On deviated and branched crack paths in Al-Li-X alloys

The origins of deviated and/or branched fatigue crack paths in Al-Li-X type alloys were investigated. The crack paths were predicted by calculating the orientation of the slip planes in single crystals with idealized textures and the associated Schmid factors. These predictions were compared with results obtained by performing fatigue crack growth (FCG) tests on compact tension specimens of the Al-Li-Cu alloy 2095 with a T-L specimen orientation. The alloy was heat treated to the T8 temper condition in which T-1(Al2CuLi) is the predominant precipitate. The material has a partially recrystallized grain structure and a deformation texture containing the {112}[111] and {110}[112] textures. The fatigue crack path was found to consist of three distinct portions: an initial straight portion close to the machined notch, a portion deviated at 22 degrees to the longitudinal direction, and a 'final fracture' portion in the longitudinal direction. Optical and SEM microscopy revealed the micromechanisms of FCG to be intergranular decohesion rupture in the portions where the crack path was in the longitudinal direction, and transgranular shear in the deviated region. The angle of deviation, 22 degrees, was in agreement with the angle of deviation predicted for a material with the {112}[111] texture. After a re-examination of the results available in the literature for similar materials, it is proposed that crack path morphology in textured Al-Li-X type alloys depends on a combination of texture, grain boundary strength, and specimen orientation. It was further concluded that the presence of delta'(Al3Li) is not a principal factor in determining crack path morphology in these alloys.