Research Status and Prospective Properties of the Al ‐ Zn ‐ Mg ‐ Cu Series Aluminum Alloys

: An Al–Zn–Mg–Cu alloy has high specific strength, good corrosion resistance, fracture toughness and fatigue resistance. It is one of the most important structural materials in the fields of aviation, aerospace, weapons and transportation; in particular, it plays a huge role in the field of aerospace. In order to optimize the strength, toughness and corrosion properties of an Al–Zn–Mg– Cu alloy, the focus of research on this alloy has always been on the alloying process. The effects of the main alloying elements, trace alloying elements and rare earth elements on the microstructure and properties of Al–Zn–Mg–Cu alloys are briefly introduced in this paper, and future research directions are proposed.


Introduction
The main elements of an Al-Zn-Mg-Cu alloy include Al, Zn, Mg and Cu; this alloy also contains trace amounts of Si and Fe (usually present as impurity elements).The types and contents of trace elements are different among different brands, which leads to different aluminum alloys having different mechanical properties.Therefore, the mechanical properties of the Al-Zn-Mg-Cu alloy could be improved by adjusting its composition, selectively introducing trace alloying elements, reducing the impurity content and realizing the reasonable distribution of the precipitate phase.Rare earth elements play a more significant role in this process due to their special electronic structure and chemical properties.However, with the increase in types of alloying elements within an alloy, the phase composition and precipitation behavior of the alloy become more complex.The component differences in local areas are significant.The corrosion cracking sensitivity increases and the contradiction between strength and corrosion performance becomes more acute.Therefore, the influence mechanisms of different elements on the properties of the alloy are discussed, and the range of action and best content of elements are researched in order to solve the contradiction between the strength, toughness and corrosion resistance of aluminum alloy.This is an urgent problem to be solved in the development process of ultra-high-strength aluminum alloy.

Main Alloying Elements
The main alloying elements in an Al-Zn-Mg-Cu alloy are Al, Zn, Mg and Cu.The MgZn2 phase is the main strengthening phase in aluminum alloy.An increase in Zn and Mg elements can improve the yield strength of the alloy.Cu provides a certain contribution to the strength of the alloy that improves the corrosion resistance [1,2].But there is a threshold for each element, for which the difference is obvious for different components of the alloy.

Effect of Different Cu Contents on an Al-Zn-Mg-Cu Alloy
Cu has a certain strengthening effect in Al-Zn-Mg-Cu alloys that has a prominent effect on their corrosion resistance.The addition of Cu can improve the age-hardening potential of the alloy and increase the density of the precipitated strengthened phase [3].However, when excessive copper was added to a 7055 aluminum alloy, the grain size gradually increased and the alloy performance decreased (Figure 1) [4].Therefore, reducing the contents of Cu within a certain range was conducive to the diffusion of more Zn and Mg into the matrix, which increased the volume fraction of the strengthening phase Mg(Zn,Al,Cu)2, thus improving the yield strength (YS) and hardness of the alloy.

Effect of Cu on Alloys at Different Zn/Mg Ratios
The effect of Cu on the properties of an Al-Zn-Mg-Cu alloy is related to the alloy's composition.In low Zn/Mg ratio alloys (such as type 7050), the addition of Cu increases the amount of reinforced precipitate phase and improves the yield strength and corrosion resistance of the alloy.However, in an Al-9.3Zn-2.4Mg-xCu-0.16Zraluminum alloy with a high Zn/Mg ratio, when copper content was decreased from 2.2% to 1.8% (mass fraction), the mechanical properties of the alloy and the resistance to spalling corrosion also significantly improved.When the Cu content was reduced to 0.8% (mass fraction), the potential difference between the grain boundary and Al matrix became larger, which resulted in corrosion of the precipitate phase and significantly deteriorated the corrosion performance (Figure 2b) [5].

Effect of Cu in a Highly Alloyed Al-Zn-Mg-Cu Alloy
In a highly alloyed Al-Zn-Mg-Cu alloy, the influence of Cu on the comprehensive properties includes grain refinement and coupling effect of residual eutectic phase.In Al-10.0Zn-2.6Mg-xCu-0.15Zralloys, on one hand, the volume fraction of the second phase increased significantly with the increase in Cu content.On the other hand, the low diffusion coefficient of Cu inhibited the dissolution of the second phase, which gradually increased the volume fraction of the residual eutectic phase after a solid solution treatment.In the hard and brittle residual eutectic phase, stress concentration easily occurred and it became a crack source, which seriously damaged the plasticity of the alloy.For an Al-10.0Zn-2.6Mg-1.0Cualloy with x = 1.0, the precipitate phase was small and the volume fraction was large (Figure 3), which exhibited the best mechanical properties [6].

Mg
Mg is the main strengthening element of an Al-Zn-Mg-Cu alloy.η(MgZn 2 )phase that is formed by Mg and Zn has a large solid solubility in the matrix.The solid solubility decreases significantly with the decrease in temperature, which have a strong age-hardening ability and was the main strengthening phase of the Al-Zn-Mg-Cu alloy.With the increase in Mg content, α(Al) grains were refined (Figure 4) and the tertiary dendrites and the T(AlCuMgZn) phases increased [7].In addition, the increase in Mg content could improve the volume fraction of the precipitate phase during aging treatment, thus improving the yield strength [8].However, high yield stress could also lead to lower deformation resistance at grain boundaries, thus promoting intergranular fracture and reducing plasticity [9].

Effect of Different Zn Contents on Al-Zn-Mg-Cu Alloys
In an Al-Zn-Mg-Cu alloy, increasing the Zn content can promote the nucleation, growth and phase precipitation transformation of the alloy.Al-Zn-Mg-Cu alloys with high Zn content had high precipitation density and fast precipitation rate, which had the advantages of improving the alloy strength and shortening the peak aging time [10].When the Zn content increased from 4.4% (mass fraction) to 7.2% (mass fraction), the ultimate tensile strength (UTS) of the Al-xZn-2.3Mg-1.7Cualloy increased from 212.15 MPa to 248.05 MPa, which is an increase of 16.92%; the elongation (elongation, EL) increased first and then decreased with the increase in Zn content.And the maximum elongation was 8.78% when x = 5.8 [11].The increase in the number of MgZn2 strengthened phases, and the changes in the shape, number and size of the intercrystalline eutectic phases improved the tensile strength.And the change in elongation was mainly due to the refinement of grain size and the transformation of the dendrite morphology from columnar dendrite to cellular dendrite [12].

Effect of Zn on Softening Behavior of Al-Zn-Mg-Cu Alloys
Zn has an important effect on the softening behavior of Al-Zn-Mg-Cu alloys.At 300 °C, there was a plateau in the static softening curve of the alloy.The increase in Zn content promoted the formation and coarsening of η phases, which also promoted the duration and softening fraction of the plateau decrease (Figure 5a,b).When the deformation temperature increased to 400 °C, the static softening curve was less affected by Zn content (Figure 5c,d) [13].The effect of Zn content on the dynamic softening behavior is shown in Figure 6.The increase in Zn led to the acceleration of work hardening at the initial deformation stage at 300 °C and the dynamic softening process enhanced with the increase in Zn content.However, at 400 °C, the high supersaturation solid solubility caused by the addition of Zn inhibited the dynamic recovery and recrystallization process, which resulted in a slow-down of the dynamic softening behavior (Figure 6c,d) [14].

Effect of Zn on Hot-Cracking Sensitivity of Al-Zn-Mg-Cu Alloys
The hot-cracking sensitivity of an Al-Zn-Mg-Cu alloy increases first and then decreases with the increase in Zn content.When the Zn content was 5% (mass fraction) and 11% (mass fraction), the hot-cracking sensitivity was low.And when the Zn content was 9% (mass fraction), the hot-cracking sensitivity was the highest [15].The thermal cracking sensitivity of the alloy was determined by grain morphology, grain size, thermal shrinkage behavior of the solidification interval and non-equilibrium eutectic content.The increase in Zn content increased the thermal shrinkage of the alloy, which resulted in a larger thermal strain during the solidification process.Although the decrease in grain size helped to reduce the thermal cracking sensitivity of the alloy, the transition from equiaxed spherules to irregular grains reduced the fluidity of the alloy (Figure 7), so the hot-cracking sensitivity of the alloy with 9% Zn content (mass fraction) was the highest.When the content of Zn was further increased to 11% (mass fraction), the grain was refined and transformed from an equiaxed crystal to a fine dendrite and the thermal shrinkage rate increased.But the significant increase in the non-equilibrium eutectic content enabled the alloy to have a better ability to heal the cracks and began to decrease the thermal crack sensitivity.8.It could be observed from the figure that a large number of small cubic precipitates were evenly distributed in the aluminum matrix and its structure was stable, which could prevent deformation at high temperature.The thermal stability of the alloy was significantly improved [16].

The Synergistic Action of Cu and Mg
Mg content was the main factor affecting the strength of over-aged Al-Zn-Mg-Cu alloy.The increase of Cu content could also enhance the strength, but to a lesser extent.In contrast, for spray formed aluminum alloy, the volume fraction of the secondary phase inside the grain was mainly determined by the Mg content and the volume fraction of the secondary phase at the grain boundary was mainly controlled by the Mg + Cu content.After solid solution treatment, coarse grain boundary phase with high Mg content was difficult to enter the single-phase Al region, so the formation of refractory phase in Mg rich (2.65%, mass fraction) aluminum alloy could be promoted by adding Cu element, which was conducive to maintaining the peak strength of the alloy in the over-aging state [8].In addition, it had been found that compared with non-Cu alloys, the yield strength of Al-4.0Mg-3.0Zn-1.5Cualuminum alloys with high Cu increased by 80.5%, while the ductility didn't decrease [17].Therefore, a balance between ductility and strength was expected to be achieved by appropriate adjustment of Cu/Mg ratio.2.4.2.Effect of Different Cu/Mg Ratio on Al-Zn-Mg-Cu Alloy The influence of Cu/Mg ratio on Al-Zn-Mg-Cu alloy was mainly reflected in two aspects: one was to affect the coarsening rate in the aging process; the other was to change the size and distribution of precipitated phase.Alloys with a low Cu/Mg ratio had a slow coarsing rate and a high tensile strength.Under T79 aging, Cu/Mg had the highest tensile strength than the lowest 2.5Mg1.5Cualloy.The alloys with a higher Cu/Mg ratio had coarser Grain boundary precipitates, GBPs) and a wider Precipitate free zone (PFZ) could effectively delay the propagation of microcracks along grain boundaries [18].With the increase of Cu/Mg ratio, the elongation of Al-Zn-Mg-Cu alloy increased significantly and the fracture morphology changed from the typical brittle fracture to the intergranular mixed fracture or ductile fracture.

Effect of Zn/Mg Ratio on the Precipitation Sequence and Phase of the Alloy
The Zn/Mg ratio was closely related to the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloy and also affected the precipitation sequence and phase of the alloy.It was generally believed that the precipitation sequence of Al-Zn-Mg-Cu alloy was as follows: SSSS → GP region → η′ phase → η′ phase (MgZn2) [19].With the decrease of Zn/Mg ratio, T′ phase replaces η′ phase, and the precipitation sequence changed to SSSS → GPI region → GPII region → intermediate phase T′ → equilibrium phase T-Mg32(Al,Zn)49 [20].

Effect of Different Zn/Mg Ratio on Al-Zn-Mg-Cu Alloy
The influence of Zn/Mg ratio on mechanical properties of Al-Zn-Mg-Cu alloy was shown in Figure 9.As the Zn/Mg ratio increased from 1.50 to 2.86, the density and strength of precipitated phase in the alloy increased and more solute atoms precipitated at grain boundaries or within grains, which led to a decrease in PFZ width and elongation.When the Zn/Mg ratio further increased from 2.86 to 10.00, the changes in strength and elongation were opposite to those before [21].In addition, the increase of Zn/Mg ratio could reduce the precipitation of equilibrium η phase during slow quenching, which resulted in the increase of enhanced precipitates after aging treatment, thus reducing the quenching sensitivity of the alloy [22].For high Zn/Mg ratio alloys, the critical cooling rate without precipitation during the cooling process was about 100 K/s.While for low Zn/Mg ratio alloys, the critical cooling rate was about 1 K/s [23].Therefore, Al-Zn-Mg-Cu alloys with high Zn/Mg ratio showed higher quenching sensitivity and were more prone to hot cracking.

Effect of Different Zn/Mg Ratio on Corrosion Resistance of Al-Zn-Mg-Cu Alloy
Electrochemical impedance spectroscopy (EIS) revealed the relationship between impedance size, phase Angle, and frequency.Electrochemical impedance spectroscopy (EIS) showed the relationship between electrochemical impedance, phase Angle, medium frequency range and corrosion resistance of electrochemical impedance spectroscopy.With the increase of Zn/Mg ratio, the arc radius and phase Angle of Nyquist curve decreased, which indicated that the corrosion resistance of the alloy decreased [24].And Al-Zn-Mg-Cu alloy with lower Zn/Mg ratio showed better surface protection and corrosion resistance [25].

Brief Summary
In conclusion, the influence of alloy elements on the precipitation process can be summarized in the following Table 1.Cu played an important role in the corrosion performance of the alloy, which could adjust the trend of Zn and Mg to decrease the corrosion resistance when the strength and hardness were increased.At the same time, Zn could make the alloy have high plasticity properties, which was expected to achieve a balance between ductility and strength.Therefore, adjusting the proportion between the main elements and finding the threshold between the proportion of elements could effectively solve the contradiction between strength and toughness and corrosion resistance.
Table 1.Effect of alloy elements on the precipitation process.

Alloy Element Content
Dissolving Process low Zn/Mg ratio α (supersaturated solid solution)

Microalloy Elements
Trace elements in Al-Zn-Mg-Cu alloys include two categories: First is impurity elements, mainly Fe and Si.A small amount of impurity elements inevitably exist in aluminum alloys and with the recycling of recycled aluminum, the content of Fe and Si in the alloys will further increase.The second is alloying elements including Ag, Ni, Ti, etc. Microalloying elements are mainly introduced to refine grain and improve alloy strength.

Effect of Fe and Si on the Properties of Al-Zn-Mg-Cu Alloys
Fe and Si were the most common impurity elements in Al-Zn-Mg-Cu alloys and generally formed coarse Al7Cu2Fe, Al3Fe, α-AlFeSi and Mg2Si phases in the alloys [26,27].The brittle and hard Al7Cu2Fe phase was not coherent with the α(Al) matrix and was easy to crack or separate from the matrix to form voids.It grew up under the loading stress and eventually formed macroscopic cracks, which led to the fracture of the alloy [28].
When Fe content increased from 0.12% (mass fraction) to 0.26% (mass fraction), the tensile strength of the alloy changed little, where the elongation decreased seriously [29].When Fe content increased from 0.0% (mass fraction) to 0.6% (mass fraction), the fracture toughness decreased from 40 MPa.m 1/2 to 30 MPa.m 1/2 [30].However, the fatigue life of Al-Zn-Mg-Cu alloys could be improved by reasonable control of Fe content.When the strain amplitude was less than 0.5%, the fatigue life of Al-Zn-Mg-Cu alloy with 0.33% (mass fraction) Fe was obviously better than that with 0.01% (mass fraction) Fe [31].The reason was that fine fe-rich phase could change the crack propagation path under low strain amplitude and increased the crack propagation resistance, which improved the fatigue life.However, when the volume fraction and size of the Fe-rich phase exceeded a certain critical value, the Fe-rich phase might still become a crack source, which would damage the fatigue property of the alloy.
The Al-4.5Zn-1.5Mg-1.0Cu-0.35Sialloy contained fine and dispersed plate-strip precipitates in the intragranular structure, which was called the GPB-II zone [32].The GPB-II zone exhibited stronger stability than η′ and T′ phases under high temperature aging, which significantly improved the alloy strength [33].After aging at 125 °C, 175 °C and 225 °C, the microhardness and strength of Al-Zn-Mg-Cu alloys containing Si were significantly higher than those without Si.[34]. Figure 10 shows the EBSD pattern of the matrix around the coarse intermetallic particles in the longitudinal section of the aging alloy, with the white line showing a low Angle boundary of 1.5° to 15° (LAGBs) and the black line showing a high Angle boundary greater than 15° (HAGBs).The local deformation zones (outlined with yellow dashed lines) of each kind of particles extending from the particle-matrix interface to the neighboring matrix were evidently observed through the EBSD micrographs.With the increase of Fe and Si content, the density of Al7Cu2Fe and Mg2Si increased.It was easier to induce recrystallization and formed recrystallized grains whose orientation was much different from the texture direction of extruded fibers, which contributed to the formation of more high Angle grain boundaries.when the mass fraction of Fe and Si increased from 0.041% and 0.024% to 0.272% and 0.134%, respectively, Recrystallization resulted in a decrease in the texture of extruded fibers from 56.8% to 53.8% [35], which decreased slightly strength.High angle grain boundaries hindered deformation more seriously and promote intergranular fracture more easily than low Angle grain boundaries.Therefore, with the increase of Fe and Si content, a large number of coarse Al7Cu2Fe and Mg2Si phases and more high Angle grain boundaries would also reduce the elongation.

Effect of Ag on the Hardness and Yield Strength of Al-Zn-Mg-Cu Alloy
Ag could significantly improve the early aging response and the overall aging response of Al-Zn-Mg-Cu alloy [36], which increased the density of fine aggregates.The aggregate density of 7075 alloy after peak aging was 1.2 × 10 24 m −3 , which increased to 3.1 × 10 24 m −3 after adding 0.3% Ag.The increase of fine T-phase and η phase density in Agcontaining alloys increased the hardness and yield strength by more than 10% [37].

Effect of Ag on the Corrosion Resistance of Al-Zn-Mg-Cu Alloy
The tensile strength of Al-4.5Zn-1.55Mg-0.12Cualloy with 0.12% Ag added was 457.6 MPa in 3.5% NaCl solution, and it was 20.5 MPa higher than that without Ag added, which loss elongation little and showed better corrosion resistance to NaCl solution (Figure 11) [38].On the one hand, Ag atoms were distributed around the grain boundary precipitated phase, which significantly reduced the potential difference between PFZ and GBPs, increased pitting potential, and reduced the sensitivity of the alloy to stress corrosion cracking.On the other hand, the addition of trace Ag could increase the size of precipitated phase and the distance between precipitated phases, which was conducive to the capture of hydrogen atoms in the grain boundary, so as to improve the stress corrosion resistance of the alloy.

Effect of Ag on the Fracture of Al-Zn-Mg-Cu Alloy
When Ag was added to Al-Zn-Mg-Cu alloys containing Ge, a small amount of Ag could effectively capture vacancy and reduced vacancy loss near the grain boundary, which effectively reduced the width of PFZ.The joint addition of Ge and Ag made Mg2Ge particles finer and more evenly distributed, which reduced the degree of stress concentration and further improved the plasticity of the alloy.After air cooling, the fracture changed from brittle intergranular fracture to ductile transgranular fracture (Figure 12) [39].In the equilibrium phase diagram of Al-Zn-Mg-Cu-Ni alloy, Ni only existed in the form of (Al + Al3Ni) eutectic [41].Generally speaking, the increase of eutectic helped to inhibit the formation of hot cracking of casting alloy, which effectively reduced the hot cracking tendency of alloy.

Effect of Ni on the Strength and Elongation of Al-Zn-Mg-Cu Alloy
In as-cast Al-Zn-Mg-Cu-Ni alloys, the amount of Al3Ni phase increased with the increase of Ni content.The Al3Ni phase was formed by L → (α-Al + Al3Ni) eutectic reaction at the last stage of solidification.The increase of Al3Ni phase on the fracture surface meant that there was more residual liquid between the primary α-Al dendrites.In this case, the initial fracture would be filled with residual eutectic liquid, which reduced the possibility of thermal cracking.When the amount of Ni was less than 0.6% (mass fraction), the improvement of grain size and shrinkage defect could improve the tensile strength and elongation at the same time.However, when the Ni content was more than 0.6% (mass fraction), the increase of alloy strength led to the decrease of the elongation(Figure 13).When the Ni content was greater than 1.2% (mass fraction), the alloy had no hot cracking tendency [42].Therefore, the strength and elongation of Al-Zn-Mg-Cu alloy could be increased by adding appropriate amount of Ni, and the thermal cracking sensitivity could be reduced, so as to improve the service performance of the material.

Effect of Ti on the Thermal Cracking Sensitivity of Al-Zn-Mg-Cu Alloy
The effects of Ti on Hot tearing sensitivity (HTS) of Al-Zn-Mg-Cu alloys were related to the content of elements.Adding 0.06% (mass fraction) Ti could reduce the hot-cracking sensitivity factor of Al-6Zn-2Mg-2Cu-Ti alloy from 17 (0.00%Ti, mass fraction) to 7 (0.06%Ti, mass fraction).But with further increase of Ti content, the hot-cracking sensitivity factor increased from 7 (0.06%Ti, mass fraction) to 13 (0.24%Ti, mass fraction) [43], which reduced the pouring performance of the alloy.

Effect of Ti on the Strength and Plasticity of Al-Zn-Mg-Cu Alloy
In addition to affecting the hot-cracking sensitivity of the alloy, 0.1% (mass fraction) Ti could also reduce the grain size and the degree of phase agglomeration during the solidification of Al-Zn-Mg-Cu alloy, thus improving the strength and plasticity of the ascast alloy.The addition of Ti changed the chemical composition of L12 precipitated phase from (Al,Zn)3Zr to (Al,Zn)3(Zr,Ti), and the lattice constant decreased from 0.4111 nm to 0.4018 nm.Because the decrease of lattice constant resulted in the lattice mismatch between the precipitated phase and Al matrix, L12 precipitated phase was refined from 18.7 ± 0.4 nm to 14.2 ± 0.4 nm [44].In addition, the addition of Ti led to the formation of Al18Mg3Ti2 metal compound phase during the solid solution process, which played an important role in the preferential nucleation of η-Mg(Zn,Cu,Al)2 precipitates in slow-cooling alloys after solid solution treatment [45].At the same time, the interface between Al18Mg3Ti2 phase and Al matrix also provided non-uniform nucleation sites for L12 precipitated phase and zinc-rich phase [46].

Brief Summary
The impurity elements Fe and Si that were controlled in a certain range could not only reduce the appearance of alloy cracks, but also improve the fatigue life and strength of the alloy.Ag played an important role in improving resistance to stress corrosion and solution corrosion.At the same time, the ductility decrease was small.Ti and Ni played a crucial role in improving strength and reducing thermal cracking sensitivity.However, the role of Ag, Ti and Ni also had a threshold and the right amount of addition could play its most effective role.And balance the relationship between mechanical properties.

Rare Earth Element
The effects of rare earth elements on Al-Zn-Mg-Cu alloys were similar, which were mainly reflected in the following aspects: on the one hand, the solubility of rare earth ele-ments in aluminum was small and they could form Al3RE phase with Al.In the solidification process, the Al3RE phase served as the heterogeneous core of Al matrix, which significantly refined the grains.On the other hand, the dispersing Al3RE phase was strongly pinned to the dislocation in the process of homogenization and solution aging, which effectively inhibited the recrystallization of the alloy and significantly increased the recrystallization temperature.In addition, rare earth elements could react with H, O, N, C, S, Si, halogen, oxide and other nonmetallic impurities in aluminum melt to form compounds.So this reduced the content of gas content, inclusions and harmful elements in liquid aluminum, which achieved the role of gas removal, slag removal and matrix purification.And it reduced porosity, porosity, slag inclusion and other defects in the ingot, so as to improve the quality of the ingot.

Refinement Effect of Sc on Al-Zn-Mg-Cu Alloy
Sc had a very low solid solubility in Al-Zn-Mg-Cu alloy and the presence of Sc led to the formation of primary Al3Sc phase, which could act as the nucleating particle of Al matrix during solidification and significantly modify the as-cast structure [47].Al3Sc phase with different primary morphologies could produce different refining effects, which would affect the mechanical properties of the alloy.Holding at 760 °C for 10 min followed by rapid cooling (at a cooling rate of 100-1000 K/s) could lead to the diversity of primary Al3Sc phase morphology, and this form of Al-Sc alloy had the best refining effect on Al-Zn-Mg-Cu-Sc-Zr alloy [48].

Effect of Sc on the ηʹ Phase Distribution and Corrosion Resistance of Al-Zn-Mg-Cu Alloys
Sc could not only affect the refining effect of Al-Zn-Mg-Cu alloy during solidification, but also could promote the uniform precipitation of η′ phase during aging, which inhibited the coarsing of η′ phase and prevented its transition to η phase [49].Compared with 7055 aluminum alloy without Sc, the addition of 0.25% (mass fraction) of Sc made 7055 aluminum alloy smaller in size and it also made η′ phase distribution more uniform, so the alloy had higher hardness, tensile strength, ductility and thermal stability.Sc microalloying could improve the corrosion potential of aluminum alloy, thus improving the corrosion resistance [50].The addition of trace Sc elements led to an increase in the proportion of grain boundaries in Al-Zn-Mg-Cu alloy, which reduced the amount of precipitated phase per unit grain boundary.Moreover, the discontinuous distribution of precipitated phase along grain boundaries reduced the anodic dissolution rate of precipitated phase, which improved the resistance of the alloy to intergranular corrosion and spalling corrosion [51].

Effect of Sc on the Damping Properties of Al-Zn-Mg-Cu Alloy
It had been shown that Sc also affected damping properties of Al-Zn-Mg-Cu alloys.The excellent damping performance of metal was due to the excellent grain boundary sliding ability of its crystal structure, and the fine grain structure always contributed to the improvement of grain boundary sliding ability.Therefore, the damping performance of Al-Zn-Mg-Cu alloy could be improved by refining the grain [52].For example, reducing grain size, promoting the formation of equiaxed crystals and introducing wetting interface between Al-Zn phase, etc. [53].7055 alloy with 0.25% (mass fraction) Sc added was further treated by friction stir, which had fine grain and equiaxed fine crystal structure and showed the best damping performance [54].

Effect of Sc on the Plastic Properties of Al-Zn-Mg-Cu Alloy
The Figure 14 showed a transmission electron micrograph obtained from the measured portion of an Al-Zn-Mg-Cu-0.10Sc-0.10Zralloy sample at a strain value of 1.10 (corresponding to 200% elongation).It could be observed that a large number of fine Al3ScxZr1- x particles with Ashby-Brown contrast were evenly distributed inside the particles (black circle) and remained almost constant in size (10-20 nm) during deformation.HRTEM image of the Al3ScxZr1−x particle was shown in Figure 14c.The Al3ScxZr1−x particle was spherical with a size of about 15 nm and was completely coherent with the aluminum matrix alloy.These coherent particles strongly fixed dislocations and grain boundaries.As shown by the arrows in Figure 14a,b, they resulted in the controlled growth of microscale grains [55].Therefore, adding an appropriate amount of Sc to the aluminum alloy could significantly improve the plastic properties of the alloy.

Refinement Effect of Zr on Al-Zn-Mg-Cu Alloy
Figure 15 shows the effects of mechanical properties of Al-5.8Zn-2.3Mg-1.7Cualloys at different Zr contents.Adding 0.05% (mass fraction) Zr had no significant effect on grain refinement of Al-Zn-Mg-Cu alloy, but the Brinell hardness reached the maximum.With the increase of Zr content, the grain refinement effect became more obvious.When the addition amount reached 0.20% (mass fraction), the spacing between secondary dendrite arms reached the minimum, and the ultimate tensile strength, yield strength and elongation at room temperature reached the maximum 296.74 MPa, 249.41 MPa and 8.26%, respectively [56].

Effect of Zr on the Quenching Sensitivity of Al-Zn-Mg-Cu Alloy
When Zr content was within a certain range, fine Al3Zr dispersion phase could be formed to increase the number of crystal nuclei and refine grains [57].For Al-Zn-Mg-Cu alloys with high cold-rolling ratio, the coherent interface between Al3Zr particles and substrate changed to semi-coherent interface after solid solution treatment, which led to nonuniform nucleation of solutes during quenching.So the quenched sensitivity increased that further increased with the cold rolling ratio.In contrast, Alloys with lower cold rolling ratios didn't show any significant quenching sensitivity [58].

Effect of Different Sc + Zr Contents on Al-Zn-Mg-Cu Alloy
The composite addition of trace Sc and Zr resulted in fine-grained strengthening, substructural strengthening, and Orowan mechanism strengthening of Al3(Sc,Zr) and Al3Zr dispersions [59].By adding 0.07% (mass fraction) Sc and 0.07% (mass fraction) Zr, the strength of the alloy increased about 133 MPa.With the addition of 0.10% (mass fraction) Sc + 0.16% (mass fraction) Zr and 0.22% (mass fraction) Sc + 0.40% (mass fraction) Zr, Al-Zn-Mg-Cu alloy could obtain better recrystallization resistance and thermal stability.And with the increase of Sc and Zr addition, Abundant nano-Al3(Sc,Zr) particles could effectively refine dislocations and subgrain boundaries(Figures 16 and 17), which showed good recrystallization resistance and precipitation strengthening effect [60].

Effect of Sc + Zr on the Thermal Stability of Al-Zn-Mg-Cu Alloy
The influence of Sc and Zr on the mechanical properties of Al-Zn-Mg-Cu alloys was mainly reflected in Al3Sc and Al3(Sc,Zr).Al3Sc was the precursor of Al3(Sc,Zr) precipitates.Susaturated solid solutions containing Sc decomposed at 250 °C, which formed Sc-rich clusters and gradually developed into Al3Sc crystal nuclei.With the increase of temperature, driven by the chemical potential gradient,Zr atoms began to diffuse over long distances and approached Al3Sc nuclei.Then they enriched around them to form Zr-rich thin layers, thus forming the Al3(Sc,Zr) core/shell structure of Al3Sc crystal nuclei + Zr-rich shells [61].The coarsening coefficient of Al3(Sc,Zr) precipitates was 3 orders of magnitude smaller than that of Al3Sc particles, which showed better thermal stability.

Effect of Sc + Zr on the Yield Strength of Al-Zn-Mg-Cu Alloy
However, there was a threshold for the beneficial effect of Sc and Zr combined addition on yield strength.The addition of trace Sc and Zr improved the hardness and yield strength of 7055-T6 alloy by strengthening the grain boundary and Al3(Sc,Zr) phase.But in Al-Zn-Mg-Cu alloy with high Cu content, Sc atoms diffused to θ-Al2Cu phase, which promoted the formation of w-AlCuSc phase [62].When the total mass fraction of Sc and Zr exceeded 0.45% (mass fraction), large w phase and coarse primary Al3(Sc,Zr) phase were formed, leading to the degradation of mechanical properties of the alloy (Table 2) [63].Sm could effectively reduce the distance between secondary dendrite arms and was a good thinning element of aluminum alloy.The addition of Sm to Al-Cu alloy could refine the Al2Cu phase in the alloy and promote its precipitation during aging [64].The ascast microstructure of Al-Zn-Mg-Cu-Zr alloy containing Sm mainly included η-(Mg(Zn,Cu,Al)2) phases, Al10Cu7Sm2 phases and Fe-rich phases, and there were also fine η phases and acicular θ(Al2Cu) phases distributed in the grain [65].The addition of Sm made the peak aging reach earlier and prolonged the peak aging time.With the increase of Sm content, the peak hardness of the alloy increased first and then decreased, and the maximum peak aging hardness was obtained at 0.3%Sm (mass fraction) [66].The effect of Sm content on the ultimate tensile strength, yield strength and elongation of Al-Zn-Mg-Cu alloy at room temperature was similar, which firstly increased and then decreased.When the mass fraction of Sm reached 0.3%, the strengthening effect was the best, and the tensile strength, yield strength and elongation were 732 MPa, 694 MPa and 9.3%, respectively.Compared with unmodified alloy, they were improved by 3.8%, 4.5% and 19%, respectively (Figure 18) [67].

Er
In Zr-containing Al-Zn-Mg-Cu alloys, Er and Zr could form dispersed Al3(Er,Zr) phases [68].Al3(Er,Zr) phase could not only serve as the strengthening phase of matrix, but also nailed grain boundaries to inhibit the migration of low-angle grain boundaries to high-angle grain boundaries [69].η phases tended to preferentially gather at the high Angle grain boundaries with higher energy, but the high Angle grain boundaries with abundant η phases were prone to anodic dissolution, which provided favorable channels for crack propagation [70].Therefore, the addition of Er could preserve more low Angle grain boundaries, thus improving the corrosion resistance of the alloy.When the Er mass fraction increases from 0% (mass fraction) to 0.10% (mass fraction), the hardness and yield strength of the alloy increased by 25.4 HV and 70 MPa, respectively, and the intergranular corrosion depth decreased by 65% [71].

La
In Al-Zn-Mg-Cu alloys, Al-Ti-B was the most commonly used refiner.When La was added to aluminum alloys containing Al-Ti-B, La mainly existed in the form of Ti2Al20La phase [72].Ti2Al20La phase was a non-thermodynamically stable phase [73], which could release rare earth elements.The rare earth could combine with TiB2 particles to form "rare earth film", which reduced the free energy of TiB2 phase and hindered the aggregation and growth of TiB2 phase, thus maintaining the long-term refining ability of Al-Ti-B.In addition, La atoms significantly promoted the nucleation and motion of di-slocation and could promote the twinning deformation at the crack tip, thus inhi-biting brittle propagation.Moreover, high concentration of La atoms was conduci-ve to promoting twin deformation and improving the plastic deformation ability o-f grain boundaries [74].When 0.3% (mass fraction) of La was added into Al-6.7Zn-2.6Mg-2.0Cu-0.1Zralloy, the tensile strength and yield strength were 583 MPa an-d 559 MPa, respectively, which were 18 MPa and 19 MPa higher than those wit-hout La modification [75].Al-Zn-Mg-Cu alloys without Ce had a high degree of Cu susaturation, which would provide more Cu atoms to form A12CuMg phase, which resulted in a more stable A12CuMg particles.After adding 0.12%Ce (mass fraction), the stable AlCuCe phase could trap a large number of Cu atoms, effectively inhibit the further formation of Al2CuMg phase [76], and it facilitated the dissolution of homogenization process.When Ce content increased to 0.3% (mass fraction), Al4Ce phase appeared in the alloy [75].In the process of solution treatment, fine Al4Ce phase could nail the grain boundary, inhibit recrystallization and refine the structure.

The Effect of Ce on the Corrosion Resistance on Al-Zn-Mg-Cu Alloy
In addition, adding Ce to Al-Zn-Mg aluminum alloy could significantly prolong the stress corrosion cracking time and reduce the stress corrosion sensitivity.When 0.04% (mass fraction) Ce was added to Al-Zn-Mg aluminum alloy, Ce atoms and Al atoms could be oxidized simultaneously to form rare-aluminum composite oxide film (Al2O3•Ce2O3 and (Al,Ce)2O3) [77], which improved the resistance of passivation film, significantly reduced the self-corrosion current density, and significantly improved the corrosion resistance of the alloy [78].The addition of 7.5% (mass fraction) Y in aluminum alloy could greatly improve the degree of grain refinement and dislocation density.And the formation of β-Al3Y intermetallic phases at grain boundaries could not only promoted nucleation, but also prevented grain growth, for which Al-7.5% (mass fraction) Y alloy could maintain fine grains in annealed state [79].In 6063 aluminum alloy, the combined addition of rare earth Y and Al-Ti-B also played a role in reducing the grain size [80].

Effect of the Phase of the Y on the Al-Zn-Mg-Cu Alloy
In Al-Zn-Mg-Cu alloys, adding Y element alone could refine dendrite substructure and increase the proportion of equiaxed crystals.The combined addition of Zr, Ti and Y could simultaneously refine the grain and dendrite substructure, and the tensile strength could reach about 700 MPa [81].The synergistic strengthening of rare earth Y with transition elements Zr and Ti might be related to the formation of Al8Cu4Y and Al3(Zr,Y) phases in the nanonetwork structure.The ideal tensile strength of Al8Cu4Y phase in (100) [001] slip system was 15.4 GPa [82], and it had good mechanical properties and thermal stability.The nanocrystalline Al8Cu4Y phase was a more ideal existence form than the conventional Al8Cu4Y phase.The strengthening of Al-Zn-Mg-Cu alloys had a wider application prospect.

Ta
The growth limiting factor Q value of Ta in aluminum alloy was 10.5 K.It had been shown that the larger the Q value is, the more obvious the grain refinement effect is [83].And Ta and Al had higher chemical activity and could form Al3Ta phase in situ.The Al3Ta phase with Al had small mismatch, which had been proven to be an efficient grain refiner for additive manufacturing [84].
Al-Zn-Mg-Cu alloy was prepared by laser powder bed melting (3D printing).The addition of 2% (mass fraction) Ta replaced Al atoms in the nano-Al2Cu phase, and Al3Ta and AlTaCu nano-phases were formed by in-situ reaction, which prevented coarsening during solidification.These nanophases could inhibit hot cracking, refine and strengthen the alloy, and accelerate the aging process by hetero-nucleation or by hindering the oriented epitaxy growth of grains, thus increasing the ultimate tensile strength from 103 ± 21 MPa to 401 ± 11 MPa (Figure 19b).The aluminum alloy with Ta had a high Angle grain boundary, which could be used as a source of dislocation in the deformation process [85].Therefore, the grain boundary surface of the alloy melted by laser powder bed had a high density of dislocation sources, which was conducive to deformation and thus to enhancing ductility.The addition of Ta transformed the fracture morphology from brittle intergranular fracture (Figure 19a) to ductile fracture (Figure 19c), and the elongation increased from 0.8 ± 0.4% to 5.1 ± 0.6%.In addition, the discontinuous ultrafine grains and grain boundaries could inhibit the expansion of corrosion path, reduce the corrosion sensitivity of grain boundaries, and improve the Cl -corrosion resistance of Al-Zn-Mg-Cu alloy [86].

Brief Summary
In summary, the rare-earth phases are summarized as follows (Table 3).Sc played an important role in refining grain, improving toughness and corrosion resistance [87,88].However, due to its high cost, it was not conducive to widespread industrial application and it was necessary to achieve the same effect by low-cost rare earth elements or composite addition.For example, the combined addition of Sc and Zr could not only promote precipitation strengthening, but also improve thermal stability.Other rare earth elements Sm, Er, La, Ce, Y and Ta all played different roles in refining grains.Among them, Er and Ce could greatly improve the corrosion resistance of the alloy.Sm and La played a role in improving the tensile strength.It is important to clarify the main functions and effects of each rare earth element.Compound addition of rare earth elements that reduced cost and balanced performance were urgent problems in the development of alloys.

Forecast
The effect of alloying on the microstructure and properties of Al-Zn-Mg-Cu alloy was reviewed.For the purpose of corrosion resistance, the ratio of Zn/Mg should be appropriately reduced to improve the corrosion resistance, and the content of Cu should be controlled to avoid the potential difference between grain boundary and Al matrix to cause the corrosion of precipitated phase.In order to achieve comprehensive energy, the ironrich phase should be made as small as possible to reduce the adverse effects of impurity elements such as Fe and Si on plasticity and fracture toughness.By adding Sc, Zr and other rare earth elements, solution strengthening, aging strengthening and fine crystal strengthening should be introduced to improve the strength and toughness of the alloy.In addition, considering the machining properties of Al-Zn-Mg-Cu alloy, the quenching sensitivity of the alloy should be reduced by adding an appropriate amount of Ge element and the hot cracking tendency should be reduced by adding Ni element.According to the research status and existing problems of alloying Al-Zn-Mg-Cu alloy, the future research directions are as follows: (1).Optimize the content of main alloy elements and control the content ratio of different main alloys, especially the content ratio of Zn/Mg to Zn/Cu.The influence mechanism of the main alloying elements on the microstructure and properties of Al-Zn-Mg-Cu alloy is studied deeply, and the action threshold of the main alloying elements is defined.
(2).Reduce the content of impurity elements to reduce the influence of impurity elements on the microstructure and properties of the alloy.The strong toughness and corrosion resistance of the alloy can be improved by adding microalloying elements as needed, especially by adjusting the content of rare earth elements and adding compound rare earth elements..The effects of precipitated phase content and distribution on the properties of Al-Zn-Mg-Cu alloys are analyzed and studied to improve the comprehensive mechanical properties of Al-Zn-Mg-Cu alloys.
(3).Combining the effects of main alloying elements and trace alloying elements, the optimal alloying ratio of elements is analyzed through experimental tests, and the mechanism is explored to further optimize the microstructure and mechanical properties of the alloy.

Figure 2 .
Figure 2. Stress-strain curves (a) of Al-9.3Zn-2.4Mg-xCu-Zr alloy and evolution of the EXCO rating as a function of immersion time for the alloys in the T6 heat treatment (b) (Reprinted with permission from Ref. [5].2023 Elsevier).

2. 3 . 4 .
Effect of Zn on Thermal Stability of Al-Zn-Mg-Cu Alloys BF TEM images and corresponding SAED modes of Al-Mg-Cu alloy with Zn addition after aging at 125 °C for 192 h were shown in the Figure

Figure 9 .
Figure 9. Mechanical properties of peak aged samples: (a) engineering stress-strain curves; (b) strength and elongation values as a function of Zn/Mg ratios (Reprinted with permission from Ref. [21].2023 Elsevier).

3. 1 . 2 .
Effect of Fe and Si on the Recrystallization Process of Al-Zn-Mg-Cu Series Alloys Fe and Si could change the mechanical properties of Al-Zn-Mg-Cu alloy by influencing the recrystallization process.The change of grain boundary orientation distribution was closely related to recrystallization caused by Particle stimulated nucleation (PSN) of Al7Cu2Fe and Mg2Si

3. 3 .
Ni 3.3.1.Effect of Ni on the Thermal Cracking Sensitivity of Al-Zn-Mg-Cu Alloy It had been shown that adding Ni to Al alloy could improve the strength at room temperature or high temperature, which had been confirmed in Al-Zn-Mg-Cu alloy [40].

4. 4 . 2 .
Effect of Sm on the Yield Strength and Elongation of Al-Zn-Mg-Cu Alloy

Figure 18 .
Figure 18.Tensile properties of the Al-Zn-Mg-Cu alloys with different Sm contents: (a) stress-strain curves, (b) relationship of ultimate tensile strength, yield strength and elongation with Sm contents (Reprinted with permission from Ref. [67].2023 Elsevier).

1 .
Effect of Y on the Effect of Al-Zn-Mg-Cu Alloy Refinement and Dislocation Density

Table 3 .
Rare-earth phase in the aluminum alloy.