Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior
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INTRODUCTION
ALUMINUM 7XXX series alloys have been extensively used for structural components in military as well as civilian aircraft.[1] However stress corrosion cracking (SCC) of the higher strength tempers in salt spray/salt fog environments continues to be a problem for 7XXX series alloys,[2,3] particularly with respect to maintenance of aging aircraft.[4] It is generally accepted that in salt water environment SCC susceptibility is the result of anodic dissolution of grain boundary precipitates which supports the generation of hydrogen through cathodic reaction, which in turn is associated with the embrittlement in Al 7XXX alloys.[5–12] Different aging treatments have been employed to improve the resistance to SCC of 7XXX aluminum alloys. Simple overaging treatments[13] and more sophisticated retrogression and re-aging (RRA)[14] are known to considerably reduce the SCC velocities and increase the SCC thresholds (K1SCC) in Al-Zn-Mg alloys containing Cu. The overaging treatments reduce the strength of the materials by 10 to 15 pct,[3,15,16] while RRA treatments maintain the strength close to the peak aged (PA) condition,[16,17] or even can result in improved strength in some cases.[18] The improvement of SCC due to aging treatments in these alloys has been attributed to various factors,[5–9] such as (i) increasing grain-boundary precipitate size, RAMASIS GOSWAMI and RONALD L. HOLTZ, Scientists, are with the Multifunctional Materials Branch, Naval Research Laboratory, Code 6351, Washington, DC. Contact e-mail: ramasis.goswami@ nrl.navy.mil STANLEY LYNCH, Scientist, is with the Defence Science and Technology Organisation, Melbourne, VIC, Australia. N.J. HENRY HOLROYD, Consultant, is from Riverside, CA 92506. STEVEN P. KNIGHT, Scientist, is with the RMIT University, Melbourne, Australia. Manuscript submitted February 16, 2012. Article published online September 28, 2012 1268—VOLUME 44A, MARCH 2013
spacing or volume fraction and increasing precipitate free zone (PFZ) width (ii) changes of microchemistry of grain boundary precipitates, and (iii) decreasing slip planarity.[8] There are still unresolved questions about which factors are important in controlling the SCC behavior, partly because the various microstructural features are not independently controllable. It has been proposed that the SCC plateau velocity of Al-Zn-Mg-Cu alloys[19] in aqueous environment decreases with increasing Cu content, as a function of aging time. It was argued that increasing Cu content of the grain boundary precipitates with aging makes the precipitate more noble.[19] Cooling rates during quenching from solution treated temperature have been shown to affect the SCC plateau velocity in works by Knight et al.[5,20] on a 7079 alloy. They argued that microchemistry, particularly the content of Cu of the grain boundary precipitate may determine alloy susceptibility to SCC in salt water environment. Several attempts[7] have been made to correlate the increase in grain boundary precipitate size and spacing with higher resistance to SCC. Re
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