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Article

Microstructure and Properties of 7050-T74 Aluminum Alloys with Different Zn/Mg Ratios

by
Daihong Xiao
1,*,
Zongzheng He
2 and
Lanping Huang
1
1
National Key Laboratory of Science and Technology on High-Strength Structural Materials, Central South University, Changsha 410083, China
2
Institute of Materials and Technology, Beijing 266699, China
*
Author to whom correspondence should be addressed.
Metals 2024, 14(11), 1226; https://doi.org/10.3390/met14111226
Submission received: 21 August 2024 / Revised: 23 October 2024 / Accepted: 24 October 2024 / Published: 27 October 2024
(This article belongs to the Section Metal Casting, Forming and Heat Treatment)
Browse Figures
Versions Notes

Abstract

:
Aluminum alloy 7050-T74 with varying zinc-to-magnesium (Zn/Mg) mass fractions was synthesized using melt casting and hot extrusion techniques. This study investigated the influence of different Zn/Mg ratios on the microstructure, mechanical properties, and corrosion resistance of the alloy. Light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile testing, and corrosion testing were employed as analytical methods. The findings indicate that as the Zn/Mg ratio increases from 2.36 to 3.84, the proportion of low-angle boundaries (LABs) within the alloys initially rises and then decreases, achieving a balance between high strength and favorable elongation. Specifically, at a Zn/Mg ratio of 2.72, the alloy exhibits a tensile strength of 641 MPa, a yield strength of 609 MPa, and an elongation of 10.1%. Additionally, increasing the Zn/Mg ratio to 2.90 slightly reduces intergranular corrosion resistance while enhancing exfoliation corrosion resistance.

1. Introduction

The Al-Zn-Mg-Cu (7xxx) series alloys are widely used in the aerospace and transportation sectors due to their superior strength among deformed aluminum alloys, while also exhibiting commendable plasticity, stress corrosion resistance, and fatigue resistance [1,2]. Key performance indicators for assessing the overall efficacy of Al-Zn-Mg-Cu alloys include strength, ductility, corrosion resistance, and damage tolerance. These properties can be effectively tailored through methods such as heat treatment, thermomechanical processing, and compositional design [2]. Notably, the strength of aluminum alloys is significantly enhanced after solution heat treatment and artificial aging (T6 temper); however, 7xxx series alloys remain highly susceptible to stress corrosion [3,4]. In contrast, while the T74 aging process (two-step over-aging) reduces certain strength attributes, it provides superior overall performance under stress conditions compared to T6-aged aluminum alloys [5]. Despite their exceptionally high strength, 7xxx series aluminum alloys continue to face challenges related to inadequate corrosion resistance and limited elongation. Copper content also has a significant impact on the microstructure and properties of 7xxx aluminum alloys [6,7,8].
A widely recognized approach to enhancing the overall performance of Al-Zn-Mg-Cu series aluminum alloys involves modifying their elemental compositions. Zinc (Zn) and magnesium (Mg) are the primary strengthening elements within these alloys [9,10,11]. After aging treatment, these elements precipitate strengthening phases, specifically η′ (MgZn2) and T (Al2Mg2Zn3), which significantly contribute to the strength of material. However, this enhancement in strength may come at the expense of plasticity, toughness, and corrosion resistance. The Zn/Mg mass ratio plays a crucial role in determining the strength, hardness, elongation, and corrosion resistance of 7xxx series aluminum alloys. The inclusion of Zn and Mg in Al-Zn-Mg-Cu series aluminum alloys is pivotal in shaping their mechanical properties and corrosion resistance. These elements influence precipitate phase formation, the precipitation hardening process, and microstructural evolution. An optimal Zn/Mg ratio enhances the strength of 7xxx aluminum alloys, but an excess of Zn can compromise both corrosion resistance and toughness. Therefore, precise control of the Zn/Mg ratio is essential in the design of high-strength Al-Zn-Mg-Cu series aluminum alloys to achieve an ideal balance between strength, toughness, and corrosion resistance. Chen et al. [12]. investigated the effects of Zn/Mg ratio on the performance and microstructure of Al-Zn-Mg alloys [11,12]. They found that as the Zn/Mg ratio decreases, both the tensile and yield strengths of the alloy increase, the amount of matrix precipitation rises, and the width of the precipitate-free zone (PFZ) narrows. Additionally, Luo et al. reported that slowly quenched aluminum alloys with a lower Zn/Mg ratio (mass %) exhibit a quenching sensitivity of 7–11%, which is higher than that of alloys with a higher ratio [13].
The aluminum alloy 7050, primarily composed of Al-(5.7-6.7) Zn-(1.9-2.6) Mg-(2.0-2.6) Cu-(0.08-0.15) Zr (mass fraction) represents an advancement over alloy 7075 through modifications in alloying element proportions and the addition of trace elements, such as zirconium (Zr), Ti, and scandium (Si) [14,15,16]. This alloy not only enhances strength and toughness but also improves corrosion resistance. Alloy 7050 is characterized by its high strength and exceptional resistance to stress corrosion cracking and exfoliation corrosion, making it particularly suitable for critical load-bearing components in aircraft structures, especially in environments that impose high stress and harsh conditions. However, relatively little research has been conducted on the influence of the Zn/Mg ratio on the microstructure and properties of 7050 aluminum alloy. Previous studies have shown that T74 aging treatment can significantly enhance the alloy’s corrosion resistance without compromising its tensile properties [17,18]. In this study, we systematically examined the effects of the Zn/Mg ratio on the processing deformation microstructure, aging microstructure, tensile properties, and corrosion properties of 7050 aluminum alloy, with particular emphasis on the T74 aging treatment.

2. Experimental

The experimental alloy was formulated with varying Zn/Mg ratios, adhering to the compositional range established for the 7050 alloy. The nominal composition of the alloy is shown in Table 1. The alloys were prepared through melting and casting by vacuum melting. The raw materials, including pure aluminum, pure magnesium, pure zinc, and master alloys of Al-50Cu and Al-5Zr, were placed in an electric resistance furnace and heated to at 760 °C. The molten metal was then poured into a cylindrical mold with a diameter of 110 mm. Following this, the ingots were subjected to homogenization annealing for 24 h at 450–470 °C. The homogenized ingots were then processed into round billets with a diameter of 98 mm and hot extruded into plates measuring 12 mm × 48 mm in cross-section (Figure 1). The plates were subjected to a solution treatment for 2 h at 470 °C, followed by water quenching. Finally, the samples underwent T74 treatment, which involved aging for 5 h at 120 °C, followed by a re-aging process for 14 h at 160 °C. The chemical composition of the alloy was analyzed by plasma spectroscopy, as shown in Table 2. It is noted that during the smelting process, the actual mass fraction of alloying elements may vary due to the partial loss of magnesium. As a result, in the subsequent experimental results and discussions, we will analyze the actual Zn/Mg ratio present in the alloy.
The tensile properties were evaluated using an Instron 3369 (Instron, Boston, MA, USA) universal testing machine equipped with a contact extensometer, at a displacement rate of 2 mm/min. The tensile specimens, measuring 80 × 6 × 2 mm (Figure 1), were extracted along the extruded direction of the plate. For each testing condition, three tensile specimens were analyzed. Intergranular corrosion (IGC) and exfoliation corrosion (EXCO) tests were conducted according to the corrosion testing standards for 7xxx series aluminum alloys, as outlined in GBT7998-2005 [19] and GBT22639-2008 [20], respectively. These standards are based on extensive experimental data and practical experience. Conducting tests according to national standards minimizes errors and enhances both the accuracy and reliability of the results. Although different corrosion conditions may lead to varying outcomes, these results alone cannot definitively determine the corrosion performance of aluminum alloys. The intergranular corrosion test was conducted by immersing the specimens in a solution consisting of 57 g/L NaCl and 10 mL/L H2O2 at a temperature of 35 °C for a duration of 6 h. EXCO tests, conducted in accordance with GB/T 22639-2008, used a solution containing 234 g/L NaCl, 50 g/L KNO3, and 6.3 mL/L HNO3 for 48 h at 25 °C. Corrosion damage observed in the cross-sections of the samples, oriented perpendicular to the extrusion direction, was characterized using light microscopy. Electrochemical corrosion experiments were performed with a CHI660-B electrochemical workstation (Chenhua Instrument Co., Ltd., Shanghai, China). The alloy sample, with a contact area of 1 cm2, was, firstly, polished to achieve a surface roughness of less than 0.1 µm, cold mounted, and sealed. One terminal of the sample was connected to the working electrode (WE), the saturated calomel electrode serving as the reference electrode (SCE), and a platinum electrode was used as the auxiliary electrode (CE). The immersion solution was 3.5% NaCl. Before measuring the Tafel curve, the open circuit potential was recorded until stabilized. The Tafel curve was then obtained within a potential range of −1.2 VSCE to −0.4 VSCE, employing a scan rate of 0.2 mV/s.
Light microscopy (RX-50M, Shandong Huayin Testing Instrument Co., Ltd., Yantai, China) samples were prepared by mechanical polishing and etched with Graff Sargent’s solution (3 g CrO3 + 16 mL HNO3 + l mL HF + 83 mL H2O). Scanning electron microscopy (SEM, MIRA4 LMH model, TESCAN, Brno, Czech Republic) was used to examine the fracture surfaces of the specimens. The NordlysMax3 detector (Oxford Instruments, Abingdon, United Kingdom) was utilized to acquire electron backscatter diffraction (EBSD) data with a step size of 1 μm. The EBSD specimens were electro-polished in a mixed solution of 10% HClO4 and 90% C2H5OH (vol.%) at a voltage of 20 V to remove the mechanically polished layer. Transmission electron microscopy (TEM) analyses were performed using a Talos F200X apparatus from Thermo Fisher Scientific, Waltham, MA, USA, equipped with a Super-X energy-dispersive X-ray spectroscopy (EDS) system, operating at 200 kV. For TEM sample preparation, the specimens were mechanically ground into 70 μm thin foils, which were then punched into 3 mm discs. These discs were subjected to twin-jet electro-polishing in a solution consisting of 30% HNO3 and 70% CH3OH (vol.%) at −30 °C and 20 V.

3. Results and Discussion

3.1. Microstructure

Figure 2 presents light microscopy images of 7050 aluminum alloys with varying Zn/Mg ratios after T74 treatment. The micrographs reveal coarse recrystallized grains interspersed with smaller recrystallized grains of various sizes across the three alloys. These coarse grains are recrystallized grains. Typically, smaller grains, with their larger grain boundary areas, provide more pathways for corrosion [21], thereby reducing the corrosion resistance of aluminum alloys. Notably, these recrystallized grains are predominantly located near residual phases. The relatively larger spacing between these residual grains hinders dislocation movement during deformation [22]. This increased energy enhances the driving force for recrystallization, allowing the deformed matrix to be consumed and new, undistorted fine equiaxed grains to grow until the deformed structure is fully transformed. Conversely, when the spacing of the second-phase particles is minimal, dislocation rearrangement is impeded, resulting in the formation of subgrain boundaries that obstruct grain boundary migration and suppress the recrystallization process [23]. For instance, in alloy 1, with a Zn/Mg ratio of 2.72 (Figure 2a), the smaller spacing of the second-phase particles correlates with a lower degree of recrystallization. In contrast, alloy 3 (Figure 2c), with larger spacing of the second-phase particles, exhibits a greater extent of recrystallization.
Figure 3 illustrates the distribution of grain orientation and differences in grain boundary orientation for the three alloys in the solution-treated state. Grain orientation refers to the spatial arrangement of each grain’s lattice in polycrystalline materials. Statistical analysis of grain orientations can reveal whether a preferred orientation exists in aluminum alloys after deformation. Most grains in the alloys transition from their original equiaxed form to elongated structures aligned with the deformation direction, forming a characteristic fibrous structure along the extrusion direction (ED). In alloy 3, following solution treatment, the emergence of equiaxed grains distinct from the elongated crystals is observed (Figure 3c), which may indicate recrystallized grains. As the Zn/Mg ratio increases, the proportion of low-angle boundaries (LABs) in the alloys initially increases and then decreases, with values of 57.0%, 68.4%, and 51.5%, respectively. Conversely, the average orientation difference among grains shows an initial decrease followed by an increase, with values of 27.4%, 24.9%, and 32.4%, respectively. Generally, a higher proportion of low-angle boundaries is associated with a lower average orientation difference between grains, suggesting a more refined substructure within the alloy [24]. Notably, at a Zn/Mg ratio of 2.72, dislocation density peaks at 5.2 × 1013 m−2, while at a Zn/Mg ratio of 3.84, the dislocation density is at its lowest, measuring 4 × 1013 m−2. When Zn atoms dissolve into aluminum alloys, they induce lattice distortion, which hinders dislocation movement by creating obstacles to their slip [25]. To overcome this resistance, dislocations require higher stress, increasing the likelihood of their accumulation within the crystal, leading to an increase in low-angle boundaries (LABs). However, the overall effect on the material is relatively minor [26]. EBSD analysis also shows that the average grain sizes of the three different Zn/Mg alloys are 76.5 μm, 65.9 μm, and 82.6 μm, respectively. The solution treatment of aluminum alloys with varying Zn/Mg ratios after extrusion deformation facilitates the re-dissolution of alloying elements into the aluminum matrix, resulting in a highly concentrated supersaturated solid solution [27]. This condition promotes subsequent aging precipitation.
Figure 4 presents the inverse pole images of 7050 aluminum alloys with varying Zn/Mg ratios following solution treatment. The three alloys exhibited distinct texture orientations. Alloy 1 shows a prominent <111> orientation along with a weak <101> fiber structure (Figure 4a). In contrast, alloy 2 demonstrates a strong <001> orientation, supplemented by some <101> and <111> fiber structures, with its texture strength increasing from 2.78 to 3.33. For alloy 3, a degree of recrystallization occurs, leading to the partial replacement of the original fiber texture with recrystallized textures. This alteration results in a change in texture orientation, predominantly characterized by strong <101> and <111> textures.
Figure 5 shows the recrystallization microstructures of the examined alloys, revealing significant variations in recrystallization morphology associated with different Zn/Mg ratios. Alloy 1 exhibits proportions of recrystallized grains, subgrains, and deformed grains at 6.92%, 43.3%, and 49.7%, respectively. The predominance of subgrains and deformed grains in alloy 1, coupled with the limited occurrence of recrystallized grains, indicates a relatively low degree of recrystallization. In contrast, alloy 2 displays proportions of 6.1% recrystallized grains, 59.4% subgrains, and 34.4% deformed grains, reflecting an increase in subgrains compared to alloy 1, while the proportion of recrystallized grains remains relatively constant. Alloy 3, characterized by a Zn/Mg ratio of 3.84, demonstrates a markedly higher proportion of recrystallized grains at 20.4%, exceeding that of the other two alloys. This observation suggests a positive correlation between an increased Zn/Mg ratio and the transformation of subgrains into recrystallized grains, thereby enhancing the overall degree of recrystallization within the alloy.
TEM images of three alloys after T74 aging treatment, with alignment along the [110]Al direction, are shown in Figure 6. In alloy 1, the precipitated phases predominantly exhibit a spherical morphology, with a small number of short rod-shaped η′ (MgZn2) phases. This is similar to what has been reported in the literature [2,5,28]. Statistical analysis of precipitate diameters in Figure 6d reveals that the average size of the precipitates in alloy 1 is 7.62 nm. When the Zn/Mg ratio is increased to 2.72, there is a notable rise in the occurrence of short rod-shaped structures, accompanied by a slight increase in the average precipitate size to 8.65 nm. Further increasing the Zn/Mg ratio to 3.84 results in a significant reduction in the number of precipitated phases, while the average size of the precipitates grows to 11.34 nm, which correlates with a decrease in the tensile strength of the alloy 3. High-resolution transmission electron microscopy (HRTEM) and corresponding fast Fourier transform (FFT) analyses show streaked diffraction spots at 1/3 (111)Al and 2/3 (111)Al positions, confirming that these rod-shaped precipitates are related to the η′ phase in the alloy. These results demonstrate a consistent trend of increasing precipitate diameter with increasing Zn/Mg ratio in the alloys, which impacts the mechanical properties.
Figure 7 illustrates the precipitate-free zone (PFZ) at the grain boundaries of alloys with varying Zn/Mg ratios. Significant differences in PFZ morphology are observed among the three types of 7050 aluminum alloys. The PFZ width in alloy 1 (Figure 7a) is relatively narrow, measuring 40 μm, with a discontinuous distribution of larger intergranular precipitates. In contrast, alloy 2 (Figure 7b) exhibits the widest PFZ, measuring 53 μm, characterized by a continuous distribution of spherical intergranular precipitates along the grain boundaries. This arrangement leads to an increased potential difference for corrosion and establishes a continuous pathway for corrosion, negatively affecting the corrosion resistance of alloy 2. Alloy 3 (Figure 7c) displays needle-like intergranular precipitates, also continuously distributed along the grain boundaries, with the narrowest PFZ width, measuring only 31 μm.

3.2. Tensile Properties

Figure 8 presents the engineering stress–strain curves and average tensile properties of 7050 aluminum alloys with varying Zn/Mg ratios after T74 aging treatment. At a Zn/Mg ratio of 2.36, alloy 1 exhibits an ultimate tensile strength (UTS) of 631 MPa, a yield strength (YS) of 580 MPa, and an elongation (EL) of 8.6%. Increasing the Zn/Mg ratio to 2.72 results in alloy 2 showing improvements, with a UTS of 641 MPa, a YS of 609 MPa, and an EL of 10.1%, representing a 17% increase in elongation compared to alloy 1. However, further elevating the Zn/Mg ratio to 3.84 causes alloy 3 to experience a decrease in UTS and YS to 597 MPa and 566 MPa, respectively, while maintaining an EL of 10%, indicating a reduction in strength. In summary, the correlation between the Zn/Mg ratio and mechanical properties shows an initial increase in UTS and YS, followed by a decrease, while EL steadily rises. Notably, alloy 2 demonstrates superior mechanical performance, achieving an optimal balance between high strength and relatively high elongation.
The fracture morphologies of the three alloys with different Zn/Mg ratios exhibit significant differences (Figure 9). Fracture occurs along the crack path in a non-directional manner. Specimens with different zinc/magnesium ratios exhibit varying Poisson effects, with significant differences in lateral displacement. The Poisson effect, which describes the tendency of an alloy to contract or expand in directions perpendicular to the applied force [29], can vary based on alloy composition, such as different zinc/magnesium ratios. Varying the zinc/magnesium ratio significantly influences the Poisson effect in these alloys by affecting their microstructure, mechanical anisotropy, ductility, and temperature sensitivity. Zn/Mg ratios can alter the grain structure of the alloys, leading to variations in mechanical behavior under stress. The fracture surface of alloy 1 demonstrates both ductile and brittle fracture characteristics, with prominent cleavage planes and several large, deep fracture zones. Adjacent to the cleavage planes, numerous small, shallow dimples are observed, and at high magnification, second-phase particles are identified at the base of these dimples. Energy-dispersive spectroscopy (EDS) analysis confirms that these particles correspond to the Al2CuMg phase. The stress concentration surrounding these second-phase particles promotes the growth and coalescence of micro-voids, leading to a dimpled fracture pattern [28]. In contrast, the fracture morphology of alloy 2 predominantly exhibits cleavage steps and large dimples, with pronounced tearing ridges evident on the fracture surface. Surrounding these tearing ridges are several larger and deeper dimples, which contribute to the alloy’s favorable plasticity and elongation. The fracture morphology of alloy 3 is primarily characterized by dimples of varying sizes, with distinct white tearing edges observed around some of the larger dimples. Additionally, some cleavage planes are noted around the smaller dimples, resembling the morphology of alloy 2. Consequently, the elongation rates for alloys 2 and 3 are comparable, measuring 10.1% and 10.0%, respectively. This observation underscores the influence of the Zn/Mg ratio on the tensile properties of the alloys and highlights the variations in their fracture morphologies.

3.3. Corrosion Behavior

To evaluate the corrosion resistance of alloys with varying Zn/Mg ratios, intergranular corrosion tests were performed according to the procedures specified in “GBT 7998-2005”. The corrosion morphology and maximum intergranular corrosion depths for the three alloys are shown in Figure 10. The results reveal that aluminum alloys with different Zn/Mg ratios exhibit varying corrosion depths, with maximum values measured at 143 μm, 166 μm, and 88.5 μm, respectively. Notably, as the Zn/Mg ratio increases, the maximum corrosion depth of the alloys initially rises and then decreases, suggesting that the intergranular corrosion resistance of the alloys undergoes an initial decline followed by an improvement. When the Zn/Mg ratio is relatively high, the nucleation rate of MgZn2 precipitates at the grain boundaries increases due to the synergistic effect of Zn and Mg, which enhances nucleation capability at these boundaries. As the precipitates nucleate and grow rapidly, solute atoms near the grain boundaries are quickly depleted, resulting in a wider precipitate-free zone (PFZ) [30]. This leads to a greater potential difference between the grain boundaries and the grain interiors, thereby reducing the corrosion resistance of the aluminum alloy.
Figure 11 presents macro images of the alloys with varying Zn/Mg ratios after 48 h of immersion and subsequent exfoliation corrosion. Exfoliation corrosion in 7xxx series aluminum alloys is characterized by the combined effects of internal stress and intergranular corrosion, with corrosion primarily occurring at the grain boundaries. When aluminum alloys are immersed in an EXCO standard solution, the initial corrosion typically begins at the grain boundaries, leading to the formation of corrosion cracks. As intergranular corrosion progresses, significant buildup of corrosion products occurs near the grain boundaries. These products occupy a larger volume than the original alloy, intensifying the peeling of the metal during the corrosion process, which further promotes corrosion and exfoliation, ultimately resulting in the exfoliation corrosion patterns shown in Figure 11. Both alloy 1 and alloy 2 exhibit extensive large-area delamination, with alloy 2 showing more severe delamination. In contrast, alloy 3 displays a reddish appearance on its surface during the corrosion process, with superficial delamination that does not penetrate deeply into the alloy. Notably, as the Zn/Mg ratio increases, the exfoliation corrosion resistance of the alloy initially decreases but later improves.
To further elucidate the influence of the Zn/Mg ratio on the corrosion performance mechanisms of the three alloys, Tafel curve measurements were performed, as illustrated in Figure 12. For alloy 1, the corrosion potential and corrosion current are recorded at −0.865V and 1.486 × 10−3 A/cm2, respectively. With an increase in the Zn/Mg ratio in alloy 2, the corrosion potential decreases to −0.893V, while the corrosion current rises to 2.931 × 10−3 A/cm2, indicating a deterioration in corrosion performance. Conversely, alloy 3 exhibits an increase in corrosion potential to −0.796 V and a reduction in corrosion current to 5.44 × 10−4 A/cm2, representing a substantial decrease by an order of magnitude compared to the alloys 1 and 2. This significant reduction in corrosion current indicates a marked enhancement in the corrosion resistance of the alloys. Electrochemical corrosion involves electron transfer reactions between metallic substrates and corrosive environments [31]. In aluminum alloys, the corrosion mechanism is predominantly characterized by redox reactions, wherein aluminum acts as the anode, undergoing oxidation and releasing electrons. Concurrently, oxygen or other oxidizing agents present in the environment accept these electrons and are reduced at the cathodic site [32]. The Tafel curve is used to examine the kinetic properties of the corrosion process by linearizing the polarization curve, focusing primarily on the corrosion potential and corrosion current to evaluate the corrosion resistance of the alloy [33].

4. Conclusions

The study focused on analyzing the microstructure, texture evolution, precipitate behavior, mechanical properties, and corrosion performance of 7050-T74 aluminum alloys with different Zn/Mg ratios. The main conclusions are as follows:
The Zn/Mg ratio affects the mechanical properties of 7050 aluminum alloy. The Zn/Mg ratio of 2.72 results in a tensile strength of 641 MPa, a yield strength of 609 MPa, and an elongation of 10.1%.
Increasing the Zn/Mg ratio to 2.90 slightly reduces pitting corrosion resistance, but improves exfoliation corrosion resistance.
The Zn/Mg ratio affects the recovery and recrystallization of 7050 aluminum alloy in the solution-aging condition. As the Zn/Mg ratio increases from 2.36 to 3.84, the proportion of low-angle boundaries (LABs) within the alloys initially rises and then decreases.

Author Contributions

Conceptualization, methodology, supervision, and writing—review and editing, D.X.; investigation, data curation, and formal analysis, Z.H.; methodology, supervision, and writing—review, L.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Pre-research Fund (No. 623020012, 412130017, 623020031).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Zhou, B.; Liu, B.; Zhang, S. The Advancement of 7XXX Series Aluminum Alloys for Aircraft Structures: A Review. Metals 2021, 11, 718–747. [Google Scholar] [CrossRef]
  2. Wang, J.; Li, F. Research Status and Prospective Properties of the Al-Zn-Mg-Cu Series Aluminum Alloys. Metals 2023, 13, 1329–1353. [Google Scholar] [CrossRef]
  3. Xie, P.; Chen, S.Y.; Chen, K.H.; Jiao, H.B.; Huang, L.P. Enhancing the Stress Corrosion Cracking Resistance of a Low-Cu Containing Al-Zn-Mg-Cu Aluminum Alloy by Step-Quench and Aging Heat Treatment. Corros. Sci. 2019, 161, 108184. [Google Scholar] [CrossRef]
  4. Tang, J.W.; Wang, Y.F.; Fujihara, H.; Shimizu, K.; Hirayama, K.; Ebihara, K.; Takeuchi, A.; Uesugi, M.; Toda, H. Stress Corrosion Cracking Induced by the Combination of External and Internal Hydrogen in Al-Zn-Mg-Cu Alloy. Scripta. Mater. 2023, 239, 115804. [Google Scholar] [CrossRef]
  5. Khan, M.B.; Wang, Y.W.; Afifi, M.A.; Malik, A.; Nazeer, F.; Yasin, G.; Bao, J.W.; Zhang, H. Microstructure and Mechanical Properties of an Al-Zn-Cu-Mg Alloy Processed by Hot Forming Processes Followed by Heat Treatments. Mater. Charact. 2019, 157, 109901. [Google Scholar] [CrossRef]
  6. Chen, Y.; Liu, Z.; Guo, H.; Liu, J.; Sheng, X. Effect of Cu on the Hot Tearing Susceptibility of Al-6Zn-2.5Mg-xCu Alloy. Int. J. Met. 2021, 15, 130–140. [Google Scholar] [CrossRef]
  7. Li, Y.; Wang, Y.; Lu, B.; Yu, W.; Wang, H.Y.; Xu, G.M.; Wang, Z.D. Effect of Cu Content and Zn/Mg Ratio on Microstructure and Mechanical Properties of Al–Zn–Mg–Cu Alloys. J. Mater. Res. Technol. 2022, 19, 3451–3460. [Google Scholar] [CrossRef]
  8. Lachowicz, M.M. Metallurgical Aspects of Corrosion Resistance of 7000 Series Aluminum Alloys-a Review. Mater. Sci.-Poland 2023, 41, 159–180. [Google Scholar] [CrossRef]
  9. Chen, Z.Y.; Mo, Y.K.; Nie, Z.R. Effect of Zn Content on the Microstructure and Properties of Super-high Strength Al-Zn-Mg-Cu Alloys. Metall. Mater. Trans. A 2013, 44, 3910–3920. [Google Scholar] [CrossRef]
  10. Gao, R.S.; Li, Y.N.; Li, Z.H.; Li, X.W.; Wen, K.; Zhang, Y.A.; Xiong, B.Q. Quantitative Relationship between Microstructure and Tensile Properties of Al-Zn-Mg-Cu Alloys with Various Alloying Degrees. J. Mater. Res. Technol. 2022, 18, 5394–5405. [Google Scholar] [CrossRef]
  11. Jiang, H.T.; Xing, H.; Yang, B.; Liang, E.Q.; Zhang, J.; Sun, B.D. Effect of Zn Content and Sc、Zr Addition on Microstructure and Mechanical Properties of Al-Zn-Mg-Cu Alloys. J. Alloys Compd. 2023, 947, 169246. [Google Scholar] [CrossRef]
  12. Chen, S.Y.; Li, J.; Hu, G.Y.; Chen, K.H.; Huang, L.P. Effect of Zn/Mg Ratios on SCC, Electrochemical Corrosion Properties and Microstructure of Al-Zn-Mg Alloy. J. Alloys Compd. 2018, 757, 259–264. [Google Scholar] [CrossRef]
  13. Wang, S.; Luo, B.; Bai, Z.; He, C.; Tan, S.; Jiang, M. Effect of Zn/Mg Ratios on Microstructure and Stress corrosion Cracking of 7005 Alloy. Materials 2019, 12, 285–297. [Google Scholar] [CrossRef]
  14. Dixit, M.; Mishra, R.S.; Sankaran, K.K. Structure-property Correlations in Al 7050 and Al 7055 High-strength Aluminum Alloys. Mater. Sci. Eng. A 2008, 478, 163–172. [Google Scholar] [CrossRef]
  15. Wang, H.B.; Gao, A.N.; Song, H.; Xu, Z.; Li, S.L. Effect of Ti Addition on Microstructures and Crack Behavior of Twin-Roll Cast-Rolled 7050 Alloy Plate. Rare Metal Mater. Eng. 2020, 49, 4055–4063. [Google Scholar]
  16. She, H.; Chu, W.; Shu, D.; Wang, J.; Sun, B.D. Effects of Silicon Content on Microstructure and Stress Corrosion Cracking Resistance of 7050 Aluminum Alloy. Trans. Nonferr. Metals Soc. China 2014, 24, 2307–2313. [Google Scholar] [CrossRef]
  17. Chen, Y.; Liu, Z.Y.; Bai, S.; Zhao, J.G.; Liu, G.H. The Effect of Multistage Aging on Mechanical Properties and Microstructure of Forged 7050 Aluminum Alloys. J. Mater. Eng. Perform. 2019, 28, 3590–3599. [Google Scholar] [CrossRef]
  18. Zheng, J.H.; Pan, R.; Li, C.; Zhang, W.; Lin, J.G.; Davies, C.M. Experimental Investigation of Multi-step Stress-relaxation-ageing of 7050 Aluminium Alloy for Different Pre-strained Conditions. Mater. Sci. Eng. A 2018, 710, 111–120. [Google Scholar] [CrossRef]
  19. GBT7998-2005; Test Method for Intergranular Corrosion of Aluminium Alloy. The Standardization Administration of China: Beijing, China, 2005.
  20. GBT22639-2008; Test Method of Exfoliation Corrosion for Wrought Aluminium and Aluminium Alloys. The Standardization Administration of China: Beijing, China, 2008.
  21. Fu, X.Q.; Ji, Y.C.; Cheng, X.Q.; Dong, C.F.; Fan, Y.; Li, X.G. Effect of Grain Size and Its Uniformity on Corrosion Resistance of Rolled 316L Stainless Steel by EBSD and TEM. Mater. Today Commun. 2020, 25, 101429. [Google Scholar] [CrossRef]
  22. Nazari, F.; Honarpisheh, M.; Zhao, H.Y. The Effect of Microstructure Parameters on the Residual Stresses in the Ultrafine-grained Sheets. Micron 2020, 132, 102843. [Google Scholar] [CrossRef]
  23. Huang, Y.K.; Zhao, Y.X.; Liu, Y.; Xiao, Z.B.; Yang, L.; Huang, Y.C. Microstructural Evolution and Phase Transformation Behavior of Al-8.1Zn-2.0Mg-1.0Cu-0.2Ag-0.15Zr Alloy During Isothermal Compression. J. Mater. Res. Technol 2024, 33, 1018–1031. [Google Scholar] [CrossRef]
  24. Choundraj, J.D.; Kacher, J. Influence of Misorientation Angle and Local Dislocation Density on β-Phase Distribution in Al 5xxx Aloys. Sci. Rep. 2022, 12, 1817. [Google Scholar] [CrossRef] [PubMed]
  25. Ryen, Ø.; jørn Holmedal, B.; Oscar Nijs, O.; Erik Nes, E.; Emma Sjölander, E.; Ekström, H.K. Strengthening Mechanisms in Solid solution Aluminum Alloys. Metall. Mater. Trans. 2006, 37, 1999–2006. [Google Scholar] [CrossRef]
  26. Liu, W.B.; Liu, Y.; Sui, H.N.; Chen, L.R.; Yu, L.; Yi, X.; Duan, H.L. Dislocation-grain Boundary Interaction in Metallic Materials: Competition between Dislocation Transmission and Dislocation Source Activation. J. Mech. Phys. Solids. 2020, 145, 104158. [Google Scholar] [CrossRef]
  27. Kim, S.H.; Lee, S.W.; Moon, B.G.; Kim, H.S.; Kim, y.m.; Park, S.H. Influence of Extrusion Temperature on Dynamic Deformation Behaviors and Mechanical Properties of Mg-8Al-0.5Zn-0.2Mn-0.3Ca-0.2Y Alloy. J. Mater. Res. Technol. 2019, 8, 5254–5270. [Google Scholar] [CrossRef]
  28. Wu, M.D.; Yao, T.; Xiao, D.H.; Yuan, S.; Li, Z.Y.; Huang, L.P.; Liu, W.S. Precipitation Behavior and Properties of an Al-8.26Zn-1.95Mg-1.89Cu-0.08Sc-0.17Zr Alloy with Different Dislocation Morphologies via Pre-treatment. J. Mater. Res. Technol. 2024, 29, 4714–4727. [Google Scholar] [CrossRef]
  29. Luo, Y.H. Improved Voigt and Reuss Formulas with the Poisson Effect. Materials 2022, 15, 5656. [Google Scholar] [CrossRef]
  30. Cai, B.; Adams, B.L.; Nelson, T.W. Relation between Precipitate-free Zone Width and Grain Boundary Type in 7075-T7 Al Alloy. Acta. Mater. 2007, 55, 1543–1553. [Google Scholar] [CrossRef]
  31. Najjar, D.; Magnin, T.; Warner, T.J. Influence of Critical Surface Defects and Localized Competition between Anodic Dissolution and Hydrogen Effects during Stress Corrosion Cracking of a 7050 Aluminium Alloy. Mater. Sci. Eng. A 1997, 238, 293–302. [Google Scholar] [CrossRef]
  32. Zou, Y.; Cao, L.; Wu, X.; Mou, C.; Tang, S.; Lin, X. Unusual Secondary Precipitation within the Primary Precipitation Free Zone Substantially Enhances the Ductility of Al-Zn-Mg-Cu Alloy. Mater. Sci. Eng. A 2023, 881, 145384–145390. [Google Scholar] [CrossRef]
  33. Peng, X.; Li, Y.; Xu, G.; Huang, J.; Yin, Z. Effect of Precipitate State on Mechanical Properties, Corrosion Behavior, and Microstructures of Al-Zn-Mg-Cu Alloy. Metals Mater. Inter. 2018, 24, 1046–1057. [Google Scholar] [CrossRef]
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Figure 1. Schematic diagram of the aluminum ingot extrusion process and tensile specimen sampling. (a) Homogenized billet, (b) Hot extrusion, (c) Sampling direction for tensile test specimens, (d) Dimension diagram of the tensile test specimen.
Figure 1. Schematic diagram of the aluminum ingot extrusion process and tensile specimen sampling. (a) Homogenized billet, (b) Hot extrusion, (c) Sampling direction for tensile test specimens, (d) Dimension diagram of the tensile test specimen.
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Figure 2. Light microscopy images of the alloys with different Zn/Mg ratios after T74 treatment. (a) Alloy 1, (b) Alloy 2, (c) Alloy 3.
Figure 2. Light microscopy images of the alloys with different Zn/Mg ratios after T74 treatment. (a) Alloy 1, (b) Alloy 2, (c) Alloy 3.
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Figure 3. Distribution of grain orientation (a,c,e), grain boundary orientation difference statistics (b,d,f) for alloy 1 (a,b), 2 (c,d), and 3 (e,f) after T74 treatment.
Figure 3. Distribution of grain orientation (a,c,e), grain boundary orientation difference statistics (b,d,f) for alloy 1 (a,b), 2 (c,d), and 3 (e,f) after T74 treatment.
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Figure 4. The inverse pole of alloys 1 (a), 2 (b), and 3 (c) after solid solution treatment.
Figure 4. The inverse pole of alloys 1 (a), 2 (b), and 3 (c) after solid solution treatment.
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Figure 5. Recrystallization images of the alloys 1 (a,b), 2 (c,d), and 3 (e,f) after solution treatment.
Figure 5. Recrystallization images of the alloys 1 (a,b), 2 (c,d), and 3 (e,f) after solution treatment.
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Figure 6. TEM images and diameter distributions of precipitates along the [110]Al direction for alloys 1 (a,d), 2 (b,e), and 3 (c,f); HRTEM image (g) and the corresponding FFT (h) taken from alloy 1; and (i) schematic of the diffraction spot model of η′ precipitation in the aluminum matrix along the [110]Al direction.
Figure 6. TEM images and diameter distributions of precipitates along the [110]Al direction for alloys 1 (a,d), 2 (b,e), and 3 (c,f); HRTEM image (g) and the corresponding FFT (h) taken from alloy 1; and (i) schematic of the diffraction spot model of η′ precipitation in the aluminum matrix along the [110]Al direction.
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Figure 7. TEM pictures of alloys 1 (a), 2 (b), and 3 (c) along the [110]Al direction.
Figure 7. TEM pictures of alloys 1 (a), 2 (b), and 3 (c) along the [110]Al direction.
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Figure 8. Room temperature mechanical properties of alloys with different Zn/Mg ratios after aging treatment: (a) stress–strain curves; (b) comparison of ultimate tensile strength, yield strength, and elongation.
Figure 8. Room temperature mechanical properties of alloys with different Zn/Mg ratios after aging treatment: (a) stress–strain curves; (b) comparison of ultimate tensile strength, yield strength, and elongation.
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Figure 9. SEM images of the fracture surfaces for alloys 1 (a,d), 2 (b,e), and 3 (c,f) following tensile testing at room temperature.
Figure 9. SEM images of the fracture surfaces for alloys 1 (a,d), 2 (b,e), and 3 (c,f) following tensile testing at room temperature.
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Figure 10. Intergranular corrosion micrographs of the alloys 1 (a), 2 (b) and 3 (c).
Figure 10. Intergranular corrosion micrographs of the alloys 1 (a), 2 (b) and 3 (c).
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Figure 11. Macroscopic images of exfoliation corrosion for alloys 1 (a), 2 (b), and 3 (c).
Figure 11. Macroscopic images of exfoliation corrosion for alloys 1 (a), 2 (b), and 3 (c).
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Figure 12. Tafel curves of the alloys 1, 2, and 3 after aging treatment.
Figure 12. Tafel curves of the alloys 1, 2, and 3 after aging treatment.
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Table 1. Nominal composition of alloys.
Table 1. Nominal composition of alloys.
AlloyElement/Mass FractionZn/Mg Ratios
ZnMgCuZrAl
Alloy 15.72.62.30.15Bal.2.19
Alloy 26.72.62.30.15Bal.2.58
Alloy 36.71.92.30.15Bal.3.53
Table 2. Chemical composition of alloys.
Table 2. Chemical composition of alloys.
AlloyElement/Mass FractionZn/Mg Ratios
ZnMgCuZrAl
Alloy 15.792.452.100.11Bal.2.36
Alloy 26.952.552.090.13Bal.2.72
Alloy 36.761.762.070.12Bal.3.84
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Xiao, D.; He, Z.; Huang, L. Microstructure and Properties of 7050-T74 Aluminum Alloys with Different Zn/Mg Ratios. Metals 2024, 14, 1226. https://doi.org/10.3390/met14111226

AMA Style

Xiao D, He Z, Huang L. Microstructure and Properties of 7050-T74 Aluminum Alloys with Different Zn/Mg Ratios. Metals. 2024; 14(11):1226. https://doi.org/10.3390/met14111226

Chicago/Turabian Style

Xiao, Daihong, Zongzheng He, and Lanping Huang. 2024. "Microstructure and Properties of 7050-T74 Aluminum Alloys with Different Zn/Mg Ratios" Metals 14, no. 11: 1226. https://doi.org/10.3390/met14111226

APA Style

Xiao, D., He, Z., & Huang, L. (2024). Microstructure and Properties of 7050-T74 Aluminum Alloys with Different Zn/Mg Ratios. Metals, 14(11), 1226. https://doi.org/10.3390/met14111226

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