Natural aging behaviors of Al-Cu-Li alloy: PLC effect, properties and microstructure evolution

To facilitate the processing and manufacturing of heat-treatable aluminum alloy components, the initial temper of the billet material is usually adjusted to the as-quenched (AQ) temper by solid solution treatment. However, for most as-quenched aluminum alloys, the obvious natural aging (NA) may occur and cause sensitive time scheduling for the subsequent forming processes. Taking the 2195 Al-Cu-Li alloy as the case material, from the aspects of Portevin-Le-Chatelier (PLC) effect, properties and microstructure evolution, this study deeply and systematically explores the NA behaviors and reveals the underlying mechanism. The combined interaction of Cu and Li solute atoms with mobile dislocations is the major mechanism for the PLC effect in this alloy. The NA induced decreasing of the interaction of solutes with the dislocations makes the number of dislocations in the PLC bands reduce, resulting in its character to change from type B to type A during NA. The Al-Cu-Li alloy exhibits a strong response at the early stage of NA, viz., the tensile strength increases rapidly in the initial 4 h and then reaches the plateau 24 h later. Major sources of strengthening in the NA state are the δ′/β' precipitates, GP zones and solute atoms. The extended age hardening model were used to predict the evolution of the YS and microstructure variables of the alloy during NA. The results indicate that the contribution of solute atoms (σ_ss), δ′/β' precipitates (σ_δ′) and GP zones (σ_GP) to strength in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. The electrochemical measurement results indicate that the NA state has a higher corrosion resistance than the AQ state. The pitting pores form associating with the negative potential of the spherical δ′ phase in NA state.