Impact of obstacles on dislocation patterning and stress correlations

By virtue of their long-ranged nature, elastic stresses due to dislocations may provide a mechanism for the spatial organization of alloys with misfitting constituents. Here, we first explore the connection between dislocation patterning and correlations in elastic stress fields associated with dislocations. Specifically, we examine the impact of obstacles, such as impurities and grain boundaries, on the pattern formation in a collection of mutually interacting dislocations. This is accomplished by characterizing the order in terms of various dislocation structure factors that reflect the tendency to form dislocation wall segments. We find that random stationary impurities frustrate wall formation without altering the characteristic length scale of the dislocation patterning, while grain boundaries promote wall formation via dislocation incorporation, and frustrate polygonization away from the boundaries. We then discuss the implication of the results of induced solute patterning in phase-separating alloys

Optimization of strength and ductility in nanotwinned ultra-fine grained Ag: Twin density and grain orientations

Nanotwinned ultrafine grained Ag thick films with different twin densities and orientations have been synthesized by magnetron sputtering with a wide-range of deposition rates. The twin boundary (TB) spacings and orientations as well as the grain size for the different deposition conditions have been characterized by both synchrotron X-ray scattering and transmission electron microscopy (TEM). Structural characterization combined with uniaxial tensile tests of the free-standing films reveals a large increase in the yield strength for films deposited at high deposition rates without any accompanying change in the TB spacing – a behavior that is not reported in the literature. We find that films deposited at lower deposition rates exhibit more randomly oriented grains with a lower overall twin density (averaged over all the grains) than the more heavily twinned grains with strong 〈1 1 1〉 fiber texture in the films deposited at higher deposition rates. The TB spacing in the twinned grains, however, does not show any significant dependence on the deposition rate. The dependence of the strength and ductility on the twin density and orientations can be described by two different soft deformation modes: (1) untwinned grains and (2) nanowinned grains that are not oriented with 〈1 1 1〉 along the growth direction. The untwinned grains provide relatively low resistance to slip, and thus decreased strength, while the nanotwinned grains that are not oriented with 〈1 1 1〉 along the growth direction are softer than nanotwinned grains that are oriented with 〈1 1 1〉 along the growth direction. We have revealed that an ultrafine-grained (150–200 nm) structure consisting of a mixture of nanotwinned (∼8–12 nm spacing) and untwined grains yields the best combination of high strength and uniform tensile ductility

Temperature dependence of surface reconstructions of Au on Pd(110)

Surface reconstructions of Au film on Pd(110) substrate are studied using a local Einstein approximation to quasiharmonic theory with the Sutton-Chen interatomic potential. Temperature dependent surface free energies for different coverages and surface structures are calculated. Experimentally observed transformations from $(1\times1)$ to $(1 \times 2)$ and $(1 \times 3)$ structures can be explained in the framework of this model. Also conditions for Stranski-Krastanov growth mode are found to comply with experiments. The domain of validity of the model neglecting mixing entropy is analyzed.Comment: 7 pages, REVTeX two-column format, 3 postscript figures available on request from [email protected] To appear in Phys. Rev. Letter

A Comparison of Tourism-Related Stressors Experienced by Residents of Three Island Destinations

Residents of island tourism destinations are subject to a variety of impacts their quality of life. One impact that has recently emerged as an issue is psychological stress. Psychological stress can often lead to negative physical and mental health outcomes. Island destinations develop along Butler’s (1980) Tourism Area Life Cycle in different ways based on geography, population, accessibility, and a variety of other variables. Oftentimes rapid development follows the creation of transportation links like a new airport or cruise port. The development of cruise ports in particular often bring rapid development with unforeseen consequences. In this study, we examine the tourism related stressors experienced by residents of three island destinations with varying levels of development and cruise tourism visitation. Thematic analysis of residents’ perceived stressors revealed that level of development and type of tourism play a role in the breadth, depth, and type of stressors perceived by residents

Crystal structure of solid Oxygen at high pressure and low temperature

Results of X-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented.A careful sample preparation and annealing around 240 K allowed to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and transforms with decreasing pressure into a monoclinic phase, which is identified as the low pressure alpha-phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha-phase increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure

Hydrogen atom in a spherical well: linear approximation

We discuss the boundary effects on a quantum system by examining the problem of a hydrogen atom in a spherical well. By using an approximation method which is linear in energy we calculate the boundary corrections to the ground-state energy and wave function. We obtain the asymptotic dependence of the ground-state energy on the radius of the well.Comment: Revised version to appear in European Journal of Physic

First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants

We present a first-principles study of the relationship between stress, temperature and electronic properties in piezoelectric ZnO. Our method is a plane wave pseudopotential implementation of density functional theory and density functional linear response within the local density approximation. We observe marked changes in the piezoelectric and dielectric constants when the material is distorted. This stress dependence is the result of strong, bond length dependent, hybridization between the O $2p$ and Zn $3d$ electrons. Our results indicate that fine tuning of the piezoelectric properties for specific device applications can be achieved by control of the ZnO lattice constant, for example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.

Consistent Anisotropic Repulsions for Simple Molecules

We extract atom-atom potentials from the effective spherical potentials that suc cessfully model Hugoniot experiments on molecular fluids, e.g., $O_2$ and $N_2$. In the case of $O_2$ the resulting potentials compare very well with the atom-atom potentials used in studies of solid-state propertie s, while for $N_2$ they are considerably softer at short distances. Ground state (T=0K) and room temperatu re calculations performed with the new $N-N$ potential resolve the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure