Scientific Events

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]

[more]

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]

[more]

FeNiMnAl alloys show a wide range of microstructures and mechanical properties, but have been little explored. Studies on four different types of microstructures in this alloy system will beoutlined: 1) ultrafine microstructures (5-50 nm), present in Fe₃₀Ni₂₀Mn₂₀Al₃₀,Fe₂₅Ni₂₅Mn₂₀Al₃₀ and Fe₃₅Ni₁₅Mn₂₅Al₂₅,which consist of (Fe, Mn)-rich B2-ordered (ordered b.c.c.) and (Ni, Al)-rich L2₁-ordered (Heusler) phases, and in Fe₃₀Ni₂₀Mn₂₅Al₂₅,which consist of (Ni, Al)-rich B2 and (Fe, Mn)-rich b.c.c. phases, with the phases aligned along <100>; 2) fine microstructures (50-70 nm), present in Fe₃₀Ni₂₀Mn₃₀Al₂₀, Fe₂₅Ni₂₅Mn₃₀Al₂₀, and Fe₂₈Ni₁₈Mn₃₃Al₂₁, which consist of alternating (Fe,Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered plates with an orientation relationship close to f.c.c.(002)//B2(002); f.c.c.(011)//B2(001); 3) coarser (0.5-1.5µm) lamellar microstructures observed in alloys with a lower aluminum content, such as Fe₃₀Ni₂₀Mn₃₅Al₁₅, that consistof alternating (Fe,Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered phases with a Kurdjumov-Sachs orientation relationship between the phases; and 4) high-entropy Fe_40.4Ni_11.3Mn_34.8Al_7.5Cr₆alloys. The microstructures and mechanical properties in these alloys have been determined as a function of annealing time, testing temperature and strainrate.  Some of the unusual mechanical behavior that has been observed will be emphasized. This research was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences (grant DE-FG02-07ER46392). [more]

The tribology community presently relies on phenomenological models to describe the various seemingly disjointed steady-state regimes of metal wear. Pure metals such as gold -- frequently used in electrical contacts - exhibit high friction and wear. In contrast, nanocrystalline metals, such as hard gold, often show much lower friction and correspondingly low wear. The engineering community has generally used a phenomenological connection between hardness and friction/wear to explain this macroscale response, and thus to guide designs. We present results of recent simulations and experiments that demonstrate a general framework for connecting materials properties (i.e. microstructural evolution) to tribological response. We present evidence that the competition between grain refinement (from cold working), grain coarsening (from stress-induced grain growth), and wear (delamination and plowing) can be used to describe transient and steady state tribological behavior of metals, alloys and composites. We will explore the seemingly disjointed steady-state friction regimes of metals and alloys, with a goal of elucidating the structure-property relationships, allowing for the engineering of tribological materials and contacts based on the kinetics of grain boundary motion. [more]