Studies of retrogression and reaging behavior in a 1441 Al-Li-Cu-Mg-Zr alloy
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I. INTRODUCTION
ALUMINUM-LITHIUM alloys are potential materials for the aerospace industry because of their attractive combination of lighter weight and higher stiffness compared to the most widely used conventional 2xxx- and 7xxxseries aluminum alloys.[1,2,3] The disadvantages of the Al-Li alloys are poor fracture toughness, low ductility, and marked anisotropy of uncrystallized products.[1,2] The low ductility and toughness of the Al-Li alloys are due to many concurrent and mutually competitive factors; mainly, the inhomogeneous nature of slip resulting from coherent matrix strengthening of ordered spherical precipitates and from the coarse equilibrium precipitates at the grain boundaries. However, the presence of Zr, Mg, and Cu in commercial Al-Li-Cu-Mg-Zr alloys causes precipitation of (Al3Zr) dispersoids and semicoherent/incoherent T1 (Al2CuLi), S, and S(Al2CuMg) phases, which results in better slip homogenization and improvement of the ductility and fracture toughness[4,5,6] of the alloys. Al-Li alloys are also susceptible to stress-corrosion cracking (SCC) in their maximum-strength peak-aged, T6, or T8 tempers. Over-aged T7 temper results in an acceptable SCC resistance, but, because of its lower strength,[7,8,9] components need to be overdesigned. A heat-treatment process known as retrogression and reaging (RRA), designated as the T77 temper, was developed by Cina et al.,[10] and it was reported that the RRA treatment improved the SCC resistance of the aluminum alloys.[11] The principal benefits of the RRA-treated alloys are increased reliability, reduced maintenance costs, and avoidance of weight penalties by the use of the standard overaged K.S. GHOSH, Assistant Professor, is in the Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal 506 004, India. Contact e-mail: [email protected]. K. DAS, Associate Professor, and U.K. CHATTERJEE, Professor, are in the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur - 721 302, India. Manuscript submitted October 6, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
T7 temper. Studies on RRA treatment applied to a 7xxx series alloy have been reported for optimizing the heat-treatment conditions[12] and for its influence on microstructural changes and precipitation behavior,[13,14,15] on dislocation densities,[16,17] on physical and strength properties,[18,19] and, finally, on the SCC behavior.[15,16,18,20–23] A similar type of work has also been initiated in Al-Li alloys,[24–27] and the results show that RRA treatments increase SCC and exfoliation corrosion resistance. While extensive work has been carried out on characterizing the RRA behavior of 7xxxseries alloys, little work is reported on characterizing the RRA behavior of Al-Li-Cu-Mg-Zr alloys. In the present work, the RRA behavior of a 1441 (Russian-grade) Al-LiCu-Mg-Zr alloy has been studied by various techniques such as hardness measurements, tensile testing, scanning electron microscope (SEM) fractography, t
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