Scientific Events

Composite voxels fornonlinear mechanical problems

Composite voxels for nonlinear 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]

In situ HR-EBSD characterization during micro-mechanical testing

In situ HR-EBSD characterization during micro-mechanical testing
Quantification of the mechanical properties of crystalline materials at micro and nano length-scales is as important as it is challenging. In situ mechanical testing inside the SEM using a micro-indenter offers the great advantage of a direct observation of the progressive deformation in materials and has been applied for many years to assess the deformation mechanisms at small scale. This is now reinforced by the capability of doing in situ EBSD and HR-EBSD in order to map the evolution of the microstructure, the stress field and the GNDs distribution in the materials at several steps during progressive deformation. We apply this technique to estimate the size of the plastic zone underneath the crack tip during micro-cantilever bending in tungsten and NiAl intermetallic for fracture toughness determination. HR-EBSD is used to map the evolution of the stress field around the notch tip and to estimate the GNDs in the plastically deformed zone. In situ EBSD has been also applied to micropillar compression in alpha Titanium in order to study the formation and evolution of compressive twins during the deformation. The results show dislocation driven twin formation and twin propagation and thickening according to the local resolve shear stress. [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: 01:30 PM - 03:00 PM (Local Time Germany)
  • 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]

Alloys for Additive Manufacturing Workshop 2016

Additive Manufacturing is a technology on the verge of widespread adoption. In some fields such as dental implants, the tooling or the aerospace industry, it is rapidly becoming state of the art for the production of highly complex and/or individualised parts. Research is currently focussing on improvement of the processes in terms of reliability and productivity. At the same time, many researchers and users come to the conclusion that a renewed effort in understanding materials behaviour during the AM processes is a key requirement to pushing AM closer to application. [more]

MPIE-Colloquium: Phase Transformations: Atom-Probe Tomography versus Modeling

MPIE-Colloquium: Phase Transformations: Atom-Probe Tomography versus Modeling
The design of Atom probe tomography (APT) at Oxford and Rouen universities for 25 years ago has been an outstanding breakthrough in the microscopy world. APT is the only analytical microscope able to provide 3D images of a material at the atomic scale [1]. Because of its ultimate spatial resolution (0.1 nm in depth, a few tenths of a nm at the sample surface), combined with its quantitativity of composition measurements, APT has played a major role in the investigation of the early stages of phase separation in solids. APT has also been the first instrument to show Cottrell atmospheres (tiny clouds of impurity atoms around dislocations in crystals) at the atomic-scale in the three dimensions of space [2]. A new breakthrough has been achieved ten years ago with the implementation of ultrafast pulsed laser (duration < 1ps) to atom probe tomography [3]. This new generation of laser-enhanced atom probe tomograph, designed in our lab and at Madison, USA, has opened the instrument to semi-conductors and oxides that are key materials in micro-electronics and nanosciences [4,5]. Correlative approaches combining TEM with APT has been shown to be crucial for more accurate APT reconstructions of microelectronics devices [6]. A key force of APT is that 3D reconstructions can be confronted at the same scale to kinetic Monte-Carlo simulations conducted on rigid lattice. This dual approach has been recently applied to phase separation in self-organised GeMn magnetic thin films [5]. In this talk, APT results will be confronted to simulations but also to analytical models dealing with precipitation kinetics (non-classical nucleation [7], coarsening in ternary systems, influence of precipitate size on their composition [8]). A recently developed analytical model dealing with nucleation, growth and coarsening in ternary systems including diffusion coupling between chemical species has revealed that the kinetic pathway does not necessarily follow the tie lines of phase diagram in agreement with APT experiments on model nickel base superalloys [9]. [1] D. Blavette, A. Bostel, J.M. Sarrau, B. Deconihout and A. Menand, 1993, Nature 363, 432 [2] D. Blavette, E. Cadel, A. Fraczkiewicz, A. Menand, 1999, Science 17, 2317 [3] B. Gault, F. Vurpillot, A. Vella, M. Gilbert, A. Menand, D. Blavette, B., 2006, Rev. Sci. Instr. 77, 043705 [4] S. Duguay, T. Philippe, F. Cristiano, D. Blavette, Applied Physics Letter (2010) 97, 242104 [5] I. Mouton, R, Larde, E. Talbot, C. Pareige, D. Blavette, JAP 115, 053515 (2014) [6] A. Grenier, R. Serra, G. Audoit, Jp Barnes, S. Duguay, D. Blavette, N. Rolland, F. Vurpillot, P. Morin, P. Gouraud, Applied Physics Letters 106, 213102 (2015) [7] T. Philippe, D. Blavette, Journal Of Chemical Physics, 135, 134508 1-3 (2011) [8] M. Bonvalet, T. Philippe, X. Sauvage, D. Blavette, Phil. Mag Vol. 94, N°26, 2956-2966 (2014) [9] M. Bonvalet, T. Philippe, X. Sauvage, D. Blavette, Acta Materialia 100 (2015) 169-177 [more]

Driving Forces and Challenges of Interfacing Functional Oxide Perovskites

Driving Forces and Challenges of Interfacing Functional Oxide Perovskites
Perovskite (ABO3) oxides are by no exaggeration an extremely versatile class of materials, exhibiting a broad spectrum of fascinating physical properties: superconductivity, ferromagnetism, ferroelectricity, multiferroic behavior. Scaling down from bulk single crystals to thin and ultrathin (few unit cell thick) epitaxial films the first question is to what extent we are capable to preserve the properties of the bulk and understand the role played by epitaxial growth in changing the physical properties. Another degree of freedom and of complexity as well arises when we interface coherently thin films of two or more chemically and physically different oxides. On one hand, the interfacing poses challenges in terms of finding the fabrication conditions that satisfy the needs of all partners. On the other hand, new physical properties may arise at the interfaces: these are the driving forces for heterostructures and superlattices of perovskite oxides, with a boom of efforts in the last decades, supported by the progress in molecular beam epitaxy and pulsed-laser deposition of complex oxides. Simultaneously the advances of high resolution and scanning transmission electron microscopy with analytical accessoires (EELS, EDX) have been of tremendous help in investigating such heterostructures with unprecedented spatial resolution, contributing directly to the understanding of the physical properties at a unit cell level. In order to underline these statements, I shall give example of the physical properties of heterostructures and superlattices of ferromagnetic and ferroelectric perovskites from my work. [more]

MPIE-Colloquium: Atomic Resolution Observations of Step Structure and Dynamics in Grain Boundaries

MPIE-Colloquium: Atomic Resolution Observations of Step Structure and Dynamics in Grain Boundaries
The development of aberration correction for electron microscopy has greatly increased our ability to characterize materials at the atomic scale. The technological advances that have extended the resolution limit to 0.5Å have also made it possible to record images with better signal-to-noise and at faster rates. In this work, atomic resolution images of moving steps in grain boundaries in gold bicrystals were obtained from extended HR(S)TEM time series by averaging between structural events. The resulting precision of atomic displacements and the small volume of material involved in the observed structures allowed a direct atom-by-atom comparison with MD simulations, revealing details of the transition beyond the temporal resolution of experimental techniques. Specifically, simulations uncovered a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. This analysis is part of a broader study of grain boundary behavior during deformation and capillary shrinkage of island grains. Dynamic observations show that the rate of shrinkage is non-parabolic, and the mechanism is controlled by step motion. Finally, some current developments in electron microscopy will be outlined with a view toward future research on interfaces in materials. [more]
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