Opposing Effects of Cu Content‐Regulated Precipitation Behavior and Grain Characteristics on Strength and Fatigue Crack Propagation in Al‐ <i>x</i> Cu‐1.3Li Alloys

Herein, the mechanical performances of as‐extruded Al‐ x Cu‐1.3Li alloys are examined in relation to the precipitation behavior and grain characteristics, which are influenced by Cu content (3.1, 3.6, 4.1, and 4.6 wt%). The results demonstrated that tensile strength of alloys exhibited a quick increase initially, followed by a plateau, and then a gradual drop during ageing process. Furthermore, tensile strength is improved by increased Cu under peak‐aged state. 4.6Cu alloy showed the highest ultimate tensile strength of 718 MPa, and yield strength of 692 MPa. Nevertheless, fatigue crack propagation (FCP) performance of alloys is adversely affected by elevated Cu under peak‐aged state. When Cu content increased from 3.1 to 4.1 wt%, FCP rate increased gradually from 9.54 × 10 −4 to 2.04 × 10 −3 mm cycle −1 (Δ K = 25–26 MPa m 1/2 ). However, in 4.6Cu alloy, FCP rate became faster and ultimately terminated with Δ K ≈ 16 MPa m 1/2 . Further, alloys with higher Cu showed finer and denser nonshearable T 1 and θ ′ precipitates, wider precipitation‐free zones, and larger discontinuous intergranular precipitates. Additionally, due to refined grains and residual secondary phases induced by excessive Cu atoms, 4.6Cu alloy possessed poorest FCP performance. Hence, 4.1Cu alloy had a more favorable correlation between tensile strength and FCP performance.