Scientific Events

Metallic materials which have enabled progress over thousands of years and are produced in huge quantities (e.g. 1.8 billion tons of steels per year), are now facing severe and in part abrupt limits set by sustainability constraints and the associated legislative measures. Accelerated demand for structural alloys in key areas such as energy, construction, infrastructure, safety, mobile communication and transportation creates growth rates of up to 200% until 2050. Yet, most of these materials are energy, greenhouse gas and pollution intense when extracted, produced and manufactured. The lecture provides an introduction to this field and reviews approaches to improve the sustainability of and through structural metallic alloys. It reports about progress in direct sustainability for different steps along the value chain including CO₂-reduced primary production; recycling; scrap-compatible alloy design; contaminant tolerance of alloys; and improved alloy longevity through corrosion protection, damage tolerance and repairability for longer product use. It is also shown how structural materials enable improved energy efficiency through reduced weight, higher thermal stability, and better mechanical properties. The respective leverage effects of the individual measures on rendering structural alloys more sustainable are described. [more]

Utilizing molecular dynamics simulations, we report a non-monotonic dependence of the shear stress on the strength of an external magnetic eld (H) in a liquid-crystalline mixture of magnetic and non-magnetic anisotropic particles.This non-monotonic behavior is in sharp contrast with the well-studied monotonic H-dependency of the shear stress in conventional ferro uids, where the shear stress increases with H until it reaches a saturation value. We relatethe origin of this non-monotonicity to the competing eects of particle alignment along the shear-induced direction, on the one hand, and the magnetic eld direction on the other hand. To isolate the role of these competing eects,we consider a two-component mixture composed of particles with eectively identical steric interactions, where the orientations of a small fraction, i.e. the magnetic ones, are coupled to the external magnetic eld. By increasing Hfrom zero, the orientations of the magnetic particles show a Freederickz-like transition and eventually start deviating from the shear-induced orientation, leading to an increase in shear stress. Upon further increase of H, a demixingof the magnetic particles, from the non-magnetic ones, occurs which leads to a drop in shear stress, hence creating a non-monotonic response to H. Unlike the equilibrium demixing phenomena reported in previous studies, the demixingobserved here is neither due to size-polydispersity nor due to a wall-induced nematic transition. Based on a simplied Onsager analysis, we rather argue that it occurs solely due to packing entropy of particles with dierent shear- or magnetic-eld-induced orientations. [more]

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]

The Max-Planck-Insititut für Eisenforschung in Düsseldorf cordially invites academic and industrial researchers to the workshop on WEM formation, taking place on October 23^nd 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 30^th. The event is financed by the BMBF through grant 03SF0535 and is free of charge. [more]

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]

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]

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]