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Yasmin Ahmed Salem
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Scientific Events

Scientific Events

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HEA symposium "High entropy and compositionally complex alloys" at DPG Spring Meeting 2019 in Regensburg

4th International Conference on Medium and High Manganese Steels

MPIE Seminar

17321 1549460332

Computational Modeling of Moving Boundary Problems

The focus of this presentation is on computational methods for moving boundary/interface problems and its applications including fracture, fluid structure interaction, inverse analysis and topology optimization. First, two computational methods for dynamic fracture will be presented, i.e. the cracking particles method (CPM) and dual-horizon peridynamics (DH-PD). These methods do neither require a representation of the crack surface and associated complex crack tracking algorithms nor criteria for crack branching and crack interactions. They also do not need to distinguish between crack nucleation and crack propagation. Complex discrete fracture patterns are the natural outcome of the simulation. The performance of these methods will be demonstrated by several benchmark problems for non-linear quasi-brittle dynamic fracture and adiabatic shear bands. Subsequently, a local partition of unity-enriched meshfree method for non-linear fracture in thin shells -- based on Kirchhoff-Love theory -- exploiting the higher order continuity of the meshfree approximation will be presented. The method does not require rotational degrees of freedom and the discretization of the director field. This also drastically simplifies the enrichment strategy accounting for the crack kinematics. Based on the meshfree thin shell formulation, an immersed particle method (IPM) for modeling fracturing thin-structures due to fluid-structure interaction is proposed. The key feature of this method is that it does not require any modifications when the structure fails and allows fluid to flow through the openings between crack surfaces naturally.The last part of the presentation focuses on inverse analysis and topology optimization with focus on computational materials design of piezoelectric/flexoelectric nanostructures and topological insulators. In the first application of piezo/flexoelectricity, we use isogeometric basis functions (NURBS or RHT-splines) in combination with level sets since C1 continuity is required for the numerical solution of the flexoelectric problem. Hence, only the electric potential and the displacement field is discretized avoiding the need of a complex mixed formulation. The level set method will be used to implicitly describe the topology of the structure. In order to update the level set function, a stabilized Hamilton-Jacobi equation is solved and an adjoint method is employed in order to determine the velocity normal to the interface of the voids/inclusions, which is related to the sensitivity of the objective function to variations in the material properties over the domain. The formulation will be presented for continua though results will also be shown for thin plates. The method will be extended to composites consisting of flexible inclusions with poor flexoelectric constants. Nonetheless, it will be shown that adding these flexible inclusions will result in a drastic increase in the energy conversion factor of the optimized flexoelectric nanostructures. In the second application, we propose a computational methodology to perform inverse design of quantum spin hall effect (QSHE)-based phononic topological insulators. We first obtain two-fold degeneracy, or a Dirac cone, in the bandstructure using a level set- based topology optimization approach. Subsequently, four-fold degeneracy, or a double Dirac cone, is obtained by using zone folding, after breaking of translational symmetry, which mimics the effect of strong spin-orbit coupling and which breaks the four-fold degeneracy resulting in a bandgap, is applied. We use the approach to perform inverse design of hexagonal unit cells of C6 and C3 symmetry. The numerical examples show that a topological domain wall with two variations of the designed metamaterials exhibit topologically protected interfacial wave propagation, and also demonstrate that larger topologically- protected bandgaps may be obtained with unit cells based on C3 symmetry. [more]

17498 1548412926

Nanoscale thermodynamics at complex oxide surfaces and interfaces for application in electronics, sensing, and energy conversion

Perovskite oxides exhibit a plethora of fascinating electronic material properties covering an exceptionally wide range of phenomena in solid state and surface physics. This has led to tremendous efforts to functionalize these materials in applications for energy technology, gas sensing, and electronics. Layered in an atomically defined epitaxial heterostructures and superlattices, diverse properties of perovskites can be combined on the nanoscale level. In such structures, even new functionality can arise at interfaces of layered materials, exhibiting properties that are absent in the bare bulk materials. In our approach, we utilize atomically-defined layer growth to obtain desired material properties. However, on top of that, we employ thermodynamic engineering of crystal defects as a unique approach to functionalize material properties at surfaces and interfaces: Even at material synthesis conditions close to perfection, device properties are often determined by imperfection, hence, by lattice disorder and crystal defects. As we discuss, we can intentionally control defect structure in nanoscale devices, by developing and utilizing thermodynamic routes to trigger surface and interface reactions in confined systems. While historically defects were seen as something to be avoided, a change of paradigm is required in the field of complex oxides today: In these materials, we can promote functionality, such as metallicity in nominally insulating compounds, by atomic defect-management. Therefore, rather than avoiding defect formation, it is an essential necessity to control and to utilize defect formation in oxides on the nanoscale. Here, we discuss fundamental aspects of lattice disorder effects in bulk oxides, and elaborate the special character of defect formation in thin films, surfaces and interfaces. Focusing on SrTiO3 as a perovskite model system, we will crosslink fundamental perspectives on lattice disorder to actual applications, addressing different examples, such as resistive switching memories, high-mobility electron gases and induced magnetism, oxygen sensors, and electro-catalysts. [more]

17511 1548419576

Opportunities for bcc refractory-metal superalloys

Reinforcement with ordered intermetallic precipitates is a potent strategy for the development of strength alongside damage tolerance and is central to the success of fcc nickel-based superalloys. Such a strategy is equally of interest within bcc-based systems for their increased melting point and acceptable cost. However, only limited studies have been made on refractory metal (RM) or titanium based alloys strengthened by ordered-bcc precipitates (e.g. B2 or L21). Are such “bcc superalloys” possible? Do they offer useful properties? In this talk, opportunities for refractory-metal-based superalloys systems will be discussed, including a review of Cr-Ni2AlTi, Mo-NiAl, Ta-(Ti,Zr)2Al(Mo,Nb) and Nb-Pd2HfAl systems together with newly developed alloys. These alloys exploit an extensive two-phase field that exists between A2 (RM,Ti) and B2 TiFe to produce nanoscale precipitate reinforced microstructures that increase strength by over 500 MPa. This work was supported through EUROfusion Researcher Grant & EPSRC Doctoral Prize Fellowships, EPSRC ‘DARE’ (darealloys.org) EP/L025213/1 and Rolls-Royce/EPSRC Strategic Partnership EP/H022309/1 and EP/H500375/1. [more]

Seminar Talk

17274 1547029112

Phase Transitions in Non-Equilibrium Metallic Systems

Nearly all classes of materials show non-equilibrium phase transitions and the first technological use of quenching metals for designing properties is documented as ~800 BC. However, the decomposition towards equilibrium is still difficult to understand due to the strong non-equilibrium kinetics. Two examples are discussed: First the decomposition of a quenched super saturated solid solution and second the decomposition of a quenched metallic melt. In the first example the technological important AlMgSi alloys are addressed. Low temperature solute clustering, its implications on aging and the effect of trace elements are discussed. Moreover, it is shown which physical pre-requisites need to be fulfilled to modify diffusion by orders of magnitude and to examine a “diffusion on demand” concept. In the second example the first solid–solid transition via melting in a metal, detected upon the decomposition of a metallic glass, is demonstrated. The transformation path is discussed under its thermodynamic and kinetic prerequisites. Moreover, the capabilities of the applied novel technique of fast scanning calorimetry is addressed. Finally, it is outlined how this technique links the two examples via its potential for in-situ measuring the non-equilibrium vacancy evolution. [more]

MPIE Colloquium

17209 1547116037

Dislocation-based Functionality in Oxides

Dislocations in oxides are typically heavily charged and are surrounded by compensating electric charges. As such they are kinetically more stable than chemical dopants. Adepalli et al. termed dislocations a means for “one-dimensional doping” [1]. As they are often introduced by mechanical methods, they may also be termed “mechanical doping” or “self-doping”, as the charges derive from local concentration of the matrix elements. In the literature dislocations have been demonstrated to enhance oxygen conductivity [1] and improve the figure of merit of thermoelectrics by reducing thermal conductivity through phonon scattering by dislocations [2]. Dislocations have been suggested to improve interfacial reaction kinetics and have been theoretically predicted to pin domain walls in ferroelectrics. In Darmstadt we have so far focused on establishing a set of techniques to introduce dislocations into single crystals at room temperature or enhanced temperature and to study (dislocation) creep. Structural investigations have been performed by dark-field X-ray diffraction, rocking curve analysis [3], TEM, NMR and EPR techniques. The first property evaluations have been done with respect to electrical and thermal conductivity and domain wall pinning. All this has to be seen with the perspective of a just developing field, with many opportunities, many obstacles and a lot of exciting uncertainty. Select examples will be provided on dislocation structures, electrical and thermal conductivity in SrTiO3 and our first attempts on dislocation creep in BaTiO3. Time provided, I will show 4 slides on the small brother field: “Elastic-deformation tuned conductivity in piezoelectric ZnO." [1] Adepalli, K. K., Kelsch, M., Merkle, R., and Maier, J., "Enhanced ionic conductivity in polycrystalline TiO2 by "one-dimensional doping''," Phys. Chem.Chem. Phys., 16[10] 4942-51 (2014). [2] S. Il Kim, K. H. Lee, H. A. Mun, S. H. Kim, S. W. Hwang, J. W. Roh, D. J. Yang, W. H. Shin, X. S. Li, Y. H. Lee, G. J. Snyder, S. W. Kim, “Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics“, Science, 348, 109-114 (2015). [3] E.A. Patterson, M. Major, W. Donner, K. Durst, K.G. Webber and J. Rödel, „Temperature dependent deformation and dislocation density in SrTiO3 single crystals”, J. Amer. Ceram. Soc., 99, 3411-120 (2016). [more]

 
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