Studies of retrogression and reaging behavior in a 1441 Al-Li-Cu-Mg-Zr alloy

Abstract

A 1441 Al-Li-Cu-Mg-Zr alloy in the peak-aged T8 temper was subjected to retrogression treatment and immediately reaged to various tempers. Transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), hardness measurements, tensile testing, scanning electron microscopy (SEM) fractography, and electrochemical polarization studies have been made to characterize the retrogression and the retrogression and reaging (RRA) behavior of the alloy. Retrogression of the T8 temper causes dissolution of δ′ (Al3Li) precipitates into solid solution, resulting in a decrease of hardness and tensile strength and an increase of ductility. Reaging the retrogressed state causes restoration of strength and ductility properties because of the reprecipitation of the δ′ phase in the matrix, confirmed by TEM, XRD, and DSC studies and image analysis. The SEM fractographs revealed predominantly intergranular fracture in all the tempers due to strain localization at the grain boundaries. The gross aging time of the RRA tempers approaches that of the over-aged T7 temper, which would contribute to an improvement of the stress-corrosion cracking (SCC) resistance while maintaining the T8 temper mechanical properties.