Nickel superalloys are used for harsh condition application cases as they have high chemo-thermomechanical stability. However, they can suffer from embrittlement due to stress corrosion cracking. This effect is difficult to observe as it can take place at long time scales. Here we propose a feasible experiment to study stress assisted grain boundary oxidation, a phenomenon that has similar mechanisms involved that can take place in laboratory compatible time scales. We observed the event using phase and diffraction contrast tomography while applying monotonic loading of the sample at 650 °C. This initial analysis shows that the experimental setup is a good candidate for the study of such degradation mechanism.

In order to collect accurate information about the widely reported Oxidation Assisted Intergranular Cracking (OAIC) mechanism of the superalloy 718 at 650 °C in air environment, we investigated a new tensile test procedure which enables to have access to a quantitative semi-continuous assessment of the damaging process of the alloy 718 during a standard tensile test. Tensile tests were carried out on a solutionized and aged alloy 718 by varying the deformation mode of the alloy during the ongoing experiment. The semi-continuous quantification of the intergranular damage was performed after a fracture surface analysis of samples tested in both Dynamic Strain Ageing (DSA) domain and DSA-subdomain where Portevin-Le Chatelier (PLC) effect occurs. This innovative testing method allows to characterize the intergranular damage that occurs when the material is stressed with an unserrated DSA deformation mode as the alloy 718 is only sensitive to OAIC in this domain. In these conditions, the onset of the intergranular cracking process was found to be around 10% of total strain. An increase in the cracking kinetic when necking occurs was also noticed. A close connection between the intensity of the intergranular damage and the cumulated strain in DSA deformation mode before the onset of the damaging process was found. The gathered information represents a significant improvement in the understanding of still debated OAIC or Stress Corrosion Cracking (SCC) mechanisms.

Thermal loadings representative of a welding cycle at a point in the Heat Affected Zone (HAZ) of an Electron Beam Welding (EBW) have been reproduced experimentally. It means that the temperature is continuously varying, first increasing (during heating) and then decreasing (during cooling) without dwell-time at the highest temperature. Tensile tests have been carried out either during heating or during cooling of the specimen. The mechanical properties of two idealized phases - T6 temper and O temper - have been estimated as a function of temperature. To validate the accuracy of this thermo-mechanical database, a comparison between measured and calculated stress-strain curve is then presented which shows that this approach may be useful to predict the tensile behaviour of this alloy at high temperature whatever the experienced temperature history representative of EBW. Finally, a comparison between numerical and experimental residual stresses is presented for a beam girth weld application.

Electron beam welding fusion line was performed on 6061-T6 aluminium plates. The thermal histories encountered in the heat affected zone were measured to calibrate a thermal finite elements model. This model has been used as entry parameter of a metallurgical model that predicts the precipitation dissolution induced softening, as well as residual elastic strains. Local deformations measured with a neutron diffraction experiment show a good agreement with the coupled modelling approach for all strain components and the "M" shaped residual strain curves, characteristic of age hardening alloys weld joints, is well reproduced.

Welding can highly modify the mechanical properties of materials due to the extreme thermal solicitations applied. For precipitation hardened materials, such as aluminium alloy 6xxx, a welding operation implies a modification of the microstructural state and, consequently, of the mechanical properties, both phenomena being highly nonlinear. The purpose of this paper is to propose a methodology to predict the post-welding mechanical properties of a welded joint. For this, three models are coupled: (i) a thermal finite element model of the welded structure that allows the prediction of the material's thermal history at every point; (ii) a precipitation model to predict the microstructural state in the joint using the thermal history; and (iii) a mechanical model to link the microstructural state to the mechanical properties, i.e. hardness, yield limit and hardening. A coupling between these models and a finite element commercial code is then performed to predict the precipitation state and mechanical properties of a 6xxx-T6 aluminium alloy after welding. To validate this methodology a tensile test is performed on a specimen extracted from a 6061-T6 welded plate. Using Digital Image Correlation, the in-plane strain fields across the weld are measured and compared with the finite element simulation of the tensile test, thereby providing good prediction.

Multi-level cyclic loading is performed on an aluminium 6061 alloy. From an initial fully precipitated T6 state, various non-isothermal heat treatments are performed, leading to various precipitation states. This paper focuses on the effect of precipitates on yield stress, and on kinematic and isotropic hardening. In parallel, the elastoplastic behaviour is modelled coupling a recently developed multi-class precipitation model to an adaptation of the classical Kocks–Mecking–Estrin formalism. In addition to the classical isotropic effect of solid solution, precipitates and dislocation forests, the proposed model takes into account the kinematic contribution of grain boundaries as well as precipitates, thus providing a new physical meaning to the Armstrong–Frederick law. The resulting cyclic stress–strain curves compare well with the experimental ones for all treatments and strain levels.

In age-hardening alloys, high-temperature processes, such as welding, can strongly modify the precipitation state, and thus degrade the associated mechanical properties. The aim of this paper is to present a coupled approach able to describe precipitation and associated yield stresses for non-isothermal treatments of a 6061 aluminium alloy. The precipitation state (in terms of volume fraction and precipitate size distribution) is modelled thanks to a recent implementation of the classical nucleation and growth theories for needle-shaped precipitates. The precipitation model is validated through small-angle neutron scattering and transmission electron microscopy experiments. The precipitation size distribution is then used as an entry parameter of a micromechanical model for the yield strength of the alloy. Predicted yield stresses are compared to tensile tests performed with various heating conditions, representative of the heat-affected zone of a welded joint.