Abstract
A detailed fractographic and microstructural study, using combined scanning and transmission electron microscopy has been made of the fracture surfaces produced by fatigue of a commercial aluminium alloy 7010:T76 in moist air, dry argon and dry oxygen. In the dry environments fracture is entirely ductile: the fracture surface is non-crystallographic and essentially striation free. In moist air fracture occurs by cleavage on {110} coupled with plastic deformation during the blunting and closing of the crack: this results in the formation of well defined surface striations. Striation topography is controlled by the local orientation of the cleavage plane to the direction of maximum tensile stress and to the slip systems of highest local shear stress. In consequence a range of striation profiles are developed. It is suggested that the cleavage component of growth is the result of hydrogen embrittlement of the matrix ahead of the crack and that dislocation transport is necessary to introduce this hydrogen.
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