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Yasmin Ahmed Salem, M.A.
Yasmin Ahmed Salem
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Phone: +49 211 6792 722
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Scientific Events

Scientific Events

Month:

Topical session “Mechanical Properties at Small Scales” at the DPG-Spring Meeting 2018

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Topical session “Mechanical Properties at Small Scales” at the DPG-Spring Meeting 2018

MPIE Colloquium

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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]

Topic day “Dislocation based plasticity – experiment vs. simulation” at “The Schöntal Symposium Dislocation-based Plasticity” of the DFG Forschergruppe FOR 1650

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Topic day “Dislocation based plasticity – experiment vs. simulation” at “The Schöntal Symposium Dislocation-based Plasticity” of the DFG Forschergruppe FOR 1650

Mechanism of Enhanced Ductility in Mg Alloys

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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

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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]

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"Mechanics meets Energy VI” symposium at Kloster Steinfeld/Eifel

Fundamentals of mechanical response

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"Fundamentals of mechanical response" at the Conference on Electronic and Advanced Materials

 
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