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 & theta;' 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 degrees C, the precipitation sequence of alloys with an Ag/Mg ratio below 0.95 was SSS & RARR;GPB zone +& delta;& PRIME;/& beta;'& RARR;S'+T1, which transformed into SSS & RARR;GP zone +& delta;& PRIME;/& beta;'& RARR;& theta;'+T1 when the Ag/Mg ratio was above 1.87. The T1, S', and & theta;' 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.