## 3 Results
### Microstructures
#### 3.2.2 TEM Microstructures  
The microstructure of the CR samples was examined in more detail using TEM. Figure 4 shows a typical bright-field TEM micrograph of an as-deformed sample. Large rod-like dispersoids with an average size of 200 nm were found in Figure 4b and identified as the T phase with a composition of Al\({}_{4}\)(CuMn) by energy-dispersive X-ray spectroscopy (EDS) analysis (Figure 4c), which was consistent with the results in the literature [18; 19; 20]. These T phase particles were retained from the ST sample and played a critical role in dislocation accumulation and microstructure refinement during cold rolling [21]. After CR, a high density of dislocations was trapped around the T phase (yellow arrows in Figure 4b) while in areas without the T phase, the dislocation density was much lower and typical dislocation cell structures were formed (Figure 4a). It is worth noting that micro/nanoscale grains (~150-200 nm) appeared in local regions (red arrows in Figure 4b). In addition, apart from the T phase, no other precipitates (such as \(\theta^{\prime}\)-Al\({}_{2}\)Cu) were found in the CR samples. Selected area electron diffraction (SAED) patterns taken along the \(<\)011\(>\) zone axis for CR samples were distorted into discontinuous rings (insert). This indicates that there was a small misorientation between the refined structures. Figure 5 shows the microstructures of the CR-373K (420 min) samples. When the CR samples were aged at 373 K for 420 min, the high dislocation density was recovered. On the other hand, precipitates preferentially nucleated and grew rapidly in areas where the dislocations were heavily tangled. However, the precipitates in severely deformed alloys were difficult to identify owing to their size and high contrast of defects (white frame in Figure 5a). Therefore, high-resolution transmission electron microscopy (HRTEM) was used to characterize the morphology of the precipitates (Figure 5b). As shown in Figure 5b, fine precipitates (red arrows) formed at this stage. The corresponding Fourier transform patterns (insert in Figure 5b) clearly show an additional diffraction pattern at the [200]  
Figure 3: Optical microstructures of the alloys: (**a,b**) PA; (**c,d**) CR; (**e,f**) CR-373K (1200 min). position, indicating that the fine precipitates were \(\theta^{\prime}\) phases, which was consistent with the results in a literature report [22]. In addition, the average width and length of \(\theta^{\prime}\) phases were only approximately 0.2-3 nm and 4-10 nm, respectively. The density of the \(\theta^{\prime}\) phase measured by HRTEM was o \(\sim 6\times 10^{14}\) m\({}^{-2}\). Compared with the CR samples, these samples had more ultrafine grains, the grain boundaries were clearer (red arrows in Figure 5a) and the grain size remained almost unchanged (\(\sim\)150-200 nm). At the same time, the continuity of the SAED patterns (taken along the \(<\)110\(>\) zone axis) had further increased indicating that the misorientation between the refined structures had increased (Figure 5c). Figure 6 shows the microstructure of a CR-373K (1200 min) sample. More ultrafine grains were observed (Figure 6a). Figure 6a also shows the SAED along the \(<\)011\(>\) zone axis for the CR-373K (1200 min) samples (insert). The diffraction patterns developed into an approximately continuous ring, indicating that the orientation angle between adjacent structures increased and the grain size decreased. Notably, nanoscale grains with an average size of approximately 50 nm appeared (red arrows in Figure 6b) near some of the particles. According to statistics, the average grain size of the  
Figure 4: **(a)** Bright-field TEM micrograph of CR samples showing typical dislocation cell structures in the aluminum matrix; **(b)** corresponding high-magnification TEM micrograph showing the high dislocation density and subgrain morphologies and boundaries; **(c)** EDS spectra for the large rod-like dispersoids in **(b)**; **(d)** the SAED pattern with the \(<\)011\(>\) zone axis.