3D characterization, modelling and tailoring of microstructure heterogeneity effects on damage and fracture of 6xxx aluminium alloys

Hannard, Florent [UCL]

Ductile fracture has been an active field of research for more than fifty years and is a major subject of interest in many technological applications, such as in the automotive, nuclear and aeronautic industry. It is well recognized in materials science that real structural metallic alloys involve significant microstructural variations that affect damage properties. However, it is very complex to integrate heterogeneity effects in predictive assessments of the integrity of engineering structures containing crack-like flaws. Ductile damage has been traditionally treated as a global process, the material being often treated with "average" or “homogenized” microstructural properties assuming a strict progression from void nucleation to coalescence. In order to close this gap, this thesis is dedicated to the 3D characterization, modelling and retroactive tailoring of the microstuctural heterogeneity effects on damage evolution focusing on 6xxx aluminium alloys. In the beginning of the thesis, the dependence of ductility on strength and on the microstructure characteristics has been determined and modelled with a micromechanics approach. The key element setting the fracture strain is the effect of particle size distribution and spatial distribution on the void nucleation and coalescence processes. Furthermore, a quantitative approach is proposed to relate the propensity to fracture anisotropy to a simple microscopic parameter characterizing the degree of anisotropy in the spatial distribution of second phase particles. Based on these results, local stirring using a friction stir processing tool has been performed in order to increase the fracture strain of an Al alloy while making it more damage isotropic. Finally, the essential work of fracture method and in situ synchrotron laminography have been applied in order to characterize the toughness properties.