Kinetics of Solid-State Reactions in Al-Li-Cu-Mg-Zr Alloys from Calorimetric Studies
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UCTION
THE attractive combination of Al-Li alloys of 5 to 10 pct decrease in density, 15 to 25 pct increase in elastic modulus, and 10 to 15 pct increase in specific strength over the widely used 2xxx and 7xxx aluminum alloys has rendered them as candidate materials for aerospace applications.[1–7] However, Al-Li alloys have unattractive fracture behavior, especially poor ductility and toughness arising due to the inhomogeneous nature of slip resulting from matrix strengthening ordered d¢ (Al3Li) and coarse equilibrium d (AlLi) grain boundary precipitates.[1,3,6,7] Al-Li alloys are also susceptible to environment-induced cracking (EIC).[8] In commercial pentanary Al-Li-Cu-Mg-Zr alloys, slip is homogenized by introducing dispersoids b¢ (Al3Zr) and semicoherent/incoherent precipitates of T1 (Al2CuLi), h¢ (Al2Cu), or S (Al2CuMg) phases, with the additions of Zr, Cu, and Mg.[1,6,7] A large number of metastable precipitate phases that occur in the alloys and the wide variety of aging treatments employed can lead to complex microstructural conditions. Thus, a good description of the microstructure is essential to the understanding of structure-property relationships. The microstructural changes that occur in these alloys upon aging and retrogression and reaging treatment have been examined by a number of investigators.[1,6,7,9,10] K.S. GHOSH, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal 506 004, India. Contact e-mail: ksghosh2001@ yahoo.co.uk K. DAS, Associate Professor, and U.K. CHATTERJEE, Emeritus Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Manuscript submitted September 19, 2006. Article published online August 4, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
For a rapid and quantitative description of solid-state phase transformations, differential scanning calorimetry (DSC) study is considered as a supplement to transmission electron microscopy (TEM) studies.[11,12] The kinetics of precipitation and dissolution reactions in aluminum-base alloys have been analyzed by various researchers using resistivity measurements and the DSC technique.[13–18] The retrogression and reaging (RRA) treatment applicable to precipitation hardenable aluminum alloys has been studied in detail to assess microstructural changes,[3,7,19] mechanical properties.[20] and EIC.[21–23] However, the DSC studies of the retrogressed and RRA tempers are scanty. In the present work, the kinetics of the precipitation and dissolution reactions of 1441 (Russian grade) and 8090 Al-Li-Cu-Mg-Zr alloys have been studied from the DSC data, and the significance of retrogression treatment that enabled determination of the kinetic parameters for the overlapping peaks of the water-quenched (WQ) states has also been discussed.
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EXPERIMENTAL PROCEDURES
A. Materials The DSC studies were carried out on the 1441 and 8090 Al-Li-Cu-Mg-Zr alloys. The compositions of the alloys are given i
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