## 2 Materials and Methods  
The pure aluminum ingot (99.8%) was placed in a 750 \({}^{\circ}\)C resistance furnace crucible. After melting, pure Zn (99.9%), pure Mg (99.9%), aluminum zirconium master alloy (Al-10 Zr), etc., were added according to the alloy design. After the master alloy (Al-1.12 Sc) had been fully melted and homogenized, it was degassed with pure argon for 30 min, left to stand for 10 min, deslagged, and cast into a mold (125 mm \(\times\) 100 mm \(\times\) 25 mm) preheated to 350 \({}^{\circ}\)C in advance.  
The compositions of the experimental alloy samples were analyzed by the optical emission spectrometer (OES). As shown in Table 1, samples were designated as alloy A(0.1 Zr): (Al-4.47 Zn-1.45 Mg-0.13 Zr) and alloy B(0.05 Sc): (Al-4.46 Zn-1.43 Mg-0.05 Sc). The main composition was in compliance with the AA7005 component specifications [20]. The cast alloys were subjected to two different homogenization heat treatments: the one-stage homogenization (470 \({}^{\circ}\)C) for 24 h, and the two-stage homogenization (410 \({}^{\circ}\)C) for 3 h and then (470 \({}^{\circ}\)C) for 20 h. The homogenized alloys were hot-rolled at 420 \({}^{\circ}\)C (25 mm to 3 mm, deformation 88%), annealing heat-treated at 420 \({}^{\circ}\)C for 2.5 h, and then cold-rolled (3 mm to 2.1 mm, cold worked 30%). Finally, the alloys were subjected to the T6 heat treatment. The sequence was as follows: the solution treatment (470 \({}^{\circ}\)C) for 1 h; water quenching [21] and then artificial aging (120 \({}^{\circ}\)C) for 24 h [21].  
An optical microscope (OM, Olympus BX60M), a field emission scanning transmission electron microscope (TEM, ULTRA-HRTEM, JEOL JEM-ARM200 FTH) and a field emission gun scanning electron microscope (Thermal Field Emission Scanning Electron Microscope (FEG-SEM) + electron backscattered diffraction (EBSD), JEOL JSM-7800F Prime) were utilized to observe the microstructures. Image analysis software (ImageJ) was used to analyze the grain size by the intercept method, and the dispersed grain size of the TEM images. The composition of the dispersed grains was analyzed with an Energy-dispersive X-ray spectroscopy (EDS). An EBSD-assisted analysis software (AZtec) was applied to analyze the diameters of the grains, the average grain area, and grain boundary angle distribution. EBSD-assisted analysis software (Channel5 Tango) was applied to quantify the microstructures of the alloy recrystallization (grain boundary angle \(>\)15\({}^{\circ}\)).  
A high temperature differential scanning calorimeter (HTDSC, NETZSCH, STA 449 F3) was used to analyze the phase change of the T6 test strip, weighing about 20 mg at 50 \({}^{\circ}\)C-630 \({}^{\circ}\)C (The heating rate was 10 \({}^{\circ}\)C/min), and an Origin Pro was utilized to calculate the DSC curves.  
The hardness of the alloys was measured with a Vickers hardness machine (HV). The tensile test of the T6 alloy was measured with a 10-ton MTS(metre-tonne-second) closed loop hydraulic servo control universal test machine at room temperature and at a constant strain rate (1.3 \(\times\) 10\({}^{-3}\) s\({}^{-1}\)). The constant extension rate of the tensile testing was 0.2 mm/min from the beginning. When the strain was stretched to 1%, the rate was increased to the final rate of 2 mm/min until the test piece broke. The test piece specifications are shown in Figure 1[22].