Effect of Temper Condition on Stress Relaxation Behavior of an Aluminum Copper Lithium Alloy
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TION
A desirable combination of lower density, high strength and toughness and excellent corrosion resistance makes third-generation aluminum-copper-lithium alloys (e.g., AA 2195) a suitable option for aerospace applications compared with the conventional age-hardenable aluminum alloys such as AA 7075 and AA 2024.[1,2] Previous generations of Al-Li alloys containing high concentrations of lithium (~ 4 to 4.5 wt pct) suffered from high anisotropy in strength, manufacturing problems due to the high level of lithium, and poor thermal stability due to precipitation of the metastable d¢ (Al3Li) phase.[1,3,4] However, due to the relatively lower amount of lithium (~ 1 to 1.8 wt pct), formation of d¢ phase is suppressed in third-generation Al-Cu-Li alloys.[5–7] The chemistry of these alloys is especially designed in such a manner that precipitation of the T1 (Al2CuLi) phase is favored. The precipitation SUMEET MISHRA, VIKRANT KUMAR BEURA, AMIT SINGH, and MANASIJ YADAVA are with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Bangalore 560012, India. NIRAJ NAYAN is with the Vikram Sarabhai Space Centre, Indian Space Research Organization, Trivandrum 695022, India. Contact e-mail: [email protected] Manuscript submitted October 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
sequence in these alloys is complicated, showing characteristics of both binary Al-Li and Al-Cu systems.[8,9] Moreover, the precipitation sequence gets further complicated because of alloying additions such as magnesium and silver.[10,11] Hence, a number of phases can be observed in the ternary Al-Cu-Li system depending upon alloy composition and heat treatment. However, of all phases, the T1 phase is known to be the most effective in terms of providing high strength with acceptable ductility.[12] The T1 precipitate is semi-coherent in nature and forms as platelets on {111} planes of the Al matrix.[13,14] These T1 precipitates have a very high aspect ratio, in the sense that it is usually ~ 1 to 2 nm thick (equivalent to stacking of five {111} Al planes) and ~ 100 nm long.[13,14] Despite coherency with the Al matrix, it has been found that it is very difficult for the T1 phase to nucleate homogenously.[6,15,16] Therefore, a small amount of plastic deformation (stretching operation) is usually applied to solutionized samples to provide uniform distribution of dislocations in the Al matrix, which in turn act as nucleating sites for T1 precipitates during aging treatment.[12] It has been shown that aging kinetics improves by one order of magnitude by applying cold work prior to aging treatment.[6,7] The cold working step ensures that T1 precipitates are distributed in a dense manner
throughout the material, leading to excellent mechanical properties.[6,7,12] In the past, T1 precipitates were considered to be non-shearable in nature.[17] Hence, the initial models to predict the yield strength increment associated with the T1 phase were based upon modified versions of the Orowan model for precipitate by-passing.
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