Characterization of retrogression and reaging behavior of 8090 Al-Li-Cu-Mg-Zr alloy
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I. INTRODUCTION
ALUMINUM alloys containing lithium are candidate alloys for aerospace industries because of their reduction in density, increase in elastic modulus, and increase in specific strength over the most widely used conventional aluminumbase alloys.[1,2,3] The mechanical properties of Al-Li alloys are strongly sensitive to processing conditions, and therefore, product quality is more difficult to control than for conventional alloys. Al-Li alloys have unattractive fracture behavior, especially poor ductility, compared to the traditional high-strength alloys and high anisotropy of nonrecrystallized products. Moreover, these alloys, such as 2xxx and 7xxx series alloys, are also susceptible to stress corrosion cracking (SCC) in their maximum strength peak aged, T6, or T8 temper. Overaging (T7) temper has an acceptable SCC resistance, but lower strength.[4,5] A nonconventional two-stage heat-treatment process, retrogression and reaging (RRA), identified by T77 designation, was developed by Cina.[6] The RRA treatment applied to 7xxx series has been reported to improve the SCC resistance, which makes the heat treatment attractive to the aerospace industries.[6,7] The low ductility and toughness of binary Al-Li alloys can be traced to many concurrent and mutually competitive factors, mainly the inhomogeneous nature of slip resulting from coherent matrix strengthening ordered spherical precipitates and the presence of coarse equilibrium precipitates at the grain boundaries.[1] However, for commercial Al-Li-Cu-Mg-Zr alloys, the presence of Zr, Mg, and Cu causes precipitation of (Al3Zr) dispersoids and semicoherent/
incoherent T1 (Al2CuLi), S, and S (Al2CuMg) precipitates. These precipitates result in better slip homogenization and has been found to improve ductility and fracture toughness.[8–11] Further, the ductility parameters in these alloys could also be improved by suitable heat treatments such as by thermomechanical treatments as well as by proper RRA treatments. Hence, the principal benefits of RRA treatments are increased reliability, reduced maintenance costs, and avoidance of weight penalties by the use of standard overaged temper. Extensive work has been done on 7xxx series alloys to optimize the heat-treatment conditions in RRA treatments[12] and to study the effects of RRA on microstructural changes and precipitation behavior,[13,14,15] dislocation densities,[16,17] physical and strength properties,[18,19] and finally on SCC behaviour.[15,20–23] A similar type of work has also been initiated on Al-Li alloys and the results show that RRA treatment increases SCC resistance and exfoliation corrosion resistance.[24–28] However, little work is reported on the characterization of RRA behavior on 8xxx series Al-Li-Cu-Mg-Zr alloys. In the present work, characterization of RRA behavior of the 8090 Al-Li-Cu-Mg-Zr base alloy by various techniques, viz. hardness and tensile testing, transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), electrochemical
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