Computer Aided Design of Advanced Turbine Airfoil Alloys for Industrial Gas Turbines in Coal Fired Environments

Recent initiatives for fuel flexibility, increased efficiency and decreased emissions in power generating industrial gas turbines (IGT's), have highlighted the need for the development of techniques to produce large single crystal or columnar grained, directionally solidified Ni-base superalloy turbine blades and vanes. In order to address the technical difficulties of producing large single crystal components, a program has been initiated to, using computational materials science, better understand how alloy composition in potential IGT alloys and solidification conditions during processing, effect castability, defect formation and environmental resistance. This program will help to identify potential routes for the development of high strength, corrosion resistant airfoil/vane alloys, which would be a benefit to all IGT's, including small IGT's and even aerospace gas turbines. During the first year, collaboration with Siemens Power Corporation (SPC), Rolls-Royce, Howmet and Solar Turbines has identified and evaluated about 50 alloy compositions that are of interest for this potential application. In addition, alloy modifications to an existing alloy (CMSX-4) were also evaluated. Collaborating with SPC and using computational software at SPC to evaluate about 50 alloy compositions identified 5 candidate alloys for experimental evaluation. The results obtained from the experimentally determined phase transformation temperatures did not compare well to the calculated values in many cases. The effects of small additions of boundary strengtheners (i.e., C, B and N) to CMSX-4 were also examined. The calculated phase transformation temperatures were somewhat closer to the experimentally determined values than for the 5 candidate alloys, discussed above. The calculated partitioning coefficients were similar for all of the CMSX-4 alloys, similar to the experimentally determined segregation behavior. In general, it appears that computational materials science has become a useful tool to help reduce the number of iterations necessary to perform laboratory experiments or alloy development. However, we clearly are not able to rely solely on computational techniques in the development of high temperature materials for IGT applications. A significant amount of experimentation will continue to be required

Thermo-mechanical processing (TMP) of Ti-48Al-2Nb-2Cr based alloys

The effects of heat treatment and deformation processing on the microstructures and properties of {gamma}-TiAl based alloys produced by ingot metallurgy (I/M) and powder metallurgy (P/M) techniques were examined. The alloy selected for this work is the second generation {gamma}-TiAl based alloy -- Ti-48Al-2Nb-2Cr (at %). Homogenization of I/M samples was performed at a variety of temperatures, followed by hot working by isothermal forging. P/M samples were prepared from gas atomized powders, consolidated by both HIP and extrusion and some of the HIPed material was then hot worked by isothermal forging. The effects of processing, heat treatment and hot working on the microstructures and properties will be discussed

The hot workability and superplasticity of Ti-48Al-2Nb-2Cr alloys

The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near-{gamma} Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at%), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range 1,100--1,300 C at strain rates ranging from 1.67 {times} 10{sup {minus}1}/sec to 1.67 {times} 10{sup {minus}4}/sec. P/M materials consolidated by either hot isostatic pressing (HIP`ing) or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability, and in some cases superplastic behavior, was observed in the materials with equiaxed microstructures without any lamellar constituents

Wie kann der Einfluss biologischer Lebensmittel auf den Menschen bewertet werden? Erfahrungen aus einer Klosterstudie.

Sofern der Wert biologischer Lebensmittel nicht ausschließlich durch ethische und ökologische Standards definiert wird, muss er in Bezug auf den Menschen erfasst werden. Die Vergleichsuntersuchung einzelner Nährstoffe ist wenig erfolgreich, da biologische Lebensmittel im Bereich wertgebender Inhaltsstoffe nicht systematisch besser sind als konventionelle Produkte (Strube & Stolz, 2005). Der Hauptgrund besteht jedoch darin, dass die quantitative Bewertung einzelner Substanzen nicht dem ganzheitlichen Ansatz der biologischen Produktion entspricht. Einige Bio-Forscher sind der Meinung, dass der Einfluss von Lebensmitteln auf die menschliche Gesundheit aus analytischer Sicht generell nicht verständlich ist (Busscher et al., 2005), sondern eher mittels kontextabhängiger Konzepte wie z.B. dem Konzept der „Inneren Qualität“ (van der Burgt et al., 2005). In der Ernährungsforschung können einzelne Nährstoffe oder Produkte sowie Gesamtdiäten aus biologischen Lebensmitteln untersucht werden. Weiters muss die Erforschung des Menschen auf verschiedenen Ebenen, die von biologischen Lebensmitteln beeinflusst werden könnten, erfolgen. Das heißt, wir müssen zwischen physischen, psychischen und geistigen Auswirkungen unterscheiden, wenn wir ein umfassendes Verständnis der Komplexität erreichen wollen. Eine zukunftsweisende Ernährungsstudie zur Erfassung des Einflusses einer biologisch-dynamischen Gesamtdiät auf das menschliche Wohlbefinden wurde durchgeführt, um erste Erfahrungen in diesem komplizierten und kaum ausgearbeiteten Forschungsbereich zu sammeln. Die Testpersonen waren 32 Nonnen in einem katholischen Kloster. In 3 aufeinander folgenden Testserien von jeweils 2 Wochen erhielten sie konventionelle Fertiggerichte, frisch zubereitete konventionelle und ebenfalls frisch zubereitete biologisch-dynamische Speisen und anschließend dasselbe in umgekehrter Reihenfolge. Am Ende jeder Testserie wurde eine breite Palette von Blutparametern analysiert sowie ein umfassender Fragebogen zum Wohlbefinden beantwortet. Der Fragebogen entsprach den Standardverfahren für die Erfassung psychologischen Wohlbefindens und körperlicher Beschwerden und enthielt Skalierungen für die spezifischen Untersuchungsthemen. Mittels des Fragebogens wurden folgende Ergebnisse gesammelt: demographische Daten, Erwartungen und Motivationen bezüglich der Ernährungsumstellung, Auswirkungen der Lebensmittel, Aktivitäten und Ruhezeiten sowie psychisches und physisches Wohlbefinden. Trotz einer signifikanten Abnahme des systolischen Blutdruckes zu Beginn der biologisch-dynamischen Diät, konnte keine weitere Veränderung der physiologischen Parameter beobachtet werden; eine deutliche Zunahme der immunologischen Aktivität wurde bei der Umstellung von Fertiggerichten auf frisch zubereitete Speisen nachgewiesen. Aus den Fragebögen ging aber hervor, dass die biologisch-dynamischen Testperioden signifikante Verbesserungen des subjektiven geistigen, psychischen und physischen Wohlbefindens bewirkten. Es gibt keine Ergebnisse, die diese subjektiven Erfahrungen im Wohlbefinden auf physiologische Daten zurückführen könnten. Der Wert solcher Ergebnisse muss daher diskutiert werden. Einerseits fehlen objektive und von emotionalen Einflüssen unabhängige Daten, andererseits könnten gerade diese emotionalen und geistigen Veränderungen die wichtigsten Ergebnisse für eine ganzheitliche Betrachtung von Organismen und auch des Menschen sein. Diese Gegebenheit müssen im Zusammenhang mit den Methoden und Ergebnissen der Klosterstudie diskutiert werden

Hydrodynamic modes of a 1D trapped Bose gas

We consider two regimes where a trapped Bose gas behaves as a one-dimensional system. In the first one the Bose gas is microscopically described by 3D mean field theory, but the trap is so elongated that it behaves as a 1D gas with respect to low frequency collective modes. In the second regime we assume that the 1D gas is truly 1D and that it is properly described by the Lieb-Liniger model. In both regimes we find the frequency of the lowest compressional mode by solving the hydrodynamic equations. This is done by making use of a method which allows to find analytical or quasi-analytical solutions of these equations for a large class of models approaching very closely the actual equation of state of the Bose gas. We find an excellent agreement with the recent results of Menotti and Stringari obtained from a sum rule approach.Comment: 15 pages, revtex, 1 figure

A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal

We propose a simple Hamiltonian to describe the motion and the merging of Dirac points in the electronic spectrum of two-dimensional electrons. This merging is a topological transition which separates a semi-metallic phase with two Dirac cones from an insulating phase with a gap. We calculate the density of states and the specific heat. The spectrum in a magnetic field B is related to the resolution of a Schrodinger equation in a double well potential. They obey the general scaling law e_n \propto B^{2/3} f_n(Delta /B^{2/3}. They evolve continuously from a sqrt{n B} to a linear (n+1/2)B dependence, with a [(n+1/2)B]^{2/3} dependence at the transition. The spectrum in the vicinity of the topological transition is very well described by a semiclassical quantization rule. This model describes continuously the coupling between valleys associated with the two Dirac points, when approaching the transition. It is applied to the tight-binding model of graphene and its generalization when one hopping parameter is varied. It remarkably reproduces the low field part of the Rammal-Hofstadter spectrum for the honeycomb lattice.Comment: 18 pages, 15 figure

Dilepton production in heavy ion collisions at intermediate energies

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons ($\rho,\omega$) is described via the excitation of nuclear resonances ($R$). The theoretical framework is an extended vector meson dominance model (eVMD). The treatment of the resonance decays $R\longmapsto NV$ with arbitrary spin is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. This ensures correct asymptotics and provides a unified description of photonic and mesonic decays. The resonance model is successfully applied to the $\omega$ production in $p+p$ reactions. The same model is applied to the dilepton production in elementary reactions ($p+p, p+d$). Corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. As a possible solution of this problem the destruction of quantum interference in a dense medium is discussed. A decoherent emission through vector mesons decays enhances the corresponding dilepton yield in heavy ion reactions. In the vicinity of the $\rho/\omega$-peak the reproduction of the data requires further a substantial collisional broadening of the $\rho$ and in particular of the $\omega$ meson.Comment: 32 pages revtex, 19 figures, to appear in PR

Multispecies virial expansions

We study the virial expansion of mixtures of countably many different types of particles. The main tool is the Lagrange–Good inversion formula, which has other applications such as counting coloured trees or studying probability generating functions in multi-type branching processes. We prove that the virial expansion converges absolutely in a domain of small densities. In addition, we establish that the virial coefficients can be expressed in terms of two-connected graphs

Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in \Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low Temp. Phys. in honour of Prof. H. von Loehneyse