Understanding the Origins of Intergranular Corrosion in Copper-Containing Al-Mg-Si Alloys
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Localized corrosion of 6xxx series Al-alloys [based on the Al-Mg-Si-(Cu) system] remains of topical interest, as pitting or intergranular corrosion (IGC) decreases the lifetime of structural components used in the auto- and aerospace sectors.[1–4] The IGC is more prevalent in Cu-containing (even as low as 0.1 wt pct) alloys as compared to Cu-free alloys,[5–21] and such IGC is posited to arise from the establishment of PFZ adjacent to grain boundaries and their coupling with grain boundary precipitates that include Q-phase (Al4Cu2 Mg8Si7) or its precursors. To date, there exist few studies that have investigated the grain boundaries of Cu-containing 6xxx series Al-alloys using analytical TEM[7,9,11–13] and no studies using atom probe
SHRAVAN K. KAIRY, Ph.D. Candidate, PAUL A. ROMETSCH, Senior Lecturer, and NICK BIRBILIS, Professor and Head, are with the Department of Materials Science and Engineering, Monash University, Clayton, VIC, 3800, Australia. Contact e-mail: [email protected] TALUKDER ALAM, Postdoctoral Research Associate, and RAJ BANERJEE, Professor, are with the Center for Advanced Research and Technology, Department of Materials Science and Engineering, University of North Texas, Denton, TX 76207. CHRIS H.J. DAVIES, Professor and Head, is with the Department of Mechanical and Aerospace Engineering, Monash University. Manuscript submitted August 30, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
tomography (APT). Svenningsen et al.[7–10] indicated that there was a continuous Cu layer and a stable (i.e., equilibrium) Q-phase precipitates (>1 lm in length) along the grain boundaries in the naturally aged (NA) condition, when the alloys were air cooled after solution treatment. The presence of simultaneous combination of such large stable precipitates and an appreciable Cu layer, which were formed at the grain boundaries during slow cooling, does not fully account for the early-stage initiation of IGC. In addition, the local depletion of solute atoms (Si, Mg and Cu) adjacent to the grain boundaries was not detected in that work. In one study however, employing scanning transmission electron microscopy (STEM), the existence of a continuous Cu layer (many nm in thickness) was purported along the grain boundaries for the UA condition,[11,12] commensurate with the resolution of the analysis in that work. In addition to the Cu layer, precipitates of 20 to 50 nm in size were reported. These precipitates were suggested to be Q¢ precipitates; however, it is difficult to ascertain with certainty as the evolution of precipitates in Cu-containing Al-Mg-Si alloys is rather complicated, with the existence of several metastable precipitates including the possibility of b¢¢ precipitates containing Cu.[22,23] In addition, the local depletion of solute atoms along the grain boundaries was uncertain with EDS from STEM. In spite of important works to date, a lack of clarity still exists and the early stages of Cu segregation in Cu-containing 6xxx Al-alloys—even in the UA condition—as well as detailed chemical characteris
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