No description available.

No description available.

Le présent travail de thèse rapporte sur les mécanismes de corrosion dans les nouveaux zicaloys, des alliages riche en zirconium élaborés sur la base de dopage en niobium pour présenter une bien meilleure tenue, lors de leur utilisation dans la chaudronnerie nucléaire (circuits de cœur et d'éléments actifs fissiles), pour une plus longue durée de vie plus sécuritaire des centrales. Dans la Partie 1 de ce mémoire, on introduit les conditions essentielles de tenue du gainage des cœurs. Ensuite la littérature est rapportée (Partie 2) plus particulièrement en ce qui concerne les zircaloys. Les méthodes expérimentales qui ont permit de tester les nouveaux alliages sont décrites au Chapitre 3 Les principes de la résistance à la corrosion des alliages de zirconium sont amenés au Chapitre 4. Plus particulièrement le rôle de la microstructure est passé en revue dans la Partie suivante. Ensuite les qualités des zircone(s) formées en surface sont questionnées au Chapitre 5, et plus en détails au Chapitre 6 quant aux caractéristiques structurales de ces oxydes. Partie 7 l'étude de ces aspects structuraux est relayée par l'impact des tensions qui y sont accumulées, pour en revenir aux nouveaux alliages dopés Nb (Partie 8) avant de tirer les Conclusions de ce travail.

Laser processing of NiTi shape memory alloys (SMA) has been identified as having great potential in surface treatment, welding, and novel performance requirement applications. However, discrepancies arise regarding whether laser processing improves or degrades the corrosion performance of NiTi-based SMAs. This is a cause for concern over the reliability of the laser processed surfaces. Prior to full scale implementation, a better understanding of oxide evolution during laser processing is required. The first part of this study concerned a systematic investigation of the surface of Ni-44.2 wt.% Ti SMA after the application of differing amounts of laser pulsing and thus energy input. Specific focus was directed on characterizing local changes in the surface oxide adjacent to laser spots. The sample local electrochemical characteristics were investigated by scanning electrochemical microscopy (SECM). The effect of laser processing on the regeneration kinetics of the redox-active mediator was analyzed through microelectrode current maps and approach curves in the feedback mode. Raman spectroscopy was used to determine the crystallinity of the oxide and potentiodynamic cyclic polarization was used to determine oxide stability. Results showed that for a small number of pulses (i.e. low total energy input) corrosion performance was determined primarily by topographical effects. However, increasing the number of pulses (i.e. higher total energy input) had a significant impact on the stability of the oxide in the heat-affected zone (HAZ) region due to the increase in crystallinity, which dictated where the corrosion initiated. In the second part of this study, post-process surface treatments that could be applied to NiTi SMA after laser processing were systematically investigated. Specific focus was directed at characterizing the crystallinity of the newly formed oxides and the stability across the entire surface (containing laser-processed regions and retained base material). Raman spectroscopy and potentiodynamic cyclic polarization were used for this analysis. Results showed that the post-process surface treatments successfully restored the corrosion performance to pre-laser-processing conditions by eliminating crystallinity in the surface oxide and reducing inhomogeneity across the surface.

No description available.

No description available.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1996. / Includes bibliographical references (leaves 227-230). / Microfiche. / xxii, 230 leaves, bound ill. 29 cm

Aluminium alloys are essential to a variety of industry sectors, particularly transport, where they are used in the production of cars and aeroplanes. However, aluminium alloys are susceptible to degradation through corrosion which can compromise the integrity of components manufactured from this material. Therefore research into the means by which these alloys degrade is important. This thesis aims to understand how one of the more potentially damaging types of corrosion, known as pitting corrosion, occurs in the important aluminium alloy 2024-T3 (AA2024-T3). In order to study this phenomenon, this thesis first characterises the alloy microstructure in detail, particularly the type and distribution of intermetallic particles since these play an important role in corrosion processes. The microstructure was studied using an electron microprobe analysis of a 5 mm x 5 mm area of AA2024-T3 and some 80,000 particles were characterised. This investigation was one of the most comprehensive studies to date of any aluminium alloy. Of the particles studied, it was found that the major types included the S and θ phases and a number of compositions based around AlCuFeMn and AlCuFeMnSi. Depletion zones were an integral feature of the alloy microstructure. Pair correlation functions were used to determine the degree of clustering and it was found that there was both inter particle as well as intra particle clustering. Inter particle clustering was observed at length scales well beyond 50 µm. A detailed study of corrosion on AA2024-T3 was undertaken by examining the surface after corrosion over a time period spanning 2.5 minutes to 120 minutes. From this investigation, a hierarchy of the localised corrosion was observed as it was very apparent that particles of particular elemental compositions were more susceptible to attack much sooner than other compositions. Larger corrosion attack sites on the surface, which were called co-operative corrosion, were attributed to intermetallic clustering affects and changes in chemical composition such as Cu-enrichment. These results were used to develop a detailed model of the initiation of stable pitting corrosion in AA2024-T3, which will lead to a better understanding on how to prevent pitting attack on commercially important aluminium alloys. AA2024-T3 is rarely used in the polished state, for real world applications is it generally finished by mechanical or chemical processing. In the final part of this thesis, the influence of clusters on metal finishing was examined using a standard aluminium chemical deoxidiser. It was found that the etch rate of this deoxidiser increased dramatically with the increase in temperature. Under certain processing conditions only the intermetallic particles are etched out and these retain the history of the spatial distribution of the clustering of the intermetallic particles. This leaves a cluster of 'holes' which could trap metal finishing solution and lead to severe subsurface attack

Thesis (B.S.)--Virginia Polytechnic Institute, 1944. / Includes bibliographical references (leaves 40-43). Also available via the Internet.

Thesis (M.S.)--Ohio University, August, 1991. / Title from PDF t.p.