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Theses

Hydrogen-induced Intergranular Fracture of Aluminum-Magnesium Alloys

Abstract : Aluminium alloys that are strengthened by alloying elements in solid solution may present a particular sensitivity to intergranular stress corrosion cracking as a result of intergranular dissolution. In Al-5Mg alloys such as AA5083, precipitation of the β-phase (Al3Mg2) at grain boundaries strongly favours intergranular fracture. Previous experimental studies revealed that local plasticity seems to play a significant role in crack initiation. Nevertheless, the exact role of crystal plasticity in the vicinity of grain boundaries is not well understood. The main goal of this doctoral thesis is two-fold: (i) to study the role of the local stress and strain fields on the mechanism of intergranular stress corrosion cracking and, based on such understanding, (ii) to develop a micro-mechanics based model to predict the onset of grain boundary cracking, through a suitably defined failure criterion, and the subsequent intergranular crack propagation. An experimental procedure based on in-situ tensile tests within the chamber of an scanning electron microscope was developed to measure the evolution of local strain fields at various microstructural scales and of lattice orientation using digital image correlation and electron backscatter diffraction (EBSD) techniques, respectively. Digital image correlation techniques were used in particular over areas comprising just a few grains up to mesoscopic regions of the polycrystal to quantify the deformation and strain fields required in the multi-scale study of intergranular fracture. From these observations, it was established that interfaces between two grains which have undergone little amount of deformation but lying within a neighbourhood of significantly deformed grains are the first to develop micro-cracks. In addition, X-Ray tomography and serial EBSD sectioning analyses revealed that cracked grain boundaries were perpendicular to the applied tensile load, where maximum tensile tractions are expected. To determine the role of local stresses and local plasticity on the mechanisms of intergranular fracture, a dislocation mechanics based crystal plasticity model was employed to describe the constitutive behaviour of each grain in the finite element model of the in-situ experiments. The model parameters were calibrated as a function of the solid solution magnesium content in the aluminium alloy. Measured EBSD maps were relied upon to define the orientation of the discrete grain regions of the in-situ specimens in the corresponding multi-scale finite element (FE) models. From the FE results, a range of threshold values of the normal grain boundary tractions needed to initiate intergranular cracks was identified. This finding is in close agreement with the predictions from an analytical solution of a simplified model of intergranular cracking based on an extension of Eshelby's theory for inclusions. Finally, a cohesive zone model calibrated with the critical grain boundary tractions and typical surface energies was added to the FE model of the polycrystal. A comparison between the experimental and numerical results reveals a good agreement with the observed experimental cracking pattern.
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Submitted on : Friday, June 22, 2012 - 4:17:27 PM
Last modification on : Friday, October 23, 2020 - 4:48:19 PM
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  • HAL Id : pastel-00711202, version 1

Citation

Édouard Pouillier. Hydrogen-induced Intergranular Fracture of Aluminum-Magnesium Alloys. Materials. École Nationale Supérieure des Mines de Paris, 2011. English. ⟨NNT : 2011ENMP0095⟩. ⟨pastel-00711202⟩

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