wt lithium, that the same impressively large compression failure strains can be achieved with DTWs as without. The DTWs form stably across the microstructure and continuously throughout straining, forming three dimensional intra granular networks, a potential strengthening mechanism. We rationalize that relatively easier lt;c a gt; slip characteristic of this alloy plastically relaxed the localized stress concentrations that DTWs can generate. This result may provide key insight and an alternative perspective towards designing formable and strong magnesium alloy

Fracture model with variable range of interaction

We introduce a fiber bundle model where the interaction among fibers is modeled by an adjustable stress-transfer function which can interpolate between the two limiting cases of load redistribution, the global and the local load sharing schemes. By varying the range of interaction several features of the model are numerically studied and a crossover from mean field to short range behavior is obtained. The properties of the two regimes and the emergence of the crossover in between are explored by numerically studying the dependence of the ultimate strength of the material on the system size, the distribution of avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we analyze the moments of the cluster size distributions to accurately determine the value at which the crossover is observed.Comment: 8 pages, 8 figures. Two columns revtex format. Final version to be published in Phys. Rev.