Investigation of the Quenching Sensitivity of the Mechanical and Corrosion Properties of 7475 Aluminum Alloy

Abstract

Based on end-quenching experiments combined with conductivity, hardness testing, and microstructural characterization, the quenching sensitivity of the mechanical and corrosion properties of 7475 aluminum alloy was investigated. The study revealed that as the quenching rate decreased, both the mechanical properties and exfoliation corrosion resistance exhibited increased quenching sensitivity. With the quenching rate decreasing from 31.9 °C/s to 2.5 °C/s, the conductivity increased by 4.1%IACS, the hardness decreased by 31%, the exfoliation corrosion grade transitioned from EC to ED, and the maximum exfoliation corrosion depth increased from 237 μm to 508 μm. As the quenching rate decreased, the η phase sequentially precipitated at recrystallized grain boundaries (RGBs), E phase particles, and subgrain boundaries (SGBs), while the T phase primarily precipitated on E phase particles. Furthermore, the significant precipitation of η and T phases led to a notable reduction in the quantity of age-precipitated phases, an increase in their size, and poor coherency with the matrix, resulting in decreased mechanical properties and a higher quenching sensitivity of the mechanical performance. Meanwhile, with the reduction in quenching rate, the size and spacing of grain boundary precipitated phases increased, the Zn and Mg contents of grain boundary precipitated phases increased, and the Precipitation Free Zone (PFZ) widened, leading to decreased exfoliation corrosion resistance and higher quenching sensitivity of the exfoliation corrosion performance.

1. Introduction

Al-Zn-Mg-Cu aluminum alloys, due to their high strength resulting from solid solution, quenching, and aging processes, are widely utilized as structural materials, being particularly extensively employed in the aerospace industry [1,2]. Quenching is a crucial step in the preparation of high-strength aluminum alloy materials. Rapid quenching leads to a high degree of supersaturated solid solution in the alloy, and subsequent aging results in the precipitation of numerous strengthening phases, thus achieving high strength. However, excessively high quenching rates often lead to elevated residual stresses [3]. Reducing the quenching rate can mitigate the residual stresses in the alloy, but this may result in decreased alloy performance. This phenomenon, where the alloy’s performance diminishes after aging as the quenching rate decreases, is referred to as quenching sensitivity [4].

In general, for high-strength aluminum alloys, as the quenching rate decreases, the quantity and size of quenching-precipitated phases increase. This leads to a reduction in the solute atom concentration and vacancy concentration in the alloy after quenching. Subsequently, the number of precipitations strengthening phases decreases upon aging, resulting in a decline in mechanical properties and an increase in the quenching sensitivity of the mechanical performance [5,6,7,8]. Liu Shengdan et al. [9] found that in 7055 aluminum alloy, the quenching sensitivity of mechanical performance increases as the quenching rate decreases. Li Peiyue et al. [10] studied the quenching sensitivity of 7050 aluminum alloy using spray end-quenching and discovered that, as the quenching rate decreases, mechanical properties decrease, and quenching sensitivity increases. Liu et al. [11] found that the quenching sensitivity of the mechanical performance of 7085 alloy increases as the quenching rate decreases. Zheng Pengcheng [12] and Ma Zhimin et al. [13] found that the quenching sensitivity of the mechanical performance of 7136 aluminum alloy increases as the quenching rate decreases.

Variations in the quenching rate also impact the size, composition, spacing of grain boundary precipitated phases, and width of the Precipitation Free Zone (PFZ), thereby exerting complex effects on the alloy’s corrosion resistance. However, there is still some controversy regarding its influence on the quenching sensitivity of localized corrosion performance. Many studies have indicated that as the quenching rate decreases, the exfoliation corrosion resistance of the alloy diminishes and the quenching sensitivity of the exfoliation corrosion performance increases. Marlaud et al. [14] found that the exfoliation corrosion resistance of 7449-T7651 alloy decreases with the decreasing quenching rate. Song et al. [15] observed an increase in the exfoliation corrosion sensitivity of the AA7050 alloy with a decreasing quenching rate. Li Dongfeng et al. [16] discovered that as the quenching rate decreased from 2160 °C/min to 100 °C/min, the exfoliation corrosion grade of Al-5Zn-3Mg-1Cu alloy sheets gradually shifted from P grade to ED grade. Liu et al. [17] revealed that the maximum exfoliation corrosion depth of AA7055 alloy gradually increases as the quenching rate decreases, indicating an ascending trend in exfoliation corrosion sensitivity. Ma et al. [13] also found an increase in the exfoliation corrosion sensitivity of 7136 aluminum alloy with a decreasing quenching rate.

In summary, it is evident that high-strength aluminum alloys exhibit not only quenching sensitivity in terms of mechanical performance, but also significant quenching sensitivity in their corrosion properties. Therefore, this study utilizes end-quenching experiments in conjunction with Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Scanning Transmission Electron Microscopy (STEM) to systematically investigate the types, nucleation sites, sizes, and morphologies of precipitated phases under different quenching rates. The study also discusses the precipitation behavior of these quenching-induced phases and their influence on the quenching sensitivity of mechanical and exfoliation corrosion performance. This research aims to provide a better understanding of the quenching precipitation behavior and the mechanisms behind quenching sensitivity in high-strength aluminum alloys.

2. Experiment

The experimental material used was a hot-rolled thick plate provided by a certain company. The casting temperature was 700 °C, followed by a 24 h homogenization heat treatment at 465 °C. The hot rolling temperature was 390 °C, with a deformation amount of 90%. The chemical composition (wt%) of the plate is shown in

Table 1. Chemical compositions of the alloy (wt%).

Zn	Mg	Cu	Cr	Fe	Si	Al
5.6	2.5	1.6	0.25	0.10	0.05	Bal

. Cross-sectional samples with dimensions of 25 mm × 25 mm were cut from the surface of the hot-rolled plate with a length of 125 mm for subsequent solution heat treatment and end-quenching. The samples were heated to 470 °C and held in an air furnace (TPS, New Columbia, PA, USA) for 2 h. They were then transferred to an end-quenching device [18] and rapidly water-cooled by spraying water onto the groove end until reaching room temperature. The quenching water temperature was approximately 20 °C. The quenched samples were subsequently subjected to artificial aging in an oil bath at 120 °C for 24 h. After aging, half of the samples were polished using sandpaper and subjected to hardness testing. The test was conducted using three parallel samples. Hardness measurements were taken along the rolling direction at 5-mm intervals, starting from the water-cooled end. The hardness tests were conducted using an HV-10B Vickers hardness tester (Suzhou Nanguang Electronic Technology, Suzhou, China) with a load of 3 kg. For exfoliation corrosion testing, slices (2 mm thick) were cut from the aged samples. The test was conducted using three parallel samples. The testing was performed following the GB/T 22639-2008 standard [19]. The area-to-volume ratio of the solution was 25 cm²/L, and the testing temperature was maintained at (25 ± 2) °C. After 48 h of corrosion, the samples were evaluated according to the standard using an EXCO solution (4 mol/L NaCl + 0.5 mol/L KNO₃ + 0.1 mol/L HNO₃). Metallographic samples were prepared from different locations and observed for corrosion using an XJP-6A metallographic microscope (Suzhou Hongtai Instrument, Suzhou, China). After coarse grinding, fine grinding, and polishing, the metallographic samples were observed, and the corrosion depth was measured under the microscope. Additionally, samples of the same size were taken, and thermocouples were embedded at different distances (3, 13, 23, 53, 78, and 98 mm) from the water-cooled end. Cooling curves were recorded during the end-quenching process at these positions, and the average cooling rates were calculated in the temperature range of 185 to 415 °C [20]. The calculated average cooling rates for the respective positions were 31.9, 17.5, 8.4, 3.3, 2.9, and 2.5 °C/s, as shown in Figure 1. Thin slices (2 mm thick) were taken and subjected to water quenching at room temperature after solution treatment, resulting in a corresponding quenching rate of 960 °C/s.

After aging, samples were extracted from the end-quenched specimens for microstructural analysis. The samples were first thinned by grinding to a thickness of approximately 60–80 μm. Circular discs with a diameter of 3 mm were then punched out. Thinning was further performed using dual-jet polishing in a solution containing 80% methanol and 20% nitric acid. The electrolyte temperature was controlled at around −25 °C using liquid nitrogen. Subsequently, the precipitated phases from the quenched samples were observed using a Tecnai G2 F20 (FEI, Eindhoven, The Netherlands) TEM and a Titan G2 60–300 (FEI, Eindhoven, The Netherlands) STEM. JMatPro 8.0 (Sente Software, London, UK) software was employed to calculate Time-Temperature-Transformation (TTT) curves and Continuous Cooling Transformation (CCT) curves.

3. Results

3.1. Electrical Conductivity and Hardness Tests

Figure 2 shows the conductivity curve. From the graph, it is evident that the alloy’s conductivity increases with an increase in the distance from the water-cooled end. When the distance from the end is less than 60 mm, the conductivity rapidly increases with the distance, followed by a smaller increment in conductivity as the distance further increases. As the distance from 3 mm to 98 mm increases, the conductivity rises from 28.9 %IACS to 33 %IACS, resulting in a conductivity difference of 4.1 %IACS between the two ends. The conductivity in the quenched state serves as a reliable indicator of the quenching-induced precipitation behavior.

Figure 3 represents the hardenability curve. From Figure 3a, it is evident that as the distance from the water-cooled end increases, the hardness gradually decreases. Within the region where the distance is less than 63 mm, the hardness value rapidly decreases with an increase in the distance, while beyond 63 mm, the change in hardness value with distance is relatively small. To further study the variations in hardness, based on the hardenability curve shown in Figure 3a, the retained hardness values were calculated, leading to the retained hardness curve depicted in Figure 3b. The trend of this curve is consistent with that of Figure 3a. Beyond a distance of 63 mm, the retained hardness value stabilizes around 70%, indicating a reduction in hardness of approximately 30%. At a distance of 98 mm, the hardness decreases by 31%.

3.2. Exfoliation Corrosion

Figure 4 presents macroscopic images of the end-quenched samples immersed in EXCO solution for different durations. The regions farther from the water-cooled end exhibit a higher quantity of bubbles and more intense reactions. After a 2-h immersion, the sample surfaces show no significant corrosion. After 6 h of immersion, slight pitting corrosion is observed on the sample surface (Figure 4). In the areas beyond 23 mm from the water-cooled end, the corrosion grade is classified as PB. As the immersion time increases, the corrosion severity intensifies. After 12 h of immersion, significant corrosion is evident on the sample surface. Noticeable exfoliation is observed in the regions far from the water-cooled end, resulting in an EB corrosion grade. Extensive corrosion products are generated (Figure 4b). With prolonged immersion, severe delamination and exfoliation occur on the surface. After 24 h of immersion, exfoliation corrosion products become prominent, particularly in the regions far from the water-cooled end (Figure 4c). In areas with distances less than 23 mm, the corrosion grade is categorized as EA, while in regions beyond 23 mm, the grade is classified as EC. Following 48-h immersion, severe exfoliation corrosion is evident, with more corrosion products in positions farther from the water-cooled end. A substantial amount of corrosion products detaches, and the corrosion extends into the deeper metal interior. In regions beyond 23 mm, the corrosion grade is classified as ED (Figure 4d).

According to the GB/T 22639-2008 standard, the exfoliation corrosion degree of the samples was rated. It is evident from the graph that corrosion becomes increasingly severe with prolonged immersion. Exfoliation corrosion is more pronounced in regions with lower quenching rates than in those with higher quenching rates. Beyond 23 mm from the water-cooled end, the severity of exfoliation corrosion is notably high, with minimal difference in corrosion grades. After 48 h of immersion, samples with quenching rates above 8.4 °C/s are rated as EC. Samples with quenching rates below 8.4 °C/s are rated as ED.

Figure 5 displays cross-sectional metallographic images of the end-quenched samples after exfoliation corrosion at different quenching positions. It is evident from the images that the corrosion depth of the samples increases with a decrease in the quenching rate. Lower quenching rates lead to more pronounced exfoliation corrosion. When the quenching rate is 8.4 °C/s, the sample surfaces exhibit typical layered exfoliation corrosion morphology. The expansion of exfoliation corrosion products creates stress that lifts the metal layers one by one, resulting in severe exfoliation. The maximum and average exfoliation corrosion depths are shown in Figure 6b. From the graph, it can be observed that as the quenching rate decreases, both the maximum and average exfoliation corrosion depths increase. Moreover, lower quenching rates correspond to a greater increase in corrosion depth. At a quenching rate of 31.9 °C/s, the maximum and average exfoliation corrosion depths are 237 μm and 203 μm, respectively. At a quenching rate of 2.5 °C/s, the maximum and average exfoliation corrosion depths are 508 μm and 418 μm, respectively.

3.3. Microstructure

Figure 7 presents TEM images of samples quenched at a rate of 960 °C/s. As observed in Figure 7a, a significant number of dispersed particles are present within the aluminum matrix. These particles exhibit irregular shapes and non-uniform sizes, ranging from circular and triangular to elongated forms, with dimensions ranging from 50 to 150 nm. Notably, these particles are non-coherent with the matrix [8]. Energy-dispersive X-ray spectroscopy (EDS) analysis results (Figure 7b) indicate that these particles consist of 81.5% Al, 2.4% Zn, 10.8% Mg, 1.8% Cu, and 3.5% Cr (atomic fractions). It is likely that the Cr-containing dispersed particles are associated with the E phase (Al₁₈Cr₂Mg₃).

Figure 8 displays TEM images of samples quenched at a rate of 31.9 °C/s. In Figure 8a, it can be observed that quenching precipitation occurs at recrystallized grain boundaries (RGBs), while no quenching precipitation is observed at subgrains (SGs) and their boundaries. Figure 8b reveals that the intragranular η phase nucleates and precipitates on E phase particles, with η phase sizes ranging from 100 to 150 nm. Simultaneously, age-related precipitates can be observed within the grain. Figure 8c demonstrates the fine and uniform dispersion of age-related precipitates within the grain’s matrix. These precipitates exhibit spherical and rod-like morphologies. For a more detailed examination of the age-related precipitates, high-resolution TEM was employed to observe from the <011> direction. The HRTEM images (Figure 8d,e) reveal that the size of the age-related η’ phases is 5–10 nm, indicating a relatively good coherence with the matrix. At this stage, the age-strengthening effect is notable, resulting in higher hardness.

Figure 9 displays HAADF-STEM images of samples quenched at a rate of 8.4 °C/s. In Figure 9a, it is evident that prominent quenching precipitates are present at RGBs, while SGs and their boundaries also exhibit quenching precipitates. Within the matrix, a substantial amount of quenching precipitates, identified as the η phase, can be observed. These precipitates exhibit plate-like shapes of uneven sizes, with some reaching lengths of up to 400 nm. Additionally, hexagonal T-phase precipitates with dimensions ranging from 100 to 200 nm are present within the matrix. Furthermore, numerous smaller quenching precipitates are distributed within the matrix, as shown in Figure 9b. Based on EDS analysis, it is revealed that the η phase primarily contains 75.7% Al, 11.5% Zn, 9.6% Mg, and 3.2% Cu (at%). The T phase mainly comprises 86.1% Al, 5.8% Zn, 6.3% Mg, and 1.8% Cu (at%).

Figure 10 presents TEM images of samples quenched at a rate of 3.3 °C/s. In Figure 10a, a significant amount of quenching precipitates, identified as the η phase, can be observed within the grains. These precipitates exhibit plate-like shapes of considerable size, with some reaching lengths of up to 500 nm. Additionally, hexagonal T phase precipitates are also present. In Figure 10b, it can be observed that a substantial quantity of η phase quenching precipitates exists within SGs and at their boundaries. The sizes of these quenching precipitates vary, with some being larger and others being much smaller.

Figure 11 depicts TEM images of samples quenched at a rate of 2.5 °C/s. In Figure 11a, an abundance of η and T phases are observed within the grains. Simultaneously, numerous η phase precipitates are observed within SGs and at their boundaries, displaying uneven sizes and being distributed along the deformation direction, as shown in Figure 11b. Observations at higher magnification in Figure 11c reveal age-related precipitates. It is evident that a noticeable Precipitation-Free Zone (PFZ) forms around a wider area of the quenching precipitates, where age-related precipitates are absent. Age-related precipitates near the quenching precipitates and grain boundaries are relatively larger and less abundant. HRTEM observations from the <011> direction (Figure 11d) indicate that the size of the age-related η’ phase is 10–20 nm, indicating relatively poor coherence with the matrix. At this stage, the age-strengthening effect is relatively weak, resulting in lower hardness and a significant hardness drop of 31% in this region.

Figure 12 illustrates the HAADF-STEM images of samples quenched at a rate of 2.5 °C/s. In Figure 12a, large-sized quenched precipitates are observed at RGBs, with sizes around 500 nm. Some of these quenched precipitates have been corroded, leaving behind dark features, indicating that the precipitates at grain boundaries are susceptible to corrosion. Precipitates are also observed at subgrain boundaries (SGBs), with sizes around 250 nm. Within the grains, in addition to the larger-sized quenched precipitates, a multitude of smaller-sized quenched precipitates are observed. From the higher magnification image in Figure 12b, it is evident that quenched precipitates of η phase are precipitating on E-phase particles, resulting in larger and less abundant age-related precipitates around them. A distinct Precipitation-Free Zone (PFZ) is observed surrounding the η phase, leading to a significant reduction in mechanical performance.

Figure 13 depicts the composition of grain boundary precipitates at different quenching rates. As shown in the figure, with decreasing quenching rates, the content of Zn, Mg, and Cu in grain boundary precipitates increases. Zn content exhibits the most rapid increase, followed by Mg, while the increase in Cu content is comparatively slower. At a quenching rate of 31.9 °C/s, the grain boundary precipitates contain lower levels of Zn, Mg, and Cu. In contrast, at a quenching rate of 2.5 °C/s, both Zn and Mg content show significant increments, and there is also a notable increase in Cu content.

3.4. TTT and CCT Curves

Figure 14 illustrates the TTT and CCT curves. In Figure 14a, the curve for the η phase is on the left side, while the curve for the T phase is on the right side. The nose temperatures for the η phase and T phase are 332.1 and 301.2 °C, respectively, corresponding to transformation times of 20.6 and 47.1 s, respectively. From the CCT curve in Figure 14b, it can be observed that during the decomposition of the supersaturated solid solution, the η phase precipitates first, followed by the T phase.

4. Discussion

The quenched precipitates and their nucleation sites at different quenching rates are summarized in

Table 2. Quenching precipitates and nucleation positions at different quenching rates.

Quenching Rate (°C/s)	RGBs	E Particle	SGBs
960	—	—	—
31.9	η	η	—
8.4	η	η, T	η
3.3	η	η, T	η
2.5	η	η, T	η

. It is evident from the table that as the quenching rate decreases, both the types of quenched precipitates and their nucleation sites increase. The quenched precipitates include the η phase and the T phase. The η phase preferentially precipitates at RGBs s, while it also precipitates on the E phase particles as well as at SGBs. Combined with Figure 14, it can be inferred that the η phase precipitates first during the quenching process, followed by the T phase.

The sizes of the η phase precipitates at different quenching rates are presented in

Table 3. Size of the η phase at different quenching rates.

Quenching Rate (°C/s)	RGBs	E Particle	SGBs
960	—	—	—
31.9	151.1 ± 55.9	128.8 ± 48.7	—
8.4	235.7 ± 88.2	185.6 ± 78.1	119.4 ± 57.3
3.3	345.2 ± 102.6	240.7 ± 90.5	167.7 ± 69.4
2.5	526.1 ± 192.5	369.8 ± 138.6	251.4 ± 102.5

. As indicated in the table, the size of the η phase precipitates gradually increases with the decreasing quenching rate. The largest η phase precipitates are observed at RGBs, followed by those on the E phase particles, while the η phase precipitates at SGBs are smaller in size.

During the quenching cooling process, the quenched precipitation phase at the grain boundaries is primarily the η phase, which mainly contains Zn, Mg, and Cu elements. The diffusion rates of Zn, Mg, and Cu in the Al solid solution are in the order Zn > Mg > Cu [21,22]. Generally, the nucleation activation energy of the η phase is not very high. At room temperature, the diffusion rate of Zn in the Al matrix is 1.64 × 10⁻¹² cm²/s, while that of Mg is 7.33 × 10⁻¹⁵ cm²/s. The activation energy for aluminum self-diffusion is 142.8 kJ/mol, for Mg in aluminum it is 135.1 kJ/mol, and for Zn in aluminum it is 106.1 kJ/mol. Therefore, it can be inferred that with increasing temperature, the diffusion of Zn atoms in the system becomes relatively easier. The η phase preferentially nucleates at the RGBs with high interfacial energy. Since the grain boundaries serve as fast diffusion paths for solute atoms such as Zn and Mg, the η phase preferentially nucleates and grows at the grain boundaries [23,24]. Subsequently, nucleation occurs on E-phase particles, and finally at the SGBs. During the alloy quenching process, a significant amount of η phase preferentially precipitates, consuming a substantial number of Zn atoms. This leads to a low local concentration of Zn atoms. Meanwhile, the high interfacial energy of the E phase particles provides a favorable site for the nucleation of the T phase, resulting in the precipitation of the T phase (Figure 9).

During quenching, precipitation occurs due to desaturation, leading to a reduction in the supersaturation of the solid solution and a weakening of the lattice distortion, thereby causing an increase in conductivity. Locations closer to the water-cooled end experience a higher quenching cooling rate, resulting in the formation of a highly supersaturated solid solution and significant lattice distortion, which leads to lower conductivity. In contrast, positions farther from the water-cooled end experience a lower quenching cooling rate, causing a significant amount of equilibrium phase to desaturate, resulting in a lower supersaturation of the solid solution and reduced lattice distortion, hence leading to higher conductivity.

As the quenching rate decreases, quenching-induced phases precipitate at both grain boundaries and within the grains, with their sizes gradually increasing. The precipitation of η and T phases consumes the Zn and Mg solute atoms in the matrix, resulting in a reduction in solute atom concentration and vacancy concentration in the alloy after quenching. Subsequently, during aging, the precipitation-strengthening phases exhibit reduced quantities and increased sizes. These phases exhibit relatively poor lattice compatibility with the matrix (Figure 11c,d), leading to weaker aging strengthening effects. Furthermore, the quenched precipitation phases at the grain boundaries and within the grains can continue to grow by absorbing surrounding solute atoms during the aging process, forming broader PFZs (Figure 12b). The broadened PFZs further contribute to a reduction in mechanical properties. Consequently, the quenching sensitivity of the mechanical properties increases as the quenching rate decreases.

As the quenching rate decreases, the alloy’s resistance to exfoliation corrosion diminishes. Considering the observed microstructural features (Figure 8a, Figure 9, Figure 12a and Figure 13), this phenomenon is mainly attributed to the increased size and spacing of grain boundary precipitates, elevated Zn and Mg content in the grain boundary precipitates, and broadening of the PFZs with decreasing quenching rates. Prior investigations have indicated that the potential of the grain boundary η phase is −1.05 V, PFZ is −0.85 V, and the potential of the matrix within grains is −0.75 V [25]. In corrosive environments, the differing corrosion potentials of various phases in the grain boundary regions of the matrix solid solution can lead to galvanic corrosion, resulting in along-grain boundary corrosion in high-strength aluminum alloys. The formation of corrosion microcells between grain boundary precipitates (η phase), PFZs, and phases within the grain can establish anodic dissolution sites. The grain boundary η phase acts as the anodic phase and is preferentially dissolved, and PFZs often function as anodic sites that are susceptible to corrosion, thereby creating pathways for along-grain boundary anodic dissolution channels [26,27,28]. As the quenching rate decreases, coarser η phase precipitates form at grain boundaries, exhibiting increased and non-uniform sizes, along with elevated Zn and Mg content in the precipitates. Simultaneously, PFZs broaden, further promoting grain boundary dissolution. This corrosion process generates substantial corrosion products during corrosion, leading to exfoliation corrosion and ultimately reducing the alloy’s resistance to exfoliation corrosion.

5. Conclusions

(1) As the quenching rate decreases, the electrical conductivity increases, and the quenching sensitivity of both the mechanical properties and exfoliation corrosion increases. With a decrease in the quenching rate from 31.9 °C/s to 2.5°C/s, the electrical conductivity increases by 4.1 %IACS, hardness decreases by 31%, and the exfoliation corrosion grade transitions from EC to ED, with the maximum exfoliation corrosion depth increasing from 237 μm to 508 μm.

(2) With a reduction in the quenching rate, the variety, nucleation sites, size, and quantity of quenching-induced precipitates increase. During quenching, η phase precipitates first, followed by the precipitation of T phase. η phase predominantly nucleates at RGBs, on E phase particles, and at SGBs, while T phase primarily forms on E phase particles.

(3) Decreasing quenching rates lead to abundant precipitation of η and T phases within the matrix and along grain boundaries. This results in a significant reduction in the quantity and an increase in the size of precipitates during aging. These precipitates also exhibit poorer coherency with the matrix, and a PFZ forms, contributing to reduced mechanical properties and heightened sensitivity to quenching-induced variations. Moreover, lower quenching rates lead to larger precipitate sizes and spacings at grain boundaries, increased Zn and Mg content in grain boundary precipitates, and broadened PFZs, all contributing to diminished resistance against exfoliation corrosion and heightened sensitivity to exfoliation corrosion performance.