Effects of Aging and Environment on Fatigue Crack Growth in Precipitation Hardened Al-Cu-Mg

Artificial aging degrades the fatigue crack growth (FCG) resistance of heat treatable aluminum alloys, particularly in the near-threshold regime. Consequently, applications of aluminum alloys in tensiondominated structures that require fatigue damage tolerance are limited to the naturally aged T3 temper. Degraded FCG resistance compromises the benefit from higher strength by artificial aging. While studies on the effect of aging on FCG in 2xxx series aluminum alloys are limited, mechanisms of fatigue damage and crack growth primarily based on investigations of 7xxx-series alloys involve interactive, competitive and highly localized (at the crack-tip) processes including (a) cyclic deformation structure and plastic strain accumulation, (b) environmental interaction, (c) roughness and corrosion product-induced closure, and (d) crack deflection and path tortuosity. FCG in aluminum alloys is enhanced by exposure to moist air, likely due to production of embrittling H by crack-tip reaction of water vapor with aluminum. Studies recognized the importance of environment, but have not included systematic studies of FCG in varying tempers of the same alloy exposed to different-controlled environments. Ultra-high vacuum conditions are particularly important in this regard given the reactivity of aluminum.