Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

International audienceDuring casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements

Spin-resolved photoemission of thin magnetic films

Magnetic properties of thin films offer a fascinating field of research. Besides the fundamental interest in physical properties, such as the magnetic moment, the Curie temperature and the dependence of the magnetization on temperature or film thickness, magnetic materials are also technically important. The understanding of ferromagnetism and the electronic structure of ferromagnets are a first step in the development of new data storage devices. To study the electronic structure of ultrathin films in the monolayer regime, it is important to use a surface sensitive experimental technique. Electrons are very well suited as a carrier of information, due to their small inelastic mean free path (IMFP) in solids. For energies in the range of 10-1000eV the IMFP of electrons is typically below 20$\mathring{A}$ (see fig. 1.1). [...] Fig. 1.1: Universal Curve of electron mean free path, after {Zangwill, 88]. Theory by Penn [Penn, 76] One of the most common techniques using electrons as a sample probe is photoelectron spectroscopy. Photons of a specific energy are absorbed by electrons in the sample. Energy conservation demands that the excited system takes the energy of the annihilated photon as a whole. If the energy of the excited electrons is high enough to overcome the work function they can be detected outside the sample and characterized with respect to their kinetic energy, momentum and spin

Elektron - Ion - Multikoinzidenzspektroskopie, ein neuer Zugang zu Innerschalenprozessen in kleinen Molekuelen

An experiment for the coincident detection of one energy-resolved electron and several ions has been developed. Synchrotron radiation from the University of Bonn electron accelerator ELSA was used to excite argon atoms and CS_2 molecules at inner shells. Electrons emitted during the complex cascade of electronic deexcitation processes were analyzed with a commercial electron spectrometer. Ions produced by the relaxation processes were detected with a time-of-flight mass spectrometer. Coincidental detection of ions and energy-selected electrons allows us to decompose the L-VV spectrum after deep inner-shell ionization into partial L-VV spectra emitted in the presence of different spectator hole configurations. These partial spectra arise on different pathways of the electronic relaxation and belong to different total ionic charges. In addition, the correlation between the energy and dissociation behaviour of CS_2"+"+ ions has been studied. Sulphur(2p)-excited CS_2 emits one Auger electron whose energy is complementary to that of the ion. Detection of the fragments in coincidenc with the electron reveals that correlation. (orig.)Available from TIB Hannover: RN 4852(97-06) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Spin-resolved photoemission of thin magnetic films

In this work we have studied the electronic and magnetic structure of Co/Cu(100) and Co/Cu(111) using spin- and angle-resolved photoemission spectroscopy, Auger electron spectroscopy and low-energy electron diffraction. The growth of cobalt on the (111) surface of copper has been studied under ultra-high vacuum conditions at room temperatures using Auger-electron spectroscopy and low-energy electron diffraction. It has been found that cobalt grows layer-by-layer (Frank van der Merve growth) on Cu(111). The overlayer grows epitaxially with the fcc structure of the substrate up to at least 5.5 monolayers (ML). (orig.)Available from TIB Hannover: RA 831(2706) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman