Lattice Green's function for crystals containing a planar interface

Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions. The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a $(10\bar{1}2)$ twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by \textit{ab initio} methods

Solute strengthening of twinning dislocations in Mg alloys

Solute strengthening of twin dislocation motion along an existing twin boundary in Mg–X (X = Al, Zn) is investigated using a new Labusch-type weak pinning model. First, the View the MathML source(101¯2) twinning dislocation structure is computed using first-principles methods. Second, the interaction energies of Al and Zn solutes with the twin boundary and twin dislocation are computed. It is shown that the interaction energies of Zn solutes scale with the Al solute energies in proportion to the misfit volume plus an additional “chemical” interaction factor, demonstrating an efficient means for estimating the solute energies of other solutes. Third, the solute–dislocation interaction energies are used in a new Labusch-type model to predict the overall solute strengthening of the twinning dislocation. New features emerge in the application of the model to twinning because of the very small Burgers vector of the twin dislocation, leading to a new functional form for the dependence of the strengthening on concentration, temperature and strain rate. Fourth, application of the model leads to parameter-free predictions that agree well with available experimental data on various Mg–Al–Zn alloys. The predicted strengthening is not large, e.g. ≈10≈10 MPa for the AZ31 alloy at room temperature, but is larger than the strengthening of basal slip by the same solutes. Overall, this work demonstrates the ability of mechanistic theories to provide a quantitative understanding of alloying effects on deformation modes in Mg

Convergence rate for numerical computation of the lattice Green's function

Flexible boundary condition methods couple an isolated defect to bulk through the bulk lattice Green's function. The inversion of the force-constant matrix for the lattice Green's function requires Fourier techniques to project out the singular subspace, corresponding to uniform displacements and forces for the infinite lattice. Three different techniques--relative displacement, elastic Green's function, and discontinuity correction--have different computational complexity for a specified numerical error. We calculate the convergence rates for elastically isotropic and anisotropic cases and compare them to analytic results. Our results confirm that the discontinuity correction is the most computationally efficient method to compute the lattice Green's function.Comment: 12 pages, 4 figure

A productivity dashboard for hospitals: an empirical study

Health information systems are key assets in managing health units’ daily operations. Nevertheless, literature is scarce concerning information systems for increasing and managing hospital productivity. This study aims at filling such gap through an empirical research based on large Portuguese hospital. Specifically, a dashboard prototype is proposed addressing productivity indicators in areas such as assistance, hospitalization, surgery, among others. This dashboard is tuned using a design science research approach where health experts successively validate the prototype. Interviews are conducted to assess the benefits of using our proposal to manage productivity on a daily basis.info:eu-repo/semantics/acceptedVersio

Analysis of dissociation of < c > and < c+a > dislocations to nucleate (1 0 -1 2) twins in Mg

A mechanism for (1 0 (1) over bar 2) twin nucleation in Mg is studied in which edge and mixed lattice dislocations dissociate into a stable twin, having at least the minimum 6-layer thickness formed by three glissile twinning dislocations, plus a residual stair rod dislocation. Continuum dislocation theory is used to compute the energy of the initial and final states of the proposed dissociation process, using the twin boundary energy computed by density functional theory. For the dislocation, the proposed dissociation is energetically favorable. An alternative dissociation path into partials on two {1 0 (1) over bar 1}-type pyramidal planes is possible, as seen in an atomistic analysis, and the continuum analysis predicts this alternative path to be more favorable than the twin process. For the dislocation, the continuum model also predicts that dissociation into the twinned structure is energetically favorable for 6-layer and thicker twins. In both and cases, the equilibrium twin length is predicted to increase with increasing applied resolved shear stress and grow unstably beyond a critical stress. Atomistic simulations of these processes are then performed. For , a twinned structure is stable under zero loading but with higher energy than the alternative dissociation on two {1 0 (1) over bar 1} planes. Under a positive applied strain of 4%, resolved on the twin plane, the twinning structure grows while under a negative applied strain of -3%, it reverts back to the alternative low-energy dissociated configuration on the pyramidal planes. For the mixed dislocation, the atomistic models predict that the dissociation into twinning dislocations does not occur spontaneously at zero applied strain but there is a stable twinned region at finite applied loads. These results demonstrate that dislocation-assisted mechanisms for twinning in Mg, initiating from lattice dislocations with large Burgers vectors, are physically feasible, and therefore twin nucleation from grain boundaries is not necessarily the dominant mechanism of twinning in Mg

Deformation modes in magnesium (0001) and (0 1 -1 1) single crystals: simulations versus experiments

Magnesium is an excellent candidate as lightweight structural material, but has strong plastic anisotropy, and the activation of, operation of, and competition between different slip and twinning systems remain active areas of research. Here, the nucleation of twinning and basal slip in Mg single-crystalline nanopillars are studied using molecular dynamics over a range of strain rates allowing for reasonable extrapolation to experimental rates. Deformation along the [0 0 0 1] direction shows tension and compression twinning at stresses predicted to be similar to 1400 and similar to 1700 MPa at a strain rate of 10(-3) s(-1). Moreover, twin nuclei are shown to be absolutely stable only above 1170 MPa. No evidence of nanotwinning is found and the twin-growth velocities are very fast (similar to 400 m s(-1)). These results do not support recently proposed mechanisms for nanotwinning. Deformation along the [0 1 (1) over bar 1] direction shows basal dislocation nucleation at stresses of 1000-1300 MPa in tension and 670-900 MPa in compression, at experimental strain rates, with one EAM potential exhibiting compression/tension asymmetry. Size effects are observed between pillars of diameters between 5 and 10 nm, which are attributable to surface stress effects driving nucleation and expected to be irrelevant at experimental pillar sizes (200 nm and above). Overall, most of the observed deformation mechanisms mirror those found in experiments but the stress levels, even when extrapolated to experimental strain rates, remain well above those found in micro-and nanopillar experiments. This indicates that deformation in the experimental specimens is controlled by the motion of pre-existing dislocations or is associated with significant stress concentrations due to surface defects

Solute Strengthening in Random Alloys

International audienceRandom solid solution alloys are a broad class of materials that are used across the entire spectrum of engineeringmetals, whether as stand-alone materials (e.g. Al-5xxx alloys) or as the matrix in precipitate-strengthening materials (e.g. Ni-based superalloys). As a result, the mechanisms of, and prediction of, strengthening in solid solutions has a long history. Many concepts have been developed and important trends identified but predictive capability has remained elusive. In recent years, a new theory has been developed that builds on one historical model, the Labusch model, in important ways that lead to a well-defined model valid for random solutions with arbitrary numbers of components and compositions. The new theory uses first-principles-computed solute/dislocation interaction energies as input, from which specific predictions emerge for the yield strength and activation volume as a function of alloy composition, temperature, and strain-rate. Being a general model for materials that otherwise have a low Peierls stress, it has broad application and has been successfully applied to Al-X alloys, Mg-Al, twinning in Mg alloys, and recently fcc High-Entropy Alloys. Here, the new theory is presented in a general and systematic manner. Approximations and limiting cases that reduce the complexity and facilitate understanding are introduced, and help relate the new model to various physical features present among the historical array of models, other recent models, and simulation studies. The quantitative predictions of the model in the various materials above is then demonstrated

First-principles core structures of < c+a > edge and screw dislocations in Mg

The core structures of edge and screw dislocations in Mg are computed using density functional theory (DFT). Both types dissociate into two 1/2 partials on the second-order pyramidal planes. These DFT results are then allowed to relax with embedded-atom and modified embedded-atom (MEAM) potentials. Only MEAM retains the general structure of the DFT predictions. The DFT core structures provide the basis for future investigations of solute effects and calibration of interatomic potentials. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved