Scientific Events

The processing of materials as well as their technologically important properties are controlled by a combination of thermodynamics--which determines equilibrium--and kinetics--how a material evolves. Mass transport in solids, where different chemical species diffuse in a material due to random motion with or without a driving force, is a fundamental kinetic process for a wide variety of materials problems: growth of precipitates in nearly every advanced structural alloy from steels to superalloys, fusing of powders to make advanced ceramics, degradation of materials from irradiation, permanent changes in shape of materials over long times at high temperatures, corrosion of materials in different chemical environments, charge/discharge cycles in batteries, migration of atoms in electric fields, and more. Mass transport is a fundamentally multiscale phenomenon driven by crystalline defects, where many individual defect displacements sum up to produce chemical distributions at larger length and time scales. State-of-the-art first principles methods make the computation of defect energies and transitions routine for crystalline systems, and upscaling from activation barriers to mesoscale mobilities requires the solution of the master equation for diffusivity. For all but the simplest cases of interstitial diffusivity, and particular approximations with vacancy-mediated diffusion on simple lattices, calculating diffusivity directly is a challenge. This leaves two choices: uncontrolled approximations to map the problem onto a simpler (solved) problem, or a stochastic method like kinetic Monte Carlo, which can be difficult to converge for cases of strong correlations. I will describe and demonstrate our new developments for direct and automated Green function solutions for transport that take full advantage of crystal symmetry. This approach has provided new predictions for light element diffusion in magnesium, "pipe diffusion" of hydrogen along dislocations cores in palladium, and the evolution of vacancies and silicon near a dislocation in nickel. I will also show our latest results for technologically relevant magnesium alloys with containing Al, Zn, and rare earth elements (Gd, Y, Nd, Ce and La), where prior theoretical models to predict diffusivity from atomic jump frequencies make uncontrolled approximations that impact their accuracy. The underlying automation also makes the extension of first-principles transport databases significantly more practical and eliminate uncontrolled approximations in the transport model. [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]

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The application process for tenure-track university faculty positions in the US is often opaque and unclear. Job listings can be broad and vague and are sometimes difficult to find; clear guidelines for cover letters, research statements, and CVs are non-existent; interview formats vary drastically between departments, even in the same university or college. But fear not! All of these obstacles are surmountable, with sufficient preparation, of course. This talk will attempt to elucidate many of the aspects of the application process. Topics that will be covered include: a brief overview of the American university structure, job responsibilities of a tenure-track professor, how to find job listings, how to determine which universities and departments to apply to, and tips for applying, interviewing, and negotiating. [more]

Nanostructured materials represent a well-established part of nanoscience today due to their tunable electrical, optical, magnetic and catalytic properties, and their potential in nanomedicine. There are some common techniques used for the investigation of nanomaterials, e.g. light scattering (DLS and NTA), scanning and transmission electron microscopy (SEM and TEM), fluorescence and IR spectroscopy and many others. X-ray powder diffraction (PXRD) with different geometrical setups is a complementary non-destructive technique for the determination of crystallographic and size-related properties of nanostructured materials. Here, some examples of PXRD measurements in different applications with the use of Rietveld analysis, including size-specific data obtained from colloid-chemical analysis, transmission and scanning electron microscopy will be presented. Several scientific questions will be addressed, like: - How can crystallite size, residual stress and texture be determined for nanostructured materials? - How is it possible to investigate a thin coating of nanomaterials? - Which advantages does a characterization of samples in temperature chamber offer? It will be shown that the non-destructive X-ray method is well suited to describe not only the crystallographic properties of nanostructural materials, but also their size, shape and inner structure with a possible atomic substitution as well as their “nano”-orientation on the surface. All these scientific answers can be received by the use of different X-ray diffractometers such Bruker D8 Advance and Panalytical Empyrean available at the facility for X-ray Diffraction of the University of Duisburg-Essen. [more]

The thermodynamics and kinetics of solute segregation in crystals is important for controlling microstructure and properties. Prime examples are the effects of solute drag on interface migration and of static strain aging on the yield stress. A fully quantitative prediction of solute segregation is difficult, however, due to the spatially varying solute-defect binding energies that are atomic in origin. Moreover, as solute segregation enhances (locally) the solute concentration, dilute approximations for the underlying thermodynamics and kinetics become questionable. We present a dynamical version of the variational gaussian method for binary alloys [1] and illustrate its potential for select problems involving solute segregation including static strain aging in Al-Mg alloys [2]. Our model adapts the recently proposed Diffusive Molecular Dynamics (DMD) model for vacancy diffusion in crystals where a phonon- free description of solids is coupled with statistical averaging over various configurations to allow for the efficient calculation of free energies. In the alloy version of the model, the free energy is minimized by optimizing the atomic positions and vibrational amplitudes while relaxational dynamics are used to evolve the solute concentration field based on the local energy landscape. We show that this model successfully describes solute redistribution over diffusive timescales. In contrast to traditional continuum diffusion treatments, atomistic effects are automatically accounted for, and full kinetic pathways of the evolution of material properties are revealed in addition to the equilibrium properties. [1] E. Dontsova, J. Rottler, C. W. Sinclair, Phys. Rev. B 90, 174102 (2014) [2] E. Dontsova, J. Rottler, C. W. Sinclair, Phys. Rev. B 91, 224103 (2015) [more]