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Magnesium alloys microstructures: coupling experiments and simulations

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If you have a question about this talk, please contact Ms Helen Gardner.

Lightweight Mg alloys constitute alternative materials of interest for many industrial sectors and in particular the transport industry. Indeed, reducing vehicle weight and thus fuel consumption can actively benefit the global efforts of the current environmental industry policies. Despite being already widely used, high-pressure die-casting and wrought alloys are still subject to intense research campaigns. The micromechanics of deformation in both type of alloys are still not fully understood and neither simulations alone or experiments alone can possibly unravel the remaining unkowns. In this presentation, two approaches will be presented making use of a coupled experimental-modeling approach:

1-Casting processes usually lead to the formation of significant amounts of gas and shrinkage microporosity, which adversely affects the mechanical properties. The application of hydrostatic pressure after casting can reduce the porosity and improve the properties but little is known about the effects on the size and morphology of the casting pores. In the present study we have used an experimental-computational approach based on X-ray Computed Tomography, image analysis and finite element analysis for the determination of the 3D porosity distribution and its evolution with hydrostatic pressure in a high pressure die-cast AZ91 Mg alloy. The corresponding reconstructed 3D pore distribution has been used as an input for finite element simulations, thus complementing experiments with numerical data difficultly accessible otherwise.

2-Wrought Mg alloys present strong textures and thus specific deformation mechanisms are preferentially activated depending on the orientation of the applied load. Developing models that can contemplate the complexity inherent to deformation of Mg alloys is now timely. In particular, a comprehensive crystal plasticity model including both twin and slip systems as well as their interactions through hardening mechanisms provides a numerical tool directly relating texture and deformation mechanisms. Here, a crystal plasticity finite element model previously developed has been expanded to represent more realistic polycrystal features considering the topological information of grains. In this new model, a 3D polycrystal is represented as a 3D Voronoi tessellation, thus allowing for the study of the local intragranular mechanical fields, as well as the specific interactions between twin nucleation/propagation, and the accompanying slip systems. The experimental calibration and validation of the model are ultimately carried out with an AZ31 rolled sheet, along with quantitative 2D and 3D EBSD characterization of the evolution of deformation twinning. Ultimately, the model demonstrates its ability at capturing some of the intrinsic micromechanisms associated to twin nucleation, growth and transmission.

This talk is part of the Engineering Department Micromechanics Seminars series.

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