Enhanced matrix precipitation of T1 (Al2CuLi) phase in AA2055 Al–Li alloy during stress aging process

Abstract

Attempts to improve the strength of Gen3 Al–Li alloys without obviously deteriorating the ductility have been directed towards enhancing the matrix precipitation of T1 phase and reducing its enrichment at sub-grain boundaries. In this study, a novel artificial aging treatment during which uniaxial tensile stress was applied has been carried out on the AA2055 Al–Li alloy while a certain temperature was maintained. The microstructure, fractography, dislocation density, and the mechanical properties were analyzed by HAADF-STEM, HRTEM, DSC, SEM, FWHM of XRD patterns, and tensile tests. It was found that matrix T1 precipitation could be enhanced at the expense of θ’ phase by the stress applied. The enrichment of T1 phase at sub-grain boundaries could be observed in all specimens. However, the degree decreased as the pre-stretch and the applied external stress imposed. A good combination between strength and ductility was obtained for the specimen aged in T8SA100 temper (2% pre-stretch, 155 °C, 24 h, 100 MPa) and the average YS, UTS, and EI were 623 MPa, 649 MPa and 10.5%, respectively. The dislocation density in the stress aged specimens was higher than that in the stress-free aged specimens, indicating that the dislocation mechanism plays an important role in the enhanced T1 matrix precipitation during stress aging. The present study suggests that the novel stress aging treatment has great potential in improving the overall performance of advanced Al–Li alloys.