Micromechanical studies and modelling of toughness in high strength aluminium alloys

In this thesis the influence of microstructure on fracture toughness is investigated for twodifferent medium/high strength Al-alloys for aerospace application. In weldable AA6156(Al-Mg-Si-Cu) alloy sheet, the quench sensitivity in toughness is assessed via enhancedKahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and aircooled materials cf. 20°C water quench material. Fractography via scanning electronmicroscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well asDifferential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM)studies, have clarified the mechanisms of the quench sensitivity with respect to toughness.Both the coverage of grain boundary decoration and precipitate free zone (PFZ) widthincrease with reduced quench rates. The failure morphology of the air cooled materialappears consistent with classical intergranular ductile failure. Coarse voiding and sheardecohesion was prevalent in 20°C water quenched material (depending on localtriaxiality), whilst the 60°C water quenched material showed a mixture of transgranularand intergranular fracture modes. The experimental toughness trends are compared tomodels in the literature and a simple new model is suggested.Fracture toughness anisotropy of AA2139 (Al-Cu-Mg), a candidate alloy for age forming,in T351 and T8 conditions has been investigated via mechanical testing of smooth andnotched specimens of different geometries, loaded in the rolling direction (L) or in thetransverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT.Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, butis substantially reduced for the T8 condition compared to the T351. Contributions tofailure behaviour have been identified with: (i) anisotropic initial void shape and growth,(ii) plastic behaviour, including isotropic/kinematic hardening and plastic anisotropy, and(iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrowcrack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crackduring propagation in the rolling direction, resulting in shorter intervoid ligaments than forcrack propagation in the transverse direction. Coalescence through shear decohesion in thecrack initiation and propagation region was found indicating the necessity to investigateand account for this mechanism. A model based in part on the Gurson-Tvergaard-Needleman approach is constructed to describe and predict deformation behaviour, crackpropagation and, in particular, toughness anisotropy. Model parameters are fitted usingmicrostructural data and data on deformation and crack propagation for a range of smalltest samples. The model accounts for the material features found in the experimental studyand its transferability has been shown by simulating tests of large M(T) samples showingstrong fracture toughness anisotropy. A parametric study shows that nucleation of smallvoids at different strains for different loading directions is crucial for a correct model oftoughness anisotropy; the combined effects of kinematic hardening and void growthanisotropy can not fully describe fracture toughness anisotropy