Abstract
Rotating bending fatigue tests were performed in distilled water and in a 3%NaCl solution using smooth specimens of 6063 aluminium alloy. The corrosion fatigue strengths were evaluated and compared with those of 2024 and 7075 alloys. Fracture mechanisms were discussed on the basis of detailed observation of crack initiation and small crack growth behaviour. The fatigue strength of 6063 alloy decreased with increasing aggressiveness of environment, but was nearly the same as those of 2024 and 7075 alloy in a 3%NaCl solution, indicating a considerably low susceptibility to the corrosive environment of 6063 alloy. There was a strong propensity that fatigue cracks were initiated at grain boundaries as environment became more aggressive. Such intergranular crack initiation was attributed to corrosive dissolution of grain boundary precipitates and a precipitation free zone. The crack growth rates of small cracks were enhanced by corrosive environments; particularly remarkable at the early stage of small crack growth following crack initiation in the 3%NaCl solution, where extensive intergranular facets were seen. Therefore, both premature crack initiation and enhanced small crack growth were responsible for the reduction in fatigue strength in the corrosive environment.
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