Fatigue crack initiation and propagation in aluminum alloys 8090 and 2124 in hydrogenous atmospheres

An Al-Li-Cu-Mg-Zr alloy, 8090-T8 (short transverse orientation), was evaluated with corrosion fatigue tests. This material was compared with 2124-T851 in dry helium and hydrogen-bearing atmospheres of dry hydrogen and water vapor saturated helium.; The microstructures of both alloys were characterized using microscopy with image analysis. The monotonic mechanical properties were determined by tensile testing. Both 8090-T8 and 2124-T851, in short transverse orientation, suffered loss of ductility from the presence of large precipitate particles. This was more severe for 8090-T8.; Fatigue crack initiation specimens were evaluated using replicas and SEM examination. Atmosphere and stress amplitude were variables. The environments were dry helium, dry hydrogen, and saturated helium at ambient temperatures. The response of the alloys to these environments was evaluated by qualitative observation of slip bands, surface crack count, and number of cycles to failure.; Low cycle corrosion fatigue tests provided data for evaluating strain hardening characteristics and determining the fatigue strength, ductility, and cyclic strength constants. Fatigue performance in inert and hydrogen-bearing environments was compared over a range of strain amplitudes. The variation of fatigue ductility coefficients and exponents with environment aggressiveness clearly shows effects by environment on strain controlled fatigue behavior, e.g. the fatigue ductility exponents changed by 50% or more between dry and saturated helium.; Load controlled fatigue under fully reversed loading, in dry helium, hydrogen, or saturated helium, provided S/N plots to illustrate the effects of aggressive environments. The presence of water vapor decreased the fatigue life of 8090-T8 by an order of magnitude at low stress amplitudes. The fatigue life of 2124-T851 decreased in dry hydrogen, but was unaffected in saturated helium. At high stress amplitudes, 2124-T851 is superior to 8090-T8 in all tested environments. At low stress amplitudes, 8090-T8 is superior in dry environments. In saturated helium, 2124-T851 is superior at all stress amplitudes.; A model was proposed to rationalize the observed effects of the aggressive hydrogenous atmosphere. This model suggests that particle-matrix decohesion is enhanced in the aggressive environments. If this is true, then particle-matrix decohesion would occur at smaller particles than in inert atmospheres. Decohesion would continue to occur at large particles.