Regulation of precipitation behavior among T1, S', and θ' phases in Al–Cu–Li-(Mg–Ag) alloys by optimizing Ag/Mg ratios

In third-generation Al–Cu–Li alloys, T1, S', and θ' precipitates are the primary strengthening phases, and the strength-ductility synergy of the alloys depends strongly on their precipitation behavior. This study aimed to regulate the precipitation behavior among these precipitates by optimizing Ag/Mg ratio in Al–Cu–Li alloys, and its influence on mechanical properties and fracture mechanism was investigated. To this end, four alloys were designed with a total content of 1.1 wt% of Ag and Mg, and an Ag/Mg ratio ranging from 0.24 to 5.63. The results shows that an increase in the Ag/Mg ratio dramatically accelerated the ageing process and shortened the peak-ageing time. At 150 °C, the precipitation sequence of alloys with an Ag/Mg ratio below 0.95 was SSS→GPB zone +δ′/β'→S'+T1, which transformed into SSS→GP zone +δ′/β'→θ'+T1 when the Ag/Mg ratio was above 1.87. The T1, S', and θ' precipitates coexisted and were dispersedly distributed among studied alloys with an Ag/Mg ratio ranging from 0.95 to 1.87. An Ag/Mg ratio of 1.87 resulted in the fastest ageing hardening response and the highest strength (YS = 705 MPa, UTS = 717 MPa) because of the highest volume fraction of T1 precipitates. Moreover, the fracture mode changed from transgranular fracture to intergranular fracture with increasing Ag/Mg ratio due to the difference in distribution and number density of these precipitates between the matrix and grain boundary.