S. Steeb

On the structure of the compounds Ag3R.E. (R.E. = Y, La, Ce, Sm, Gd, Dy, Ho, Er)

Abstract The intermetallic compounds Ag3R.E. with R.E. = Y, La, Ce, Sm, Gd, Dy, Ho and Er are isotypical with Ag3Pu, the hexagonal elementary cell containing 64 atoms. A figure is given, which shows the smooth variation of the mean atomic volume vs, the concentration in at.% in the eight systems Ag-R.E. concerned. The results of high-temperature investigations of the phase Ag3Sm are given. Thermal expansion coefficients are: Δa ΔT = 1.5 × 10 −4 kX/ ° C and Δc ΔT = 1.7 × 10 −4 kX/ ° C .

Small angle x-ray and neutron scattering for the investigation of the medium-range structure of metallic glasses

Abstract A review is presented on the application of small angle X-ray (SAXS) and neutron (SANS) scattering to the investigation of the medium-range structure in metallic glasses in the as-quenched state as well as after thermal treatments. Scattering experiments revealed that the medium-range structure is characterized by compositional inhomogeneities occurring on a very extended size scale from some ten up to at least some thousand Angstroms. Various types of inhomogeneities are discussed as possible sources for the observed small angle scattering effects. The development of a uniform picture for the medium-range structure and consequently also for the atomic scale structure is complicated by the indications that in fact different inhomogeneities coexist in metallic glasses. Scale invariance of the scattering suggests dilation symmetry of the medium-range structure with fractally rough surfaces.

Diffraction studies of liquid and amorphous alloys

Abstract X-ray, neutron, and electron diffraction experiments permit the evaluation of the total structure factor S as a function of the momentum transfer Q, which yield important information about the structure of liquid and amorphous alloys. A rise of S (Q) at small values of Q, i.e., small-angle-scattering, indicates a tendency for segregation. A premaximum below the first peak in S (Q) is usually a sign for the tendency of compound formation. Detailed structural information is obtained from the so-called partial structure factors S ij (Q).

Das system silber-gadolinium

Zusammenfassung Der Aufbau des Zustandsbildes Silber-Gadolinium wurde mit Hilfe von thermischen Analysen, Gefugeuntersuchungen und rontgenographischen Methoden untersucht. Demnach existieren drei intermetallische Verbindungen, namlich Ag3Gd, Ag2Gd und AgGd. Fur die Verbindungen Ag3Gd und AgGd wurden offene Schmelz-punktsmaxima bei 975 und 1100 °C gefunden, wahrend Ag2Gd bei 830 °C peritektisch aus Ag3Gd und Schmelze gebildet wird. Ag3Gd hat eine hexagonal dichteste Kugel-packung (Ag3Pu-Typ), Ag2Gd ist tetragonal (MoSi2-Typ) und AgGd hat eine ku-bisch-raumzentrierte Gitterstruktur (CsCl-Typ). Durch die Verbindungen Ag3Gd und AgGd ist das gesamte System in drei Teilsysteme untergliedert. Die auf der Agreichen und auf der Gd-reichen Seite liegenden Teilsysteme sind einfach eutektisch mit den eutektischen Punkten bei 11 und 77,5 At.% Gd und den dazu gehorenden eutektischen Temperaturen von 780 und 790 °C. In dem dazwischen liegenden Teil-system Ag3Gd-AgGd findet sowohl eine peritektische als auch eine eutektische Reaktion statt; fur die betreffenden Temperaturen wurden etwa 830 und 825 °C und fur die ausgezeichneten Konzentrationen 35 und 37,5 At.% Gd ermittelt. Die maximale Loslichkeit von Gd in festem Silber liegt unterhalb von 2 At.%. Der Aufbau des Systems Silber-Gadolinium zeigt grosse Ahnlichkeit mit den bereits fruher beschriebenen Systemen des Silbers mit Yttrium1, Samarium1, Holmium2 und Erbium3.

An X-ray and neutron scattering investigation of precursor derived Si24C43N33 ceramics

Abstract X-ray and neutron wide angle scattering studies of amorphous Si 24 C 43 N 33 ceramics, annealed at different temperatures, were performed using contrast variation by isotopic substitution ( nat N ⇔ 15 N). The total pair correlation functions exhibit distinct peaks, which can be correlated with two phases, namely amorphous Si 3 N 4 and amorphous carbon. Peaks due to Si–C and C–N bonds were not observed. Reverse Monte Carlo simulation yields partial pair correlation functions which are consistent with the phase separation model.

Ermittlung der Struktur des Tantalsuboxydes TaOy (Ta32O9) mittels Elektronenbeugung

Zusammenfassung Durch Beugung von Elektronenstrahlen an elektrolytisch abgedunnten einkristallinen Proben von Tantal, die unter 5 × 10−4 Torr Sauerstoffpartialdruck bei einer Temperatur von 300°C isotherm isobar begast wurden, erhalt man Uberstrukturdiagramme, welche der Phase TaOy zugeschrieben werden. Die Auswertung dieser Diagramme ergibt, dass die Phase TaOy im kubischen System kristallisiert ( a = 13.4 A , entsprechend 4aTa) und die Zusammensetzung Ta32O9 besitzt. Achtzehn Sauerstoffatome bilden jeweils eine dichte Gruppe, die aus acht primitiven Wurfeln mit der Kantenlange a 8 besteht. Die Mittelpunkte der Gruppen bilden ein kubisch raumzentriertes Gitter mit Kantenlange a.

Structure of binary metallic glasses

Abstract The short range atomic structure of binary amorphous alloys is described by the interatomic distances and the partial coordination numbers. These quantities are deduced from partial structure factors via pair correlation functions. Some methods to determine these partial functions are described. A review of experimental results obtained with metal-metalloid as well as metal-metal alloys is given. The amorphous alloys from the NiNb system are treated more in detail. Some special topics such as the dependence of the amorphous structure on the production process, refractory metallic glasses, hydrogen in metallic glasses, modelling, and triplet correlation functions are discussed.

Structure and Density of Gold-Cesium-Melts. II. Neutron Diffraction with Molten Gold-Cesium-alloys

Recently with Au-Cs-melts measurements of some physical properties were done such as electrical conductivity,1 thermoelectric power,1 electromigration,2 and magnetic susceptibility.3 According to t...

Bestimmung der Zwischengitterverteilung von Sauerstoffatomen mittels Elektronenbeugung an einkristallinen Urandioxydschichten

Evaporated monocrystalline films of uranium dioxide display superstructure diffraction diagrams when they are heated up to 250–600° C in the electron diffraction camera and then exposed to oxygen at 10−3 torr pressure. Interpretation of the diagrams shows that under the specified experimental conditions a superstructure is formed the unit cell of which contains 4 × 4 × 1 unit cells of the UO2 structure (a=5.47 A). This superlattice cell with its basic 64 uranium and 128 oxygen atoms contains a further 12 oxygen atoms, so that the composition is UO2.19. The exact positions of these additional atoms is specified and the conclusion is drawn that during oxidation from UO2 to UO2.19 the additional oxygen atoms are not distributed uniformly through the lattice, but that groups of three closely neighbouring atoms are formed; the balance of attractive and repulsive forces is such that the additional atoms do not lie exactly in the ideal interstitial positions.

Short range order for a molten Mg-Bi-alloy with 30 at % Bi

Abstract Using Keatings assumption, from neutron- and X-ray diffraction experiments the partial structure factors SNN and SCC as well as the short range order parameter α1 = −0.44 were calculated for a molten Mg-Bi-alloy containing 30 at % Bi.