Optimization of Solution Treatment of Cast Al-7Si-0.3Mg and Al-8Si-3Cu-0.5Mg Alloys

The influence of solidification rate on the solution-treatment response has been investigated for an Al-7Si-0.3Mg alloy and an Al-8Si-3Cu-0.5Mg alloy. The concentrations of Mg, Cu, and Si in the matrix after different solution-treatment times were measured using a wavelength dispersive spectrometer. All Mg dissolves into the matrix for the Al-Si-Mg alloy when solution treated at 803 K (530 °C) because the π-Fe phase is unstable and transforms into short β-Fe plates which release Mg. The Q-Al₅Mg₈Cu₂Si₆ phase do not dissolve completely at 768 K (495 °C) in the Al-Si-Cu-Mg alloy and the concentration in the matrix reached 0.22 to 0.25 wt pct Mg. The distance between π-Fe phases and Al₂Cu phases was found to determine the solution-treatment time needed for dissolution and homogenization for the Al-Si-Mg alloy and Al-Si-Cu-Mg alloy, respectively. From the distance between the phases, a dimensionless diffusion time was calculated which can be used to estimate the solution-treatment times needed for different coarsenesses of the microstructure. A model was developed to describe the dissolution and homogenization processes.     

Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct Fe 319-type alloys

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the β-Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like β-Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the β-Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energydispersive X-ray (EDX) analysis.     

Evolution of microstructure and tensile strength of rapidly solidified Al-4.7 pct Zn-2.5 pct Mg-0.25 pct Zr-X wt pct Mn alloys

Analytical transmission electron microscopy and thermal analysis of as-extruded Al-4.7 pct Zn-2.5 pct Mg-0.2 pct Zr-X wt pct Mn alloys, with Mn contents ranging from 0.5 to 2.5 wt pct, were carried out to elucidate the microstructural change and accompanying mechanical properties during subsequent heat treatments. The as-extruded alloy was fabricated from rapidly solidified powder and consisted of a fine, metastable manganese dispersoid and the ternary eutectic T phase (Al₂Mg₃Zn₃). Solution heat treatment resulted in the formation of the stable Al₆Mn phase and complete dissolution of the T phase. Formation of stable Al₆Mn was made by two routes: by phase transition from metastable Mn dispersoids which already existed, and from the supersaturated solid solution by homogeneous nucleation. The density of the Al₆Mn phase increased with the addition of manganese, while the shape and average size remained unchanged. A significant increase in the hardness was observed to coincide with the formation of the Al₆Mn phase. Similarly, the tensile strength increased further after the aging treatment, and the increment was constant over the content of Mn in the alloy, which was explained by the contribution from the same amount of precipitates, MgZn₂. Results of thermal analysis indicated that the dissolution of the T phase started near 180 °C and that formation of Al₆Mn occurred at about 400 °C, suggesting that further enhancement of strength is possible with the modification of the heat-treatment schedule.     

Microstructure and Mechanical Properties of Mg-7Al-2Sn Alloy Processed by Super Vacuum Die-Casting

The microstructural evolution of Mg-7Al-2Sn (AT72) alloy processed by super vacuum die-casting and heat treated at various conditions was studied. The results showed that the dendritic microstructure in the as-cast AT72 alloy consisted of α-Mg, Mg₂Sn, and Mg₁₇Al₁₂ phases. After solution treatment at temperatures ranging from 663 K to 703 K (390 °C to 430 °C), the Mg₁₇Al₁₂ phase dissolved into the Mg matrix entirely, while the Mg₂Sn phase partially dissolved into matrix. An average grain size of about 40 μm in the alloy could be achieved after solution treatment at 683 K (410 °C) for 16 hours. A large amount of lath-shaped precipitates of Mg₂Sn and Mg₁₇Al₁₂ was observed in the aged AT72 alloy. The results of tensile property evaluation at room temperature showed that the ductility of the solution-treated alloy was dramatically improved, in comparison with the as-cast alloy. In the peak aged condition, the tensile strength of the alloy was increased, which was attributed to the deposition of fine Mg₁₇Al₁₂ and Mg₂Sn precipitates during the aging treatment.     

Microstructural evolution and mechanical properties of AC8A/Al2O3 short-fiber composites after thermal exposure at 150 °C to 350 °C

The microstructural evolution and mechanical properties of an AC8A/12 vol Pct A1₂O₃ (sf) composite fabricated by squeeze casting were characterized. Thermal treatments included the normal T6 temper and thermal exposure at 150 °C, 250 °C, 300 °C, and 350 °C for 400 hours. The predominant strengthening phase in the matrix appeared to be β′ (Mg₂Si) needles. Bulk pure Si particles and dendrites were commonly seen. Large particles, termed asB-type phase, might include hexagonal Al₃(Ni, Cu, Fe, Si, Mg)₂ and orthorhombic Al₃(Ni, Cu, Fe, Si, Mg) phases. Both the Si andB dispersoids were not obviously affected by artificial aging at 150 °C to 350 °C. In certain cases, large cubic β (Mg₂Si) particles, hexagonalQ′ orQ (Al₄Cu₂Mg₈Si₇) precipitates, and numerous small Al particles inside Si dispersoids were also seen. No interfacial reaction product was observed along the fiber/ matrix interface even after long exposure at 350 °C. Amorphous SiO₂ gels, which were used as a binder during fabrication, were occasionally observed. The tensile and fatigue behavior of the AC8A alloys and composites after the preceding thermal exposures were evaluated over the temperature range of 25 °C to 350 °C. The composites showed similar strength as the matrix alloy at room temperature but exhibited higher strength at temperatures above 250 °C, with the sacrifice of the lower ductility. The strength levels of both the alloys and composites were significantly reduced after long thermal exposure, especially for temperatures higher than 250 °C. The loss of strength after long-term exposure at elevated temperatures may be attributed to age-softening of the matrix.     

Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment

The dissolution and melting of Al₂Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al₂Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al₂Cu phase. Dissolution of the (Al + Al₂Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al₂Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al₂Cu). Unlike the eutectic, fragments of the blocky Al₂Cu phase are still observed in the matrix, even after 24 hours at 540 °C.     

Phase equilibria in Al-rich Al-Mg-Sc-Zn alloys at 430 and 300°C

Optical microscopy and X-ray diffraction and electron microprobe analyses are used to study Al-Mg-Sc-Zn alloys annealed at 430 and 300°C. The Al-based solid solution is found to be in equilibrium only with binary and ternary phases of the corresponding systems; these are Al₃Sc, β(Al₃Mg₂), η(MgZn₂), and τ(Al₂Mg₃Zn₃). Sections are constructed for the isothermal tetrahedra of the Al-Mg-Sc-Zn phase diagram that correspond to a scandium content of 0.5% and magnesium and zinc contents of up to 20%.     

Determination of the isothermal sections of the Al-Ni-Si ternary system at 750 °C and 850 °C

The phase equilibria of the Al-Ni-Si ternary system at 850 °C and 750 °C have been investigated using scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA). Isothermal sections at 850 °C and 750 °C were constructed based on experimental data from 53 alloys heat treated at 850 °C for 1200 hours and at 750 °C for 1440 hours, respectively. The phase equilibria among the following intermetallics and solid-solution phases are described: Ll₂-Ni₃(Al,Si), B2-NiAl, Ni₅Si₂, δ-Ni₂Si, ϑ-Ni₂Si(τ ₄), Ni₃Si₂, NiSi, NiSi₂, Ni₂Al₃, NiAl₃, Ni₂AlSi(τ ₂), Ni₃Al₆Si(τ ₃), Ni₁₆AlSi₉(τ ₅), the fcc solid solution, and the diamond (Si) phase. In addition, a phase, temporarily designated as Ni₅(Al,Si)₃(τ ₆), was observed for the first time at both 750 °C and 850 °C. This phase is probably the stabilization of Ni₅Al₃ by Si to higher temperatures than the binary Ni₅Al₃, which is only stable at <∼700 °C.     

Diffusion Brazing of Al6061/15 Vol. Pct Al2O3p Using a Cu-Sn Interlayer

Diffusion brazing of Al-6061 alloy containing 15 vol. pct Al₂O₃ particles was attempted using Cu-Sn interlayer. Joint formation was attributed to the solid-state interdiffusion of Cu and Sn followed by eutectic formation and subsequent isothermal solidification. Examination of the joint region using scanning electron microprobe analyzer (EPMA), wavelength dispersive spectroscopy (WDS) and X-ray diffraction (XRD) showed the formation of intermetallic phases such as Al₇Cu₃Mg₃, Mg₂Cu₆Al₅, Cu₃Sn, and Mg₂Sn. The results indicated an increase in joint strength with increasing bonding time giving the highest joint shear strength of 94 MPa at a bonding duration of 3 hours.     

Transformation of Alumina Inclusions by Calcium Treatment

The objectives of this study were to investigate reactions of calcium with Al₂O₃ by different model experiments both on the laboratory and on the industrial scale. Experiments with solid Al₂O₃ and CaO were performed between 1350 °C and 1600 °C. Reaction rate constants were determined based on scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) observations of reaction products and weight measurements of the Al₂O₃ reacted via dissolution of the CaO bearing phases from the specimens after the annealing period. The results showed that the formation of calcium aluminate phases proceeded rapidly at temperatures greater than 1405 °C when a liquid calcium aluminate was formed. In the lowest temperature range (1350 °C–1405 °C), when the formation of liquid phase ceased, the reaction rate was several orders of magnitude lower. Industrial trials including Ca-alloy injection into steel, sampling and SEM/EDS analyses, as well as an inclusion rating in the samples show the concept of rapid transformation of the alumina inclusions with Ca treatment.     

The thermodynamic properties of solid Al−Mg alloys

By measurements of the electromotive force of reversible galvanic cells of the form Mg (solid)/MgCl₂ in fused LiCl/KCl eutectic/Mg _x Al _y (solid) between 370° and 550°C, values have been determined for the free energies, entropies, and heats of formation of all the solid phases of the Al?Mg binary alloy system. The aluminum-rich solid solution appears to be essentially regular with a small positive heat of formation, while the magnesium-rich terminal solution shows negative heats and entropies of formation. The intermediate phases β, γ, and ε all exhibit moderate negative heats and large negative excess entropies of formation; in the case of β and γ these properties pass through pronounced minima, at compositions within their homogeneity ranges, indicative of highly ordered stoichiometric conditions. The results are discussed in relation to known phase structures.     

Structural and phase transformations in α + β brasses

The effects of the chemical composition of α + β brasses and heat treatment conditions on the amount, morphology, and chemical composition of the phases forming these brasses have been studied. Apart from the main phases (an α solid solution of alloying elements in copper and a β phase based on the electronic compound CuZn), all brasses under study are shown to contain M ₅Si₃ (M = Fe, Mn, Ni) silicides and particles of free lead (1–2 vol %). The amount and morphology of α and β grains can be controlled by heat treatment, whereas the quantitative characteristics of the silicides are mainly determined by the chemical composition of the alloy. Cu-Zn-Mn-Al-Fe-Si brasses can be additionally hardened due to the formation of ultradispersed Mn₅Si₃ silicides and martensite upon quenching and due to the partial dissolution of these particles and the formation of a bainite structure upon subsequent tempering at 260–270°C.     

Phase equilibria of the ternary Al-Cu-Ni system and interfacial reactions of related systems at 800 °C

A series of Al-Cu-Ni alloys of various compositions were made and annealed at 800 °C. The equilibrium phases were studied by metallography, X-ray diffraction (XRD) analysis, and electron probe microanalysis. The isothermal section of the ternary Al-Cu-Ni system at 800 °C was then determined based on these experimental results and the available phase relationship knowledge of the three constituent binary systems. No ternary compound was found. All three phases, AlNi₃, AlNi, and Al₃Ni₂, have very high ternary solubility, especially the AlNi phase, which almost reaches the binary Al-Cu side. However, no continuous solid solution was formed between the AlNi phase and any of the binary Al-Cu phases. Interfacial reactions of Al/Ni, Al/Cu, Al-Cu/Ni, and Al-Ni/Cu at 800 °C were investigated by using reaction couple techniques. The results showed that Al₃Ni and Al₃Ni₂ phases were formed in the Al/Ni couples; β-AlCu₄, γ ₁-Al₄Cu₉, and ɛ ₂-Al₂Cu₃ phases were formed in the Al/Cu couples. As for the results in the Al-2 at. pct Ni/Cu, Al-5 at. pct Ni/Cu, and Al-2 at. pct Cu/Ni, Al-4.5 at. pct Cu/Ni, and Al-6 at. pct Cu/Ni were similar to those in the binary Al/Cu and Al/Ni couples, respectively. A different reaction path was found in the Al-7.5 at. pct Cu/Ni couples, and an AlNi solid solution layer was formed instead of the Al₃Ni and Al₃Ni₂ phases.     

Etude par analyse en profondeur de l'interdiffusion en couches minces du couple AlCu: Cinetique de croissance des phases entre 200 et 300

This paper deals with thin layers interdiffusion of AlCu at annealing temperatures in the range of 200–300°C. The use of a method for depth analysis based upon the thermal ionization of sputtered material allows one to determine the composition of phases from the very beginning of their formation. Departure from stoichometry of about 1 % are measured for most observed phases. Comparison with thermal equilibrium diagram known at 600°C confirms the existence of the γ phase and shows that the γ₂ phase is in fact a mixture of two compounds Cu₃₂Al₁₉ and Cu₉Al₄. The main results to be mentioned are large diffusion coefficients (~ 10⁻¹⁶cm²/s at 220°C) and the formation of well defined compounds, with no appreciable composition range, growing in successive strates parallel to the plan of the interface of diffusion.     

Kinetics of dissolution of synthetic heazlewoodite (Ni3S2) in nitric acid solutions

The dissolution rate of heazlewoodite in nitric acid solution has been determined. The effects of nitric acid concentration, temperature, particle size, stirring intensity and addition of Cu²⁺ ions have been investigated. Solid residues after leaching were examined by SEM, X-ray diffraction and chemical analysis. In the solutions containing less than 2.0 M HNO₃, dissolution was observed to be completely inhibited after 30 min leaching time, and the rate of hydrogen sulphide production was faster than its oxidation to S⁰ and HSO₄^?. In 3 M HNO₃, an abrupt increase in dissolution rate of Ni₃S₂ was found. Two different regions of the dissolution of heazlewoodite were observed below and above 50°C. At temperatures below 50°C, the dissolution rate was very slow, even in 3.0 M HNO₃ solution, and H₂S gas was evolved. Above 50°C, the dissolution rate rapidly increased. Over the temperature interval 60–90°C in 3.0 M HNO₃ dissolution followed a linear rate law, and the activation energy was found to be 42.1 kJ mol^?1. Most of the oxidized sulphide ion was found in the solution as sulphate. The leaching rate was independent of stirring speed. The rate-controlling step of the Ni₃S₂ dissolution is the oxidation of hydrogen sulphide to elemental sulphur or sulphate ions on the Ni₃S₂ surface. Addition of small amounts of Cu²⁺ ions to the nitric acid acted as catalyst for the dissolution of Ni₃S₂. Bubbling air through the leach suspension increased the dissolution rate of Ni₃S₂ in solutions containing less than 2.0 M HNO₃.     

High-rate sputter deposition and heat treatment of thick Nb-Al-Ge and Nb-Al superconductors

Crystal structures, microstructures and critical temperatures were determined for Nb-Al-Ge and Nb-Al sputter-deposits in order to evaluate their dependence on sputter-deposition conditions, heat treatment procedures and composition. High-rate sputter deposition techniques were used to make the deposits at rates up to 1 Μm/min. Compositions studied were Nb₃(Al_0.6Ge_0.4), Nb₃(Al_0.75Ge_0.25), Nb₃Al, Nb_2.52(Al_0.84)Ge_0.16), Nb_2.33Al, Nb_3.07(Al_0.75Ge_0.25), and Nb_4.15(Al_0.71Ge_0.29). The investigation indicated it is feasible to make practical A-15 phase superconductors by high-rate sputter deposition. Of all the deposits studied, Nb₃(Al_0.75Ge_0.25),i.e., Nb₂Al₃Ge, deposited at 15°C and heat treated at 750°C for 1 to 5 days had the highest critical temperature (18.5 K), and it had a very high critical current density;e.g., 4.4 × 10⁵ A/cm² at 100 kOe and 4.2 K. Deposits having the highest critical temperatures consisted only of undecomposed metastable A-15 phase. The high current density was attributed to the presence of very small A-15 phase grains, which were observed to be about 350å in diameter by transmission electron microscopy. The crystal structures for deposits made at 15°C were not always clearly defined, but probably were all body-centered-cubic. Body-centered-cubic phases were transformed by heat treatment for short times at 550°C to 850°C to an A-15 phase that was supersaturated with Al and Ge. If heat treatment temperatures were too high or heat treatment times were too long, however, minor phases formed as the A-15 phase decomposed. Nb₃(Al_0.75Ge_0.25) deposits made at elevated temperatures (485°C and 750°C) predominantly consisted of the A-15 phase, but the presence of minor phases even before heat treatment indicated Al and Ge were not completely retained in the A-15 phase solid solution during deposition. Deposits made with-20 V substrate-anode bias had nearly the same composition as the sputtering target, but deposits made with -50 V and -75 V bias had significantly lower Al and Ge contents than the target. The compactness of the A-15 crystal structure relative to the bcc structure was noted in a comparison of average atomic volumes for the two structures in Nb₃Al. The average atomic volume in the A-15 phase formed by heat treatment was 1.36 pct less than the average atomic volume in the bcc phase that existed before heat treatment in sputter-deposited Nb₃Al made at 15°C.     

Effect of scandium on the phase composition and hardening of casting aluminum alloys of the Al–Ca–Si system

The phase composition of the Al–Ca–Si–Sc system is investigated in aluminum corner uisng computational (Thermo-Calc) and experimental (optical microscopy, scanning electron microscopy, and electron probe microanalysis) methods. The influence of annealing on the structure and hardness of alloys containing 0.3 wt % Sc is investigated in the region up to 550°C. It is shown that the maximum in the hardening curve caused by the isolation of nanoparticles of the Al₃Sc (L1₂) is attained after annealing at temperatures of 300–350°C in alloys belonging to the phase region (Al) + Al₄Ca + Al₂Si₂Ca ((Al) is the aluminum-based solid solution). Scandium completely enters the (Al) composition in alloys of this region, while the silicon concentration is minimal in it. On the other hand, hardening is almost absent in alloys from the (Al) + (Si) + Al₂Si₂Ca phase region. The possibility in principle to form the casting alloys based on the (Al) + Al₄Ca + Al₂Si₂Ca eutectic hardened without quenching is substantiated.