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Scientific Events

Scientific Events

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4th International Conference on Medium and High Manganese Steels

MPIE Colloquium

12953 1519312993

Nanoindentation for Investigating Dynamics of Shear Bands in Metallic Glasses

Deformation in metallic glasses occurs by initiation and propagation of multiple thin shear bands. This mode is rather difficult to analyse since generally, a single band soon propagates to a large extent in the specimen leading to a catastrophic failure. Exceptions are for example in creep tests under very low stress and moderate temperature or in confined deformation tests. We used instrumented nano-indentations to perform series of independent experiments at room temperature on a Mg65Cu12.5Ni12.5(Ce75La25)10 metallic glass. Loading part of the curves shows serrations which size and duration were measured using an automatic procedure. To make analyses consistent, data were considered only in the domain with similar strain rates, in the range of 1 to 0.3 s-1. Times between successive serrations follow a normal distribution characterizing a random occurrence of deformation burst in the glass. It was then conjectured, first that serration occurs through activation of appropriate zone in the glass that should naturally scale with a multiple of an elementary domain size characterizing the deformation mechanism. Second, as activated zones leading to serration are very few in the glass, the model should be described by the Poisson statistics. Data analyses reveals that serration size are well fitted by a Poisson distribution. The model predict an elementary size which scale with that of the activation volume of 3 atoms, measured from creep test at constant load in the same series of experiments. Eventually, energy dissipated during serration is analyzed as to define shear bands dynamics characteristics.Depending on time, I shall present the use of nano-indention for investigating dynamics of nanoporous metallic materials deformation. N. Thurieau, L. Perriere, M. Laurent-Brocq, Y. Champion, J. Appl. Phys., 118 (2015) 204302. [more]

Mechanism of Enhanced Ductility in Mg Alloys

12983 1519115173

Mechanism of Enhanced Ductility in Mg Alloys

Pure Mg has low ductility due to strong plastic anisotropy and due to a transition of <c+a> pyramidaldislocations to a sessile basal-oriented structure [1]. Alloying generally improves ductility; for instance, Mg-3wt.%RE (RE=Y, Tb, Dy, Ho, Er) alloys show relatively high ductility [2], and typically larger than mostcommercial Mg-Al-Zn alloys at similar grain sizes. Possible concepts for ductility in alloys include thereduction of plastic anisotropy due to solute strengthening of basal slip, the nucleation of <c+a> from basal I1stacking faults, the prevention of the detrimental <c+a> transformation to sessile structures, and the weakeningof strong basal texture by some solute/particle mechanisms. Here, we introduce a new mechanism ofpyramidal cross-slip from the lower-energy Pyr. II plane to the higher energy Pyr. I plane as the key toductility in Mg and alloys [3]. Certain alloying elements reduce the energy difference between Pyr. I and IIscrew dislocations, accelerating cross-slip that then leads to rapid dislocation multiplication and alleviates theeffects of the undesirable pyramidal-to-basal dissocation. A theory for the cross-slip energy barrier ispresented, and first-principles density functional theory (DFT) calculations, following methods in [4], are usedto compute the necessary pyramidal stacking fault energies as a function of solute type for many solutes in thedilute concentration limit. Predictions of the theory then demonstrate why Rare Earth solutes are highlyeffective at very low concentrations, and generally capture the trends in ductility and texture evolution acrossthe full range of Mg alloys studied to date. The new mechanism then points in directions for achievingenhanced ductility across a range of non-RE alloys.[1] Z. Wu, W.A. Curtin, Nature 526 (2015) 62-67[2] S. Sandlobes, et al., Acta Materialia 59 (2011) 429-439; Acta Materialia 70 (2014) 92–104[3] Z. Wu, R. Ahmad, B. Yin, S. Sandlobes, and W. A. Curtin, Science 359, 447-452 (2018).[4] B. Yin, Z. Wu, and W. A. Curtin, Acta Materialia 136 (2017) 249-261. [more]

Early stages of high temperature oxidation and sulphidation studied by synchrotron X-ray diffraction and spectroscopy

12681 1517215921

Early stages of high temperature oxidation and sulphidation studied by synchrotron X-ray diffraction and spectroscopy

Ferritic high temperature alloys are widely used as boiler tube and heat exchanger materials in thermal power plants. All technologies have in common that the applied materials are exposed to different temperatures, process pressures and reactive atmospheres which lead to a change of the material properties and a further degradation of the material. Material changes caused by ageing in highly corrosive and toxic gases such as SO2 are mainly studied ex situ after the reaction is finished.The presentation will focus on a novel approach to study high temperature oxidation and sulphidation of alloys aged in a strongly corrosive environment in real time by energy dispersive X-ray diffraction (EDXRD). A special designed corrosion reactor was used to combine high temperature gas corrosion experiments with the collection of diffraction pattern. For this technique high energetic white X-ray radiation (10-100 keV) was used instead of conventional monochromatic radiation. It enables us to study crystallization procedures on short and medium time scales (1 min < t < 24 h) as a function of process time.X-ray diffraction is not phase sensitive for structural very similar oxide phases such as Fe2O3 and Cr2O3. To enlighten the formation mechanism of protective Cr2O3 at high temperature in corrosive atmosphere for different ferritic alloys an experimental setup for X-ray absorption near edge structure spectroscopy (XANES) in corrosive environment was developed and put into operation. The presentation will provide an overview of the possibilities of high temperature corrosion analysis using synchrotron-based X-ray diffraction and spectroscopy techniques. [more]

Atomistic modeling of grain boundary segregation in transition metals

11840 1518598215

Atomistic modeling of grain boundary segregation in transition metals

[more]

11010 1504763231

4th NRW-APT User Meeting

The Max-Planck-Institut für Eisenforschung GmbH in Düsseldorf is organizing the 4th NRW-APT user meeting on November the 23rd 2017 and we would like to invite you and your research colleagues to participate in this event. This meeting will bring together scientists from North Rhine-Westphalia dealing with APT technique or correlating APT with other techniques. We want to discuss problems and share knowledge regarding sample preparation, measurement conditions, data reconstruction & analysis, etc. If you and your colleagues would like to attend this event, then please register before November 6th 2017. There are limited places only. We are looking forward to see you in Düsseldorf! [more]

MPIE Colloquium

11436 1509365877

Insights into the Role of Mechanics on Diffusion-Controlled Phase Transformations using Phase Field Models

The role played by the microstructures ensuing from phase transformations on the mechanical properties is now very well documented and thoroughly studied, in particular in metallic alloys. Although there is also a large number of works devoted to the reverse, i.e. the influence of mechanics on the microstructure formation and evolution, a general picture is still lacking, in particular when the phase transformations are diffusion-controlled. Indeed, drawing such a general picture requires to address at the same time the issues related to both mechanical behavior and phase transformations, as well as to address new issues arising from the tight coupling between evolving interfaces and evolving strain/stress fields.Besides the well-known modification of two-phase thermodynamic equilibrium by elasticity, as explored by Larché and Cahn (e.g. [10]) and Voorhees and Johnson (e.g. [9]), the trends are less obvious concerning the morphological evolutions when the chemical and mechanical driving forces are competing to reduce the overall free energy of the materials.In this contribution, an attempt will be made to draw some trends by examining several situations in different types of alloys where the different contributions that mechanics encompasses are decoupled. For that purpose, we will resort to extensive calculations with phase field models that have been specifically developed [1,7].First we will discuss the role of elasticity on the shape selection of precipitates, beyond the classical results of hard precipitates in a soft matrix against soft precipitates in a hard matrix, that are relevant only for isotropic elasticity.Indeed, we will show how the anisotropy of the elastic energy arising from either the moduli or the eigenstrain is crucial for the shape selection, even for diffusion-controlled transformation at high temperatures where it is usually believed that elasticity is totally relaxed by plasticity. The examples supporting our analysis will concern cuboidal ordered precipitates in Ni-base superalloys [6] and acicular structures in alloys exhibiting allotropic transformation such as Ti alloys, steels or brass [3].In a second part, we will show that in many cases, plasticity does not relax totally the elastic strain associated with the phase transformations [4]. As a direct consequence, plasticity may not change qualitatively the shape evolutions driven by elasticity, as it will be illustrated on the rafting of the ordered precipitates in Ni-base superalloys [5] and on the acicular structures [4], although it can change the kinetics of the processes. However, we will show that in some cases, plasticity may induce shape bifurcations [5,8] that are difficult to infer with simplified qualitative arguments, as usually done in the literature on diffusion-controlled phase transformations.We will conclude with a few open questions that we have been able to identify thanks to our phase field calculations, such as the inheritance of plastic strain by the growing phases [2].[1] K. Ammar, B. Appolaire, G. Cailletaud, and S. Forest. Combining phase field approach and homogenization methods for modelling phase transformation in elastoplastic media. European Journal of Computational Mechanics, 18(5-6):485–523, 2009.[2] K. Ammar, B. Appolaire, S. Forest, M. Cottura, Y. Le Bouar, and A. Finel. Modelling inheritance of plastic deformation during migration of phase boundaries using a phase field method. Meccanica, 49:2699–2717, 2014.[3] M. Cottura, B. Appolaire, A. Finel, and Y. Le Bouar. Phase field study of acicular growth: Role of elasticity in Widmanstätten structure. Acta Materialia, 72:200–210, 2014.[4] M. Cottura, B. Appolaire, A. Finel, and Y. Le Bouar. Plastic relaxation during diffusion-controlled growth of Widmanstätten plates. Scripta Materialia, 108:117–121, 2015.[5] M. Cottura, B. Appolaire, A. Finel, and Y. Le Bouar. Coupling the phase field method for diffusive transformations with dislocation density-based crystal plasticity: Application to Ni-based superalloys. Journal of the Mechanics and Physics of Solids, 94:473–489, 2016.[6] M. Cottura, Y. Le Bouar, B. Appolaire, and A. Finel. Rôle of elastic inhomogeneity in the development of cuboidal microstructures in Ni-based superalloys. Acta Materialia, 94:15–25, 2015.[7] M. Cottura, Y. Le Bouar, A. Finel, B. Appolaire, K. Ammar, and S. Forest. A phase field model incorporating strain gradient viscoplasticity: Application to rafting in Ni-base superalloys. Journal of the Mechanics and Physics of Solids, 60:1243–1256, 2012.[8] V. de Rancourt, K. Ammar, B. Appolaire, and S. Forest. Homogenization of viscoplastic constitutive laws within a phase field approach. Journal of the Mechanics and Physics of Solids, 88:291–319, 2016.[9] W.C. Johnson and P.W. Voorhees. Interfacial stress, interfacial energy, and phase equilibria in binary alloys. Journal of Statistical Physics, 95(5-6):1281– 1309, 1999.[10] F. Larché and J.W. Cahn. Thermochemical equilibrium of multiphase solids under stress. Acta Metallurgica, 26(10):1579–1589, 1978. [more]

 
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