Scientific Events

The aim of modern material modeling is the realistic prediction of the behavior of materials and construction parts by numerical simulation. Experimental investigations prove that the microstructure and thus the mechanical properties may vary under loads. It is thus essential to describe the load-dependent microstructure in these cases by material models to close the system of fundamental physical equations. One elegant way for the derivation of such material models is given by the Hamilton principle which belongs to the class of variational, energy-based modeling strategies. The talk starts with fundamental investigations for modeling the simple harmonic oscillator. Afterwards, the presented modeling concept is generalized to the Hamilton principle which is also applicable to deformable solids with evolving microstructure. As first example for such materials, phase transformations in solids are modeled. The numerical results are compared to experimental observations and an industrially relevant application is presented. In the last part of the talk, the universal character of the Hamilton principle is demonstrated by solving the inverse problem of topology optimization. To this end, a growth approach as observed in biological processes is presented which computes component structures with minimal weight at maximum stiffness. [more]

A lot of research effort has now been dedicated to the study of complex multicomponent alloys (more commonly called High Entropy Alloys HEA). This family of materials introduced in 2004 breaks with the traditional alloying concept, since they explore the domain of concentrated solid solution(s) of +5 elements. Several studies sprovide fundamental understanding on the structure and the mechanical properties of some of these alloys, mostly fcc [1–3]. If the results are promising, as for example the incredible fracture toughness of FeCoCrMnNi at low temperatures [4], recent papers suggest that equiatomic fcc alloys with less than 5 elements, or non-equiatomic fcc concentrated alloys also display great, or even greater mechanical properties [2,5,6]. The sub-family of bcc complex multicomponent alloys has been less investigated. Therefore, a multi-scale characterization of a model bcc multicomponent alloy with composition Ti₂₀Zr₂₀Hf₂₀Nb₂₀Ta₂₀ is performed. After optimization of the microstructure, investigated by SEM (EBSD), TEM and EXAFS, the mechanical properties of the alloy are studied during both tensile/relaxations tests and shear tests. Deformation mechanisms are discussed in terms of activation volume and flow stress partitioning, interpreted with the help of microstructural investigations by transmission electron microscopy. Finally, the “HEA” concept is coupled with the chemical design based on electronic parameters Bo and Md used in Ti-alloys. This concept, first introduced by Morigana was successfully used to help predicting the structure stability, and hence the mechanical behavior – dislocation glide, twinning induced plasticity (TWIP) or transformation induced plasticity (TRIP) – of Ti-rich alloys [7,8]. The studied composition Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅ displays a large ductility of 20% and an increased work-hardening [9]. It confirms that extending the concept of “HEAs” to non-equiatomic compositions can be highly beneficial and that the design strategy developed for Ti-alloys can be used with great results in concentrated alloys. [1] F. Otto, A. Dlouhý, C. Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy, Acta Mater. 61 (2013) 5743–5755. doi:http://dx.doi.org/10.1016/j.actamat.2013.06.018. [2] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428–441. doi:http://dx.doi.org/10.1016/j.actamat.2014.08.026. [3] C. Varvenne, A. Luque, W.A. Curtin, Theory of strengthening in fcc high entropy alloys, Acta Mater. 118 (2016) 164–176. doi:http://dx.doi.org/10.1016/j.actamat.2016.07.040. [4] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science. 345 (2014) 1153–1158. doi:10.1126/science.1254581. [5] Y. Deng, C.C. Tasan, K.G. Pradeep, H. Springer, A. Kostka, D. Raabe, Design of a twinning-induced plasticity high entropy alloy, Acta Mater. 94 (2015) 124–133. doi:10.1016/j.actamat.2015.04.014. [6] Z. Li, K.G. Pradeep, Y. Deng, D. Raabe, C.C. Tasan, Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off, Nature. advance online publication (2016). http://dx.doi.org/10.1038/nature17981. [7] M. Abdel-Hady, K. Hinoshita, M. Morinaga, General approach to phase stability and elastic properties of β-type Ti-alloys using electronic parameters, Scr. Mater. 55 (2006) 477–480. doi:http://dx.doi.org/10.1016/j.scriptamat.2006.04.022. [8] M. Marteleur, F. Sun, T. Gloriant, P. Vermaut, P.J. Jacques, F. Prima, On the design of new β-metastable titanium alloys with improved work hardening rate thanks to simultaneous TRIP and TWIP effects, Scr. Mater. 66 (2012) 749–752. doi:http://dx.doi.org/10.1016/j.scriptamat.2012.01.049. [9] L. Lilensten, J.-P. Couzinié, J. Bourgon, L. Perrière, G. Dirras, F. Prima, I. Guillot, Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity, Mater. Res. Lett. 5 (2017) 110–116. doi:10.1080/21663831.2016.1221861. [more]

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]

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]

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]

Although magnesium is the lightest structural metal and has a great potential to be utilized in lightweight constructions, e.g. in automotive engineering, the use of wrought magnesium alloys is limited due to, inter alia, a high mechanical anisotropy and poor room temperature formability. Against this background, understanding the underlying physical mechanisms and microstructural changes in the material during processing is crucial in order to overcome the difficulties associated with the limited ductility by innovative processing, microstructure and alloy design. In order to isolate and access specific mechanisms of plasticity, model experiments on single crystal provide an invaluable tool, as they permit a much clearer and forthright analysis compared to conventional polycrystal studies. Specifically oriented single crystals of various orientations were subjected to channel-die plane strain compression at room and elevated temperatures. The microstructure and texture evolution were characterized experimentally with respect to the deformation behavior. Pure Mg crystals of ‘hard’ orientations that were compressed along the c-axis displayed limited room temperature ductility, although pyramidal 〈c+a〉 slip was readily activated, and fractured along crystallographic {112 ̅4} planes as a result of highly localized shear. A two stage work hardening behavior was observed in ‘soft’ Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was correlated with the occurrence of anomalous extension twinning that formed as a result of deformation heterogeneity and constituted obstacles for dislocation glide. The interaction between slip and twinning was further investigated by performing in-situ simple shear experiments on Mg bicrystals. It was shown that slip transmission takes place across different twin boundaries with basal slip being readily transmitted through a whole twin, which contradicts a classical Hall-Petch type hardening. The amount of twinning shear for {101 ̅2} twins in Mg was measured experimentally and discussed in terms of the shear-coupled grain boundary migration by considering the formal dislocation content of the respective twin boundaries. The coupling factor that equals the amount of twinning shear was found to result from a combination of two elementary coupling modes, i.e. the correct formal description of the twin boundary comprises two arrays of dislocations with 〈101 ̅0〉 and [0001] type Burgers vectors. [more]