Microstructural characterization of rapidly solidified Al-Ta alloys

Two Al-rich Al-Ta alloys containing by weight 3 and 6 pct Ta have been rapidly solidified from the melt using the ‘gun’ technique. The microstructures and the crystal structures of the phases in the as-solidified as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized. A supersaturated solid solution was obtained in both the alloys in the as-solidified condition indicating a solid solubility extension of Ta in Al to almost 6 wt pct. The supersaturated solid solutions formed in both the alloys have been found to be quite stable up to 673 K (for 1 hour). Annealing at higher temperatures resulted in the formation of rod-shaped precipitates inside the grains and massive precipitates along grain boundaries. The rod-shaped precipitates arranged in a regular pattern constitute a new metastable intermediate phase Al₇Ta having an ordered structure. The massive precipitates which form along grain boundaries constitute the equilibrium Al₃Ta phase with a tetragonal crystal structure. The transformation behavior and the morphology of the transformation products are detailed in this paper.     

Effect of dysprosium on the kinetics and structural transformations during the decomposition of the supersaturated solid solution in magnesium–samarium alloys

The effect of dysprosium added in the amounts such that it does not form an individual phase in equilibrium with solid magnesium on the decomposition of the supersaturated magnesium solid solution in Mg–Sm alloys is studied. The presence of dysprosium in Mg–Sm alloys is found to retard the decomposition of the supersaturated magnesium solid solution and to increase the hardening effect upon aging. When these alloys are aged, dysprosium is partly retained in the magnesium solid solution and partly enters into the compositions of the phases that form during the decomposition of the solid solution and are characteristic of Mg–Sm alloys.     

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.     

Combined Effect of Calcium and Silicon on the Phase Composition and Structure of Al–10%Mg Alloy

Phase transformations in the Al–Ca–Mg–Si system in the region of aluminum–magnesium alloys are investigated using the Thermo-Calc program. The liquidus projection of the quaternary system is constructed with a Mg content of 10% and it is shown that phases Al4Ca, Mg₂Si, and Al₂CaSi₂ can crystallize (in addition to the aluminum solid solution (Al)) depending on the calcium and silicon concentrations. The crystallization character of quaternary alloys is investigated with the help of a polythermal cross section calculated at concentrations of 10% Mg and 84% Al. Based on the analysis of phase transformations occurring in alloys of this section, the presence of the Al–Al₂CaSi₂–Mg₂Si quasi-ternary section in the Al–Ca–Mg–Si system was assumed. Three experimental alloys were considered from a quantitative analysis of the phase composition, notably, Al–10% Ca–10% Mg–2% Si, Al–4% Ca–10% Mg–2% Si, and Al–3% Ca–10% Mg–1% Si. Metallographic investigations and electron-probe microanalysis were performed using a TESCAN Vega 3 scanning electron microscope. Critical temperatures are determined using a DSC Setaram Setsys Evolution differential calorimeter. The experimental results agree well with the calculated data; in particular, a peak at t ~ 450°C is revealed for all alloys in curves of the nonequilibrium solidus and invariant eutectic reaction L → (Al) + Al₄Ca + Mg₂Si + Al₃Mg₂. It is established that the structure of the Al–3% Ca–10% Mg–1% Si alloy is closest to the eutectic alloy. It is no worse that the AMg10 alloy in regards to density and corrosion resistance and even surpasses it in hardness, which allows us to consider this alloy as the basis for the development of a new cast material: “natural composites.”     

Influence of small additives of calcium on the structure and properties of ML5 alloy (AZ91)

The influence of the calcium additive (from 0.1 to 1.0 wt %) on the phase composition and solidus temperature of ML5 magnesium alloy is investigated. Calcium transfers into the intermetallic compound of the variable composition during alloy crystallization. This compound contains Al (53.4–57.4%), Ca (42.6–42.8%), and Mg (0.002–3.8%) and is transformed with a decrease in temperature into the Al₂Ca compound. The influence of calcium on the amount of phases Mg₁₇Al₁₂ and Al₂Ca and its distribution in the casting and thermally treated ML5 alloy structures are investigated. It is revealed with the help of the electron probe microanalysis that calcium and aluminum are concentrated along the boundaries of the magnesium solid solution. It is shown that, in order to acquire satisfactory mechanical properties and pouring of calcium-containing magnesium alloys should be performed according to the production process preventing the contamination of metal of the coarse inclusions. It is established that small additives of calcium (up to 1 wt %) increase the ignition temperature and lower the alloy oxidability at elevated temperatures (up to 715°C). The influence of the sulfur hexafluoride (SF6) for the calcium loss during flux-free melting was established.     

Structure and stability of the precipitates in melt spun ternary Al-transition-metal alloys

The decomposition of supersaturated solid solution of ternary, melt spun Al-transition-metal alloys has been examined using analytical transmission electron microscopy. It has been found that the presence of small amounts of iron in AlV, AlMo and AlCr alloys can give rise to a fine dispersion of P-phase precipitate in the grain centres. It has been shown that the P-phase is quasicrystalline phase and has an orientation relationship with Al matrix as: i2|〈001〉_Al, 〈τ²τ1〉_Al, i3|〈111〉_Al, 〈τ²10〉_Al; i5|〈τ10〉_Al, τ = (1 + √5)/2. The stability of quasicrystals is significantly improved by the presence of iron in A1-transition-metal alloys. The effect of iron on stabilising the quasicrystal precipitates is believed to be due to the fact that iron replaces the oversized transition metal (TM) atoms V, Cr and Mo in the smaller TM sites, resulting in a reduction of structural stress of quasicrystalline phase. The significance of these observations on the structure and stability of quasicrystal precipitates is discussed in terms of the development of high temperature dispersion strengthening Al-based alloys with transition metals.     

Galvanic cell measurements on supersaturated activities of oxygen in Fe-Al-M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) melts

Using a mullite (3Al₂O₃ · 2SiO₂)-tube and ZrO₂-9 mol pct MgO-plug type solid electrolyte galvanic cells, the activities of supersaturated oxygen in Fe-0.0017 to 0.41 mass pct Al-M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) alloys were measured as a function of total Al or M contents at 1873 K in an alumina crucible. Based on these results, the effects of alloying elements on the supersaturated oxygen activity with respect to alumina precipitation were studied. In the Fe-Al-M (M=C, Te, Mn, Cr, and Si) alloys, the supersaturated oxygen activities for a given Al level approach the equilibrium values with increasing contents of alloying elements. However, the oxygen activities for a given Al level are still supersaturated in the Fe-Al-M (M=Ti, Zr, and Ce) alloys even in the presence of considerable amounts of the alloying elements.     

Study of the phase equilibria and the decomposition of a supersaturated solid solution in magnesium alloys with aluminum, calcium, and manganese

Magnesium alloys Mg-Al-Ca-Mn having 1 wt % Al, up to 1.8 wt % Mn, and up to 1.6 wt % Ca are studied by optical microscopy, electron microprobe analysis, and electrical resistivity and microhardness measurements. The phase equilibria corresponding to the Al, Ca, and Mn concentration ranges under study were determined and sections of the isothermal tetrahedrons of the Mg-Al-Ca-Mn phase diagram in the Mn-rich region at 450 and 300°C have been constructed. As was found, manganese does not affect the decomposition kinetics of the magnesium supersaturated solid solution in ternary Mg-Al-Ca alloys but increases their hardness.     

Microstructure development during rapid solidification of highly supersaturated Cu-Co alloys

We report a detailed microstructure analysis of rapidly quenched Cu_1−xCo_x (0.1 ⩽ × ⩽ 0.5) alloys. The chemical homogeneity of the alloys was investigated on nanometer scale using a combination of AP/FIM- and TEM-studies. Cu-Co alloys with a Co concentration up to 10 at.% Co can be prepared homogeneously by rapid quenching. For larger Co contents, the microstructure is determined by a competition between polymorphic solidification, nucleation of the Co-rich solid solution from the melt, and spinodal decomposition of the unstable supersaturated solid solution. The microstructures are discussed in terms of the kinetics of the Cu-Co system, provided quantitatively by recent thermodynamic calculations.     

Decomposition kinetics of the supersaturated solid solution in Al-Mg2Si alloys with scandium,zirconium, and hafnium additions

The decomposition kinetics of the aluminum-based supersaturated solid solution in Al-Mg₂Si alloys with some transition metal additions is studied by hardness and electrical resistivity measurements, optical microscopy, and scanning electron microscopy. The effect of only scandium or scandium along with zirconium and hafnium on the hardening of the Al-Mg-Si alloys after heat treatment, which includes quenching, natural aging, and artificial aging, is revealed. The artificial aging temperature at which the hardening is maximal is chosen.     

Precipitation in a rapidly solidified and aged NiAlMo alloy

The early stages of decomposition of a highly supersaturated nickel-base alloy have been studied using transmission electron microscopy, scanning transmission electron microscopy and X-ray diffraction. The material was produced as a metastable solid solution by chill-block melt-spinning. On ageing the material exhibited a number of decomposition products appearing in series or concomitantly. Some of the decomposition products of this alloy, Ni₄Mo, Ni₃Mo (D0₂₂) and Ni₂Mo, are related to those found in NiMo binary alloys. α-Mo formed during solidification was distinguished from that formed by precipitation in the solid state by orientation relationships.     

Effect of scandium on the phase composition and mechanical properties of ABM alloys

The effect of scandium on the composition and mechanical properties of ABM-1 alloys (Al-30% Be-5% Mg) is studied. The scandium content is varied from 0.1 to 0.5 wt %. It is established that, in the studied part of the Al-Be-Mg-Sc system, an aluminum solid solution (Al) and the ScBe₁₃ compound are in equilibrium with a beryllium solid solution (Be). Magnesium dissolves in both the aluminum component and the ScBe₁₃ compound. The strengthening effect related to the decomposition of the solid solution and the precipitation of Al₃Sc cannot be extended to the strengthening of ABM-type alloys. Additions of 0.1–0.15 wt % Sc only weakly improve the mechanical properties of the alloys due to the refinement of beryllium-component grains. At high scandium contents, the strength increases insignificantly due to primary precipitation of ScBe₁₃ and the plasticity decreases simultaneously.     

Study of aged Mg-Sm alloys subjected to severe plastic deformation

The effect of severe plastic deformation by high-pressure torsion on the structure and properties of aged magnesium alloys containing 2.8–5.5 wt % Sm (the maximum solubility of samarium in solid magnesium is 5.8 wt %) are studied. The severe plastic deformation leads to substantial strengthening caused by the formation of a submicrocrystalline structure along with strengthening caused by the decomposition of a supersaturated magnesium-based solid solution.     

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%.     

Study on solid solution and aging process of AZ91D magnesium alloy with cerium

The influence of Ce on solid solution and aging process of AZ91D magnesium alloy was analyzed.The results showed that the decomposition of β-Mg17Al12 phase in AZ91D magnesium alloy at 420 ℃ could be completed within 12 h,while this process in the Ce-containing alloy required more time.In subsequent aging process at 175 ℃,Ce obviously delayed the aging process of AZglD.It was inferred that the influence of Ce on process of solid solution and aging was relative to the Ce that existed in β-Mg17Al12 phase of original structure in the form of solid solution,and the interaction of the Ce and Al was an important factor to get process of solution and aging slowly.     

Research of magnesium alloys of the Mg-Sm-Y system subjected to severe plastic deformation and subsequent heat treatment

The effect of severe plastic deformation and subsequent aging on the structure and properties of cast and hardened magnesium alloys containing samarium and yttrium is studied. Severe plastic deformation leads to additional hardening both before and after aging. Hardening is achieved by the formation of nanocrystalline structure in quenched alloys or submicrocrystalline structure in cast alloys. Severe plastic deformation results in the rapid decomposition of a supersaturated magnesium solid solution and additional hardening due to the formation of nano-sized particles of hardening phases.     

Diffusion-induced dislocations in RSP AlMo

Samples of melt-spun AlMo alloys containing either 2.2at.% Mo or 0.7 at.% Mo have been examined, using analytical electron microscopy, in the as-quenched state and after various annealing treatments. It has been found that in the alloys containing 2.2 at.% Mo, dislocation networks are produced and Al₂Mo precipitates are formed during annealing, whereas although Al₁₂Mo is precipitated, in the more dilute 0.7 at.% Mo alloy, no change in dislocation density occurs. The dislocations in the 2.2 at.% Mo samples separate regions of crystal, which still contain the supersaturated solid solution, from the regions where the Mo has been removed from solution. Removal of the Mo takes place only when the dislocations move by climb into the Mo-rich regions. These observations are discussed in terms of the generation of dislocations during diffusion with particular emphasis on the role of coherent and partially coherent interfaces.     

Phase Equilibria in the Mg – Al – Ca System (Region 50-100 mass% Mg)

Phase equilibria in the ternary system Mg – Al – Ca in the composition range 50-100 mass% Mg were studied by the methods of differential thermal, xray diffraction, electron-probe and microscopic analysis. The projection of the liquidus surface on the concentration triangle, isothermal section at 150°C and polythermal sections at 4.5, 8.5, and 16 mass% Al were constructed. It was determined that additions of Al and Ca decrease the liquidus temperature of magnesium alloys (from 650 to 438°C). It is shown that the three-phase region + 2Ca> + 17Al₁₂> exists at 150°C with the corresponding two-phase fields. The temperature dependence of the homogeneity range of the Mgbased solid solution was determined, and also the temperatures of the phase transformations which occur in the investigated range of compositions in the system.     

Effect of Samarium on the Properties of Mg–Y–Gd–Zr Alloys

The microstructure, the aging kinetics, and the strength properties of Mg–Y–Gd–Zr cast alloys, in particular, a samarium-alloyed IMV7-1 alloy, at room and high (250, 300°C) temperatures after homogenization without and with subsequent aging are studied. Alloying with samarium accelerates the decomposition of the supersaturated magnesium solid solution and enhances the properties of the Mg–Y–Gd–Zr alloys.     

Solid solution decomposition mechanisms in cast and microcrystalline Al-Sc alloys: III. Analysis of experimental data

The results of experimental studies of the decomposition of the solid solution in cast and microcrystalline (MC) Al-X wt % Mg-0.22 wt % Sc-0.15 wt % Zr (X = 0, 1.5, 4.5) aluminum alloys that were described in part I of this work are analyzed. The data on the electrical resistivity of the alloys are used to determine the volume fraction of Al₃Sc(Zr) particles precipitating in the temperature range 240–400°C. The model developed in part II of this work is used to reveal the mechanism of particle precipitation in the cast and MC alloys. It is shown that the particles in the cast and MC alloys precipitate mainly on dislocations. The differences in the decomposition kinetics of the solid solution in the cast and MC alloys are explained by different mechanisms of dislocation structure recovery mechanisms occurring in them.