Bismuth | EPFL Graph Search

vignette|upright=1.6|Cristaux oxydés de bismuth.|alt=Masse cristallisée fortement colorée de bismuth oxydé en surface. Le bismuth est l'élément chimique de numéro atomique 83, de symbole Bi. C'est le cinquième et dernier élément du groupe des pnictogènes (groupe ). Il est toxique mais moins que le plomb qu'il tend donc à remplacer pour certains usages, et ce n'est pas un oligo-élément : il n'a aucun rôle physiologique connu. Réputé très faiblement présent dans les organismes animaux, sa cinétique environnementale et dans les organismes a été peu étudiée (contrairement à celle d'autres métaux lourds proches). Il peut être théoriquement rapproché du phosphore P, de l'antimoine Sb et de l'arsenic As, appartenant au , mais aussi de l'étain Sn et du plomb Pb appartenant au . Les éléments successifs du (azote N, phosphore P, arsenic As, antimoine Sb et bismuth) montrent une tendance croissante à former des sulfures stables plutôt que des oxydes : le bismuth est nettement chalcophile.

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

EPFL2014

Post-irradiation analysis of the tantalum container of an ISOLDE LBE target

CERN-ISOLDE operates a range of oxides, carbides, refractory metal foils and liquid metal targets for the production of radioactive ion beams. Following irradiation with a pulsed beam of 1 GeV and 1.4 GeV protons at temperatures reaching 600 degrees C, the tantalum container of a liquid lead bismuth eutectic (LBE) target was examined. A thin layer of Pb/Bi was observed on the inner surface of the container. A sample of the surface prepared using the focused ion beam technique was investigated using SEM and EDX. Results show a higher concentration of bismuth at the interface with tantalum and micron-sized cracks in the tantalum filled with LBE. Implications of these results for the lifetime of the target container which has been known to fail under pulsed beam operation are discussed. (C) 2011 Elsevier B.V. All rights reserved.

Elsevier Science Bv2012

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

EPFL2014