Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution

doi:10.1007/s40195-015-0293-x

Abstract

Abstract:

The S-N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages: the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.

Key words: Corrosion fatigue, Intergranular corrosion, Microstructure, 7A85 aluminum alloy

Zuo-Yan Ye, Dao-Xin Liu, Xiao-Hua Zhang, Xiao-Ming Zhang, Ming-Xia Lei, Zhi Yang. Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 1047-1054.

Figures/Tables 11

Fig. 1 Schematic drawing of the fatigue sample and device for the corrosion fatigue

Fig. 2 S-N curves of 7A85 aluminum alloy

Fig. 3 SEM fractographies of 7A85 aluminum alloy fatigue sample: a, b tested in air, c, d tested in 3.5 wt% NaCl solution

Fig. 4 Fracture morphologies at the distance of 500 μm from the initiation site: a tested in air, b tested in 3.5 wt% NaCl solution

Fig. 5 Fracture morphologies at the distance of 1500 μm from the initiation site: a tested in air, b tested in 3.5 wt% NaCl solution

Fig. 6 SEM image of a corrosion pit on the corrosion fatigue sample (the dark arrows point to the microcracks)

Fig. 7 Cross-sectional morphology of the samples after IGC test

Fig. 8 Matrix precipitates of 7A85 aluminum alloy: a bright-field image, b high-resolution electron micrograph

Fig. 9 Grain boundary structure of 7A85 aluminum alloy

Fig. 10 Selected area electron diffraction pattern of GBP

Fig. 11 Four regions corresponded to the four stages of corrosion fatigue on fracture surface

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