Strain localization in deformed polycrystalline materials is of major importance for a large number of physical phenomena occurring during the lifetime of a material, such as fatigue, rupture or corrosion cracking. The early state of plastic deformation of a material is of strong interest on the one hand because it is encountered during severe loading conditions, and on the other hand because particularly strong strain heterogeneities occur, as only some grains deform plastically.
The heterogeneities of plastic deformation of polycrystals is due to several factors. Of course, the attributes of a grain somewhat dictates its deformation (intrinsic effect), but neighbour interactions are also important (extrinsic effect). As a result, both the stress and strain fields are strongly heterogeneous, with typical variations of the same order as the average, imposed values, forming intense “hot spots”. Numerical simulations have shown that, interestingly, while the stress hot spots are typically limited to the volume of individual grains, the strain hot spots tend to appear as 2 to 3-grain long bands oriented at 45° with respect to the loading direction. It should be noticed that the direct experimental observation of such features is virtually impossible in 3D. <br /> <br />The Project aims at analysing in 3D the plastic deformation in a polycrystal deformed in tension. We will employ a combination of 3D experimental and numerical techniques, for which new developments are needed. The deformation field and its correlation with the microstructure will finally be analysed in detail.
In polycrystalline materials, plastic deformation lead to crystal lattice reorientations. In the past few years, lattice reorientations have been the object of intense research, in particular in the context of large deformations associated to shaping operations, where they lead to a signifi cant change of strength and / or to stored energy heterogeneities. Lattice orientations and reorientations are traditionally analysed in 2D using electrons from example in a Scanning Electron Microscope (SEM), but also in 3D using high-energy X-rays or neutrons. Such methods can also be used to determine the local distortion of the crystal lattice itself, leading to the elastic strain and therefore to the stress.
Several variants of synchrotron X-ray diff raction techniques can be used for 3D analyses, some of which are: (i) 3D X-ray diff raction microscopy (3D-XRD), which provides grain average (and dispersion) values can be obtained: grain centroids, lattice orientations and reorientations, lattice orientation distributions and elastic strains, (ii) diff raction contrast tomography (DCT), which is primarily dedicated to microstructure imaging of undeformed microstructures, and (iii) 6D-DCT, which is dedicated to orientation mapping of moderately deformed microstructures. Simulation techniques include the crystal plasticity finite element method, by which the complete stress, strain and lattice reorientation fields in a deformed polycrystal can be computed, given a crystal behaviour, a polycrystalline structure and loading conditions. These techniques will be used together in this work for detailed analyses.
The project, which is in progress, is divided into 4 main tasks:
- Development of a new methodology to determine strain field from reorientation fields;
- Synchrotron X-ray experiments by 3D-XRD and 6D-DCT;
- Crystal plasticity finite-element simulations;
- Analysis of the 3D deformation field by comparison between experiment and simulation.
The main originalities of the project are:
- The use and extension of state-of-the-art synchrotron experimental methods to characterize the reorientation field and in a deformed polycrystal;
- The link between the reorientation field and the deformation field;
- The direct comparison between experiment and simulation at grain scale;
- The detailed analysis of the deformation field and its correlation with the microstructural features.
This will provide important information on the local behaviour of the material to be incorporated in polycrystal deformation models.
work in progress
Strain localization in deformed polycrystalline materials is of major importance for a large number of physical phenomena occurring during the lifetime of a material, such as fatigue, rupture or corrosion cracking. The elastic-plastic transition of a material is of strong interest on the one hand because it constitutes severe loading conditions, and on the other hand because particularly strong strain heterogeneities occur, as only some grains deform plastically. In this project, we will first develop a methodology for 3D strain measurements in polycrystalline materials. Two materials will be used: Al-0.1wt%Mn and 316L steel, which are expected to develop different local behaviours. As strains cannot be measured directly in 3D, we will adopt an inverse approach, consisting of deriving the plastic strains from one of their microstructural consequences: the local lattice reorientations. Two high-energy X-ray diffraction techniques will be used for measuring lattice reorientations in 3D during deformation: 3D-XRD and 6D-DCT. For validation of the method, a 2D configuration will also be adopted, where the lattice reorientations at the surface of a sample will be followed by high angular resolution EBSD and the real, surface deformation measured by standard digital image correlation techniques. The strains measured in the 3D case will be compared to full-field crystal plasticity finite element simulations using the experimental microstructure. Using both the experimental and simulation results, the strain heterogeneity will then be analysed at gradually increasing strains, by focusing on specific aspects: the early state of plastic deformation and the influence of elastic anisotropy, the spatial distribution of the strain "hot spots" and grain interaction effect, the strain propagation as strain increases and relation to orientation gradients.
Monsieur Romain Quey (Laboratoire Georges Friedel)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LGF CNRS Laboratoire Georges Friedel
Help of the ANR 169,478 euros
Beginning and duration of the scientific project: September 2015 - 48 Months