Optimization of aging regimes of alloys in the Al−Mg system for stabilizing the structure and mechanical properties

The previous study of the authors has resulted in the development of an Al−10% Mg composition alloyed additionally by zinc, manganese, antimony, and zirconium. The alloy was to be cast into sand molds and had σ_r=380-400 N/mm², σ_0.2=240-260 N/mm², and δ=10–12%. The main difficulty in the development and use of the given alloy consisted in attaining stable properties in the operating process. The present paper is devoted to this problem. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 32–34, August, 1997.     

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.     

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.     

Effect of Al addition on microstructure and mechanical properties of Mg−Gd−Zn alloys

The microstructure evolution and mechanical properties of Mg?15.3Gd?1Zn alloys with different Al contents (0, 0.4, 0.7 and 1.0 wt.%) were investigated. Microstructural analysis indicates that the addition of 0.4 wt.% Al facilitates the formation of 18R-LPSO phase (Mg₁₂Gd(Al, Zn)) in the Mg?Gd?Zn alloy. The contents of Al₁₁Gd₃ and Al₂Gd increase with the increase of Al content, while the content of (Mg, Zn)₃Gd decreases. After homogenization treatment, (Mg, Zn)₃Gd, 18R-LPSO and some Al₁₁Gd₃ phases are transformed into the high-temperature stable 14H-LPSO phases. The particulate Al?Gd phases can stimulate the nucleation of dynamic recrystallization by the particle simulated nucleation (PSN) mechanism. The tensile strength of the as-rolled alloys is improved remarkably due to the grain refinement and the fiber-like reinforcement of LPSO phase. The precipitation of the β′ phase in the peak-aged alloys can significantly improve the strength. The peak-aged alloy containing 0.4 wt.% Al achieves excellent mechanical properties and the UTS, YS and elongation are 458 MPa, 375 MPa and 6.2%, respectively.     

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.     

Microstructure and mechanical properties of squeeze-cast Al−5.0Mg−3.0Zn−1.0Cu alloys in solution-treated and aged conditions

The microstructure and mechanical properties at different depths of squeeze-cast, solution-treated and aged Al−5.0Mg−3.0Zn−1.0Cu alloy were investigated. For squeeze-cast alloy, from casting surface to interior, the grain size of α(Al) matrix and width of T-Mg₃₂(AlZnCu)₄₉ phase increase significantly, while the volume fraction of T phase decreases. The related mechanical properties including ultimate tensile strength (UTS) and elongation decrease from 243.7 MPa and 2.3% to 217.9 MPa and 1.4%, respectively. After solution treatment at 470 °C for 36 h, T phase is dissolved into matrix, and the grain size increases so that the UTS and elongation from surface to interior are respectively reduced from 387.8 MPa and 18.6% to 348.9 MPa and 13.9%. After further peak-aging at 120 °C for 24 h, numerous G.P. II zone and η′ phase precipitate in matrix. Consequently, UTS values of the surface and interior increase to 449.5 and 421.4 MPa, while elongation values decrease to 12.5% and 8.1%, respectively.     

Tensile properties and strengthening mechanisms of Mg−Zn−Zr alloys

Six Mg−Zn−Zr alloys, with and without RE additionswere tested at room temperature and 473 K. Yield stresses of parts of the six alloys were calculated theoretically. Microstructures of the specimens before and after heat treatment were compared in order to determine the reason for the decrease in yield stresses at room temperature. The strengthening mechanisms at different temperatures were discussed. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

Reversal of the negative natural aging effect in Al–Mg–Si alloys

The effect of solute clustering on subsequent precipitation has been investigated for three different Al–Mg–Si alloys with similar Mg/Si ratios but different Mg + Si contents. Hardness measurements were carried out (i) during various natural aging (NA) times at room temperature, and (ii) after a fixed artificial aging (AA) heat treatment preceded by various NA times. Comparison between the two hardness plots stressed an intimate connection between solute clusters and precipitates: for the NA times investigated (between a few seconds and several months), transient phenomena with short (less than 10 h) lifetimes were observed, both occurring at earlier times and with higher relative importance for increasing solute concentration. Upon NA + AA, these transient phenomena were found to be responsible for the well-known “negative NA effect”, as evidenced by a close correlation between the respective lifetimes. In particular, the negative NA effect itself was found to be transient, and was reversed upon long storage times by the dominance of a process beneficial to precipitation. Transmission electron microscopy studies of selected alloy conditions ensured that the recorded hardness variations reflected variations in the alloy microstructure. Attempts were made to couple the transient clustering phenomena with solute strain field interactions. Preliminary calculations following this line of thought suggested a competitive nature of both Mg–Mg and Mg–Si interactions, in contrast with the usual view of Mg–Mg repulsion. An alloy hardness optimization procedure as a function of storage time, temperature and alloy composition has been suggested.     

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al−Zn−Mg−Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al₃(Sc,Zr) and Al₃Zr dispersoids. The volume of Al₃Zr particles is less than that of Al₃(Sc,Zr) particles, but the distribution of Al₃(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.     

Dependence of the thermal conductivity of alloys of the Al−Mg system on the composition and temperature

It is a common practice to study the dependencies of the physical properties of alloys on the temperature and the composition without generalizing the results. It is more logical to study these dependences complexly, i.e., as fragments of “composition-temperature-property” diagrams (whole diagrams in the ideal case). Today’s mathematical and computer possibilities provide processing of the dependences of any property on the temperature and the composition even for multicomponent systems. Mathematical analogs replace the “composition— property” graphical diagrams. The present work generalizes data on the thermal conductivity of 11 alloys of the Al−Mg system that contain 1 to 14% Mg in the temperature range of 20–350°C. The results are obtained in the form of a single regression equation that describes the data on the thermal conductivity within the range 86–190 W/(m·K) with a standard deviation of 0.7%. The choice of the regression equation is based on the existence of an analogy between heat transfer and electric transfer in metallic systems and on dependences of the electrical resistivity on the temperature and the concentration of the alloying elements known from solid-state physics. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 13–15, June, 1998.     

Fatigue crack growth and wear behaviors in rheocast Al−Si−Mg alloys

Rheocast aluminum alloys, which consist of globular α-Al cells, refined grains and eutectic Si particle, were used to investigate fatigue crack growth and wear characteristics. The Si particles were systematically varied from coarse and acicular shapes to small and globular ones. At low ΔK fatigue crack growth rates decreased in samples consisting of acicular Si particles of large grain size, which induced a large amount of crack closure. Large and acicular Si particles were easily cracked and separated the particle/matrix interface, which promoted to fracture at smaller ΔK. On the other hand, small Si particles made fatigue crack grow, even at a high ΔK region, and increased the fracture toughness of the alloy. However, in the wear test, small eutectic Si particles were pulled out by friction force during sliding wear and the wear loss amount increased with increase in sliding distance.     

Effects of Li content on microstructure and mechanical properties of as-cast Mg−xLi−3Al−2Zn−0.5Y alloys

The effects of Li content on the microstructure and mechanical properties of the as-cast Mg?xLi?3Al?2Zn? 0.5Y (LAZx32-0.5Y) alloys were investigated by XRD, SEM, TEM, hardness tester and universal testing machine. The results show that the matrix of the alloy transforms from α-Mg to α-Mg+β-Li and then to β-Li when the Li content increases from 4% to 14% (mass fraction). All LAZx32-0.5Y alloys contain AlLi and Al₂Y, while MgLi₂Al appears only in the alloy containing the β-Li matrix. As the Li content increases, the content of AlLi and MgLi₂Al gradually increases, while the content of Al₂Y does not change much. As the Li content increases from 4% to 10%, the ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually decrease while the elongation gradually increases. The corresponding fracture mechanism changes from cleavage fracture to quasi-cleavage fracture and then to microporous aggregation fracture. This is mainly attributed to the decrease of α-Mg and the increase of β-Li in the alloy. When the Li content continues to increase to 10% and 14%, the yield strength, ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually increase, while the elongation decreases sharply, which is mainly attributed to the nano-scale MgLi₂Al uniformly distributed in the β-Li matrix.     

Effects of Mg content on microstructure and mechanical properties of low Zn-containing Al−xMg−3Zn−1Cu cast alloys

Effects of Mg content on the microstructure and mechanical properties of low Zn-containing Al?xMg? 3Zn?1Cu cast alloys (x=3?5, wt.%) were investigated. As Mg content increased in the as-cast alloys, the grains were refined due to enhanced growth restriction, and the formation of η-Mg(AlZnCu)₂ and S-Al₂CuMg phases was inhibited while the formation of T-Mg₃₂(AlZnCu)₄₉ phase was promoted when Mg content exceeded 4 wt.%. The increase of Mg content encumbered the solution kinetics by increasing the size of eutectic phase but accelerated and enhanced the age-hardening through expediting precipitation kinetics and elevating the number density of the precipitates. As Mg content increased, the yield strength and tensile strength of the as-cast, solution-treated and peak-aged alloys were severally improved, while the elongation of the alloys decreased. The tensile strength and elongation of the peak-aged Al?5Mg?3Zn?1Cu alloy exceed 500 MPa and 5%, respectively. Precipitation strengthening implemented by T′ precipitates is the predominant strengthening mechanism in the peak-aged alloys and is enhanced by increasing Mg content.     

Effect of microalloying on the structure and properties of alloys in the Al−Si−Cu system

Conclusions  

Predicting the size- and temperature-dependent shapes of precipitates in Al–Zn alloys

We study theoretically the size versus shape versus temperature relation of precipitates in Al–Zn via quantum-mechanical first-principles simulations. Our parameter-free model, based on a mixed-space cluster expansion, allows the prediction of the experimentally observed size and temperature dependences of the precipitate shape. We find that aging experiments can be explained in terms of equilibrium shapes. The precipitates change from a nearly spherical to a more ellipsoidal/hexagonal shape with increasing size and decreasing temperature. They always flatten in the [111] direction, which can be interpreted as a consequence of a mechanical instability of face-centered cubic Zn when rhombohedrally distorted along [111] and a strong anisotropy of the chemical energy. The excellent agreement between experiment and theory shows that our model can be used to quantitatively predict precipitate shapes and sizes.     

Solute clustering in Al–Cu–Mg alloys during the early stages of elevated temperature ageing

The evolution of atomistic-level nanostructure during the early stages of elevated temperature ageing of rapid hardening (RH) Al–Cu–Mg alloys has been characterised by a combination of atom probe tomography (APT), transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). APT analysis confirms that significant dispersions of small solute clusters form during ageing for 60 s at 150 °C. No zone-like precipitate structures were observed by TEM and APT examinations. These small clusters are believed to be responsible for the RH effect. Careful quantitative APT analysis reveals that a high density of Cu–Mg clusters with high Mg:Cu ratio gives the most potent strengthening response. Positron annihilation measurements also show that Cu–Mg clusters provide additional sites for vacancy stabilisation.     

Effect of non-isothermal aging on microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys

The evolution of microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys during non-isothermal aging was studied. The mechanical properties of the alloy were tested by stretching at room temperature. The results show that in the non-isothermal aging process, when the alloy is cooled to 140 °C, the ultimate tensile strength of the alloy reaches a maximum value of 582 MPa and the elongation is 11.9%. The microstructure was tested through a transmission electron microscope, and the experimental results show that the GP zones and η’ phases are the main strengthening precipitates. At the cooling stage, when the temperature dropped to 180 °C, the GP zones were precipitated again. Besides, the experimental results show that the main strengthening phase during non-isothermal aging is η’ phases.     

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.