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The anisotropic material behaviour of a recrystallized AA2198 Al-Cu-Li alloy in T3 and T8 conditions was assessed by proportional loading of uniaxial-tension specimens in rolling (L), transverse (T) and diagonal (D) orientations. The width and longitudinal strains were measured to quantify plastic anisotropy. Notched-tension samples were tested in L and T directions. The material showed weak anisotropy in terms of stress strain curves and a moderate plastic anisotropy, consistent with its texture obtained by EBSD. An anisotropic Bron-Besson type material model was identified using this data base and a micro shear-only (SO) test. The model succeeded in predicting the behaviour of micro specimens for proportional tension-only (TO) loading and non-proportional load path changes including 'shear to tension' (ST) as well as 'tension to shear' (TS) tests. The non-proportional loading was achieved using a newly designed cross shaped sample. It was loaded in one direction, unloaded and subsequently loaded in the orthogonal direction till fracture. The average stretch to fracture of both alloys measured by a four point frame optical extensometer decreased by 29 % and 16 % for T3 and T8 respectively for the 'shear to tension' experiment compared to the proportional TO experiment. The average stretch to fracture of 'tension to shear' tests was reduced by 10 % for 2198T3 and hardly reduced for 2198T8 compared to the stretch to fracture of the SO tests, but subject to strong scatter. FE simulations showed local accumulated strain to fracture values that were similar for all loading histories for the T8 condition (0.73 − 0.84). Lower strain to fracture values were found in T3 condition (0.45 − 0.73), despite the enhanced macroscopic ductility in tension. This was attributed to larger less localized plastic zones, especially for the ST test. The ductility scatter was attributed to necking and damage development in tension that can affect strain localization, associated fracture path and ductility, as observed by DIC and fractography.

Three-dimensional (3D) time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging is a promising tool in the analysis of surface modifications of alloy matrix and intermetallic particles induced by localized corrosion due to its good spatial resolution and very high sensitivity to chemical identification especially involving light mass elements like Li. In this paper, ToF-SIMS 3D chemical mapping combined with scanning electron microscopy was performed to study the localized corrosion of Al-Cu-Li alloy (AA2050-T8) in a region around an Al-Cu-Fe-Mn intermetallic particles (IMPs) in 0. 1 M NaCl solution. Initiation of corrosion was locally observed on the IMPs and between IMPs and matrix substrate. Moreover, a modified oxide/hydroxide layer was observed over IMPs compared to the matrix. This new corrosion layer was enriched with chlorides and Cu hydroxides. ToF-SIMS 3D imaging revealed that Li was present over the alloy matrix, while no Li was present over the IMPs in regions where localized corrosion occurred.

Al-Cu-Li alloys are considered as advanced materials for the aircraft manufacturing due to their high mechanical properties and low weight. However, Al-Cu-Li alloys, similarly to other Al alloys, are susceptible to corrosion due to the presence of intermetallic particles (IMP) such as Al-Cu-Fe-Mn particles, T1 phase (Al2CuLi). These IMPs can lead to different surface modifications of the alloy during the alloy treatment and/or exposure to various working conditions. The objective of this PhD thesis was to understand the surface modifications induced by thermal treatments, the corrosion behaviour of bare Al-Cu-Li alloy (AA2050) and the corrosion protection efficiency of the alloy after chemical treatments (degreasing, pickling and TCP deposition).This PhD thesis is composed of three parts. In the first part, the surface composition of the as received AA2050-T8 and the alloy surface modifications induced by thermal oxidation at 190°C, using in situ X-ray photoelectron spectroscopy (XPS), were performed. The formation of a mixed Al-Li oxide layer was demonstrated after sample exposure to oxygen at low pressure. Post mortem time-of-flight secondary ion mass spectrometry (ToF-SIMS) 3D chemical mapping and ion depth profile analyses showed Mg segregation at the grain boundaries and Ag enrichment at the oxide/metal interface. In the second part, the pitting corrosion of the AA2050-T8 alloy exposed to the aggressive chloride-based electrolytes was studied in details by ToF-SIMS 2D and 3D chemical mapping and scanning electron microscopy (SEM). The formation of an oxide-hydroxide layer with different composition and higher thickness was evidenced over the IMPs than over the alloy matrix. Moreover, the behaviour of Li in the oxide was also discussed. In the third part, the surface chemical treatments including degreasing, pickling and conversion coating (TCP) and related surface chemical modifications were studied using XPS, ToF-SIMS and SEM. An improved corrosion behaviour on TCP coated alloy was revealed. A strong influence of copper enrichment on the quality, composition and corrosion performance of the TCP was demonstrated., Les alliages Al-Cu-Li sont considérés comme des matériaux de pointe dans le domaine de l’aéronautique en raison de leurs excellentes propriétés mécaniques et de leur faible poids. Cependant, ces alliages, comme d’autres alliages à base d’aluminium, sont sensibles à la corrosion en raison de la présence de particules intermétalliques (IMP) telles que les particules Al-Cu-Fe-Mn, constituant la phase T1 (Al2CuLi). Ces particules intermétalliques peuvent conduire à des modifications de surface pendant le traitement de l’alliage et lors de l’exposition à diverses conditions de travail. L’objectif de cette thèse de doctorat est de comprendre les modifications de surface induites par les traitements thermiques, le comportement de l’alliage nu Al-Cu-Li (AA2050) vis-à-vis de la corrosion et l’efficacité de la protection contre la corrosion après traitements chimiques (dégraissage, décapage et dépôt TCP).Cette thèse de doctorat est composée de trois parties. Dans la première partie, la composition de surface de l’AA2050-T8 tel que reçu et les modifications de surface de l’alliage induites par l’oxydation thermique à 190°C, ont été étudiées in situ à l’aide de la spectroscopie de photoémission induites par rayons X (XPS). La formation d’une couche mixte d’oxyde d’Al-Li a été démontrée après exposition de l’échantillon à de l’oxygène à basse pression. L’analyse de la cartographie chimique 3D post mortem par spectrométrie de masse d’ions secondaires en temps de vol (ToF-SIMS) a révélé la ségrégation de Mg aux joints de grains et l’enrichissement en Ag à l’interface oxyde/métal. Dans la deuxième partie, la corrosion de l’alliage AA2050-T8 exposé à des électrolytes agressifs à base de chlorure a été étudiée en détail par ToF-SIMS 2D et 3D et microscopie électronique à balayage (MEB). Une couche d’oxyde-hydroxyde de composition différente et d’épaisseur plus élevée a été démontré au-dessus des IMPs par rapport à celle formée sur la matrice de l’alliage. En outre, le comportement de Li dans l’oxyde a également été discuté. Dans la troisième partie, les traitements chimiques de surface, y compris le dégraissage, le décapage et le revêtement de conversion (TCP) et les modifications chimiques de surface associées, ont été étudiés à l’aide des techniques XPS, ToF-SIMS et MEB. L’amélioration du comportement de corrosion sur l’alliage traité TCP a été révélée. Une forte influence de l’enrichissement en cuivre sur la qualité, la composition et les performances vis-à-vis de la corrosion du traitement TCP a été démontrée.

International audience; This paper presents a systematic study where the distribution of precipitate microstructures is mapped in the cross-section of a friction stir weld made with an AA2050 Al-Cu-Li alloy in the naturally aged temper, as well as the evolution of this microstructure during subsequent post-welding heat treatment (PWHT). This study is carried out using spatially resolved small-angle X-ray scattering, supported by transmission electron microscopy, differential scanning calorimetry and microhardness mapping. The as-welded microstructure is dominated by solute clusters, while very little precipitation has taken place during the welding operation. During PWHT, the precipitation kinetics in the different zones of the weld is mainly controlled by the local dislocation density inherited from welding, and by the amount of solute available for precipitation, which depends on the volume fraction of welding-induced intermetallics. Pre-deforming the weld before the PWHT results in a very effective strength recovery and a nearly homogeneous distribution of hardness. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; High-resolution in situ synchrotron X-ray laminography combined with digital volume correlation (DVC) is used to measure the damage and plastic strain fields ahead of a notch introduced within a 2198 Al-Cu-Li alloy sheet. Synchrotron laminography is a technique specifically developed for three-dimensional (3-D) imaging of laterally extended sheet specimens with micrometre resolution. DVC is carried out using the 3-D image contrast caused by iron-rich intermetallic particles present in the alloy. The alloy is recrystallized and tested in T8 artificial ageing condition involving relatively low work hardening. Inclined strain localization bands are shown to develop at 800 µm from the notch prior to the onset of damage. Damage in this region results mainly from the nucleation of voids on micrometric intermetallic particles and occurs at a very late stage of deformation, followed by very limited void growth. The accumulation of strain in the slanted localization band is found to be a steady process, whereas the crack propagation is a sudden process. Standard 3-D FE calculations using either von Mises plasticity or Gurson's model do not capture the plastic localization process.