3D incipient Polycrystal Plasticity – 3DiPolyPlast

Understanding the deformation processes leading to failure in polycrystalline materials is one of the key challenges in materials science. Significant progress has been achieved over the past decades, thanks to both cutting-edge experimental characterization techniques and computational methods. Still, the localization of plasticity in slip bands and the propagation of plasticity through a polycrystalline aggregate are not fully understood: one of the major difficulties lies in the capacity to perform in-situ bulk observations covering all relevant length scales from atomistic to the grain aggregate scale.

3DiPolyPlast aims at responding to this challenge by combining state-of-the-art electron- and synchrotron-based characterization techniques, and multi-scale crystal plasticity simulations. This joint approach will be applied to pure nickel which is a model crystallographic system for elementary plastic deformation processes. The onset of plasticity, both at the surface (using digital image correlation and electron back-scatter diffraction) and in the bulk (using advanced X-ray diffraction imaging techniques) of the same polycrystalline sample volumes will be observed. With the upgrade of the ESRF synchrotron storage ring repeated 3D bulk observations at small strain increments and improved resolution (down to 100 nm) will, for the first time, become possible.

The experimental observations will be compared to the simulation results performed on digital copies (“clones”) of the measured 3D grain microstructures. In the proposed multi-scale simulation approach, discrete Dislocation Dynamics (DD) simulations will be used in order to better model the individual and collective behaviour of dislocations at the mesoscale. Stress concentration at the origin of strain localization in slip bands is naturally reproduced by DD simulations. Modeling the boundaries in such simulations is non-trivial and will be addressed in the framework of the Discrete-Continuous Model (DCM) which couples crystal plasticity finite element calculations carried out on the full polycrystalline aggregate with DD simulations inside a single grain.

More specifically, measured 3D grain maps of each sample will be turned into a mesh suitable for DCM and CP-FEM calculations. A classical CP constitutive law will be used in all the grains, but the ones identified experimentally as the grains where a first slip band is recognized. Careful comparison with in situ experiments will be made to validate the fidelity of the DCM simulation and to reproduce the formation of a slip band. In addition, these simulations will provide the mechanical field generated by the slip bands in the vicinity of grain boundaries to analyze the propagation of plasticity from one grain to the next. Repeated simulations with different experimental configurations (grain orientation and boundary conditions for DCM) will be carried out to build a large data set of various micro-mechanical situations. This work will provide an avenue towards an improved and physically motivated description of plastic strain localization in slip bands and propagation of plasticity throughout 3D grain aggregate.

In summary, the key objectives and ambitions of the project are:
1. Pushing the frontier of experimental characterization of bulk plasticity
2. Determining the contribution of slip band/localization in plastic strain of individual grains
3. Identifying mechanisms governing the propagation of plastic strain in the polycrystal
4. Advancing image-based mesoscale modelling of crystal plasticity