Soutenance de thèse : Zheheng LIU

Characterization of plastic strain localization in polycrystalline materials by means of 3D X-ray diffraction imaging techniques

Doctorant : Zheheng LIU

Laboratoire INSA : MATEIS

École doctorale : ED34 : Matériaux de Lyon

This thesis is part of the ANR 3DiPolyPlast project devoted to an in-depth study of the mechanisms leading to plastic strain localization in pure metals. 2D surface observations, 3D volume characterization, and numerical simulations are carried out in three simultaneous PhD projects. This thesis focuses on 3D characterization of strain localization using a combination of Topo-Tomography (TT) and Diffraction Contrast Tomography (DCT) - two synchrotron radiation near-field imaging techniques allowing for
1 micron spatial and 0.02-degree angular resolution. Two material systems have been studied: Ti-7Al alloy and pure Ni. In both cases the 6D algorithm is used to reconstruct the orientation field during initial stages of plastic deformation, which in turn is analysed for signatures of strain localization. Both the data acquisition scheme for TT and the 6D / 5D reconstruction algorithms for DCT and TT scans have been optimized. Theoretical analysis for the TT scan is performed to explore the limits of its reconstruction capacities and the methods to enhance them. Simulated diffraction data are used to study joint reconstructions of DCT and TT scans, illustrating possibilities and limitations of the optimized reconstruction framework. Contrary to Ti7Al, exhibiting clearly resolvable orientation contrasts in vicinity of slip bands, the diffraction data from pure Ni could not reveal slip bands, probably due to the weak and diffuse character of slip in this pure material. However, distinct structures in the orientation field were observed in directions perpendicular to the primary slip plane.