A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys
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INTRODUCTION
HIGH-strength aluminum alloys are well known to be susceptible to stress-corrosion cracking (SCC) in aqueous solutions. Anodic dissolution [1 7] could be supposed to be the mechanism from the fact that SCC is accompanied by preferential dissolution of grain boundary precipitates, which are more anodic than the matrix, and by the localized dissolution of the plastically deformed region within the precipitate free zone. ~,2~ Recently, many authors t8-18~suggested that hydrogen embrittlement is involved in the SCC of the 7000-series aluminum alloys based on the following results: the change of SCC susceptibility with loading mode, t91 a similar potential dependence of SCC susceptibility to that of hydrogen permeability/~~ and a reduction in tensile ductility on pre-exposure to humid environments, tls,19~ Despite these observations, it is still not clear whether SCC is caused by anodic dissolution or by hydrogen embrittlement. This may be due to the amphoteric property t2~ of aluminum as well as difficulty in observing direct evidence of the SCC mechanism in aqueous solutions. There is no doubt that SCC behavior is influenced in a complex manner by many variables, such as stress state (plane stress and plane strain), material parameters (chemical composition, strength, microstructure, etc.), and environmental factors (species, electrochemical potential, test temperature, etc.). Both anodic dissolution and hydrogen embrittlement are, in principle, distinct from the view of a fracture criterion. As a consequence, their SCC responses should be occasionally distinctive for specific variables. In addition, both mechanisms can simultaneously operate in a material/environment system; SEONG-MIN LEE and YOUNG-GAB CHUN, Graduate Students, and SU-IL PYUN, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Chongyangni, Seoul, Korea. Manuscript submitted July 25, 1990. METALLURGICAL TRANSACTIONS A
however, it is supposed that either of them can predominantly operate in SCC under a certain combination of experimental variables. Considering these facts, it seems inappropriate to investigate the mechanism by using un-notched specimens for which the stress state is not clearly defined as a crack advances. There are generally two distinguishable stages of stresscorrosion (SC) crack propagation for a precracked specimen. Stage I is more complex than stage II from the aspect of propagation kinetics, but the identification of the kinetics for stage II crack propagation provides an essential link for understanding the SCC mechanism. On the other hand, it is stage I crack propagation that determines the K~scc level. Thus, in this work, we will pay attention to the measurements of the threshold stress intensity due to SCC, K~scc, and the stage II crack propagation rate, (da/dt)n. The terms Ktscc and (da/dt)~i are regarded as measures of susceptibility to SC crack propagation in stage I and stage II, respectively. It should be noted that K~scc is an eq
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