Scientific Events

Room: Seminarraum 1

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

MPIE Colloquium
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]

New concepts in electrochemistry – from magnetic structuring of macroscopic layers to single nanoparticle analysis

New concepts in electrochemistry – from magnetic structuring of macroscopic layers to single nanoparticle analysis
Electrochemistry is a well-established technique for the electrodeposition of thin films for corrosion protection or of 3D structures for integrated circuits. It is also key to most approaches for sustainable energy conversion and storage and it is widely utilized in sensors for the detection and quantification of ions and biomolecules. In this presentation novel concepts adopting classical electrochemical methods for the fabrication and characterization of magnetic materials at the micro- and nanoscale will be presented.First the influence of magnetic fields on electrochemical deposition will be discussed using the magnetic-field assisted fabrication of structured electrodeposits in the milli- and micrometer range as an example. The relevant magnetic forces and their effect on local mass transport control will be discussed.[1,2]Electrochemistry will then be highlighted as a powerful tool for the characterization of magnetic nanoparticles beyond conventional imaging methods. For superparamagnetic Fe3O4 core Au shell nanoparticles electrochemical analysis of the particle coating quality will be shown.[3] Advancing from this, single nanoparticle electrochemistry will be presented as a new method that provides hitherto inaccessible insights into magnetic field effects on single nanoparticles in suspensions. Thus, magnetic field enhanced particle agglomeration and altered particle corrosion dynamics can be detected on a single particle level.[4]Fig. 1: Magnetic field assisted structuring of electrodeposits (left) and electrochemical characterization of magnetic core shell nanoparticles (right).References:[1] K. Tschulik, C. Cierpka, A. Gebert, L. Schultz, C.J. Kähler, M. Uhlemann, , Anal. Chem. 2011, 83, 3275–3281.[2] K. Ngamchuea, K. Tschulik, R. G. Compton, Nano Res. 2015, 8, 3293–3306.[3] K. Tschulik, K. Ngamchuea, C. Ziegler, M. G. Beier, C. Damm, A. Eychmueller, R. G. Compton, Adv. Funct. Mater. 2015, 25, 5149–5158.[4] K. Tschulik, R. G. Compton, Phys. Chem. Chem. Phys. 2014, 16, 13909–13913. [more]

MULTICOMPONENT AND HIGH-ENTROPY ALLOYS

Conventional strategy for developing metallurgical alloys is to select the main component based on a primary property requirement, and to use alloying additions to confer secondary properties. This strategy has led to the development of many successful alloys based on a single main component with a mix of different alloying additions to provide a balance of required in-service properties. Typical examples include high temperature Ni superalloys, wrought Al alloys and corrosion resistant stainless steels. However, conventional alloy development strategy leads to an enormous amount of knowledge about alloys based on one component, but little or no knowledge about alloys containing several main components in approximately equal proportions. Theories for the occurrence, structure and properties of crystalline phases are similarly restricted to alloys based on one or two main components. Information and understanding is highly developed about alloys close to the corners and edges of a multicomponent phase diagram, with much less known about alloys in the centre of the diagram. This talk describes a range of other multicomponent alloying strategies and gives a number of examples of high-entropy and other multicomponent alloys. [more]

Hydrogen Storage Technology at the Helmholtz Zentrum Geesthacht

The use of fossil fuels as energy supply is growing increasingly problematic both from the point of view of environmental emissions and energy sustainability. As an alternative to fossil fuels, hydrogen is widely regarded as a key element for a potential energy solution. In this respect, hydrogen storage technologies are considered a key roadblock towards the use of H2 as energy carrier. Among the methods available to store hydrogen, solid-state storage appears to be a very interesting alternative, showing for example the highest volumetric storage densities and high safety. Within the Helmholtz “Advanced Engineering Materials” Programme, the Department of Nanotechnology focusses on the development of both nanostructured hydrogen storage materials and hydrogen storage systems. A detailed account of the actual and future research activities in the field of hydrogen technology at the Helmholtz-Zentrum Geesthacht will be presented. [more]

NANO-HITEN - Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

NANO-HITEN - Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides
A ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability. Manganese in a content of 1.5 % and molybdenum in a content of 0.2 % were added to 0.04 % carbon Ti- bearing steel in order to lower austenite–ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09 % Ti steel. Microstructure of the 0.09 %Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09 % Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite–ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to con- tain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCl-type by X-ray diffractometry. The calculated amount of precipitation- strengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hot- rolled high strength sheet steel exhibited excellent stretch flange formability. [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]

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

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]
Show more
Go to Editor View