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The effect of Zn/Mg ratio on the microstructure and properties of Al-Zn-Mg-Cu alloys under single-step and double-step aging processes were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), as well as hardness, room temperature tensile, and fracture toughness testing. During single-step aging, as the Zn/Mg ratio increases, the aging response of the alloys accelerates. However, the peak hardness and strength gradually decreased while the average size of precipitates increased. During double-step aging, as the secondary aging time increases, the strength and hardness of the alloys gradually decrease. Additionally, the higher the Zn/Mg ratio, the faster the strength and hardness decline. Under the T74 temper, the alloy with higher Zn/Mg ratios exhibits lower hardness and strength but better fracture toughness and larger precipitate sizes. A simple method was proposed to statistically determine the critical diameter size of GPII zone and η′ phase, which was approximately 4.47 nm. A strength model was developed to predict the yield strength of alloys, considering factors such as solid solution strengthening, grain boundary strengthening, and precipitation strengthening. The model demonstrates good predictive accuracy for both peak-aged and T74 temper alloys, providing valuable guidance for alloy design and microstructure regulation.

The microstructure of Al-Cu-Li alloys is influenced by the alloying contents, which in turn affect the ultimate performance. This study examined the microstructure characteristics of Al-Cu-Li alloys with varying Li contents (1.05, 1.30, and 1.66 wt%) in as-cast state, as well as microstructure evolution during homogenization treatments. The investigation employed a range of analytical techniques, including optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), differential scanning calorimeter (DSC), focused ion beam (FIB), transmission electron microscopy (TEM), and thermodynamic calculation software. The findings indicate that the eutectic phase exhibited significant variations with the Li content changing. Three distinct low-melting phases were identified in 1.05Li alloy, including Ag-containing Al2CuMg, Al7Cu4Li, and Al2Cu. As Li content was elevated to 1.30 wt%, there was a noticeable drop in the quantities of Ag-containing Al2CuMg and Al7Cu4Li phases. Conversely, the Al2Cu phase increased, and a few Al2CuLi phases emerged. When the Li raised to 1.66 wt%, an increased Al2CuLi phase was discovered, which existed along the dendritic edges. However, there were only little Ag-containing Al2CuMg phase, and no Al7Cu4Li phase detected. The initial melting temperatures for the four soluble eutectic phases were systematically arranged in the following order: Ag-containing Al2CuMg

The content of Zn and Mg directly influences the microstructure characteristics of Al–Zn–Mg–Cu alloys. In the present study, the solidification phases of three as-cast Al–Zn–Mg–Cu alloys with different Zn/Mg ratio (1.5, 1.8 and 2.1 at.%, successively) were thoroughly investigated, and their evolutions during homogenizations were analyzed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results showed that serious dendritic segregation existed in the as-cast microstructures. In the as-cast alloys with three different Zn/Mg ratio, the secondary phases are represented by non-equilibrium intermetallic phases (Mg(Zn,Cu,Al)2 and Al2Cu) and precipitate MgZn2 phase. Thereinto, the Mg(Zn,Cu,Al)2 phase possessed a majority, which was determined through computational analysis of solidification processes. With the increase of Zn/Mg ratio, the amount of Mg(Zn,Cu,Al)2 phase continuously decreased, while the content of Al2Cu phase showed only minor variation. As the Zn/Mg ratio increased, the strength and hardness of the as-cast alloy gradually improved. The Zn/Mg ratio has a significant impact on the dissolution and transformation of the secondary phases during homogenization process. As the Zn/Mg ratio increased, the melting point of the as-cast eutectic gradually decreased. Additionally, due to the higher diffusion rate of Zn compared to Mg and Cu elements, the dissolution rate of the secondary phase increased accordingly. At relatively low temperatures (380, 410, and 440 °C), phase transformation from the Mg(Zn,Cu,Al)2 phase to the Al2CuMg phase occurred in all three alloys. However, phase transformation was only present in Low/Mid-Zn/Mg ratio alloys at 470 °C. These differences are attributed to the varying diffusion rates of Zn, Mg, and Cu elements during the homogenization process. The implementation of a two-stage homogenization process effectively addresses the issue of residual Al2CuMg phase in Low/Mid-Zn/Mg ratio alloys.

In this study, the effect of Sc content on the microstructural characteristics and phase evolution of an Al–Cu–Li alloy under as-cast and homogenization conditions was investigated. The results show the grain refinement effect was distinct when the content of Sc was 0.16 wt%. However, further increased in Sc content resulted in a limited decrease in grain size. Besides, In the 0.16Sc alloy, the flocculent phase with primary Al3(Sc,Zr) phase coexisting with W (Al8-xCu4+xSc) phase appeared, and in the 0.32Sc alloy, the additional blocky primary Al3(Sc,Zr) phase was observed. With the increased of Sc content, the proportion of primary Al3(Sc,Zr) and W phases gradually increased, and the number and size of Al2Cu and Ag-containing Al2CuMg phases at grain boundaries gradually decreased. Two different morphologies of the primary W phase, flocculent and long strip shaped, were revealed within the Sc-containing alloy. The formation of the core-shell structure formed by Al3(Sc,Zr) particles and W phase was generated by the peritectic-eutectic reaction (Liquid + Al3(Sc,Zr)→α-Al + W) and the long strip of W phase was generated by the eutectic reaction (Liquid→α-Al + Al2Cu + W). During the homogenization process, the Al2Cu and Ag-containing Al2CuMg phases were completely re-dissolved after the two previous stages of homogenization, and only a small amount of the spheroidal residual W phase persisted. A subsequent low temperature treatment was employed to allow the Sc atoms that had diffused into the Al matrix to precipitate as Al3(Sc,Zr) dispersoids, resulting in a higher number density of Al3(Sc,Zr) particles for more efficiently utilizing the Sc elements.

A quantitative relationship between microstructure and tensile properties of Al–Zn–Mg–Cu alloys with various alloying degrees was investigated in this study. Three kinds of high strength Al–Zn–Mg–Cu alloys with little strength difference were used for the microstructural characteristic and statistical analysis. A strength model with high accuracy was established through the systematical calculation of strengthening theories, which was validated by the microstructure and ultimate tensile strength of a testing sample. The model gives the precise ratio of the contribution of precipitation hardening and even the component achieved from the bypass mechanism. Moreover, guidance on the regulation of precipitation phases was proposed from the strength model. It can be disclosed that the alloys will show the best strength when the precipitates of 7xxx series Al alloy are distributed near the right of critical radius, which is identified as 2.5 nm in this model.