Compositional characterization of Cu-rich phase particles present in as-cast Al-Cu-Mg(-Li) alloys containing Ag

The effects of small additions of Ag on the aging of wrought Al-Cu-Mg(-Li) alloys, involving widespread nucleation of Cu-rich Ω (and T₁) phase precipitates, are known. This article examines the influence of small additions of Ag on the nature of Cu-rich θ-Al₂Cu, S-Al₂CuMg, and T₁-Al₂CuLi phases present in appropriate as-cast Al-Cu, Al-Cu-Mg, and Al-Cu-Li-Mg alloys. Using a combination of light microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), it is shown that neither independent nor combined additions of Ag and Mg to the binary Al-Cu alloy alter the composition of the θ phase; however, there are differences in the ways the θ phase is evolved in Al-Cu-Mg alloys with and without Ag. Ag additions to the Al-Cu-Mg alloy result in the formation of an Al-Cu-Mg-based ternary phase, having a Cu content similar to that of the θ phase and containing small amounts of Mg; rapid rates of cooling cause the retention of this phase in the as-cast alloy. Relatively large amounts of both Ag and Mg are always located in the peripheral regions of such a phase. This phase is readily replaced by θ phase upon annealing at 450 °C. The S phase of the ternary Al-Cu-Mg system is identified in this alloy and is found to dissolve small amounts of Ag. In the case of the Al-Cu-Li-Mg-Ag alloy, two major changes are observed: both Ag and Mg are always present in relatively large amounts in the peripheral regions of the T₁ phase, and the S-phase particles are once again found to dissolve small amounts of Ag. These results are discussed in light of the known compositional features of the precipitates formed in the artificially aged Al-Cu-Mg(-Li)-Ag alloys, to reveal that examination of phases present in the as-cast microstructure is a contributory step toward determining the locations of trace alloy additions in the phase precipitates of interest.     

The influence of alloy composition on precipitates of the Al-Mg-Si system

To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.     

The titanium-rich end of the Ti-Cd phase diagram

Ti-Cd alloys containing up to 30 at. pct Cd have been prepared by diffusing cadmium from the vapor phase into pure titanium. Phase relations in these alloys have been explored by metallographic and X-ray techniques. Cadmium has quite a large solubility in theβ phase of titanium at 1000°C. Addition of cadmium decreases theα-β transformation temperature, forming a eutectoid at approximately 785°C. The solubility of cadmium inα titanium at the eutectoid temperature is approximately 6.5 at. pct, decreasing with decreasing temperature. The phase in equilibrium with saturatedα titanium is an intermetallic compound based on the composition Ti₂Cd.     

Creep resistance and high-temperature metallurgical stability of titanium alloys containing gallium

Tensile properties up to 1100°F and the creep resistance at 1000°F were correlated with composition for twelve complex developmental titanium alloys with additions of Al, Ga, Sn, Mo, Zr, and Si. Creep resistance for these alloys in the β heat-treated condition was found to be strongly dependent on the totalα stabilizer content and the silicon concentration. The creep activation energy for a Ti-4.5 Al-2 Sn-3 Zr-3 Ga-1 Mo-0.5 Si alloy, established over the 900° to 1100°F temperature range, was about 100 kcal per g-mole. This high creep activation energy is hypothesized to result from dispersion strengthening within theα matrix by the Ti₃ X (X = Al, Ga, Sn) phase and pinning of the interplatelet and priorβ grain boundaries by the Zr₅Si₃ phase. Both phases were identified by transmission electron microscopy in these respective locations. Metallurgical instability, as evidenced by decreased fracture toughness, is also shown to be relatable to the totalα stabilizer content. The activation energy for the embrittlement process is about 45 kcal per g-mole. which approximates that for interdiffusion of gallium inα titanium.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Thermodynamics of the α-AuZn and β′-AuZn phases

Thermodynamic data of theα-AuZn alloys recently reported in the literature are found to follow closely the sub-regular solution model. Values of the two parameters for this model are found to increase linearly with temperature. Using theα-phase data, the established(α + β′) phase boundaries, the thermodynamic activities of Zn in theβ′-phase, the theoretical equations of Chang and co-workers for ordered phases and the calorimetric value of the stoichiometric alloy, the thermodynamic properties of β₁-AuZn alloys are obtained. It is found that the thermal entropy of the β₁-AuZn phase is essentially independent of composition, verifying one of the assumptions made in the theoretical equations of Chang and co-workers. The large discrepancies between the three sets of experimental data for β′-AuZn phases are discussed and best consistent results are reported.     

Phase evolution in laser-deposited titanium-chromium alloys

Ti-Cr alloys have been laser deposited from powder feedstock consisting of a blend of elemental powders using the laser-engineered net-shaping (LENS) process. The microstructure of the as-deposited Ti-Cr alloys primarily consists of a metastable bcc matrix of β-Ti(Cr) with precipitates along the grain boundaries. The grain-boundary precipitates have been identified to be of three types, essentially pure hcp Ti, an alloyed hcp phase designated α-Ti(Cr), and the C14 TiCr₂ Laves phase. Initial stages of decomposition, visible within the β matrix, suggest a spinodal clustering process resulting in a fine dispersion of second phases. Diffraction studies have revealed the presence of fine precipitates of α within the β matrix. The evidence for the precipitation of the metastable ω phase within the β matrix is not strong. The phase evolution in the LENS-deposited Ti-Cr alloy has been discussed in the context of rapid solidification and the enthalpy of mixing of the elemental powders.     

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct). Formerly Research Assistant, Department of Materials Science, University of Southern California     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Interdiffusion structures and paths for multiphase Fe-Ni-Al diffusion couples at 1000 °C

Diffusion studies were carried out in the Fe-Ni-Al system at 1000 °C using solid-solid diffusion couples assembled with β (B₂), γ (fcc) single phase, and (β + γ) two-phase alloys. The diffusion couples were encapsulated in quartz tubes under vacuum and annealed for 48 hours. The diffusion structures were examined by optical and scanning electron microscopy. For all β vs (β + γ) couples, growth of the β phase was observed as the (β + γ) two-phase region recessed with the dissolution of the γ phase. For multiphase couples assembled with two (β + γ) terminal alloys, demixing of the (β + γ) two-phase alloys occurred with the formation of single-phase β and γ layers. The development of an interphase boundary between the (β + β′) two-phase region and the γ phase is reported for the first time for a Fe-Ni-Al diffusion couple assembled with single-phase, β, and γ terminal alloys. Various diffusion structures for the couples were related to their diffusion paths constructed from concentration profiles determined by electron probe microanalysis. Interdiffusion fluxes of individual components were determined directly from the experimental concentration profiles and examined in light of diffusional interactions and the development of zero-flux planes and flux reversals. In addition, the boundaries for the miscibility gap between the ordered β and disordered β′ phases of the Fe-Ni-Al system at 1000 °C were determined with the aid of diffusion couples that developed β and β′ phases in the diffusion zone.     

On the fracture and fatigue properties of Mo-Mo<Subscript>3</Subscript>Si-Mo<Subscript>5</Subscript>SiB<Subscript>2</Subscript> refractory intermetallic alloys at ambient to elevated temperatures (25 °C to 1300 °C)

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo₃Si and Mo₅SiB₂ (T₂), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest” resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo₅Si₃. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi₂, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost 12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi₂, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.     

Lattice misfits in four binary Ni-Base γ/γ1 alloys at ambient and elevated temperatures

High-temperature X-ray diffractometry was used to determine thein situlattice parameters,a _γ anda _γ′, and lattice misfits, δ = (a _γ′, -a _γ)/a _γ, of the matrix (γ) and dispersed γ′-type (Ni₃X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 °C. Concentrated alloys containing large volume fractions of theγ′ phase (∼0.40 to 0.50) were aged at 700 °C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ± 4 pct. The absolute values of the lattice constants of theγ andγ′ phases, at compositions corresponding to thermodynamic equilibrium at about 700 °C, are in excellent agreement with data from the literature, with the exception of Ni₃Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, δ decreases to 0.00612 at 679 °C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, δ exhibits a stronger temperature dependence, changing to-0.00285 at 683 °C (Ni-Si) and to 0.00424 at 680 °C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in δ observed are attributable to differences between the thermal expansion coefficients of theγ andγ′ phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 δC. Empirical equations are presented that accurately describe the temperature dependences ofa _γ,a _γ′, and δ over the range of temperatures of this investigation.     

Microstructural study of the titanium alloy Ti-15Mo-2.7Nb-3Al-0.2Si (TIMETAL 21S)

A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si-0.15O (in wt pct)), is a potential matrix material for advanced titanium matrix composites for elevated temperature use. In order to develop a perspective on the microstructural stability of this alloy, the influence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied using optical and transmission electron microscopy (TEM). Depending on the specific thermal treatment, a number of phases, includingα,ω- type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized areas could be present in the microstructure of an annealed bulk alloy, but the microstructure of annealed sheet alloy was fully recrystallized. The mixed structure of the bulk alloy, developed as a result of inhomogeneous deformation, could not be removed by heat treatment alone at 900 °C. Athermalω-type phase formed in this alloy upon quenching from the solution treatment temperature (900 °C). Silicide precipitates were also found in the quenched sample. Thermal analysis was used to determine theβ transus and silicide solvus as close to 815 °C and 1025 °C, respectively. In solution-treated and quenched samples, a high-temperature aging at 600 °C resulted in the precipitation ofα phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystallized regions. During low-temperature aging (350 °C), the ellipsoidalω-type phase persisted in the recrystallized areas even after 100 hours, whereas a high density ofα precipitates developed in the nonrecrystallized areas within only 3 hours. The observed behavior in precipitation may be related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging. Theα precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-temperature aging) always obeyed the Burgers orientation relationship. With respect to the microstructure, TIMETAL 21S is similar to other solute-lean, metastableβ titanium alloys.     

Transformations in a Ti-24AI-15Nb alloy: Part I. Phase equilibria and microstructure

A variety of heat treatments have been employed to explore phase equilibria and the development of microstructure in a Ti-24Al-15Nb alloy. These include solution treatments both above and below the β-transus, followed by controlled cooling and aging at temperatures high enough to preclude ω-phase formation. The phase fieldsβ, β _o, α₂ + β_o, α₂ +β _o + O, andβ _o + O have been identified in the alloy, and schematic time-temperature-transformation (TTT) curves are proposed for continuous cooling transformations from theβ phase. The composition of the α₂ andβ _o phases in the α₂ +β _o region and of the α₂,β _o and O phases in the ternary phase field have been obtained by analytical electron microscopy.     

Microstructural Evolution in Niobium-Based Alloys

Nb-Ti-Cr-Si-Al-X alloys have been considered as candidate materials for high-temperature structural applications in aero engines. Using a multicomponent approach, attempts are being made to obtain good strength with adequate oxidation resistance at the expected operating temperatures (∼1073 through 1273 K) in these alloys. In the microstructure of these alloys, which essentially consist of the silicide phase distributed in the bcc β matrix, the presence or absence of the Laves phase can bring about considerable changes in the mechanical properties. The present article reports on the instability observed in the β phase and also on the formation of the Laves phase in the β matrix. The extent of the chemical separation and elements responsible for this separation has been determined. Different morphologies of the Laves phase have been obtained by aging at different temperatures for various durations and the structure, chemistry, and volume fraction of these phases have been determined. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.

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A sintering study on the β-spodumene-based glass ceramics prepared from gel-derived precursor powders with LiF additive

Beta-spodumene (Li₂O·Al₂O₃·4SiO₂, LAS) powders were prepared by a sol-gel process using Si(OC₂H₅)₄, Al(OC₄H₉)₃, and LiNO₃ as precursors and LiF as a sintering aid agent. Dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED) were utilized to study the sintering, phase transformation, microstructure, and properties of the β-spodumene glass-ceramics prepared from the gel-derived precursor powders with and without LiF additives. For the LAS precursor powders containing no LiF, the only crystalline phase obtained was β-spodumene. For the pellets containing less than 4 wt pct LiF and sintered at 1050 °C for 5 hours the crystalline phases were β-spodumene and β-eucryptite (Li₂O·Al₂O₃·2SiO₂). When the LiF content was 5 wt pct and the sintering process was carried out at 1050 °C for 5 hours, the crystalline phases were β-spodumene, β-eucryptite (triclinic), and eucryptite (rhombohedral (hex.)) phases. With the LiF additive increased from 0.5 to 4 wt pct and sintering at 1050 °C for 5 hours, the open porosity of the sintered bodies decrease from 30 to 2.1 pct. The grains size is about to 4 to 5 μm when pellect LAS compact contains LiF 3 wt pct as sintered at 1050 °C for 5 hours. The grains size grew to 8 to 25 μm with a remarkable discontinuous grain growth for pellet LAS compact contain LiF 5 wt pct sintered at 1050 °C for 5 hours. Relative densities greater than 90 pct could be obtained for the LAS precursor powders with LiF > 2 wt pct when sintered at 1050 °C for 5 hours. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 × 10⁻⁷ to 5.2 × 10⁻⁷/°C (25 °C to 900 °C) as the LiF addition increased from 0 to 5 wt pct.     

Phase transformations in an Fe-7.8Al-29.5Mn-1.5Si-1.05C alloy

Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)₃AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from 550 °C to 825 °C, aγ → coarse (Fe,Mn)₃AlC carbide + DO₃ reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)₃AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)₃AlC carbide reaction or aγ →α + coarse (Fe,Mn)₃AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

The influence of alloy composition on precipitates of the Al−Mg−Si system

To study how changes in solute elements affect precipitation, six Al−Mg−Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.