NEMOLIGHT · Novel Experimental and Modelling approach for Optimisation of Light alloys

Novel Experimental and Modelling approach for Optimisation of Light alloys

Project context and objectives The NEMOLIGHT project aimed at devising a new multidisciplinary approach, combining experimental and modelling approaches, to address the question of alloy optimisation. The application sought was light alloys development, especially for transport applications (terrestrial or aerospace), and most of the work performed was therefore concerning high-strength aluminium. The strength of such Al alloys is mainly controlled by nanoscale precipitation, so that precipitation was the main scientific subject of the NEMOLIGHT project. However the approach is much more general, and can be applied to most metallurgical systems. The strategy devised for this purpose on the subject was first to do proof-of-concept studies on model systems, which showed the potential of using together advanced characterisation and modelling tools. Then three main topics were tackled, which were identified as both practically important, scientifically mature and requiring the type of interdisciplinary approach developed in this project. Finally, on a more long-term basis, the project has built the foundations for a new strategy for alloy optimisation using combinatorial material science. Like any mobility project, the impact of the NEMOLIGHT project goes well beyond the scientific production at the end of the reporting period. It has been a unique opportunity to foster a long-term relationship between the incoming and outgoing institutions (Grenoble Institute of Technology and Monash University) on the subject of metal physics. These two teams, which are each internationally recognised in the field of light alloys, and master complementary experimental tools, have reached a much higher level of collaboration than existed before the NEMOLIGHT project. This collaboration has already had a positive impact on the light alloy research carried out in France. Socio-economic impact In terms of socio-economic impact of the work, this project occurred at a time where a fierce competition between materials exists in the transportation industry. This competition is particularly strong in the aerospace civil industry where composite materials have taken a large fraction of the share of airplanes to the expense of metallic light alloys. New alloy development is a strategic response of the European light alloy metallurgy industry to answer these challenges. In this context, the new European-based Aluminium producer Constellium, which is co-world leader with the US company Alcoa in the aerospace sector, has developed new Al-Cu-Li alloys (named AIRWARETM) that offer a step increase in their combination of properties for the aerospace market. We believe that the approach developed in the NEMOLIGHT project is one important step in optimising this new generation of alloys to stay at the forefront of Aluminium alloy technology. A PhD funded by this company has actually started following the end of the project to exploit this new strategy. Project work 1. Methodology development: the keyword was the use of a multidisciplinary approach for nanoscale precipitation characterisation. -i) improvement of the Small-Angle Scattering (SAS) technique applied to metallic alloys, particularly for non-isotropic precipitates, whose structural characterisation was still challenging in the former understanding of the technique (case study: platelet precipitates in Al-Cu-Li); -ii) the first coupled use of SAS and Nuclear Magnetic Resonance to study nanoscale clustering in Al alloys. The method developed has resulted in the improvement of the understanding of clustering in Al-Cu-Mg; -iii) the development of high resolution synchrotron diffraction to determine the crystal structure of nanoscale precipitates (case study: Al-Cu-Mg); -iv) the coupling between SAS and electrochemical testing to improve the understanding of the influence of precipitation on corrosion of high strength light alloys (case study: Al-Zn-Mg-Cu). 2. Microstructural characterisation of industrial alloys during complex processing routes: the focus of the project has been on the interactions between plastic deformation and precipitation, with several sub-projects: -i) precipitation on dislocations and related kinetics on newly developed aerospace alloys of the Al-Cu-Li family. The collaboration within the project has focused on coupling advanced electron microscopy and synchrotron radiation work to understand the effect of alloy composition on precipitation kinetics, while in parallel a study in collaboration with University of Sydney has focused on the use of Atom Probe Tomography to understand the nucleation mechanisms of the Al2CuLi-T1 phase on dislocations. -ii) study of the shearing mechanisms of the T1-Al2CuLi phase by dislocations by state-of-the-art atomic resolution electron microscopy in Al-Cu-Li alloys. The use of the Monash Centre for Electron Microscopy facilities has enabled the best-to-date characterisation of T1 plates sheared by dislocations with atomic scale resolution. It brings new light to the strengthening potential of these aerospace alloys; -iii) study of dynamic precipitation in Al-Zn-Mg-Cu alloys under creep conditions; using in-situ small angle scattering under the combination of stress and temperature, we have achieved the first study of the coupling between creep and precipitate evolution, which is crucial for the understanding of the evolution of microstructures in service conditions; -iv) study of dynamic precipitation in Al-Zn-Mg-Cu alloys under fatigue conditions. Following preliminary work conducted at Monash University, we have pursued a collaboration to evaluate the possibility of dynamic precipitation to improve the fatigue performance of high strength Aluminium alloys; -v) effect of precipitates on plastic deformation in Al-Zn-Mg-Cu alloys. This work has led to the development of physically-based constitutive laws for plastic deformation of precipitate-containing light alloys. 3. Modelling of microstructure development along complex processing routes on model and industrial alloys: the so-called "class" precipitation model has been implemented and adapted to the competitive precipitation of several phases (metastable vs. stable, or within multi-constituent alloys). A robust model is now available, that can be applied to any stoechiometric phase of known thermodynamic properties. The capability of this model has been demonstrated on a model system, namely Al-Mg-Li. It has been shown to be a very efficient tool for calibrating the thermodynamic properties of an alloy system when applied to specific non-isothermal heat treatments. The model is now under application on the data obtained on Cu-Co diffusion couples. 4: Electrochemical behaviour and corrosion resistance: a study has been launched, to evaluate the relationship between the precipitate microstructure in the Al-Zn-Mg alloy family and the electrochemical behaviour as evidenced by the statistics of pitting events in a de-aerated corrosive aqueous solution. We have shown that in the absence of Cu a critical size exists, above which the number of pitting events drastically increases, showing a direct influence of the precipitates. However, in Cu-containing alloys, precipitation is accompanied by a Cu enrichment, which cancels this effect. 5. Combinatorial materials science: alloy development requires extensive optimisation of compositions in a multi-variable space (most modern alloys include at least five constituents). Making an optimisation by making individual alloys is both too costly and practically impossible. One of the outcomes of the NEMOLIGHT project is a methodology to make diffusion couples between dissimilar alloys to provide a continuous segment of composition space in alloy composition. Using synchrotron X-ray radiation scanning across the samples (diffraction and small-angle scattering) a method has been devised to exploit the potential of these composition gradients by quantifying the nanoscale microstructures as a function of the position, which translates into a chemical composition. This methodology is believed to offer a great potential for being applied to alloy optimisation.

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