Effect of Natural Aging and Cold Working on Microstructures and Mechanical Properties of Al-4.6Cu-0.5Mg-0.5Ag alloy
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THE precipitation-hardened Al-Cu-Mg-Ag alloys have been widely applied in the aviation and military industries. The Al-5.6Cu-0.45Mg-0.45Ag-0.3Mn-0.18Zr (wt. pct) alloy offers the best combination of mechanical strength (522 MPa) and fracture toughness (40.6 MPam1/2), with good resistance to creep at elevated temperature,[1] being used as a material for construction of the supersonic transport aircraft. The main precipitation strengthening phases of AlCu-Mg-Ag alloy are h¢ and X. The tetragonal structure of h¢ (CuAl2) phase, with a = b = 0.404 nm and c = 0.580 nm, is the primary strengthening phase of Al-Cu alloy. The h¢ forms rectangular or octagonal plates parallel and semi-coherent to the {100}a planes of the a matrix.[2] Adding small amounts of Mg and Ag to the Al-Cu alloy promotes the formation of a fine and uniform dispersion phase with a hexagonal plate-like precipitate on the {111}a planes of the a matrix.[3] This precipitate is known as the X phase with a composition close to CuAl2. The phase transformations sequence of X phase formation is as follows: supersaturated solid solution, Mg-Ag clusters, X phase, and finally h phase.[4] The X phase, precipitating on the primary {111}a slip plane of the aluminum alloy, is more helpful to YU-TE CHEN, PhD Candidate, is with the Department of Mechanical Engineering, National Central University, No. 300, Jhongda Rd., Jhongli, Taoyuan 32001, Taiwan (ROC). SHENGLONG LEE and JING-CHIE LIN, Professors, are with the Institute of Materials Science and Engineering, National Central University, and also with the Department of Mechanical Engineering, National Central University. Contact e-mail: [email protected] HUIYUN BOR, Associate Professor, is with Materials and Electro-Optics Research Division, Chung-Sheng Institute of Science and Technology, No.481, Chung-Cheng Rd., Chiaan Village, Lungtan, Taoyuan 32500. Manuscript submitted January 12, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
strengthening than the h¢ phase. The X phase has a face-centered orthorhombic structure, with a = 0.496 nm, b = 0.859 nm, and c = 0.848 nm.[5] Its hightemperature stability improves the high-temperature strength of Al-4.6Cu-Mg-Ag alloy.[6] Owing to the existence of the X phase in Al-Cu-Mg alloy, Ag atoms are not necessary for precipitation of X phase, but Ag can encourage X to precipitate.[7] There is no detection of X phase in Al-Cu or Al-Cu-Ag alloy, inferring that Mg element is essential to the precipitation of X phase. The strong interaction between Mg and Ag atoms results in the generation of Mg-Ag clusters, which act as the nucleation sites of X phase.[8] Hono et al. found that Mg-Ag co-clusters were generated in Al-CuMg-Ag alloy upon aging for 15 seconds at 453 K (180 C), and the tiny X phase appeared upon aging for 30 seconds at 453 K (180 C). A prior investigation[10] also revealed that Mg-Ag clustering was generated in an Al-4.6Cu-Mg-Ag alloy upon aging for 24 hours at room temperature immediately after quenching, benefiting the precipitation of X phase during artificial
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