Effect of Temper on Environmental Fatigue Crack Propagation in 7000-Series Aluminum Alloys

Abstract

In contrast to stress corrosion, the effect of overaging temper on environmental fatigue crack propagation rate in the 7000-series aluminum alloy/aqueous-chloride solution system is nil, moderately beneficial, or detrimental depending on loading frequency. For both peak and overaged AA7075 plate, Paris regime fatigue crack growth rates are accelerated up to 10-fold by cyclic loading in 3.5% NaCl solution compared to fatigue in moist air for the SL-crack orientation. Crack growth is intersubgranular for the T6 microstructure, but transgranular-brittle for the T7 case. Cracking at low f is dominated by corrosion product induced crack closure that is enhanced by overaging. At intermediate to high f, environmental fatigue is due to hydrogen embrittlement and rate limited by H diffusion over a crack tip process zone distance established by local stress. The relationship between plateau crack growth rate and critical frequency for f-dependent EFCP is predicted by a H diffusion model. Plateau regime transgranular-brittle EFCP in overaged AA7075 is slower than intersubgranular cracking in the T6 microstructure, but persists to higher f CRIT , requiring either rapid H diffusion or reduced crack tip H for process zone embrittlement of the overaged case.