The Corrosion Fatigue Behavior of a High Purity Al-Zn-Mg-Cu Alloy

摘要

The effect of hydrogen on the corrosion fatigue behavior of an Al-5Zn-2.5Mg-1.5Cu alloy was investigated. In the study, a constant mean load (207 MN/sq m), S-N type test method was used to monitor the fatigue response under air and .5N NaC1 environments. In both cases, specific experiments were conducted in an attempt to determine the operating failure mechanism. In the air fatigue case, all specimens failed in a transgranular mode. The cyclic stress-strain response of the alloy indicated that the alloy hardened as the fatigue process proceeded to the crack initiation stage. TEM examination of the dislocation substructure indicated a large density of dislocation dipoles in the slip bands. These observations are consistent with debris accumulation model suggested by Duquette and Swann. The model proposes that the accumulation of dipoles within the slip band and will exert an increasing cleavage stress across the slip plane. By using a conservative estimate of the dipole density it is possible to show that the dipole stress may reach levels in excess of 100 MN/sq m. Since this stress is thought to be additive to the applied stress levels, the model provides a plausible failure mechanism. In addition to the laboratory air environment, tests were also performed in an aerated, .5N NaCl solution. In comparison to the air fatigue properties, the tests conducted under open circuit conditions (-.750v vs SCE) resulted in a marked decrease in the fatigue response. These failures were characterizedby intergranular initiation leading to a transgranular cleavage type propagation mode. It was found that moderate polarization (to -1.3v SCE) produced little change in the fatigue response.