HREM study and structure modeling of the η′ phase,the hardening precipitates in commercial Al–Zn–Mg alloys

The structure of the η′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η-MgZn₂ phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.     

Transmission electron microscopy study of the early stage of precipitates in aged Al–Li–Cu alloys

We report here on the early stage of the precipitation behavior of aged Al–Li–Cu alloys using transmission electron microscopy. The δ′ phase (Al₃Li, L1₂ structure (Structurbericht)) was found to exist in the as-quenched specimen containing 2.4 wt%-Li, whereas no precipitation was observed in the as-quenched specimen with 1.6 wt%-Li. After aging at 100 °C for 3 h, GP-I zones nucleated homogeneously in both specimens; but no δ′ phase was detected in the 1.6 wt%-Li alloy at that stage. As the aging proceeds, the δ′ phase nucleates and grows on the GP-I zones. The composite structure, in which the GP-I zone is flanked by a pair of lenticular δ′ particles, is stable in 180–200 °C. The facing lenticular δ′ particles on a GP-I zone were systematically found to be anti-phase with respect to each other.     

Dependency of the thermal and electrical conductivity on temperatures and compositions of Zn in the Al−Zn alloys

The variations of thermal conductivity (K) with temperature for Al–xZn (x = 5, 10, 20, 30, 50 and 60 wt. %) alloys were measured by using a radial heat flow furnace. The variations of electrical conductivity (σ) of solid phases with temperature for the studied alloys were determined from the Wiedemann-Franz and Smith-Palmer equations by using the measured values of K from the plots of K. The thermal temperature coefficient (α_TTC) and the electrical temperature coefficient (α_ETC) were obtained. Dependency of the α_TTC and α_ETC on the composition of Zn in the Al?Zn alloys was also investigated. According to the present experimental results, K of Al–Zn alloys linearly decrease with increasing temperatures up to the melting temperature for each composition and exponentially decrease with the increasing Zn content. On the other hand, the σ of Al based Al-Zn alloys exponentially decrease with increasing temperature and Zn content.     

Trans-interface-diffusion-controlled coarsening of γ′ precipitates in ternary Ni–Al–Cr alloys

Published data on the coarsening behavior of γ′ precipitates in three ternary Ni–Al–Cr alloys are examined in light of the theory of trans-interface-diffusion-controlled (TIDC) coarsening, in which the kinetics is controlled by diffusion through the coherent precipitate–matrix interface. The experimental data are independent of the equilibrium γ′ volume fraction, as expected for TIDC coarsening. Kinetics of the type 〈r〉ⁿ ∝ t for the growth of precipitates of average radius 〈r〉, and X_∞Al and X_∞Cr ∝ t^–1/n for the variations of the far-field matrix solute concentrations, X_∞Al,Cr, with aging time, t, are characteristic of TIDC coarsening. The temporal exponent n ≈ 2.4 was obtained from the fitting of published particle size distributions. Based on correlation coefficients, the dependencies of 〈r〉ⁿ on t and X_∞Al,Cr on t^?1/n were comparable for n = 2.4 and n = 3 (the temporal exponent for matrix-diffusion-controlled coarsening). The dependencies of volume fraction, f, and number density, N_v, on t are also compared with theoretical predictions. Using a thermodynamic model of the Ni–Al–Cr phase diagram, the interfacial free energy, σ, was estimated from analysis of the data; σ varies from ～14.5 to 19 mJ m^–2 in the three alloys. Interfacial diffusion coefficients, also obtained from analysis of the data, are greater than those in the γ′ phase but smaller than those in the γ phase, which is consistent with the demands of the TIDC theory. Comparisons with the results from previously published work are noted and all discrepancies are discussed.     

Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys

The effects of adding 0.02 or 0.06 at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al₃(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300 °C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al₃(Sc,Zr) (L1₂) precipitates in the high-Si alloy subjected to double aging (8 h/300 °C for Sc precipitation and 32 days/400 °C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al₃(Sc,Zr) (L1₂) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al₃Sc; and (iii) Si partitions preferentially to Al₃Sc (L1₂) rather than Al₃Zr (L1₂).     

On the formation of faceted Al3Zr (β′) precipitates in Al–Li–Cu–Mg–Zr alloys

Trace additions of Zr to Al alloys inhibit recrystallization through the formation of spherical and coherent Al₃Zr (β′) precipitates. Recently, observations have been made of faceted β′ precipitates in several hot deformed Al alloys, although no systematic experimental study of either the causes of the formation of such precipitates or their orientation relationships with the Al matrix has so far been reported. A detailed examination of the orientation relationships shows that the cube-on-cube orientation relationship existing between spherical, coherent β′ precipitates and the Al matrix does not hold good for the faceted β′ particles and that the faceted β′ particles are twin-related with the matrix. It is shown that the twin-related β′ particles are not incoherent, but bound by large facets fully coherent with the matrix, and that such particles are associated with fairly significant coherency strains. A probable shape of the faceted β′ is also described.     

GP-zones in Al–Zn–Mg alloys and their role in artificial aging

The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated <110> domains. The structural relation to the η′-precipitate is discussed.     

The crystal structure of the equilibrium Φ phase in Mg–Zn–Al casting alloys

The crystal structure of the equilibrium intermetallic Φ phase formed in a Mg–Zn–Al casting alloy has been characterised using transmission electron microscopy. Electron diffraction patterns recorded from particles of the Φ phase in the casting alloy can be well indexed according to a primitive orthorhombic unit cell, with lattice parameters a=0.90 nm, b=1.70 nm, and c=1.97 nm. Examination of the whole pattern symmetry of principal zone axis diffraction patterns indicates a space group of Pbcm. A model for the decoration of the unit cell of the Φ phase is proposed, in which the Mg₅(Zn,Al)₁₂ Friauf polyhedron is the key structural unit. The Zn and Al atoms are all in icosahedral coordination, but their icosahedral shells are distorted due to the presence of Mg atoms. A total of 84 Mg atoms and 68 Zn/Al atoms can be accommodated in the orthorhombic unit cell, resulting in a formula of Mg₂₁(Zn,Al)₁₇ that is consistent with the composition obtained experimentally. Computer simulations of electron diffraction patterns provide very good agreement with experimental observations.     

High-strain-rate superplasticity of the Al–Zn–Mg–Cu alloys with Fe and Ni additions

During high-strain-rate superplastic deformation, superplasticity indices, and the microstructure of two Al–Zn–Mg–Cu–Zr alloys with additions of nickel and iron, which contain equal volume fractions of eutectic particles of Al₃Ni or Al₉FeNi, have been compared. It has been shown that the alloys exhibit superplasticity with 300–800% elongations at the strain rates of 1 × 10^–2–1 × 10^–1 s^–1. The differences in the kinetics of alloy recrystallization in the course of heating and deformation at different temperatures and rates of the superplastic deformation, which are related to the various parameters of the particles of the eutectic phases, have been found. At strain rates higher than 4 × 10^–2, in the alloy with Fe and Ni, a partially nonrecrystallized structure is retained up to material failure and, in the alloy with Ni, a completely recrystallized structure is formed at rates of up to 1 × 10^–1 s^–1.     

Transmission electron microscopy study of the evolution of precipitates in aged Al–Li–Cu alloys: the θ′ and T1 phases

We have investigated the structures of the θ′ and T₁ precipitates in Al–1.6wt%Li–3.2wt%Cu and Al–2.4wt%Li–3.2wt%Cu alloys aged at 220 °C. The θ′ precipitates in the 1.6 wt%-Li alloy are those known for the Al–Cu binary system (a=0.40 and c=0.58 nm); whereas those in the 2.4 wt%-Li alloy exhibited two atypical structures. One, named a type I T_B′ plate in this study, is isostructural to the known θ phase with a large c value of about 0.64 nm, having a habit plane parallel to the matrix {1 0 0}_α; the other, type II T_B′, is characterized by a=0.41 and c=0.61 nm, having a habit plane inclined at about 20° with {1 0 0}_α, while maintaining a coherent interface. Also images of {1 1 1} precipitates in the 1.6 wt%-Li alloy revealed a continuous change from the T₁ phase (c=0.935 nm), to a structure with c=0.87–0.90 nm. The image and small lattice parameter suggest that this {1 1 1} precipitate is likely to be the Ω phase.     

Phases and microstructures of high Zn-containing Al–Zn–Mg–Cu alloys

Phases and microstructures of three high Zncontaining Al–Zn–Mg–Cu alloys were investigated by means of thermodynamic calculation method, optica microscopy(OM), scanning electron microscopy(SEM)energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) analysis. The results indicate that similar dendritic network morphologies are found in these three Al–Zn–Mg–Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg(Cu, Zn, Al)_2 eutectic phases, and fine intermetallic compounds g(MgZn_2). Both of as-cast 7095 and 7136 aluminum alloys involve a(Al)eutectic Al/Mg(Cu, Zn, Al)_2, intermetallic g(MgZn_2), and h(Al_2Cu). During homogenization at 450 °C, fine g(MgZn_2) can dissolve into matrix absolutely. After homogenization at 450 °C for 24 h, Mg(Cu, Zn, Al)_2 phase in 7136 alloy transforms into S(Al_2Cu Mg) while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zncontaining Al–Zn–Mg–Cu alloys.     

Study of precipitation in aged binary Mg–Al and ternary Mg–Al–Zn alloys using 27Al NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy of ²⁷Al was used to study the development of precipitation in aged Mg–6 wt%Al, Mg–9 wt%Al and Mg–9 wt%Al–(x) wt%Zn alloys. The ²⁷Al spectra for the aged alloys consist of two peaks; one from the aluminium in solid solution and the other from aluminium in the precipitate phase. The proportion of aluminium atoms in the matrix and precipitate phases was measured, as a function of time at temperature, using the relative intensities of peaks. The nucleation of the continuous precipitates was found to be highly dependent on the initial supersaturation and it is proposed that it is a homogeneous process. The Austin–Rickett relation successfully models the amount of continuous precipitation with aging time; the kinetics is consistent with one-dimensional and interface-controlled growth. Changes in composition of the matrix and precipitate phases were correlated with the ²⁷Al Knight shift characterising these phases. The Knight shift data from a series of ternary Mg–9 wt%Al–(x)wt%Zn alloys indicates that the Zn segregates to the precipitate phase during precipitation.     

A simulation study of the shape of β′ precipitates in Mg–Y and Mg–Gd alloys

The metastable β′ phase is a key strengthening precipitate phase in a range of Mg–RE (RE: rare-earth elements) based alloys. The morphology of the β′ precipitates changes from a faceted and nearly equiaxed shape in Mg–Y alloys to a truncated lenticular shape in Mg–Gd alloys. In this work, we study effects of interfacial energy and coherency elastic strain energy on the morphology of β′ precipitates in binary Mg–Y and Mg–Gd alloys using a combination of first-principles calculations and phase-field simulations. Without any free-fitting parameters and using the first-principles calculations, CALPHAD databases and experimental characterizations as model inputs (lattice parameters of the β′ phase, elastic constants and chemical free energy of Mg matrix and interfacial energies of the coherent β′/Mg matrix interfaces), the phase-field simulations predict equilibrium shapes of β′ precipitates of different sizes that agree well with experimental observations. Factors causing the difference in the equilibrium shape of β′-Mg₇Y and β′-Mg₇Gd precipitates are identified, and possible approaches to increase the aspect ratio of the β′ precipitates and thus to enhance the strength of Mg–RE alloys are discussed.     

Theoretical design and distribution control of precipitates and solute elements in Al−Zn−Mg−Cu alloys with heterostructure

In order to simultaneously improve strength and formability, an analytical model for the concentration distribution of precipitates and solute elements is established and used to theoretically design and control the heterogeneous microstructure of Al−Zn−Mg−Cu alloys. The results show that the dissolution of precipitates is mainly affected by particle size and heat treatment temperature, the heterogeneous distribution level of solute elements diffused in the alloy matrix mainly depends on the grain size, while the heat treatment temperature only has an obvious effect on the concentration distribution in the larger grains, and the experimental results of Al−Zn−Mg−Cu alloy are in good agreement with the theoretical model predictions of precipitates and solute element concentration distribution. Controlling the concentration distribution of precipitates and solute elements in Al−Zn−Mg−Cu alloys is the premise of accurately constructing heterogeneous microstructure in micro-domains, which can be used to significantly improve the formability of Al−Zn−Mg−Cu alloys with a heterostructure.     

Formation of microstructure and the superplasticity of Al–Mg-based alloys

The influence of a 3–10% content of magnesium in Al–Mg–Mn(Cr) alloys on the characteristics of the microstructure of sheet blanks and their superplasticity has been examined. It has been shown that the minimum size of grains and the best superplasticity are characteristic of the alloy that contains about 7% magnesium and is additionally alloyed simultaneously with manganese and chromium. An increase in the content of magnesium leads to the formation of conglomerates of particles of a chromium–manganese phase and, as a result, to a coarsening of the grain structure and a deterioration of superplasticity.     

Crystal structure and stability of complex precipitate phases in Al–Cu–Mg–(Si) and Al–Zn–Mg alloys

We demonstrate how first-principles total energy calculations may be used to elucidate both the crystal structures and formation enthalpies of complex precipitates in multicomponent Al alloys. For the precipitates, S(Al–Cu–Mg), η′ (Al–Zn–Mg), and Q(Al–Cu–Mg–Si), energetics were computed for each of the models of the crystal structures available in the literature allowing a critical assessment of the validity of the models. In all three systems, energetics were also calculated for solid solution phases as well as other key phases (e.g., equilibrium phases, GP zones) in each precipitation sequence. For both the S and η′ phases, we find that recently proposed structures (based on electron microscopy) produce unreasonably high energies, and thus we suggest that these models should be re-evaluated. However, for all three precipitates, we find that structures based on X-ray diffraction refinements provide both reasonable energetics and structural parameters, and therefore the first-principles results lend support to these structural refinements. Further, we predict energy-lowering site occupations and stoichiometries of the precipitate phases, where experimental information is incomplete. This work suggests that first-principles total energy calculations can be used in the future as a complementary technique with diffraction or microscopy for studying precipitate structures and stabilities.     

On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.