Peak-aged Condition Research Articles

Enhanced long-term thermal stability and mechanical properties of twin-roll cast Al–Mg–Si alloys with Mn and Cu additions

The long-term thermal stability of Al–Mg–Si alloy is of great significance for its engineering application. In this work, the effects of Mn and Cu on the microstructure and mechanical properties of the twin-roll cast (TRC) AA6005 alloy after peak aging and long-term thermal exposure were studied. The results show that the addition of Mn introduces Al(FeMn)Si dispersoids, which improves the mechanical properties of the alloy under the peak aging condition by dispersion strengthening. After thermal exposure at 150 °C for 1000 h, the Al(FeMn)Si dispersoids have good thermal stability while do not significantly improve the mechanical properties. The reason may be related to the decreased number density of the β″ phase after alloying with Mn. The addition of Cu changes the aging precipitation sequence, and the needle-like pre-Q′ phase is formed during artificial aging. Moreover, due to the low diffusion rate of Cu in the Al matrix, Cu atoms can better stabilize the structure and then improve the coarsening resistance of the precipitates, and therefore the Cu-containing precipitates exhibit better thermal stability than the β″ phase. After thermal exposure at 150 °C for 1000 h, the yield strength of the 0.5Cu alloy decreases by only ∼12 MPa (from ∼295 to ∼283 MPa), which is significantly lower than that of the AA6005 alloy (∼41 MPa, from ∼274 to ∼233 MPa). The yield strength of the 0.25Mn0.25Cu alloy decreases by ∼27 MPa (from ∼294 to ∼267 MPa) and is more suitable for industrial production as its composition is within the range of commercial 6xxx aluminum alloys. The results would provide a strategy for the preparation of Al–Mg–Si alloys with high thermal stability.

Precipitation behavior of an Al7SiMg alloy processed by laser powder bed fusion during non-isothermal and isothermal heat treatments

Laser powder bed fusion (LPBF) is drawing great interest in industry due to its ability of producing complex-shaped components with superior mechanical properties and unique microstructures. Al7SiMg alloy is one of the most used aluminum alloys in LPBF due to its easy processing window. Direct T5 aging is often applied to LPBF-processed Al7SiMg alloy to obtain better mechanical performances over T6 heat treatment. However, after T5 heat treatment, a comprehensive understanding of the precipitation sequence of the strengthening phase in LPBF-processed Al7SiMg alloy is still lacking. In the present study, a high-resolution TEM examination of the precipitation behavior of Al7SiMg alloy processed by LPBF during heating without and with isothermal holding was conducted to further understand the precipitation sequence of Si precipitates and the associated microstructure evolution. During the heating process, irregular and needle-shaped pure Si precipitates were observed which shared the same [011] zone axis with the aluminum matrix. After analyzing the phase transformation process during heating, the following precipitation sequence is proposed: Si in supersaturated solid solution → Si-rich clusters → Si-rich intermediate phases → pure Si precipitates. During isothermal holding at 160 ℃ (T5), nano-scale Si-rich clusters were observed at the peak aging condition. When the holding time extended to 24 h, larger Si-rich clusters with reduced coherence were found. Strain fields caused by these Si-rich clusters are believed to strengthen the LPBF-processed Al7SiMg alloy after T5.

Effect of Aging on the Ballistic Performance of AA 7017 Alloy


 This study presents the effect of aging on microstructure, texture, mechanical and ballistic properties of AA-7017 aluminium alloy. AA-7017 alloy is subjected to three different aging namely under-aging, peak-aging and over-aging. Significant difference is observed in the mechanical properties after aging. The alloy exhibits maximum strength and hardness in the peak-aged condition. From texture measurements, it is noticed that the overall intensity of orientation distribution function increases from under-aged to peak-aged condition. The AA-7017 aged plates are impacted with 7.62 mm steel core projectiles at 820±10 m/s to evaluate the ballistic performance. It is noticed that AA-7017 alloy displays best ballistic resistance in peak-aged condition. Distorted material flow lines and adiabatic shear band induced cracking is detected in post ballistic microstructure. The ballistic performance of AA-7017 alloy is correlated with the variations in the microstructure and mechanical properties with aging

Enhancing mechanical properties of 7055 Al alloy by adding 1 wt.% Mg and tailoring the precipitations

The 7055-Mg alloy with the chemical composition 1 wt.% Mg higher than that of 7055 aluminum alloy was prepared by the powder metallurgy route. The mechanical strength of the 7055-Mg alloy was significantly improved by comparison with the 7055 alloy doping with other elements and particles. The microstructural and mechanical property evolutions of the alloy were systematically studied. The results reveal that the sample fabricated in this work has fine grains and a uniform microstructure. The addition of 1 wt.% Mg promoted the precipitation of η’ phase, which greatly shortened the PA time of 7055 alloy and improved the preparation efficiency of the alloy. The peak-aging (PA) state was obtained by ageing at 120 ℃ for only 16 h after solution treatment. It was also found that the volume fractions and sizes of η’ phases could be tailored by post heat treatment, thus optimizing the mechanical properties of samples. The 120 ℃/16 h sample has an excellent combination of the high ultimate tensile strength (UTS) of 701 MPa and good elongation of 6.7%. After 64 h aging, the alloy showed a slight tendency of over aging. However, the UTS of 120 °C/64 h sample is still up to 684 MPa, indicating that the alloy has excellent over-aging resistance. The main strengthening mode of the alloy is Orowan dislocation bypass mechanism. This study provides an effective approach for improving the mechanical properties of 7055 alloy and expects to inspire the composition design of aluminum alloy.

Effect of low-temperature annealing on electrical resistivity and mechanical properties of laser-powder bed fused AlSi10Mg alloy

The AlSi10Mg sample was additive-manufactured using laser powder bed fusion (L-PBF), and subsequent low-temperature annealing (LTA) at 413K or 473 K was applied. Vickers hardness tests show the peak at around 160 HV by 473 K annealing and around 145 HV by 413 K annealing. The flow stress of tensile and compression deformations are also affected by LTA, and peak aging condition gives the highest 0.2% proof stress. 0.2% proof stress obtained by tensile tests is around 270 MPa, whereas that obtained by compression tests is between 326 MPa and 386 MPa. Electrical resistivity at 293 K changes from approximately 80 nΩm down to around 60 nΩm, whereas that at 77 K changes from around 40 nΩm down to around 20 nΩm. The estimated concentration of solute Si in the as-built specimen is between 2.5 mass% and 2.9 mass%, corresponding to the previous results using energy-dispersive X-ray spectroscopy. The lattice constant obtained from XRD increased from 0.4043 nm up to 0.4048 nm by LTA. Both electrical resistivity and XRD measurement indicate a decrease in the concentration of solute atoms. Microstructure observations revealed numerous nano-sized precipitates of Si phase with a plate-like or granular morphologies in α-Al grains due to LTA.

On anisotropic tensile mechanical behavior of Al–Cu–Li AA2198 alloy under different ageing conditions

The anisotropic mechanical behavior of Al–Cu–Li (AA2198) alloy under different artificial ageing conditions as well as for different thicknesses of the material was examined in the present investigation. Material characterization was performed for three different sheet directions, namely, longitudinal (0°), diagonal (45°) and transverse (90°) to the rolling direction. The results showed that the grain structure did not have significant changes by applying different artificial ageing times. In T3 heat treatment condition, only δ′ (Al3Li) phase was observed, while with increasing the ageing time, the major precipitates were T1 (Al2CuLi) phases. The latter was found to increase in size with artificial aging time. Almost similar yield strength values were observed for all investigated thicknesses, while the higher thickness specimens showed higher elongation at fracture values. Anisotropy was slightly higher at T3 condition, while the lowest anisotropy degree was noticed at the peak-aged condition. The specimens extracted in the diagonal direction showed high variation in mechanical properties, when compared with the respective results at the other two investigated directions, independently from the thickness of the specimens as well as from heat treatment condition. The smaller thickness specimens presented lower elongation at fracture values and for all investigated sheet directions, since they undergo plane stress conditions. Analysis of work hardening behavior showed that Bauschinger effect, which was not observed in T3 condition, became increasingly more prominent with artificial ageing time in all directions investigated.

Enhanced precipitation kinetics in non-stretched Al-Cu-Li-Mg-Ag-(Sc)-(Zr) alloys

This work investigates the impact of Sc and Zr on the precipitation kinetics in an Al-Cu-Li alloy in the absence of pre-stretching. Modern Al-Cu-Li alloys usually require pre-stretching to accelerate the formation kinetics of the T1 precipitates. A combination of in-situ Small-Angle X-ray Scattering (SAXS) and hardness are used to monitor the precipitation kinetics in an Al-Cu-Li alloy with and without Sc and Zr. The presence of Al3(Sc, Zr) dispersoids is found to significantly accelerate the precipitation kinetics. Surprisingly, transmission electron microscopy (TEM) reveals the absence of T1 precipitates in the peak aged condition and instead reveals that θ′ precipitates dominate the microstructure. Atom probe tomography (APT) and density functional theory (DFT) calculations reveals that the Al3Zr-shell of the dispersoids is a preferred heterogeneous nucleation sites of θ′ precipitates. Additions of Sc and Zr can hence be a suitable solution to promote precipitation in Al-Cu-Li alloys where the pre-stretching is not applicable.

Effect of Zn addition on microstructure and mechanical properties of Mg-3Y-2Nd-0.5Zr alloy

The effect of 0.5wt.% Zn addition on the microstructure and mechanical properties of Mg-3Y-2Nd-0.5Zr (WE32) alloy was investigated. The results indicate that WE32-0.5Zn alloy takes 48 h to reach peak hardness after solid solution treatment at 525 °C and aging at 200 °C, 10 h earlier than WE32 alloy, which implies an accelerated aging precipitation kinetics owing to the addition of 0.5wt.% Zn. A large quantity of fine-rod and rectangular block-like Zn-Zr precipitates in the α-Mg matrix are formed in the WE32-0.5Zn alloy, and numerous needle-like β1 phases are distributed at both ends of the Zn-Zr precipitates at peak-aged condition. In peak-aged condition, the ultimate tensile strength considerably increases from 263.2 MPa (WE32) to 309.6 MPa (WE32-0.5Zn), and the elongation dramatically increases from 4.3% (WE32) to 8.9% (WE32-0.5Zn). The β′ and β1 phases are the main precipitates of the WE32-0.5Zn alloy peak-aged at 200 °C. The β′ and β1 phases easily nucleate at the Zn-Zr precipitates, and the β1 phases are particularly likely to nucleate and grow at the interface between the two ends of the Zn-Zr precipitates, which accelerates aging precipitation kinetics and leads to a shorter time to achieve peak aging.

Effects of Mg2Sn precipitation on the age-hardening and deformation behaviour of a Mg-Sn-Al-Zn alloy

The influences of Mg2Sn precipitation on the age-hardening and deformation behaviour of a Mg-4Sn-3Al-1Zn (wt.%) alloy have been systematically investigated via the assistance of analytical electron microscopy and X-Ray Computed Tomography (XCT) techniques. The Mg-4Sn-3Al-1Zn alloy shows a moderate hardening response during the ageing heat treatment at 175 °C, and a ∼35% increment of yield tensile strength was achieved with significantly deteriorated tensile elongation under the peak-aged condition. Nanosized Mg2Sn precipitates are uniformly distributed within grains, improving tensile strength properties. In addition, coarsened Mg2Sn precipitates can be observed along with precipitate-free zones (PFZs) at grain boundaries. During tensile deformation of the solution-treated alloy, plastic strain tends to be localized in the slip band array regions in grain interiors. For the as-aged alloy, strain localization primarily occurred in the PFZs. The degraded tensile ductility was associated with the initiation of early intergranular microcracks caused by coarse grain boundary Mg2Sn precipitates and PFZs. The results provide insights and implications for the development of affordable Mg alloys with a balanced combination of strength and ductility.

Age-hardening behavior, corrosion mechanisms, and passive film structure of nanocrystalline Al-V supersaturated solid solution

• Synthesis of a nanocrystalline Al-5at.%V alloy with high solid solubility of V by high-energy ball milling • Study on the age hardening behavior of the ball milled Al-V alloy • Corrosion behavior as a function of age hardening time and temperature • Study of the passive film and pits using transmission electron microscope • Microstructural features causing passive film breakdown and subsequent pit growth or repassivation The effect of age-hardening on microstructure, hardness, and corrosion of an Al-5at.%V alloy, produced using high-energy ball milling and subsequent cold compaction, has been investigated. The alloy exhibited a grain size below 100 nm and extremely high solid solubility of V in Al (3.1 at.%). The age-hardening was carried out at 150, 200, and 250 ℃. The peak-aged condition of 150 ℃ demonstrated the highest hardness—transpired from grain refinement, precipitation, and solid solution hardening. The corrosion resistance of the Al-5at.%V alloy was studied as a function of aging conditions. The peak-aged condition retained the corrosion resistance while it deteriorated in the over-aged condition. Nonetheless, the corrosion resistance of the ball-milled Al alloys in all the aging conditions was superior to that of pure Al. The passive film structure and origin of corrosion were studied using scanning/transmission electron microscopy (S/TEM). The high corrosion resistance of the alloy was attributed to the V enrichment at the film/metal interface and deposition of V on the cathodic phases, which suppresses the dissolution of Al within the pit and therefore promotes repassivation in the early stages of corrosion.

Effect of Al Content on Microstructure and Properties of Cu-14Ni-xAl Alloys

This paper focuses on the influence of Al content on the microstructure and properties of Cu-14Ni-xAl alloys, with the mass fraction of aluminum element ranging from 0 to 5% as the object. First, the evolution of Vickers hardness and electrical conductivity of Cu-14Ni-xAl alloys during aging were tested. Then a comparative study of the friction and wear behavior of Cu-14Ni-xAl alloys with different Al contents were studied. The results show that the alloy with Al content of 3% possesses the best mechanical and friction performance, with the hardness of HV 275.0 and friction coefficient of 0.7 in the peak-aged state. Microstructure analysis show that the precipitates are denser and the volume fraction of aging induced precipitates are more in the Cu-14Ni-3Al alloy, compared with that in the other studied alloys. Finally, the correlations between the Al content, microstructure, the mechanical and tribological properties of the Cu-Ni-Al alloys were further discussed.

HRTEM Characterization of an Age-Hardened Mg–Ca Binary Alloy

A Mg–2.8 mass% Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg2Ca phase (with C14 structure), which precipitated in the primary α-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg2Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {-1010}α columnar plane of the α-Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the α-Mg matrix for aging time of 3 h, which corresponded to the peak-aged condition. Furthermore, during the coarsening process, misfit dislocations were introduced on the planar surface of the precipitates, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.

Effect of La addition on microstructure, mechanical behavior, strengthening and toughening mechanisms of cast Mg-Gd-Zn alloy

High strength cast Mg-RE (magnesium-rare earth) alloys usually show a poor balance between strength and ductility, typically high strength with low ductility. Grain coarsening during solution treatment is one of the reasons for the reduced ductility. La element has very limited solubility in Mg and the La-containing second-phase is expected to pin the grain boundaries and restrict the significant grain coarsening during solution treatment. Therefore, different amounts of La addition are introduced to the Mg-Gd-Zn casting alloy and the microstructure and mechanical properties are systematically studied. The addition of La results in the formation of the Mg12La second-phase along the grain boundaries during solution treatment, significantly increasing the number density of the β’ precipitates while slightly reducing their sizes. The Mg12La phase along the grain boundaries restricts grain coarsening, resulting in very low grain coarsening rates in the La-containing alloys. Compared with high strength cast Mg-RE alloys in literature, the present La-containing alloys indicate better balance between strength and ductility. The 0.4 La alloy exhibits the highest yield strength (YS) of 247 MPa and ultimate tensile strength (UTS) of 397 MPa with moderate elongation (El) of 5.8% in the under-aged condition; while the 0.2 La alloy with the highest product of strength and elongation (PSE) of 4.63 GPa·% presents a good combination of high strength and outstanding elongation (YS: 245 MPa; UTS: 383 MPa; El: 12.1%) in the peak-aged condition. Micro-alloying with La element is expected to be an effective strategy to improve the strength and ductility of cast Mg alloys simultaneously.

Microstructure evolution and mechanical properties of the Al-Cu-Mg-Ag alloy during non-isothermal aging

In this work, the effect of non-isothermal aging treatment with the heating rate of 20, 40 and 60 °C/h on the precipitation behavior and mechanical properties of the Al-Cu-Mg-Ag alloy were investigated by differential scanning calorimetry (DSC) analysis, transmission electron microscopy (TEM) and high-angle annular dark field – scanning transmission electron microscopy (HAADF-STEM) observation, as well as hardness and tensile tests. The activation energy and Avrami exponent associated with the precipitation of Ω phase were calculated based on the non-isothermal DSC experiments with various heating rates. The size distribution and number density of Ω plates in the α-Al matrix under peak-aged condition were quantitatively counted. Besides, A non-isothermal aging kinetics model proposed was applied to evaluate the heating aging treatment (HAT) process of the Al-Cu-Mg-Ag alloy. Results indicate that HAT modifies the size and density of Ω phase, thereby enhancing the mechanical properties of the Al-Cu-Mg-Ag alloy. HAT promotes the precipitation of Ω phase and shortens the aging time for the alloy to reach the peak-aged condition. It is interesting that the size distribution of Ω plates shifts to larger values with the increase of the heating rate during the HAT process.

Dual role of trace elements in magnesium dissolved age hardened A356 alloy on microstructure and peak micro hardness

The present work reflects the effects of trace addition of copper (up to 1 wt.%) and zinc (0.5 wt.%) on the microstructure and hardness property of heat treated A356 (Al-7Si) alloy. Small amount of zinc and copper was introduced into A356 in both atomic form (alloy) and powder form (composite) and its age hardening behaviour were investigated. To enhance the wettability during composite fabrication and solid solution strengthening in alloys, a small quantity of magnesium (1 wt.%) was added. The main objective of this study is to introduce lower melting point zinc reinforcement into A356 matrix alloy by copper coating of zinc particles and then reinforcing it into A356 matrix. All stir cast specimens were subjected to T6 treatment by solutionizing at 520 °C for 2 h followed by 60 °C water quench and then aging at 100 and 200 °C to determine peak hardness value. Microstructure analysis showed that a minimum 1 wt.% copper was required to form Al2Cu intermetallic phase during solidification resulting in finer grain structure with hardness improvement. Under as-cast conditions, a maximum hardness of 85 VHN was obtained in 1 wt.% copper reinforced composite. Under peak aged conditions, aging at 100 °C showed 116% hardness improvement in 1 wt.% Cu reinforced composite.

Effect of trace Zn addition on microstructure and corrosion behavior of AA2024 under different aging treatments

The effect of trace Zn (0.67 wt%) addition on the microstructure and corrosion behavior of AA2024 under different aging treatments was investigated. The results reveal that, with trace Zn addition, the corrosion depth decreases at the natural aging condition, but increases at the artificial aging treatment (both the peak aging condition and over aging condition). The corrosion morphology of the Zn-containing alloy under different aging treatments is also different from that of the Zn-free alloy. Compared with the Zn-free alloy, the dealloying process of the S phase in the Zn-containing alloy is slower, which are not conducive to the formation of pitting corrosion. Transmission electron microscope shows that trace Zn addition can promote the nucleation of the intragranular Guinier-Preston-Bagaryatski zones which transform into S phases in the subsequent aging process. Compared with the Zn-free alloy, the S phases of the Zn-containing alloy are coarser at the over aging condition. Moreover, details of how the corrosion morphology of Zn-containing alloys changed and the relationship between the microstructure and the corrosion behavior were discussed. • Trace Zn inhibits the dealloying of Intermetallic S particles. • The local corrosion depth at the natural aging condition is small and the local corrosion depth at the artificial aging condition is large in the AA2024 with trace Zn compared with the Zn free alloy. • Zn segregated at the S-Al interface has been characterized. • Trace Zn promotes the growth and coarsening of grain boundary S phase particles in the AA2024 under artificial aging treatments. • Zn promotes the coarsening of S phase at the grain boundary and the aggregation of Zn at the S-Al interface works together on the local corrosion morphology of the alloy.

Effects of pre-aging on microstructure evolution and deformation mechanisms of hot extruded Mg–6Zn–1Gd–1Er Mg alloys

Abstract The influence of pre-aging treatment on the microstructure, texture and mechanical properties of the Mg–6Zn–1Gd–1Er (wt.%) alloy was investigated. The microstructure analysis shows that the presence of pre-aging is beneficial to { 10 1 ¯ 2 } twin nucleation at the early stage of extrusion and inhibits the growth of twins and promotes the formation of [ 10 1 ¯ 0 ] -fiber texture components, thus accelerating the complex process of recrystallization. In the middle stage of extrusion, the extruded samples under the condition of solid solution were subjected to dynamic precipitation during severe shear deformation. The precipitation of the second phase particles followed the particle stimulating nucleation (PSN) mechanism, which increased the volume fraction of DRX grains during extrusion. In the extruded samples under the peak-aged condition, the particles appear dissolved during the severe shear deformation strain, which slows down the DRX process. In the later stage of extrusion, the small rod-shaped particles followed the PSN mechanism, and finally formed the strong fiber texture. The extruded alloy exhibits the strongest mechanical properties under the peak-aged state, with ultimate tensile strength (UTS) of 346 MPa, tensile yield strength (TYS) of 217 MPa, and elongation to failure (EL) of 13.6%. The improvement of mechanical properties is mainly attributed to the existence of strong fiber texture, small rod-shaped and block-shaped phases.

Dissimilarity of physical and mechanical properties of Al-(0.2–17.9 wt%) Si automotive alloys in under ageing, peak ageing, and over ageing conditions

The current study investigates the impact of under, peak, and over-aging on the mechanical and physical characteristics of Al-based Si-doped automotive alloys. The Al–Si–Cu–Mg alloys with 0.2, 3.5, 6.1, 12.7, and 17.9 wt% Si are cast in a conventional manner, followed by homogenizing, solutionizing, quenching, and ageing. According to the experimental data, peak-aged alloys' increased hardness is mostly caused by the production of hard and brittle metastable Al2Cu, Mg2Si, and iron-rich precipitates inside the Al matrix. A higher level of Si-rich intermetallics makes the alloy of superior strength as it hinders the dislocation movement. Nevertheless, the tensile strength decreases beyond the eutectic composition as the formation of primary Si throughout the alloy distorts the mechanical properties. Additionally, peak-aged conditions show a decreased percentage of elongation and impact energy because of the substantial production of various fine precipitates along the grains. At the over-aged state, the strength drops drastically for precipitation coarsening as coarse particles reduce the effective pinning effect letting the dislocations move. Similarly, the alloys do not offer enough strength in under-aged conditions as the elements remain in a solid solution but are not precipitated. Thermal conductivity decreases with the Si content for a higher level of precipitates. The over-aged condition shows the highest values due to the alloys' stress relieving and grain coarsening effect. An analysis of the alloys' microstructure shows that the solution treatment enhances the distribution of silicon grains. The microstructure of the grain in the over-aged stage reveals that it has totally recrystallized, for which, compared to the eutectic composition, the primary Si is more common in the alloys.

Microstructural Influences Caused by Different Aging Strategies on the Strain-Dependent Damping of the High-Strength Aluminum Alloy AA7075

The present study focused on the influence of different aging conditions on the strain-dependent damping of the high-strength aluminum alloy AA7075. For this purpose, different artificial aging strategies were carried out after solution heat treatment with subsequent water quenching to identify correlations between microstructural evolution, hardness development, and individual material damping. The resulting material damping was measured using an experimental setup based on the principle of electromagnetic feedback. Scanning transmission electron microscopy (STEM) investigations were carried out using a scanning electron microscope (SEM) to characterize the material’s microstructure. Depending on the aging conditions, the damping investigations revealed specific characteristic behaviors in the strain-dependent range from 1 × 10−7 to 0.002. Peak aging conditions showed lower damping than the overaged conditions but resulted in the highest hardness. The hardness decreased with increasing aging time or temperature.

Effect of Cu Additions on the Evolution of Eta-prime Precipitates in Aged AA 7075 Al–Zn–Mg–Cu Alloys

In the present study, after solid solution treatment, four different artificial aging treatments (100, 120, 140 and 160 °C) were performed on Al-5.98Zn-2.86Mg-1.61Cu (wt.%) alloy, denoted as 7075-LCu, and Al-5.91Zn-2.83Mg-1.98Cu (wt.%) alloy, denoted as 7075-HCu. Peak aging conditions were determined for each aging temperature at various hold time intervals of up to 24 h. It was found that both alloys possessed the optimal strengths after artificial aging at 120 °C for 24 h. Under this condition, the ultimate tensile strengths (UTSs) were 618 MPa (7075-LCu) and 623 MPa (7075-HCu), respectively. Moreover, a method was used to calculate the average sizes and number density of the major strengthening precipitates, η′, under peak aging conditions in these two alloys from transmission electron microscopy (TEM) images and electron energy loss spectroscopy (EELS). The above results indicated that for the 7075-LCu and 7075-HCu samples with the optimal UTS strengths, the former possessed an average thickness of 2.15 nm, and a number density of 3.27 × 1017 cm−3; the latter, 2.04 nm and 3.52 × 1017 cm−3.