Archive March 2020

CPFEM simulations of grain size effect in FCC polycrystals: a new approach based on surface GND density

A multiscale modeling methodology involving discrete dislocation dynamics (DDD) and crystal plasticity finite element method (CPFEM) is used to study the physical origin and to simulate the grain size effect in FCC polycrystalline plasticity. This model is based on the dislocation density storage–recovery framework, expanded on the scale of slip systems. DDD simulations are used to establish a constitutive law incorporating the main dislocation mechanisms controlling strain hardening in monotonically deformed FCC polycrystals. This is achieved by calculating key quantities controlling the accumulation of the forest dislocation density within the grains and the polarized dislocation density at the grain boundaries during plastic deformation. The model is then integrated into the CPFEM at the polycrystalline aggregate scale to compute short- and long-range internal stresses within the grains. These simulations quantitatively reproduce the deformation curves of FCC polycrystals as a function of grain size. Because of its predictive ability to reproduce the Hall-Petch law, the proposed framework has a great potential for further applications.

Speaker: Maoyuan Jiang

Date and Location: Monday 09/03/20 14h00, LEM meeting room (E2.01.20), Châtillon.

Orientation imaging at the onset of plastic deformation

Diffraction Contrast Tomography (DCT) is a near-field X-ray diffraction technique for the inspection of ductile materials at the micron scale. It has traditionally been used for the study of undeformed polycrystalline materials with grain sizes of a few tenths of microns. It uses a box-sized monochromatic X-ray beam, which allows it to scan large regions of millimeter sized sample (with up to thousands of grains) in a relatively short time.
Recent work has introduced sub-grain orientation reconstruction (6D-DCT), which has made DCT a viable tool for the reconstruction of slightly deformed materials.
Topo-tomography (TT) is also a near-field X-ray diffraction technique, which, on the other hand, allows to focus on a single grain with a high-resolution detector and to obtain sub-micron level shape information.
In this talk, we will first present how the data is acquired and reconstructed in modern DCT and TT acquisitions. Then, we will present their 6D and 5D extensions (respectively) for the reconstruction of sub-grain level orientation information. Finally, we will discuss future applications, including the combined use of DCT and TT data in a single 6D reconstruction for the investigation of slip bands formation at the onset of deformation.

Speaker: Dr Nicola Viganò

Date and Location: Friday 21/02/20, 14h00 LEM meeting room (E2.01.20), Châtillon.