Micromechanical studies and modelling of toughness in high strength aluminium alloys

Abstract

In this thesis the influence of microstructure on fracture toughness is investigated for two
different 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 enhanced
Kahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and air
cooled materials cf. 20°C water quench material. Fractography via scanning electron
microscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well as
Differential 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) width
increase with reduced quench rates. The failure morphology of the air cooled material
appears consistent with classical intergranular ductile failure. Coarse voiding and shear
decohesion was prevalent in 20°C water quenched material (depending on local
triaxiality), whilst the 60°C water quenched material showed a mixture of transgranular
and intergranular fracture modes. The experimental toughness trends are compared to
models 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 and
notched specimens of different geometries, loaded in the rolling direction (L) or in the
transverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT.
Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, but
is substantially reduced for the T8 condition compared to the T351. Contributions to
failure 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 narrow
crack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crack
during propagation in the rolling direction, resulting in shorter intervoid ligaments than for
crack propagation in the transverse direction. Coalescence through shear decohesion in the
crack initiation and propagation region was found indicating the necessity to investigate
and account for this mechanism. A model based in part on the Gurson-Tvergaard-
Needleman approach is constructed to describe and predict deformation behaviour, crack
propagation and, in particular, toughness anisotropy. Model parameters are fitted using
microstructural data and data on deformation and crack propagation for a range of small
test samples. The model accounts for the material features found in the experimental study
and its transferability has been shown by simulating tests of large M(T) samples showing
strong fracture toughness anisotropy. A parametric study shows that nucleation of small
voids at different strains for different loading directions is crucial for a correct model of
toughness anisotropy; the combined effects of kinematic hardening and void growth
anisotropy can not fully describe fracture toughness anisotropy.

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