Full three-dimensional experimental and numerical analysis of local mechanical fields in a polycrystal: crystal plasticity and grain boundary sliding micromechanisms in halite. – MicroNaSel

The understanding of the relationships between the mechanical response of polycrystalline materials and their microstructure is a major challenge for the development of new efficient manufactured materials or the mastered use of natural ones. The development of such a micromechanical approach cannot avoid experimental investigations to both feed and validate models, either in terms of elementary deformation mechanism, of pertinent scales of interactions and heterogeneities, or of microstructure evolutions. This conviction has motivated two decades of developments of experimental techniques for the microscopic analysis of materials, such as for instance mechanical tests combined with optical or electron microscopy, or diffraction techniques. Most of these investigations are however performed at the surface of the samples and suffer from a potential non representativity of the mechanical responses at the surface with respect to bulk.

To get rid of such a limitation, the project aims first at extending, through several technological challenges, such local measurements to fully three-dimensional analyses, not only of the inital microstructures but also of their evolutions and, last but not least, of their mechanical response in terms of stresses and strains fields. The second aim will be to develop a methodology which will combine such full field 3D measurements to numerical simulations at the same scale by means of up-to-date computational tools. Finally, we intend to compare the results of this new approach with those of more standard surface investigations, in order to validate or improve them. In particular, the efficiency of recently proposed algorithms for the 3D reconstruction of polycrystalline microstructures from 2D surfacic observations will be investigated.

This general approach will be applied to a simple model material, halite, which exhibits various advantages:
- its low cost, the possibility to control its microstructure and its easy manipulation
- its good adequacy with all experimental techniques to be used
- the variety of its deformation mechanisms and their similarity with those of many structural metals
- its intrinsic interest as the constitutive material of geological formations which are candidates for underground repositories.
A particularity of this material is its ability to deform, depending on its microstructure and the thermodynamic loading conditions, either through intragranular dislocation slip or grain boundary sliding, or both. An important component of the project, based on the new combined 3D experimental and modelling methodology, will be to understand, describe, and characterize this grain boundary sliding mechanism, which is far less known than standard dislocation plasticity.

To reach our goals, many cutting-edge experimental techniques will be used and the innovative developments required for some of them are technological challenges by themselves. The main components are :
- the 3D characterisation of crystalline microstructures by means of Diffraction Contrast Tomography (DCT), extended in order to get access to subgrain misorientations and elastic strain distributions
- volumetric full field strain measurements by means of 3D digital image correlation, applied for the first time to crystalline materials at grain scale.
- the quantification of kinematic discontinuities at grain boundaries over representative domains, both in 2D and 3D
- the generation of realistic complex microstructures, which conform to statistical information from surface investigations
- intense parallel finite element computations of microstructures
- microstructure computation by means of Fast Fourier Tranform-based techniques, modified to account for real experimental boundary conditions.

But the main originality of the project is the combination of all these tools, with the ambition to be the seed of a new generation of experimental micromechanics.