Micromechanical studies and modelling of toughness in high strength aluminium alloys

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Morgeneyer, Thilo F

Advisors dc:contributor.advisor

• Sinclair, Ian
• Starink, Marco