Impact of Annealing Prior to Solution Treatment on Aging Precipitates and Intergranular Corrosion Behavior of Al-Cu-Li A
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INTRODUCTION
LITHIUM-CONTAINING aluminum alloys possess a low density, high specific strength, and low fatiguecrack growth rates, making them widely suited for aircraft and aerospace applications—where they are presently used industrially.[1,2] Over the last century, at least three generations of Al-Cu-Li alloys have been developed,[2,3] and one of the most significant characteristics of incumbent third-generation Al-Cu-Li alloys is the microalloying addition of elements including Mg, Ag, Mn, and Zn.[2,4] ZHI-HAO YE, WEN-XIN CAI, XIANG-RONG CHEN, and ZI-QIAO ZHENG are with the School of Materials Science and Engineering, Central South University, Changsha 410083, China. JIN-FENG LI is with the School of Materials Science and Engineering, Central South University and also with the Key Laboratory of Nonferrous Materials Science and Engineering of Ministry of Education, Changsha 410083, China. Contact e-mail: [email protected] RUI-FENG ZHANG and NICK BIRBILIS are with the Department of Materials Science and Engineering, Monash University, Clayton, VIC, 3800, Australia. Contact e-mail: [email protected] YONG-LAI CHEN, XU-HU ZHANG, and PENG-CHENG MA are with the Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, China. Manuscript submitted March 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Al-Cu-Li alloys are precipitation-hardening alloys where the main strengthening precipitate is T1 (Al2CuLi), while precise composition and processing may also stimulate h¢ (Al2Cu), d¢ (Al3Li), and S¢ (Al2CuMg). Elements including Zr, Sc, and Mn have also been explored in Al-Cu-Li alloys to form dispersoid particles such as Al3Zr, Al3Sc, and Al20Cu2Mn3, helping control the grain structure during homogenization and hot rolling.[2] The T1 phase, the precipitation and volume fraction of which may be affected by various factors, is the most effective strengthening precipitate in Al-Cu-Li alloys.[5,6] Previous works have reported that a suitable Cu/Li ratio and aging parameters can facilitate the precipitation of T1 phase.[7–9] Cassada and Gable et al.[10–12] proposed that the T1 precipitates preferentially nucleated at dislocations and subgrain boundaries, and therefore plastic deformation prior to aging can increase the number of nucleation site. Recently, Dorin et al.[13] reported an interesting phenomenon that the distribution of hardness caused by T1 precipitates in 2198 Al-Cu-Li alloy was heterogeneous, and this heterogeneity was strongly affected by the grain structure (crystallographic texture). The electrochemical behavior of T1 precipitates is also diverse from that of other secondary phases and the matrix. Therefore, the aging process with the precipitation of T1 phase, in addition to the necessary solute
segregation of Li which accompanies such precipitation, has a significant influence on the corrosion behavior of Al-Cu-Li alloys.[4] The effect of T1 precipitates in localized corrosion of Al-Cu-Li alloy has been previously studied.[14,15] It was also reported that the subgrain
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