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Yasmin Ahmed Salem, M.A.
Yasmin Ahmed Salem
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Scientific Events

Scientific Events

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Molecular dynamics on the diffusive time scale

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Molecular dynamics on the diffusive time scale

We formulate a theory of non-equilibrium statistical thermodynamics for ensembles of atoms or molecules. The theory is an application of Jayne's maximum entropy principle, which allows the statistical treatment of systems away from equilibrium. In particular, neither temperature nor atomic fractions are required to be uniform but instead are allowed to take different values from particle to particle. In addition, following the Coleman-Noll method of continuum thermodynamics we derive a dissipation inequality expressed in terms of discrete thermodynamic fluxes and forces. This discrete dissipation inequality effectively sets the structure for discrete kinetic potentials that couple the microscopic field rates to the corresponding driving forces, thus resulting in a closed set of equations governing the evolution of the system. We complement the general theory with a variational meanfield theory that provides a basis for the formulation of computationally tractable approximations. We present several validation cases, concerned with equilibrium properties of alloys, heat conduction in silicon nanowires, hydrogen desorption from palladium thin films and segregation/precipitation in alloys, that demonstrate the range and scope of the method and assess its fidelity and predictiveness. These validation cases are characterized by the need or desirability to account for atomic-level properties while simultaneously entailing time scales much longer than those accessible to direct molecular dynamics. The ability of simple meanfield models and discrete kinetic laws to reproduce equilibrium properties and long-term behavior of complex systems is remarkable. [more]

MPIE Workshop: Mechanisms of White Etching Matter Formation

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MPIE Workshop: Mechanisms of White Etching Matter Formation

The Max-Planck-Insititut für Eisenforschung in Düsseldorf cordially invites academic and industrial researchers to the workshop on WEM formation, taking place on October 23nd 2018. This workshop will focus on the fundamental materials scientific processes behind this phenomenon. For this we have invited a number of speakers from complementary fields that are crucial for understanding the phenomenon. Topics will range from WEM formation mechanisms in bearings and rails, over WEM generation by heat, surface machining and high pressure torsion, and the role of hydrogen and electric current, to the remarkable resistance of high nitrogen steels to WEC failure. Participants must register till September 30th. The event is financed by the BMBF through grant 03SF0535 and is free of charge. [more]

MPIE Colloquium

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Phase-field Modeling of Polycrystalline Structures: From Needle Crystals to Spherulites Phase-field Modeling of Polycrystalline Structures: From Needle Crystals to Spherulites

Results in modeling complex polycrystalline structures by phase-field models that monitor the local crystallographic by scalar or vector orientation fields will be reviewed. The applied models incorporate homogeneous and heterogeneous nucleation of growth centers, and several mechanisms to form new grains at the perimeter of growing crystals, a phenomenon termed growth front nucleation. Examples for PF modeling of such complex polycrystalline structures are shown as impinging symmetric dendrites, polycrystalline growth forms (ranging from disordered dendrites to spherulitic patterns), and various eutectic structures, including spiraling two-phase dendrites. Simulations exploring possible control of solidification patterns in thin films via external fields, confined geometry, particle additives, scratching/piercing the films, etc. are also displayed. Advantages, problems, and possible solutions associated with quantitative PF simulations are discussed briefly. [more]

Predicting solute segregation kinetics and properties in binary alloys from a dynamical variational gaussian model

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Predicting solute segregation kinetics and properties in binary alloys from a dynamical variational gaussian model

The thermodynamics and kinetics of solute segregation in crystals is important for controlling microstructure and properties. Prime examples are the effects of solute drag on interface migration and of static strain aging on the yield stress. A fully quantitative prediction of solute segregation is difficult, however, due to the spatially varying solute-defect binding energies that are atomic in origin. Moreover, as solute segregation enhances (locally) the solute concentration, dilute approximations for the underlying thermodynamics and kinetics become questionable. We present a dynamical version of the variational gaussian method for binary alloys [1] and illustrate its potential for select problems involving solute segregation including static strain aging in Al-Mg alloys [2]. Our model adapts the recently proposed Diffusive Molecular Dynamics (DMD) model for vacancy diffusion in crystals where a phonon- free description of solids is coupled with statistical averaging over various configurations to allow for the efficient calculation of free energies. In the alloy version of the model, the free energy is minimized by optimizing the atomic positions and vibrational amplitudes while relaxational dynamics are used to evolve the solute concentration field based on the local energy landscape. We show that this model successfully describes solute redistribution over diffusive timescales. In contrast to traditional continuum diffusion treatments, atomistic effects are automatically accounted for, and full kinetic pathways of the evolution of material properties are revealed in addition to the equilibrium properties. [1] E. Dontsova, J. Rottler, C. W. Sinclair, Phys. Rev. B 90, 174102 (2014) [2] E. Dontsova, J. Rottler, C. W. Sinclair, Phys. Rev. B 91, 224103 (2015) [more]

5th International Symposium on Computational Mechanics of Polycrystals, CMCn 2016 and first DAMASK User Meeting

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5th International Symposium on Computational Mechanics of Polycrystals, CMCn 2016 and first DAMASK User Meeting

The Max-Planck-Institut für Eisenforschung in Düsseldorf is organizing the 5th International Symposium on Computational Mechanics of Polycrystals and we would like to invite you and your research colleagues to participate in this event. This symposium is part of a biannual series of symposia that originated from the first joint research group established between the Max Planck Society and the Fraunhofer Society on the Computational Mechanics of Polycrystals. This year the symposium is combined with the first DAMASK User Meeting. DAMASK is the multi-physics simulation software developed at MPIE. If you and your colleagues would like to attend this event, then please register online before July 1st 2016. We emphasize that registration is mandatory and that there are limited places only. Many thanks and hope to see you in Düsseldorf! [more]

Magnetic Material Modeling for Numerical Simulation of Electrical Machines

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MagneticMaterial Modeling for Numerical Simulation of Electrical Machines

The development of energy efficient electrical machines requires accurate knowledge of the magnetic material behavior, i.e., iron loss components  and magnetizability, already in the design stage. In addition, knowledge on the magnetic property deterioration due to induced  residual  stresses occurring during the manufacturing as well as due to applied mechanical  stresses during the operation of the electrical machine is indispensable for the contemporary machine-design.In general, the modeling can be approached at different length scales, i.e., from quantum mechanics at the atomic level and micromagnetics at the sub-micrometer length scale to continuum modeling at the ultra-millimeter scale. The difficulty to apply micromag- netic approaches in the numerical simulation of electrical machines is given both, by the tremendous  need of computational effort as well as the difficulty to consider the inter- action with effects present at the macroscale such as, e.g., residual  stresses or non-local eddy currents.A more modern view of such aspects is to regard materials  as multilevel structures, where structural features at all length scales play a significant role. Multiscale modeling is the field of solving such problems that have important features at multiple spatial and/or temporal scales.  It allows calculating material properties on one level using information or models from other levels. In the light of this, this presentation will give an overview on the current modeling  approaches applied at the Institute of Electrical Machines (IEM) for soft magnetic materials in the simulation of rotating electrical machines.  Particular attention will be paid to the effect of residual  as well as applied  mechanical stress on the magnetic behavior occurring at the various steps of machine manufacturing and during machine operation.Selected References[1] N. Leuning, S. Steentjes, M. Schulte, W. Bleck, and K. Hameyer, ”Effect of elastic and plastic tensile mechanical loading on the magnetic properties of NGO elec- trical steel,” Journal of Magnetism and Magnetic Materials, vol.  417, pp.  42-48, November 2016.[2] S. Elfgen,  S. Steentjes, S. B¨ohmer, D. Franck, and K. Hameyer, ”Continuous Local Material Model for Cut Edge Effects in Soft Magnetic Materials,” IEEE Transac- tions on Magnetics, vol. 52, no. 5, pp. 1-4, May 2016.[3] N. Leuning, S. Steentjes, M. Schulte, W. Bleck, and K. Hameyer, ”Effect of Mate- rial Processing and Imposed Mechanical  Stress on the Magnetic, Mechanical, and Microstructural Properties of High-Silicon Electrical Steel,” steel research interna-tional, to appear, 2016.  [more]

Composite voxels for nonlinear mechanical problems

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Composite voxels fornonlinear mechanical problems

Two-scale simulations of components classically  rely upon finite element simulations  on boundary- and interface-fitted  meshes on both the macro and the micro scale. For complex microstructures fast and memory-efficient  solvers posed on regular voxels grids, in particular the FFT-based homogenization method [1], provide a powerful  alternative to FE simulations on unstructured  meshes and can be used to replace the micro-solver [2, 3]. Since representative volume elements of the microstructure  consist of up to 80003  voxels, even this micro-solver reaches its limits for nonlinear elastic computations.This talk focuses on the composite voxel technique [4], where sub-voxels  are merged into bigger voxels to which an effective material law based on laminates is assigned. Due to the down-sampled grid, both the memory requirements and the computational effort are severely reduced. We discuss the extensions of linear elastic ideas [4, 5] to the physically non-linear setting  and assess the accuracy  of reconstructed solution fields by comparing them to direct full-resolution computations.References[1] H. Moulinec and P. Suquet. A numerical method for computing the overall response of nonlinear composites with complex microstructure.Computer Methods in Applied Mechanics and Engineering, 157(1-2):69–94, 1998. [2] J. Spahn, H. Andra, M. Kabel, and R. Mueller.A multiscale approach for modeling pro- gressive damage of composite materials using fast Fourier transforms. Computer Methods in Applied Mechanics and Engineering, 268(0):871 – 883, 2014. [3] J. Kochmann,  S. Wulfinghoff, S. Reese, J. R. Mianroodi,  and B. Svendsen.  Two-scale FEFFT- and phase-field-based computational modeling of bulk microstructural evolution and macroscopic material behavior. Computer Methods in Applied Mechanics and Engi- neering, 305:89 – 110, 2016. [4] M. Kabel, D. Merkert, and M. Schneider. Use of composite voxels in FFT-based homog- enization. Computer Methods in Applied Mechanics and Engineering, 294(0):168–188,2015. [5] L. Gelebart and F. Ouaki. Filtering Material Properties to Improve FFT-based Methodsfor Numerical Homogenization.  J. Comput. Phys., 294(C):90–95, 2015. [more]

 
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