*3.2. DSC Analysis*

An enlarged view of the DSC traces for the experimental alloys in the range of 150 ◦C to 350 ◦C is presented in Figure 1. The exothermic peak at 250 ◦C represents the precipitation of θ' and Ω strengthening phase [24]. The enthalpy changes of the NA0d, NA1d, and NA4d alloys were 1.35, 1.83, and 1.85 W/g, respectively, showing that natural aging could significantly increase the exothermic heating. However, there was no obvious increase in exothermic heat while the increased period of natural aging time from 24 h to 96 h.

**Figure 1.** Differential scanning calorimetry (DSC) profile of different naturally aged A201 alloys.

#### *3.3. Mechanical Properties*

The mechanical properties of the alloys produced under different heat treatment conditions are shown in Table 4. The hardness of the alloy after solution treatment then water quenching was 33.5HRB, which increased gradually following natural aging. The percentage changes [(HNA−H0)/H0 × 100%] of the alloys, NA1d and NA4d, were 40% and 54%, indicating that there was no obvious promotion of the clustering of solute atoms with an increase in the natural aging time from 24 h to 96 h, which was consistent with the electrical conductivity and DSC analysis results.


**Table 4.** Mechanical properties of alloys under different heat treatment conditions.

\* Standard deviations are listed in parentheses.

After T7 tempering, with the precipitation of the strengthening phases θ' and Ω, there was an increase in the hardness of all the alloys, NA0d, NA1d, and NA4d, to 71HRB. Evidenced that the clustering of the solute atoms remained at the same level regardless of whether natural aging was adopted or not, and that lattice distortion would be eliminated after T7 tempering. The percentage changes [(HT7−HNA)/HNA × 100%] of the alloys, NA0d, NA1d, and NA4d, were 112%, 53%, and 30%. Obviously, the hardness did not increase as much with the extension of the natural aging time from 24 h to 96 h. In addition, there was no difference in the yielding stress (YS), ultimate tensile stress (UTS), or elongation (EL) whether natural aging was adopted or not. The YS, UTS, and EL of the three alloys were approximately 320 MPa, 397 MPa, and 3.5%, respectively, after T7 tempering.
