Effect of Loading History on Stress Corrosion Cracking of 7075-T651 Aluminum Alloy in Saline Aqueous Environment
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HIGH strength Al-Zn-Mg-Cu aluminum alloys (7XXX series) are widely used in heavily loaded aircraft structures due to their high strength-to-density ratio. However, this series of aluminum alloys is potentially susceptible to stress corrosion cracking (SCC), particularly in aqueous solutions containing chloride ions. The susceptibility of aluminum alloys to SCC in a corrosive environment can be evaluated using the precracked specimens in terms of the threshold stress intensity factor for SCC (KISCC) and the crack growth rate (da/dt).[1] For a precracked specimen of a commercial high strength aluminum alloy, the curve of the crack growth rate (da/dt) vs stress intensity factor (K) generally exhibits two distinguishable stages.[2–4] The crack propagation rate increases sharply with stress intensity factor in stage I, while it is independent of stress intensity in stage II. Lee et al.[4] suggested that SCC was dominated by anodic dissolution (AD) in stage I and by hydrogen embrittlement (HE) in stage II. Endo et al.[5] pointed out that KISCC, the crack growth rate in stage II, and SCC mechanism depended on specimen thickness, which determined the constraint condition (plane stress or plane strain condition) at the crack tips. With the increase of specimen thickness, KISCC decreased, while the crack growth rate in stage II increased and became dependent on the stress intensity factor.[5] JIXI ZHANG, Research Fellow, MAJID BEHROOZ, Graduate Assistant, and YANYAO JIANG, Professor, are with the Department of Mechanical Engineering, University of Nevada, Reno, NV 89557. Contact e-mail: [email protected] SERGIY KALNAUS, formerly Graduate Assistant, Department of Mechanical Engineering, University of Nevada, is Research Associate, with Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted October 23, 2009. Article published online November 23, 2010 448—VOLUME 42A, FEBRUARY 2011
SCC of aluminum alloys is strongly sensitive to environmental, metallurgical, and mechanical factors. The influence of various environmental variables, such as the humidity of air, viscosity, temperature, acidity of the corrosive medium, and the type and concentration of anions, on the SCC behavior of high strength aluminum alloys has been studied.[2–4,6–8] Speidel[2] reported that the rate of SCC growth in an aluminum alloy in air increased monotonically as humidity of air was increased. Different effects of various anions on SCC kinetics in aluminum alloys were also demonstrated.[3] Le et al.[7,8] investigated the SCC behavior of 7075-T651 in various electrolytes. It was found that an increase in the chloride ion content of environment up to 0.6 M enhanced the SCC growth rate, and the cracking was slower at higher concentrations. Great efforts have been made to increase the stress corrosion resistance (SCR) of high strength aluminum alloys by optimization of microstructure via heat treatment.[9–25] Overaging of 7xxx series showed high SCR but a considerable loss in strength.[9] The retrogre
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