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An up to date review of the stress corrosion cracking performance of wrought high strength aluminum alloys is presented. Special emphasis is placed on factors of microstructure and its directionality and the effect of ancillary alloying elements, and environmental factors.
The major findings from a comprehensive study on the role of microstructure on the susceptibility to environmental embrittlement of high strength aluminum alloys are presented and discussed. Most of the studies used commercial 7075, or a high purity equiaxed version, HP7075, or a similar powder version 7090. Through the innovative use of loading mode and straining electrode test, stress corrosion cracking was shown to be controlled by the introduction and internal distribution of hydrogen, particularly to grain boundaries. This was the case for the underaged and peak aged microstructures, with the latter being the most susceptible. The SCC behavior of the even more resistant overaged microstructure was shown to be controlled by anodic dissolution processes, predominantly associated with slip bands. Aluminum alloys were also shown to be susceptible to embrittlement under conditions of cathodic polarization correcting a widely held belief that concomitant anodic processes are required. Studies using controlled microstructures were able to rank the importance of different microstructural features to help develop alloy design strategies for more environmentally resistant alloys--the most effective features are those grain interior precipitates which promote fine, homogeneous slip either by reduced particle cutting or enhanced dislocation generation; fine, grain boundary precipitate free zones were found not to be very important, except in their role in reducing the local strength of the boundary region.
The report summarizes information from selected European papers and lectures that were published or presented between mid 1967 and July 1, 1968. Subjects discussed include: the nature of stress-corrosion, stress-corrosion in AlMg, AlMgZn, and AlMgSi alloys and testing for susceptibility to stress-corrosion cracking. (Author).
The stress corrosion cracking resistance of high strength, wrought aluminum alloys in a seacoast atmosphere was investigated and the results were compared with those obtained in laboratory tests. Round tensile specimens taken from the short transverse grain direction of aluminum plate and stressed up to 100 percent of their yield strengths were exposed to the seacoast and to alternate immersion in salt water and synthetic seawater. Maximum exposure periods of one year at the seacoast, 0.3 or 0.7 of a month for alternate immersion in salt water, and three months for synthetic seawater were indicated for aluminum alloys to avoid false indications of stress corrosion cracking failure resulting from pitting. Correlation of the results was very good among the three test media using the selected exposure periods. It is concluded that either of the laboratory test media is suitable for evaluating the stress corrosion cracking performance of aluminum alloys in seacoast atmosphere. Humphries, T. S. and Nelson, E. E. Marshall Space Flight Center NASA-TM-82393