The effect of copper content and heat treatment on the stress corrosion characteristics of Ai-6Zn-2Mg-X Cu alloys
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HIGH strength aluminum alloys, which are extensively used in aircraft structures, are susceptible to environment-induced subcritical crack growth under sustained stress ~ which is often called stress corrosion cracking (SCC). Considerable effort has been directed towards determining the causes of such failures by examining the microstructure, composition, aging treatment, minor alloy additions, environmental factors and dislocation arrangements.l-6 Addition of copper to high strength A1-Zn-Mg alloys has been found to simultaneously improve the yield strength and stress corrosion cracking resistance.l-Z An overaging heat treatment may also be used to greatly enhance the stress corrosion resistance without unacceptable strength penalties in AI-Zn-Mg alloys containing copper. This program was designed to systematically investigate the effects of copper additions on the stress corrosion cracking behavior of 7XXX-type alloys. The influence of copper on the deformation behavior and electrochemical characteristics was correlated with SCC kinetics of four A1-6 pct Zn-2 pct Mg-X pct Cu alloys with almost identical grain structure, texture, and degree of recrystallization. The deformation behavior of the alloys in the T651 condition has been described previously,7 and depends on the copper content. The degree of coherency of the strengthening precipitates in A1-6Zn-2Mg alloys can be modified by additions of copper. 8 Copper participates in the precipitation process during aging and changes the character of the precipitates. The addition of copper not only increases the volume fraction of strengthening precipitates, but also increases the number of partially coherent and incoherent precipitates and thus the homogeneity of the deformation. Consequently, in the T651 condition, the strengthening precipitates are sheared in the copper-lean alloys resulting in inhomoB. SARKAR is Research Scientist, Exxon Enterprises, Materials Division, Old Buncombe at Poplar, Greer, SC 29651. M. MAREK and E. A. STARKE, JR. are Associate Pzofessor of Metallurgy and Director, respectively, Fracture & Fatigue Research Laboratory, School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332. Manuscript submitted October 6, 1980.
geneous deformation and looped in the copper-rich alloys resulting in more homogeneous deformation. On the other hand, when the alloys are aged to the T7X51 condition, the strengthening precipitates are partially coherent and/ or incoherent in all alloys and deformation occurs by a looping mechanism regardless of the copper content. 9,~~Thus, the A1-6Zn2Mg-XCu system offers the possibility of studyng the effects of both copper content and deformation mode on the SCC behavior of 7XXX-type alloys. EXPERIMENTAL PROCEDURES Four A1-Zn-Mg alloys with different copper additions were prepared at the ALCOA Technical Center, Alcoa, Pennsylvania. The compositions of the alloys, containing similar amounts of Zn, Mg, and Zr, are shown in Table I. They were hot rolled by a procedure designed to produce identical grai
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