Scientific Events

Room: Seminarraum 1 Host: Prof. Dierk Raabe Location: Max-Planck-Institut für Eisenforschung GmbH
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]

Molecular dynamics on the diffusive time scale

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]

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

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]

Deformationmechanisms of TWIP steel: from micro-pillars to bulk samples

Deformation mechanisms of TWIP steel: from micro-pillars to bulk samples
  • Date: Jul 6, 2016
  • Time: 13:30 - 15:00
  • Speaker: Prof. Mingxin HUANG
  • Department of Mechanical Engineering, The University of Hong Kong, Hong KongShort biography of the speaker Dr. Mingxin HUANG is currently an Associate Professor at The University of Hong Kong, Hong Kong. His research interests focus on two areas: (1) fundamentals of microstructure-property relationship and phase transformation of advanced steels, and (2) development of lightweight materials for automotive applications. Both experimental and modelling works are involved in his research. His research projects have been well funded by government funding agents as well as industries from Europe and China (e.g. ArcelorMittal France, General Motors, Ansteel, Baosteel). Dr. Huang received his Bachelor as well as Master degrees from Shanghai Jiao Tong University (SJTU) in 2002 and 2004, respectively, and his PhD in 2008 from Delft University of Technology (TU Delft), The Netherlands. From 2008 to 2010, he worked as a research engineer at ArcelorMittal R&D centre in Maizieres-les-Metz, France. His research work in ArcelorMittal focused on the development of new advanced steels for automotive applications. Dr. Huang joined University of Hong Kong in 2010 as an Assistant Professor and was promoted to Associate Professor with tenure in 2016. Dr. Huang has published 50+ journal papers on major international journals in his field such as Acta Materialia and Scripta Materialia. Dr. Huang is an editorial board member of Materials Science and Technology, the Key Reader for Metallurgical and Materials Transactions A and has received twice “Outstanding Reviewer of Scripta Materialia” awards.
  • Location: Max-Planck-Institut für Eisenforschung GmbH
  • Room: Seminarraum 1
  • Host: Prof. Dierk Raabe
Twinning-induced plasticity (TWIP) steels have excellent combinationof strength and ductility and are potential lightweight materials forautomotive applications. Understanding the deformation mechanisms in TWIPsteels is essential for the successful application of TWIP steels. The firstpart of this work is to employ micron-sized single crystalline pillars toinvestigate the nucleation and growth mechanism of deformation twins. It isfound that the nucleation and growth of deformation twins are due to emissionand glide of successive partial dislocations. A physical model is proposed tosimulate the nucleation and growth of deformation twins. The second part of thepresentation discusses the deformationmechanism of bulk samples. Deformation mechanism of bulk samples at high strainrates will be discussed firstly. By synchrotron X-raydiffraction experiments, the present work demonstrates that a higher strainrate leads to a lower dislocation density and a lower twinning probability,which is opposite to other fcc metals. Furthermore, it has been demonstratedthat the contribution of twins to the flow stress is very limited. Instead,dislocations strengthening via forest hardening accounts for up to 90% of theflow stress. In other words, the contribution of twins to flow stress of TWIPsteels may have been overestimated in the existing literature. Finally, thepresent talk will discuss a nanotwinned steelwhich is manufactured by a simple thermomechanical treatment consisting of coldrolling and recovery annealing and possesses a high yield strength (1450 MPa)and considerable uniform tensile elongation (20%).   [more]

Mathematical and physical simulation of a funnel thin slab continuous casting machine of a Mexican plant

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