Al–Zn–Mg alloys with different ratios of Zn/Mg have been investigated to understand the influence of the Zn/Mg ratios on age-hardening, microstructures and precipitates. Alloys containing higher ratios of Zn to Mg revealed higher peak hardness, ultimate tensile strengths, 0.2% proof stresses and higher number densities of precipitates per unit area. A square shaped T′-phase has been observed in the matrices of low Zn/Mg ratio alloys aged at 423 K with the aid of transmission electron microscope (TEM) images and its selected area electron diffraction (SAED) patterns, while SAED patterns of η′ and/or η-phase have been detected in high Zn/Mg ratio alloys. When aging temperature is lowered to 393 K, η′/η-phase have been again observed in the matrix, though no T′-phase was detected in low Zn/Mg ratio alloys. The aging condition in the present work has turned out to be on over aged condition for low Zn/Mg ratio alloys with coarse precipitates of T′-phase in the matrix, because those alloys attained lower peak hardness, tensile strength and higher elongation than high Zn/Mg ratio alloys which have fine η′/η-phase.

Early stage natural aging behaviors of Al–1.0%Mg₂Si–0.35%Cu and Al–1.0%Mg₂Si–0.35%Ag alloys were investigated by magnetization and electrical resistivity measurements at a constant temperature between 250 and 320 K. The electrical resistivity of Al–1.6%Mg₂Si was also measured for the comparison. Time dependent magnetization curves for the samples show a rapid increase within the first 24 h. The observed transition time from the clustering stage 1 to the clustering stage 2, which was estimated from the rapid increase point of magnetization and electrical resistivity, has been found to be delayed by adding Cu or Ag to Al–Mg–Si alloys. This phenomenon is explained by the cluster formation kinetics that the added Cu or Ag strongly trapped quenched-in excess vacancies and thus retarded diffusions of the solute elements of Mg and Si. Activation energies for the cluster formation were evaluated by Arrhenius plots of the transition times against measuring temperatures. The effect of Cu or Ag addition on the formation of clusters is discussed in terms of activation energies.

Although several thermodynamic modelling have been carried out for the Al–Mg alloy system, no previous thermodynamic descriptions could calculate the metastable phase diagram of L1₂-type ordered GP zones (Al₃Mg). In this study, Calphad-type thermodynamic assessment of Al–Mg binary alloys was performed to determine not only the miscibility gap but also the spinodal lines of GP zones. The Gibbs energies of fcc-Al solid solution and ordered GP zones were expressed by a four-sublattice model (split compound energy formalism), and a complete thermodynamic description was obtained from available experimental data; e.g. miscibility gap of GP zones or equilibrium phases, and thermal quantities of mixing enthalpy and activity. The calculated miscibility gap and spinodal lines could satisfactorily reproduce experimentally reported reliable data, and thus the equilibrium between fcc-Al solid solution and ordered GP zones was successfully evaluated for the first time.

In this study, the method of high-pressure torsion (HPT) was applied for grain refinement of A2024/Al composites. Diffusion bonding was carried out at 773 K for 5 h to form the composite in advance. Disk samples with 10 mm diameter and 1 mm thickness were processed by HPT under a pressure of 6 GPa. The chemical composition of the diffusion treated Al layer was determined to Al–2.5%Cu–1.3%Mg by EDS analysis. Microstructural observations revealed that the grain size was refined to ∼180 nm in the thickness center and ∼440 nm in the surface side. Tensile tests showed that the ultimate tensile strength reached 710 MPa with a total elongation of 9%. Extra strengthening was attained by aging at 423 K after HPT processing, leading to an ultimate tensile strength of 760 MPa at the peak-aged condition.

Age-hardenable Al–Mg–Si, Al–Mg–Ge, and Al–Zn–Mg alloys including Cu were investigated by transmission electron microscopy to understand extra diffraction spots that appear in their selected area electron diffraction patterns. These alloys containing Cu exhibit similar extra diffracted spots to each other with diffracted spots or streaks for Al matrix and major precipitates in each alloy. The extra spots were not observed in Cu-free alloys. The initial cluster, which is based on the β″-phase in the Al–Mg–Si alloy, is proposed to be MgSi(/Ge)Mg, CuMgSi(/Ge), AlCuMg, and AlZnMg, while the second clusters, which consist of three initial clusters including anti-phase boundary short-range order, are proposed for Cu-containing alloys.