FATIGUE OF ALUMINUM LITHIUM ALLOYS

Aluminium-lithium alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary Al-Li, experimental Al-Li-Cu, and near commercial Al-Li-Cu-Zr and Al-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the low-cycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principally due to deflected and tortuous crack path morphologies, induced by the shearable nature of coherent delta' precipitates, crystallographic texture, and anisotropic grain structures. Environmental fatigue resistance is comparable with 2000 series alloys and better than 7075-type alloys. The accelerated growth of small fatigue cracks, strong anisotropy, poor short-transverse properties, and a sensitivity to compression overloads are the principal disadvantages of Al-Li alloys.