Plasticity, Recrystallization and Phase Transformation in (Poly-)Crystalline Materials (PRET)
The work aims at better understanding, simulating and controlling the elementary mechanisms active at the grain and grain boundary scale in polycrystalline materials, during conventional or innovative thermomechanical treatments. They mainly concern the mechanisms of plasticity (work hardening, restoration, structuring of dislocations), recrystallization (sprouting and growth of grains, migration of grain boundaries) and phase transformation (sprouting and growth), which are active in a large temperature range, and possibly coupled. If the final objective is the confrontation between experiments and simulations (better understanding for better prediction), the efforts of the OR focus on the development of innovations in coupled observations.
On the plasticity side
The efforts are focused on the quantification of the stored deformation energy by XRD and on the precise measurement of the crystal deformation and rotation fields, using coupled AFM / EBSD measurements. These measurements can then be used to feed crystal plasticity models developed at several scales, mainly in the SIMEON RO.
Measurement and calculation of dislocation densities after 25% tension in the two main texture components of a 73% and 96% rolled copper sample and then annealed. Values estimated from the tensile curves are also shown (PolyXexp). The size of the symbols is proportional to the measurement error. It is shown that, when the material is textured (96%), the XRD allows to distinguish the main texture components and the measurements performed (XRD) are in better agreement with the simulations than those from the EBSD measurement of local disorientations (KAM).
Bacroix et al, Acta Materialia, 2018, 160: 121-136.
Topographic and derivative MFA images obtained on copper (a,b) and iron (c,d) after 3% plastic deformation. Examples of single and double slip in both cases. These measurements allow to identify the active mechanisms and to quantify the slip quantities.
Kahloun et al., Int. J. Plasticity, 84 (0216), pp. 277-298.
On the recrystallization side
The main focus is on the role of grain boundaries and in particular on the study of their migration in the presence of a stored deformation energy gradient. Studies carried out on Al and Ti have thus allowed to better understand the mechanism of SIBM (Strain Induced Boundary Migration) and to obtain very large grains in slightly deformed plates. Moreover, the quantification of this stored energy by polycrystalline calculation has allowed to explain the texture evolutions observed during annealing in Fe-Si steels according to the parameters of the thermomechanical treatment (strain rate, temperature and annealing time in particular).
EBSD maps (Image Quality and disorientations on the left, orientations on the right) for a FeSi2.4 steel rolled then annealed at 700° or 760°. It can be seen that the final microstructure and texture depend strongly on the annealing temperature. The appearance of a fiber texture a = {h,1,1}<1/h,1,2>, associated with optimal magnetic properties, is explained by the high shear observed in some grains, associated with a strong increase in stored energy, which favors the germination of new grains.
Bacroix et al., 2019, J. Phys., Conf. Ser. 1270 012007 https://doi.org/10.1088/1742-6596/1270/1/012007.
Visualization of plasticity-induced grain boundary migration (SIBM) in a polycrystalline aluminum sample, deformed by 3% in tension and annealed in the SEM at 400°C for 5 hours. The grain boundaries in red correspond to the deformed state and the grains in color to the annealed state after deformation. These observations allowed us to measure the mobility of some classes of grain boundaries and to validate the classical mobility laws.
Beucia et al., Int. J. Plasticity, 115 (2019), 29-55.
On the phase transformation side
The aim is also to better understand the active mechanisms in different materials, and in particular the possible selection of variants according to the thermomechanical treatment undergone, in order to predict the final texture and its impact on the mechanical properties of alloys.
Study of microstructures obtained in a martensitic stainless steel (CX13) before and after hardening treatment by phase transformation. The microstructures are very similar but the analysis of the disorientations shows that the selection of the variants is not strictly the same in both cases and induces a differentiated global anisotropy.
Santos et al. (2021), à paraître.
