Fig.1 Ageing-hardening curves of the alloy aged at different conditions: a single-step ageing, b double-step ageing

Fig.2 HAADF-STEM images of the alloy natural aged for 60 days, viewed along a <100>Al direction: a an overview of the precipitates, b a Cu-monolayer GPI zone, c a δ′-GPI-δ′ precipitate composite in an “anti-phase” relation

Fig.3 Microstructures of samples ageing at 200°C for various times: a-c 2, 12 and 72 h, respectively, viewed along <100>Al. d δ′-θ′-δ′ composite phase as indicated by arrow in b. e, f 12 and 72 h, respectively, viewed along <011>Al Summarizing our observations and that reported for the samples aged at 180°C [6], the precipitation sequence in an Al-Cu-Li-Mg alloy treated with one-step artificial ageing at about 180-200°C can be considered as follows: SSSS → GPI + δ′-GPI-δ′ + GPB + T1 → δ′-θ′-δ′ + GPB + T1 + σ + S → GPB + T1 + σ + S.

Fig.4 HAADF-STEM images of the samples treated with NA60 days plus ageing at 180°C for various times: a 0.5 h, b 28 h, c 52 h, viewed along <100>Al

Fig.5 Diameter distributions of GPI zones and δ′-GPI-δ′ composite precipitates observed a in the sample natural aged for 60 days, and b in the sample first natural aged for 60 days and then artificial-aged at 180°C for 0.5 h

Fig.6 Aberration-corrected HAADF-STEM images of different precipitates: a σ-phase in the sample two-step-aged at 180°C for 28 h, b GPB zones in the same sample as a, c GPB zones the sample two-step-aged at 180°C for 52 h, viewed along a <100>Al direction

Fig.7 HAADF-STEM images of samples treated with NA60 days plus ageing at 180°C for various times: a 28 h, b 52 h, viewed along a <011>Al direction

Fig.8 Diameter distributions of T1-precipitates in the samples first natural aged for 60 days and then artificial-aged at 180°C for 28 h a, 52 h b

Fig.9 A typical HAADF-STEM images of the NA60 days sample aged at 200°C for 0.5 h, viewed along a <100>Al direction

Fig.10 HAADF-STEM images of precipitates in the samples aged at 200°C for 6, 28 and 72 h, respectively: a-c viewed along a <100>Al direction, d-f viewed along a <011>Al direction

Fig.11 <100>Al HAADF-STEM images showing the evolution of S-precipitates in the NA60 days sample aged at 200°C: a a S2-precipitate observed at 6 h, b a S4-precipitate observed at 28 h, c a large S-precipitate observed at 72 h, in which Li atoms segregate at its interfaces with the Al-matrix

Fig.12 Diameter distributions of T1-precipitates in the samples first natural aged for 60 days and then artificial-aged at 200°C for 6 h a, 28 h b, 72 h c

Fig.13 Aberration-corrected <112>Al HAADF-STEM images revealing different atomic structures of the T1-phase in different stages of its evolution: a the initial stage of GP T1 zone; b a transitional stage from GP T1 zone to T1-precipitate; c the T1-precipitate with 1 unit cell in thickness; d the T1-precipitate with 2 unit cells in thickness