Evolution of Grain Boundary Precipitates in an Al-Cu-Li Alloy During Aging
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ast two decades have seen a growing interest in lithium-containing Al alloys as structural materials, particularly in the context of aerospace applications.[1,2] Modern generation Li-containing Al alloys are nominally based on the Al-Cu-Li system (where wt pct Cu>wt pct Li, typically Cu:Li~1.5 to 3 (in wt pct)) and present good specific mechanical properties as well as high resistance to uniform corrosion.[1–3] High strength and toughness combination may be achieved in the Al-Cu-Li system owing to the formation of T1 (Al2CuLi) strengthening precipitates, as opposed to d¢ (Al3Li) or h¢ (Al2Cu) precipitates, with d¢ and h¢ favored in cases where the Cu:Li ratio is either too high or too low.[4] Modern Al-Cu-Li alloys nominally also include Ag additions (~0.3 wt pct), in addition to alloyed Mg and Zn, and grain refining elements Mn and Zr—which also contribute to overall alloy performance.[3,5,6] In the case of Al-Cu-Li alloys, the precipitation sequence and kinetics have been extensively studied,[7,8] NOE´MIE OTT, Postdoctoral Researcher, SHRAVAN K. KAIRY and YUANMING YAN, Ph.D. Candidates, and NICK BIRBILIS, Professor and Head of Department of MSE, are with the Department of Materials Science and Engineering, Monash University, 22 Alliance Lane, Clayton, VIC, 3800, Australia. Contact e-mail: noemie.ott@ monash.edu Manuscript submitted July 21, 2016. Article published online November 4, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
where it was shown that T1 precipitates as plates in {111}Al planes.[9–11] The addition of Mg to the Al-Cu-Li system has been reported to lead to the formation of Cuand Mg-rich phases during early aging, such as Guinier–Preston–Bagaryatskii (GPB) zones; however, it can also lead to the formation of additional phases, namely S and S¢ phases (Al2CuMg).[12,13] S¢ phase precipitates as laths on {210}Al planes.[10,14–19] It is noted that due to the similar structure of S¢ and S phases,[14] no clear distinction may be made between these two phases in the Al-Cu-Li system. Additionally, plate-shaped h¢ phase (Al2Cu) has also been previously reported to form in the {001}Al plane.[10,11,20] The addition of Ag to the Al-Cu-Li alloy system may lead to the presence of X phase (an Al2Cu variant) in {111}Al, favoring X over h¢.[11,21] Zr additions to the Al-Cu-Li system serve to retard recrystallization by the formation of the L12 ordered b¢ (Al3Zr).[22,23] Despite the excellent studies to date regarding the precipitation sequence in the Al-Cu-Li alloy family and how this relates to mechanical properties,[24,25] the evolution of the grain boundary microstructure as a function of aging has received less attention. While grain boundary microstructure has little or negligible effect on strengthening, the microstructure and microchemistry at grain boundaries play the key role in dictating the susceptibility of these alloys to IGC and stress corrosion cracking (SCC)[13]; as such, unique grain boundary investigations merit study. Investigations to date have been well summarized by Holroyd,[13] with recent work a
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