Ab-initio simulation and experimental validation of beta-titanium alloys

In this progress report we present a new approach to the ab-initio guided bottom up design of beta-Ti alloys for biomedical applications using a quantum mechanical simulation method in conjunction with experiments. Parameter-free density functional theory calculations are used to provide theoretical guidance in selecting and optimizing Ti-based alloys with respect to three constraints: (i) the use of non-toxic alloy elements; (ii) the stabilization of the body centered cubic beta phase at room temperature; (iii) the reduction of the elastic stiffness compared to existing Ti-based alloys. Following the theoretical predictions, the alloys of interest are cast and characterized with respect to their crystallographic structure, microstructure, texture, and elastic stiffness. Due to the complexity of the ab initio calculations, the simulations have been focused on a set of binary systems of Ti with two different high melting bcc metals, namely, Nb and Mo. Various levels of model approximations to describe mechanical and thermodynamic properties are tested and critically evaluated. The experiments are conducted both, on some of the binary alloys and on two more complex engineering alloy variants, namely, Ti-35wt.%Nb-7wt.%Zr-5wt.%Ta and a Ti-20wt.%Mo-7wt.%Zr-5wt.%Ta.Comment: 23 pages, progress report on ab initio alloy desig

Mental Illness’: Ontology, Etiology and Philosophy as‘Cure'

This essay defines the ontology of mental illness or mental disorder in nonbiomedical terms, as consisting of problematic propositional mental content rather than organic brain malfunction. This allows for a causal theory of mental disorder to be located within the parameters of existential difficulties rather than biological pathology, and contradicts the argument in defence of the necessity of psychotropic medications for the alleviation of mental distress. This in turn supports the argument that mental disorders can be treated, if not cured, by means of philosophyEste ensayo define la ontología de la enfermedad o desorden mental en términos no biomédicos, la cual se funda en contenidos proposicionales mentales antes que en una disfunción orgánica del cerebro. Esto permite localizar a la teoría causalística del desorden mental dentro de parámetros de dificultades existenciales antes que dentro de los de la patología biológica y contradice el argumento que defiende la necesidad de los psicofármacos para el alivio de la tensión mental. Esto apoya el argumento de que los desórdenes mentales pueden ser tratados, si bien no curados, a través de la filosofía

Acoustic anemometry and thermometry

Acoustic travel-time measurement is a method for remote sensing of the atmosphere. The temperature-dependent sound speed as well as the flow field can be detected by measuring the travel time of a defined acoustic signal between a sound source and a receiver when the distance between them is known. In this study the properties of the flow field are reconstructed using reciprocal sound rays to separate the directionindependent sound speed from the effective sound velocity including the flow velocity component in direction of the sound path. The measurements are taken on a horizontal scale of about 2 m x 2 m. By measurements in interiors, where no flow of air exists, the temperature can be determined with an accuracy of 0.6°C and the flow component in direction of the sound path with an accuracy of 0.3 m/s. If flow of air exists the measurements gets complicated because the phase shifts, which have been detected by the receivers, cannot be corrected like it was possible without the influence of flow

Representations of J-central J-Potapov functions in both nondegenerate and degenerate cases

AbstractLet J be an m×m signature matrix (i.e. J∗=J and J2=Im) and let D:={z∈C:|z|<1}. Denote PJ(D) the class of all J-Potapov functions in D, i.e. the set of all meromorphic m×m matrix-valued functions f in D with J-contractive values at all points of D at which f is holomorphic. Further, denote PJ,0(D) the subclass of all f∈PJ(D) which are holomorphic at the origin. Let f∈PJ,0(D), and let f(w)=∑j=0∞Ajwj be the Taylor series representation of f in some neighborhood of 0. Then it was proved in [B. Fritzsche, B. Kirstein, U. Raabe, On the structure of J-Potapov sequences, Linear Algebra Appl., in press] that for each n∈N the matrix An can be described by its position in a matrix ball depending on the sequence (Aj)j=0n-1. The J-Potapov function f is called J-central if there exists some k∈N such that for each integer j⩾k the matrix Aj coincides with the center of the corresponding matrix ball.In this paper, we derive left and right quotient representations of matrix polynomials for J-central J-Potapov functions in D. Moreover, we obtain recurrent formulas for the matrix polynomials involved in these quotient representations

Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C kappa carbides

Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires the knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In the present work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nano-sized kappa carbides (Fe,Mn)3AlC by atom probe tomography (APT) in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magentic disorder in these carbides. Secondly, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures

Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy

© 2018 Acta Materialia Inc. The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed

Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires

AbstractA multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations

Long run real exchange rate determinants: evidence from eight new member states, 1993-2003

In this paper, we estimate bilateral equilibrium real exchange rates for a group of eight new eu member states against the euro, using new and sophisticated panel-cointegration techniques. We document a stable significant positive link between productivity levels and the corresponding real exchange rate levels and a stable significant and negative link between openness and the real exchange rate. We find real exchange rate misalignments to be small and weakly mean-reverting. In the context of entry into erm-ii and emu for most of these countries over time, the results stress the importance of allowing countries to adjust to inflation pressure and real exchange rate appreciation, either through nominal appreciation or temporarily higher domestic inflation. Journal of comparative economics35 (1) (2007) 87–107

Advanced data mining in field ion microscopy

Field ion microscopy (FIM) allows to image individual surface atoms by exploiting the effect of an intense electric field. Widespread use of atomic resolution imaging by FIM has been hampered by a lack of efficient image processing/data extraction tools. Recent advances in imaging and data mining techniques have renewed the interest in using FIM in conjunction with automated detection of atoms and lattice defects for materials characterization. After a brief overview of existing routines, we review the use of machine learning (ML) approaches for data extraction with the aim to catalyze new data-driven insights into high electrical field physics. Apart from exploring various supervised and unsupervised ML algorithms in this context, we also employ advanced image processing routines for data extraction from large sets of FIM images. The outcomes and limitations of such routines are discussed, and we conclude with the possible application of energy minimization schemes to the extracted point clouds as a way of improving the spatial resolution of FIM

Shear-induced anisotropic decay of correlations in hard-sphere colloidal glasses

Spatial correlations of microscopic fluctuations are investigated via real-space experiments and computer simulations of colloidal glasses under steady shear. It is shown that while the distribution of one-particle fluctuations is always isotropic regardless of the relative importance of shear as compared to thermal fluctuations, their spatial correlations show a marked sensitivity to the competition between shear-induced and thermally activated relaxation. Correlations are isotropic in the thermally dominated regime, but develop strong anisotropy as shear dominates the dynamics of microscopic fluctuations. We discuss the relevance of this observation for a better understanding of flow heterogeneity in sheared amorphous solids.Comment: 6 pages, 4 figure