Microstructural evolution and properties of electromagnetic cast-rolled novel Al–Li alloy under different heat treatment procedures

A large number of dislocations and dislocation tangles are induced in the Al–Li alloy after 5% pre-stretching deformation before aging treatment, which increases the density of the defects in α(Al) matrix and affects the competitive precipitation relationship and precipitation behavior of δ′ (Al3Li), S′ (Al2CuMg) and T1 (Al2CuLi) precipitates during the aging process. The increase of crystal defects such as dislocations provides superior nucleation sites for the heterogeneous nucleation and promotes the precipitation of T1 phase, greatly improving the mechanical properties of the alloy. The uniform and dense precipitation of T1 phase in the grain reduces the potential difference between the grains and the grain boundaries. Therefore, not only does it reduce the driving force of intergranular corrosion (IGC), but also decreases the difference of the corrosion rate between the grain boundary and intragranular. These are beneficial to slow down the corrosion process and promote uniform corrosion. The intermittently distributed grain boundary precipitates (GBPs) in the T8 treated alloy cut off the continuous corrosion channel at grain boundaries and hinder the IGC process. Furthermore, the width of the precipitation free zones (PFZs) in the T8 treated alloy is narrow. Hence, the alloy exhibits excellent IGC resistance and superior corrosion protection performance under T8 single aging treatment. The pre-aging process in low temperature before T8 single aging treatment greatly enhances the precipitation driving force of T1 phase and effectively promotes the precipitation of T1 phase in the subsequent high-temperature aging process, which further increases the strength and improves the IGC resistance of the alloy.