A FFT-based approach for Mesoscale Field Dislocation Mechanics: application to grain size effect in polycrystals

International audienceA crystal elasto-viscoplastic FFT formulation coupled with the Mesoscale Field Dislocation Mechanics (MFDM) theory is presented. This MFDM-EVPFFT approach accounts for plastic flow and hardening from densities of geometrically necessary dislocations (GND) in addition to statistically stored dislocations (SSD). It is shown on 3D periodic Voronoi polycrystals that GND densities modify both intra-granular incompatible fields and stresses, which are at the origin of a grain size dependent flow stress of the Hall-Petch type

Advanced characterization techniques for high-angular and high-spatial resolutions in the scanning electron microscope

High-angular resolution electron diffraction-based techniques aim at measuring relative lattice rotations and elastic strains with an accuracy about 1.10-4 (<0.01°) in the scanning electron microscope (SEM). These metrics are essential for the fine characterization of deformation structures in terms of grain internal disorientations and geometrically necessary dislocation densities. To this purpose, relative deformations between electron diffraction patterns are retrieved with subpixel accuracy using digital image correlation (DIC) techniques. Here, a novel DIC approach is proposed. It relies on a linear homography [1], i.e., a geometric transformation often met in photogrammetry to model projections. The method is implemented in ATEX-software [2], developed at the University of Lorraine. Its performances are illustrated from both a semi-conductor and a metal. First, lattice rotation and elastic strain fields are investigated in the vicinity of a giant screw dislocation in GaN single crystal using the electron backscattered diffraction technique (Fig. 1). Second, the proposed method is coupled with the on-axis Transmission Kikuchi Diffraction (TKD) configuration to characterize a nanocrystalline aluminium obtained by severe plastic deformation. On-axis TKD consists in observing a thin foil in transmission in the SEM, using a scintillator is placed beneath the specimen, perpendicularly to the electron beam. Thanks to this coupling, high-spatial (3-6 nm) and high-angular (~0.01°) resolutions are simultaneously achieved in SEM. [3]

Dislocation transport and intermittency in the plasticity of crystalline solids

International audienceWhen envisioned at the relevant length scale, plasticity of crystalline solids consists in the transport of dislocations through the lattice. In this paper, transport of dislocations is evidenced by experimental data gathered from high-resolution extensometry carried out on copper single crystals in tension. Spatiotemporal kinematic fields display spatial correlation through characteristic lines intermittently covered by plastic activity. Intermittency shows temporal correlation and power-law distribution of avalanche size. Interpretation of this phenomenon is proposed within the framework of a field dislocation theory attacking the combined problem of dislocation transport and long-range internal stress field development. Intermittency and transport properties show remarkable independence from sample size, aspect ratio, loading rate, and strain-rate sensitivity of the flow stress

Electron-phonon heat transfer in giant vortex states

We examine energy relaxation of nonequilibrium quasiparticles (QPs) in different vortex configurations in "dirty" s-wave superconductors (SCs). The heat flow from the electronic subsystem to phonons in a mesoscopic SC disk with a radius of the order of several coherence lengths is calculated both in the Meissner and in the giant vortex states using the Usadel approach. The recombination process is shown to be strongly affected by interplay of the subgap states, located in the vortex core and in the region at the sample edge where the spectral gap Eg is reduced by the Meissner currents. In order to uncover the physical origin of the results, we develop a semiquantitative analytical approximation based on the combination of homogeneous solutions of Usadel equations in Meissner and vortex states of a mesoscopic SC disk and analytically calculate the corresponding spatially resolved electron-phonon heat rates. Our approach provides important information about nonequilibrium QPs cooling by the magnetic field-induced traps in various mesoscopic SC devices

Sequential localization of a complex electron fluid

Complex and correlated quantum systems with promise for new functionality often involve entwined electronic degrees of freedom. In such materials, highly unusual properties emerge and could be the result of electron localization. Here, a cubic heavy fermion metal governed by spins and orbitals is chosen as a model system for this physics. Its properties are found to originate from surprisingly simple low-energy behavior, with two distinct localization transitions driven by a single degree of freedom at a time. This result is unexpected, but we are able to understand it by advancing the notion of sequential destruction of an SU(4) spin-orbital-coupled Kondo entanglement. Our results implicate electron localization as a unified framework for strongly correlated materials and suggest ways to exploit multiple degrees of freedom for quantum engineering.Comment: 21 pages, 4 figures (preprint format

Theory and Applications of X-ray Standing Waves in Real Crystals

Theoretical aspects of x-ray standing wave method for investigation of the real structure of crystals are considered in this review paper. Starting from the general approach of the secondary radiation yield from deformed crystals this theory is applied to different concreat cases. Various models of deformed crystals like: bicrystal model, multilayer model, crystals with extended deformation field are considered in detailes. Peculiarities of x-ray standing wave behavior in different scattering geometries (Bragg, Laue) are analysed in detailes. New possibilities to solve the phase problem with x-ray standing wave method are discussed in the review. General theoretical approaches are illustrated with a big number of experimental results.Comment: 101 pages, 43 figures, 3 table