Fig. 1. (a) Age-hardening curves of the Al-Cu alloy aged at 200 °C and 250 °C for different time, respectively, and the inset shows the details of early-stage of aging. (b) Tensile stress-strain curves of samples over-aged at 200 °C for 32 h and 250 °C for 10 h, respectively.

Fig. 2. HAADF-STEM images of the precipitates viewed along the [010]Al direction in the Al-Cu alloy aged at 250 °C for 10 h. (a) Overview of precipitates morphology, in which θ′HTP precipitates are pointed by red arrows; (b) Atomic-resolution image of a typical θ′-phase precipitate with its atomic structure model superimposed. (c) A θ′-phase precipitate with an integer number of unit cells in its thickness. The red and green arrows indicate the Cu and Al atoms at the interfaces, respectively.

Fig. 3. HAADF-STEM image simulation for θ′-phase precipitates covered by the Al-layers above and below. (a) The simulation supercell containing three θ′-precipitates (orange) marked as A, B and C. The incident beam is along the Y-axis. (b) The simulated HAADF-STEM image, in which the image areas A, B, and C correspond to the 3 precipitate portions A, B, and C in (a). (c) The projected atomic structure of the supercell in (a). The inset is a magnified unit cell with arrows indicating the Al atoms projected from the covering Al-matrix rather than from the θ′-phase structure. As such, not all atom columns can be resolved in HAADF-STEM images.

Fig. 4. HAADF-STEM images of the precipitates viewed along the [010]Al direction in the Al-Cu alloy aged at 250 °C for 10 h. (a) Atomic-resolution image of a typical θ′HTP-phase precipitate. (b) A θ′HTP-phase precipitate portion attached to a θ′-phase precipitate.

Fig. 5. HAADF-STEM images of typical θ′HTP-phase precipitates with their structure models attached below, viewed along the [010]Al direction. (a) An early-stage θ′HTP-1-precipitate. (b) A typical θ′HTP-precipitate consisting of both θ′HTP1-phase (Al5Cu) and θ′HTP2-phase (Al4Cu2) lamellas. The insets are the simulated HAADF-STEM images using the structure models indicated below in (b).

Fig. 6. HAADF-STEM image simulations for the θ′HTP-precipitates covered by the Al-layers above and below. (a) The simulation supercell containing three θ′HTP-precipitates (orange) marked as A, B and C. The incident beam is along the Y-axis. (b) The simulated HAADF-STEM image of the supercell containing three θ′HTP1-precipitates (Al5Cu), in which the image areas A, B, and C correspond to the 3 precipitate portions A, B, and C in (a). (c) The simulated HAADF-STEM image of the supercell containing three θ′HTP2-precipitates (Al4Cu2), in which the image areas A, B, and C correspond to the 3 precipitate portions A, B, and C in (a).

Fig. 7. HAADF-STEM images of precipitates in the alloy aged at 250 °C for 5 min, viewed along a <010>Al direction. (a) Overall microstructure. (b) Typical morphology and distribution of the two types of precipitates with colored arrows and frames indicating θ′HTP and θ′ precipitates, respectively. (c) Atomic-resolution images of a typical θ′HTP-phase precipitate with an inserted magnified image of the red framed area indicating its atomic structure, where Al atoms are marked in blue and Cu in red. (d-e) High resolution images of the two framed areas in a precipitate platelet marked in (b).

Fig. 8. The size distributions of precipitates formed at 250 °C for different times. (a) 5 min, (b)10 h, (c) 120 h.

Fig. 9. HAADF-STEM images of the Al-Cu alloy aged at 200 °C: (a, b) low magnification images of precipitates after aging for 16 h (a) and 32 h (b), respectively. (c, d) Atomic-resolution images of a typical θ"-phase precipitate whose unit-cell includes 6 Al and 2 Cu atoms (c), observed in (a), and a θ′HTP-precipitate (d), observed in (b). All images are taken along a < 010>Al direction. Note that the image shown in (c) was taken with the F20 HRTEM microscope that has a much poor resolution as compared with that of the aberration-corrected one.

Fig. 10. HAADF-STEM atomic-resolution images of the Al-Cu alloy aged at 200 °C: (a) A θ′-phase precipitate with the interfaces of “GPI-like structure”, which is associated with a sideward GPI zone, forming a sideward one-unit-cell-thick θ"-precipitate. (b) A θ′-phase precipitate with the same interface structure as that of the θ′HTP-phase, associated with a sideward half-unit-cell-thick θ′HTP-precipitate. All images are taken along a < 010>Al direction.

Fig. 11. (a) Calculated average formation enthalpies per atom (ΔH) of the bulk θ′HTP1, θ′HTP2, θ′ and θ′′ phases, illustratively plotted against their structures. (b) Calculated formation energies of the 4 bulk phases including lattice vibration entropy deduced from phonon calculation, plotted against temperature in oK.

Fig. 12. HAADF-STEM images of two θ′/θ′HTP composite precipitates viewed along a <010>Al direction: (a) sideward composite precipitates and (b) edge-ward composite precipitates, with red arrows point to Cu atoms and green ones to Al atoms.

Fig. 13. Illustration of structure models by the supercells including interfaces, viewed along a <010>Al direction, for the precipitates embedded in the Al-matrix: (a) Al/θ′HTP1/Al, (b) Al/θ′HTP2/Al and (c) Al/θ′/Al. Formation enthalpies (ΔH) calculated with the supercells are list on the right.

Fig. 14. Three possible interfacial structures of the θ-precipitate embedded in the Al-matrix, illustrated by supercells viewed along a <010>Al direction: (a) with the (100)θ′HTP-Cu-Al plane interfaces, (b) with the (100)θ′-Cu plane interfaces and (c) with the (100)GPI-Cu plane interfaces. Their total formation enthalpies (ΔH) and relative total energy differences (ΔE) calculated with the supercells are list on the right.

Fig. 15. An atomic-resolution HAADF-STEM image of two phases with an irregular border beside which a θ′HTP-precipitate at the up-left is being transformed and being merged into a θ′-phase precipitate.

Fig. 16. Schematic illustration for the phase evolution/transformation mechanism from a θ′HTP- precipitate to a θ′-phase precipitate without having to change its morphology, lattice orientation and interfacial structure: (a) overview of the evolution and (b) genetic links between the θ′HTP-2(Al4Cu2)-structure and the θ′(Al2Cu = Al4Cu2)-structure, indicating that only partial Cu and Al columns (marked in yellow ellipses) need opposite shifts of a/2 in the height (z) to accomplish the transformation between the two structures. Cu(0) --> Cu(1/2) and Al(0) --> Al(1/2) represent that the Cu and Al atoms at the zero-height shift (along the viewing direction) by half a lattice vector to the a/2-height. vice versa.