*3.6. Transmission Electron Microscopy (TEM) Characterization*

The Schematic diagram of the calculated diffraction pattern of Ω and θ' strengthening phases along [011]<sup>α</sup> zone axis were shown in Figure 4 [27]. Strengthening phases Ω and θ' have a similar composition to CuAl2, but with a different crystal structure. As shown in Figure 5d, the selected area electron diffraction patterns (SAED) along [011]<sup>α</sup> zone axis evidenced Ω and θ' precipitation strengthening phases exist in A201 alloys after T7 tempered. The bright streaks and (101) diffraction spot of Ω phase and (100) diffraction spot of θ' phase were found at (200)<sup>α</sup> position. The TEM bright field images of the grain boundaries of A201-T7 alloys shown in Figure 5 reveal the presence of precipitation at the grain boundaries. The needle-like strengthening phases Ω and θ' inside the grains can also be seen. The primary precipitation phase of A201-T7 alloys was Ω phases, and minor θ' phases with different orientation can also be observed. It seems that natural aging did not affect the size of precipitation phases, Ω and θ', after T7 heat treatment, just as shown in Figure 5d–f. The maximum length of Ω and θ' phase of three different natural aged alloys (unaged, 24 h, 96 h) were around 100 nm and 70 nm, separately. The NA0d alloy had significant prefer-precipitation during T7 tempering [28]. The oversaturation of solute atoms near the grain boundaries would diffuse to the grain boundaries, forming coarse and discontinuous precipitation phases along with a 200 nm wide precipitation free zone (PFZ) as shown in Figure 5a. However, for NA1d and NA4d, no obvious PFZ could be found at the grain boundaries, as shown in Figure 5b,c. This might be due to the clustering of solute atoms (Cu, Mg), which occurred during natural aging, which would directly transfer to the precipitation phases (Ω and θ') during T7 tempering, thus inhibiting the formation of PFZ.

**Figure 3.** Fracture surface of A201-T7 alloys after slow strain rate testing: (**a**) NA0d sample in air; (**b**) NA1d sample in air; (**c**) NA4d sample in air; (**d**) NA0d sample in salt water; (**e**) NA1d sample in salt water; (**f**) NA4d sample in salt water.

**Figure 4.** Schematic diagram of the diffraction pattern of θ' and Ω strengthening phases along [011]<sup>α</sup> zone in A201-T7 alloy.

**Figure 5.** TEM bright field images and the selected area electron diffraction pattern (SAED) of: (**a**,**d**) Alloy NA0d; (**b**,**e**) Alloy NA1d; (**c**,**f**) Alloy NA4d after T7 tempering.

It is worth noting that the PFZ has the lowest potential compared to CuAl2 phases (θ' and Ω) and α matrix in the 2XX series and 2XXX series Al-Cu-Mg alloys [29]. To lower the galvanic corrosion effect, the inhibition of the formation of PFZ can help prevent SCC [30]. The discussion above is supported by the polarization test and slow strain rate test results, indicating that naturally aged alloys have better SCC resistance than unaged alloys.
