Localized corrosion susceptibility of Al-Li-Cu-Mg-Zn alloy AF/C458 due to interrupted quenching from solutionizing tempe
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L-LI alloys have never truly fulfilled their early promise as lightweight, 7XXX alloy replacements for airframe structural components. Poor short-transverse (S-T) mechanical properties and unpredictable fatigue-crack growth behavior, among other reasons, made their use unfeasible in product forms other than a thin sheet. While Al-Li alloy development efforts have waned since a peak in the mid to late 1980s, advances in processing and gains in properties continue to be made, and Al-Li alloys have been placed in service. Recently, an Al-Li alloy (AA 2195, Al-4Cu-1Li) was selected for use as a primary structural material for the space shuttle’s superlightweight external fuel tank.[1] Although this is a cryogenic tankage application, it is a significant instance of the use of an Al-Li alloy in a performance-critical aerospace application. Over the past several years, the United States Air Force has led exploration of new Al-Li-Cu-Zn-Mg alloy compositions that, when combined with proper thermomechanical J.E. KERTZ, Graduate Research Assistant, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. P.I. GOUMA, Assistant Professor, is with the Department of Materials Science and Engineering, SUNY at Stony Brook, Stony Brook, NY 11794-2275. R.G. BUCHHEIT, Associate Professor, is with the Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1187. Manuscript submitted November 20, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
processing regimens, produce thick plate products with isotropic mechanical properties, high S-T fracture toughnesses, and excellent stress-corrosion-cracking (SSC) resistance.[2,3,4] One of these alloys, designated AF/C458, has a nominal composition of Al-1.8Li-2.7Cu-0.6Zn0.3Mg-0.3Mn-0.04Zr on a weight percent basis and is the subject of the current study. With all plate processing, quench sensitivity is an issue that can affect a variety of properties including intergranular attack (IGA), intersubgranular attack (ISGA) (also known as, interfragmentary attack), and SCC susceptibility. Friction-stir welding can also induce thermal transients that alter alloy microstructures in the heat-affected zone in a way that increases susceptibility to IGA and SCC. In unrecrystallized Al-2Li-3Cu alloys, this susceptibility is often associated with T1 precipitation on subgrain boundaries.[5] In this study, we have examined the pitting, IGA, and ISGA behavior of this alloy as a function of the time and temperature combinations that are relevant to quenching of a thick plate from high-temperature thermal processing or to friction-stir welding situations. The localized corrosion behavior of AF/C458 has been mapped using interrupted quenching experiments for temperatures ranging from 480 ⬚C to 230 ⬚C and times ranging from 5 to 1000 seconds. Results show that the domains of IGA and ISGA are practically coincident, suggesting that attack on the two different types of boundaries is linked
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