Effect of retrogression and reaging treatments on the microstructure of Ai-7075-T651
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I.
INTRODUCTION
HIGHstrength A1-Zn-Mg alloys are susceptible to intergranular stress corrosion cracking (SCC). Numerous studies have been performed to understand the mechanism of SCC for the purpose of enhancing the resistance of these alloys to it without deteriorating their high strength. The degree of susceptibility to SCC is known to depend upon the aging conditions. Most of the studies to date have been directed toward the role of grain boundary precipitates,~-13 precipitate free zones (PFZ), ~1-16and matrix precipitates ~6'17on the susceptibility to SCC. Despite the extent of this research, there remains considerable controversy over the influence and relative importance of the various microstructural features. Recent studies 7-~3 have focused on the role of grain boundary precipitates, using as a basis the earlier original findings of Dix and his co-workers t~2'3 that such precipitates could be subject to preferential anodic dissolution due to their anodic electrochemical nature compared with that of the solid solution. The overaging heat treatment T7X has been traditionally applied to reduce the susceptibility of commercial A1-7075T651 to SCC. 18.~9This heat treatment, however, necessarily leads to a sacrifice of the maximum strength of this alloy. In recent years a new heat treatment technique that substantially increases the resistance of 7075-T6 to SCC without significant loss of the 7075-T6 strength has been developed. 2~ This so-called Retrogression and Reaging (RRA) treatment produces a several-fold increase in the threshold stress for SCC over that of 7075-T6. The RRA treatment consists of a short time retrogression anneal applied to J.K. PARK, formerly Visiting Assistant Research Engineer in the Department of Materials Science and Engineering, University of California, Los Angeles, CA, is now Assistant Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Seoul, Korea. A.J. ARDELL is Chairman and Professor, Department of Materials Science and Engineering, 6531 Boelter Hall, University of California, Los Angeles, CA 90024. Manuscript submitted September 19, 1983.
METALLURGICALTRANSACTIONS A
7075-T6 in a temperature range within the two-phase (a + r/) region of the phase diagram, followed by water quenching and a final reaging treatment equivalent to the original T6 temper. The original work on RRA 2~ has been confirmed subsequently by Wallace et al. ,22 who further demonstrated that the RRA heat treatment can be applied to 7075-T6 material in thicknesses approaching those of commercial components. The potential payoff of RRA is that the resistance to SCC of components fabricated from this alloy and currently in service can be improved at minimal cost, thereby conserving both materials and energy. The results of a recent study by Swanson et al. 23 indicate that the RRA treatment may be effective in inhibiting crack initiation under stress corrosion conditions, but probably has no influence on crack propagation. This follows from their obser
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