Design using randomness: a new dimension for metallurgy

High entropy alloys add a new dimension, atomic-scale randomness and the associated scale-dependent composition fluctuations, to the traditional metallurgical axes of time-temperature-composition-microstructure. Alloy performance is controlled by the energies and motion of defects (dislocations, grain boundaries, vacancies, cracks, ...). Randomness at the atomic scale can introduce new length and energy scales that can control defect behavior, and hence control alloy properties. The axis of atomic-scale randomness combined with the huge compositional space in multicomponent alloys thus enables, in tandem with still-valid traditional principles, a new broader alloy design strategy that may help achieve the multi-performance requirements of many engineering applications.Comment: 7 pages, 4 figure

Dislocation Cross-Slip in Face-Centered Cubic Solid Solution Alloys

The mechanical strength of metals depends on their resistance against various microscopic deformation processes. In ductile metals, the most important process is shearing of the crystal lattice by dislocations. One of the fundamental aspects of dislocation motion is cross-slip of screw dislocations, the process by which they change their glide plane. In Face-Centered Cubic (FCC) metals, cross-slip is supposed to play a role in dislocation structuring, work hardening, recovery, fatigue, etc. Most prior studies on cross-slip in FCC metals focused on pure metals. There have been few studies of solute effects on cross-slip, which are important for engineering alloys. Here, the effects of substitutional solutes are studied using atomistic simulations and statistical modeling. In the first part of the thesis, the mechanism and energy of cross-slip of short (40 Burgers vectors long) dislocations in Ni-Al, Al-Mg and Cu-Ni alloys are determined using atomistic calculations. These calculations are carried out with real random alloys and with "average" alloys, where the real atom types are replaced by a single average type. By comparison, it is shown that cross-slip is controlled by fluctuations in the solute concentration, i.e. the activation energy for cross-slip is a distributed variable with a large variance around the mean value. The latter changes only little with concentration. Most importantly, activation energies that are significantly lower than the mean value can be observed in random alloys. A linear correlation between the activation energy and the energy difference between the state of the dislocation before and after cross-slip is observed. An analytical, parameter-free model of this energy difference is developed, which takes random changes in solute-dislocation and solute-solute binding energies into account. Thus, it is possible to predict the distribution of activation energies for nucleation of cross-slip. In the second part, cross-slip of long (10^2-10^3 Burgers vectors) dislocations is studied using a random walk model. Cross-slip is seen as a discrete process, where one Burgers vector long subsegments of the dislocation cross-slip one after another. Associated with each step is a random energy due to random changes in solute binding energies, as well as a deterministic energy change due to constriction formation and stress effects. The random walk model allows the calculation of the activation energy distribution for arbitrary dislocation lengths and stresses. Cross-slip of long dislocations is unlikely at zero stress, due to increasing frequency of high activation energies with increasing length. However, an external stress eliminates these high barriers. Cross-slip then becomes a weakest-link problem. Like in the case of short dislocations, activation energies that are significantly lower than average-alloy estimates can be observed in real random alloys

Psychoanalytic and cognitive-behavior therapy of chronic depression : study protocol for a randomized controlled trial

Background: Despite limited effectiveness of short-term psychotherapy for chronic depression, there is a lack of trials of long-term psychotherapy. Our study is the first to determine the effectiveness of controlled long-term psychodynamic and cognitive-behavioral (CBT) treatments and to assess the effects of preferential vs. randomized assessment. Methods/design: Patients are assigned to treatment according to their preference or randomized (if they have no clear preference). Up to 80 sessions of psychodynamic or psychoanalytically oriented treatments (PAT) or up to 60 sessions of CBT are offered during the first year in the study. After the first year, PAT can be continued according to the ‘naturalistic’ usual method of treating such patients within the system of German health care (normally from 240 up to 300 sessions over two to three years). CBT therapists may extend their treatment up to 80 sessions, but focus mainly maintenance and relapse prevention. We plan to recruit a total of 240 patients (60 per arm). A total of 11 assessments are conducted throughout treatment and up to three years after initiation of treatment. The primary outcome measures are the Quick Inventory of Depressive Symptoms (QIDS, independent clinician rating) and the Beck Depression Inventory (BDI) after the first year. Discussion: We combine a naturalistic approach with randomized controlled trials(RCTs)to investigate how effectively chronic depression can be treated on an outpatient basis by the two forms of treatment reimbursed in the German healthcare system and we will determine the effects of treatment preference vs. randomization

Studieneingangstests und Studienerfolg. Mögliche Zusammenhänge am Beispiel zweier Hochschulen

Tests zu Studienbeginn werden an vielen Hochschulen durchgeführt, um die mathematischen Fähigkeiten der Studienanfänger zu ermitteln. Am Beispiel der Daten von der Universität Kassel und der Fachhochschule Aachen werden Einflüsse von Vorkursen und Zusammenhänge der Ergebnisse von Studieneingangstests und Mathematikklausuren der ersten Semester untersucht. Es zeigt sich ein durchaus unterschiedliches Bild – in Abhängigkeit der konkreten Rahmenbedingungen

Nonverbal Semantics Test (NVST)—A Novel Diagnostic Tool to Assess Semantic Processing Deficits: Application to Persons with Aphasia after Cerebrovascular Accident

Assessment of semantic processing capacities often relies on verbal tasks which are, however, sensitive to impairments at several language processing levels. Especially for persons with aphasia there is a strong need for a tool that measures semantic processing skills independent of verbal abilities. Furthermore, in order to assess a patient’s potential for using alternative means of communication in cases of severe aphasia, semantic processing should be assessed in different nonverbal conditions. The Nonverbal Semantics Test (NVST) is a tool that captures semantic processing capacities through three tasks—Semantic Sorting, Drawing, and Pantomime. The main aim of the current study was to investigate the relationship between the NVST and measures of standard neurolinguistic assessment. Fifty-one persons with aphasia caused by left hemisphere brain damage were administered the NVST as well as the Aachen Aphasia Test (AAT). A principal component analysis (PCA) was conducted across all AAT and NVST subtests. The analysis resulted in a two-factor model that captured 69% of the variance of the original data, with all linguistic tasks loading high on one factor and the NVST subtests loading high on the other. These findings suggest that nonverbal tasks assessing semantic processing capacities should be administered alongside standard neurolinguistic aphasia tests. View Full-Tex

Thermodynamic properties of average-atom interatomic potentials for alloys

The atomistic mechanisms of deformation in multicomponent random alloys are challenging to model because of their extensive structural and compositional disorder. For embedded-atom-method interatomic potentials, a formal averaging procedure can generate an average-atom EAM potential and this average-atom potential has recently been shown to accurately predict many zero-temperature properties of the true random alloy. Here, the finite-temperature thermodynamic properties of the average-atom potential are investigated to determine if the average-atom potential can represent the true random alloy Helmholtz free energy as well as important finite-temperature properties. Using a thermodynamic integration approach, the average-atom system is found to have an entropy difference of at most 0.05 k B/atom relative to the true random alloy over a wide temperature range, as demonstrated on FeNiCr and Ni85Al15 model alloys. Lattice constants, and thus thermal expansion, and elastic constants are also well-predicted (within a few percent) by the average-atom potential over a wide temperature range. The largest differences between the average atom and true random alloy are found in the zero temperature properties, which reflect the role of local structural disorder in the true random alloy. Thus, the average-atom potential is a valuable strategy for modeling alloys at finite temperatures

Acoustical and optical determination of mechanical properties of inorganically-bound foundry core materials

Inorganically-bound sand cores are used in many light-metal foundries to form cavities in the cast part, which cannot be realised by the mould itself. To enable FEM simulations with core materials, their mechanical properties have to be measured. In this article, we adapt methods to determine the Young’s and shear modulus, the Poisson ratio and the fracture strain of sand cores. This allows us to fully parametrise an ideal brittle FEM model. We found that the Young’s and shear modulus can be obtained acoustically via the impulse excitation technique. The fracture strain was measured with a high-speed camera and a digital image correlation algorithm

A test stand for quantifying the core gas release and the gas permeability of inorganically-bound foundry cores

Environmental and work safety aspects necessitate a radical change in the foundry industry. Organic binder systems for foundry sand cores create toxic combustion products and are, therefore, more and more often substituted by inorganic binder systems. While providing an environmental advantage by mainly releasing water vapor, inorganic binder systems impose new challenges for the casting process. The gas release of inorganically-bound foundry cores can lead to increased gas porosity in the cast parts and thus to high scrap rates. The present work aims to gain more understanding of the gas generation and transport in inorganic sand binder systems. We developed a test stand to measure the temperature-dependent core gas release in inorganically-bound foundry cores and their gas permeability. Samples were prepared in a core blowing process and analyzed using the test stand. The measurement results are in good agreement with validation experiments and existing literature

Dislocation cross-slip in fcc solid solution alloys

Cross-slip is a fundamental process of screw dislocation motion and plays an important role in the evolution of work hardening and dislocation structuring in metals. Cross-slip has been widely studied in pure FCC metals but rarely in FCC solid solutions. Here, the cross-slip transition path in solid solutions is calculated using atomistic methods for three representative systems of Ni-Al, Cu-Ni and Al-Mg over a range of solute concentrations. Studies using both true random alloys and their corresponding average alloy counterparts allow for the independent assessment of the roles of (i) fluctuations in the spatial solute distribution in the true random alloy randomness and (ii) average alloy properties such as stacking fault energy. The results show that the solute fluctuations dominate the activation energy barrier, i.e. there are large sample-to-sample variations around the average activation barrier. The variations in activation barrier correlate linearly with the energy difference between the initial and final states. The distribution of this energy difference can be computed analytically in terms of the solute/dislocation interaction energies. Thus, the distribution of cross-slip activation energies can be accurately determined from a parameter-free analytic model. The implications of the statistical distribution of activation energies on the rate of cross-slip in real alloys are then identified. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved