Al-Sc Alloys Research Articles

CREEP DAMAGE OF Al AND Al-Sc ALLOY PROCESSED BY ECAP

Experiments were conducted to determine an effect of equal-channel angular pressing (ECAP) on the creep damage and the microstructure of pure aluminium and an Al-Sc alloy. The coarse-grained materials in their as-received states were subjected to ECAP at room temperature using a die, which had a 90° angle between the channels. Creep tests in tension were performed on the as-pressed samples at 473 K under an applied stress range between 10 to 50 MPa. For comparison purposes, some creep tests were performed also on both investigated materials in their as-received states. Following ECAP and creep testing samples were prepared for examination by scanning electron microscopy equipped with an electron back scattering diffraction unit. The observation of the free surfaces of the ECAP samples after creep exposure showed the occurrence of mesoscopic shear bands exceeding considerably an average grain size. The formation of these bands could be influenced by inhomogeneity of microstructure created during ECAP. It was observed the cavities and microcracks nucleate and propagate along the mesoscopic shear bands. Their growth and coalescence lead to ductile fracture mode with dimples on the creep fracture surface. The evolution of creep damage and fracture of ultrafine-grained specimens could be influenced by synergetic effects of grain boundary sliding, formation of shear bands which were oriented along the direction of the last ECAP pass and cavitation.

Processing Al-Sc Alloys at Liquid Aluminum Cathode in KF-AlF3 Molten Salt

Al-Sc Alloys have showed many advanced properties for a wide range of engineering applications, which is usually produced using thermal reduction process. This work presents a study on electrochemical preparation of Al-Sc alloys at liquid aluminum cathode in KF-AlF3 (CR=1.3) molten salt at 750℃. Cyclic voltammetry and linear sweep voltammetry were employed to investigate the electrode reaction. The electro-reduction of Sc(III) takes place on the surface of the aluminum liquid cathode at more positive potential values (0.6V) than at the inert Mo cathode. SEM and Inductively Coupled Plasma (ICP) analysis of the product indicated that Al-Sc alloys were made of α-Al and Al3Sc phases; when the anode current density was 0.5~2.0A/cm2, Sc content was 0.44~0.73%. The obtained results show a promising method to make Al-Sc alloy electrochemically.

Drawability of thin magnetron sputtered Al–Zr foils in micro deep drawing

Deep drawing provides a great application potential for the manufacturing of parts with complex shapes, even when it is scaled to the micro range. Two different foils out of the Al–Zr alloy with a thickness of about 15 μm were manufactured using a magnetron-sputtering process by applying substrate temperatures of 310 K and 433 K, respectively. These foils were used as blank material in micro deep drawing with a punch diameter of 0.75 mm to investigate the formability. Even though the materials show small ultimate strains in tensile tests, deep drawing was carried out successfully. Limit drawing ratios of 1.8 and at least 1.7 were reached for the material produced with a substrate temperature of 310 K and 433 K, respectively. These are even better results than those realized with Al-Sc alloys in former investigations. Comparison with deep drawing results of pure aluminum produced by conventional rolling shows the same achievable limit drawing ratio. This illustrates the good suitability of magnetron sputtering as a foil manufacturing process for micro sheet forming operations.

Measuring Electrical Conductivity of Al-Sc Alloy by Application of Continuously Varying Cell Constant Technique

The electrical conductivity of the molten salts of Na3AlF6-LiF-Sc2O3 system with different compositions was measured at different temperatures by the continuously varying cell constant technique. The main influence factors on electrical conductivity were analyzed. Experiment results showed that the technique of the electrical conductivity measurement is accurate and reliable and the result’s relative error is just 0.67% in comparison with those in relevant literature. With temperature rising, electrolyte conductivity increases at a rate of about 0.03S/cm for 1°C. And we also found that the conductivity increases slightly with the addition of lithium fluoride and adding scandium oxide makes the conductivity decrease slightly and more addition doesn’t cause significant effect. It was proved that the technique can measure accurately the electrical conductivity of aluminium electrolyte and other high-temperature molten salts.

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.

Solid solution decomposition mechanisms in cast and microcrystalline Al-Sc alloys: II. Model for the decomposition of a solid solution during the formation of coherent second-phase particles

A model is proposed to calculate the kinetic parameters of the precipitation of coherent second-phase particles from a supersaturated solid solution. This model is based on the assumption of barrier-free nucleation of coherent particles. In this case, the decomposition rate of a solid solution and the particle precipitation kinetic parameters are mainly determined by the characteristics of nucleus diffusion growth. The decomposition kinetics of a solid solution during the precipitation of coherent particles is described by the Johnson-Mehl-Avrami-Kolmogorov equation W = 1 − exp[-(t/τ)n], where W is the degree of decomposition and τ = τ0exp(Q/kBT) is the decomposition time. The values of parameters n and Q characterizing the decomposition kinetics are determined. Expressions are derived to describe the dependences of the volume fraction and size of precipitating coherent particles on the supersaturation of a solid solution, the annealing temperature and time, and the thermodynamic and diffusion parameters of a material.

The effect of Sc additions on the microstructure and age hardening behaviour of as cast Al–Sc alloys

The grain refinement effect and the ageing behaviour of Al–0.5wt.% Sc, Al–0.7wt.% Sc, and Al–1wt.% Sc alloys are studied on the basis of optic microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) observations and hardness measurements. In Al–Sc alloys the higher grain refinement is observed for Sc contents greater than 0.5wt.% accompanied by a notorious morphology modification, from coarse columnar grains to a fine perfect equiaxed structure. The as cast structures are characterised by a rich supersaturated solid solution in Sc, that promotes a great age hardening response at 250°C and 300°C. The age hardening curves also demonstrate a low overageing kinetics for all the alloys. Although the higher Sc content in solid solution for the alloys with 0.7 and 1wt.% Sc, the age hardening response of all the Al–Sc alloys remains similar. The direct age hardening response of the as cast Al–0.5wt.% Sc is shown to be greater than the solutionised and age hardened alloy.

Processing Al-Sc Alloys at Liquid Aluminum Cathode in KF-AlF3 Molten Salt

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Effects of scandium additions on mechanical properties of cellular Al-based foams

By using titanium hydride as the blowing agent and Ca particles as the thickening agent, the cellular Al-0.24wt.%Sc and Al-0.41wt.%Sc foams with a porosity ranging from 48 to 86% were successfully fabricated via the melt-foaming method. The compressive behaviors and effects of minor Sc additions on the strengthening mechanism of the cellular Al–Sc alloy foams were investigated and discussed. The results show that minor Sc additions can dramatically improve the compressive strength of the cellular Al foams due to the grain refinement and pinning effect from the Al3Sc precipitates homogeneously distributed in the matrix after heat treatment, implying that the Sc addition coupled with proper heat treatment is an effective way to improve the mechanical performance for Al-based alloy foams.

A mathematical model coupled to CALPHAD to predict precipitation kinetics for multicomponent aluminum alloys

A mathematical model was developed to simulate the precipitation kinetics during heat treatment of multicomponent aluminum alloys. The model is based on the general numerical framework proposed by Kampmann and Wagner, and features a full coupling with CALPHAD software for the evaluation of the Gibbs–Thomson effect. It also does not rely on the assumption that precipitate phase composition is stoichiometric or uniform, and is therefore applicable for predicting complex precipitation kinetics encountered in industrial practices. Applications of the model to various aging treatments of binary Al–Sc alloys and a ternary Al–Sc–Zr alloy were conducted. It was found that the model predictions for extended time coarsening kinetics are in good agreement with the analytical Lifshitz–Slyozov–Wagner coarsening theory. Its ability to reproduce the complex precipitation pathways in multicomponent alloys was demonstrated by simulation of the precipitation kinetics for an Al–0.09at.% Sc–0.03at.% Zr alloy. Comparison of the simulation results with experimental measurement has also highlighted research directions that require further effort.

Solid solution decomposition mechanisms in cast and microcrystalline Al-Sc alloys: I. Experimental studies

The structure and the electrical and mechanical properties of Al-Mg-0.22 wt % Sc-0.15 wt % Zr alloys with various magnesium contents (0, 1.5, 4.5 wt %) are experimentally studied during the decomposition of the solid solution of scandium and zirconium in the states after solidification from a melt (cast ingots) and after subsequent multicycle equal-channel angular pressing (microcrystalline structure). The dependences of electrical resistivity ρ, microhardness HV, macroelasticity limit σ0, and yield strength σy on the annealing temperature and time are analyzed.

Creep properties and precipitate evolution in Al–Li alloys microalloyed with Sc and Yb

A dilute Al–Sc alloy (Al–0.12 Sc, at.%, Al–Sc), its counterpart with a Li addition (Al–2.9 Li–0.11 Sc, at.%, Al–Li–Sc), as well as a quaternary alloy (Al–5.53 Li–0.048 Sc–0.009 Yb, at.%, Al–Li–Sc–Yb) were isothermally aged at 325 °C, and in some cases isochronally aged to 450 °C. As the α′-Al3(Li,Sc) and Al3(Li,Sc,Yb) precipitates, with L12 structure, coarsen in the two Li-containing alloys, their Li and Yb concentrations decrease and their Sc concentration increases. A significant interfacial excess of Li also segregates at the α-Al matrix/α′-Al3Sc(Li,Sc,Yb) precipitate interface: 5.99 ± 0.05 atoms nm−2 in Al–Li–Sc and 13.2 ± 0.4 atoms nm−2 in Al–Li–Sc–Yb after aging isochronally to 450 °C. During compression creep at 300 °C, the aged alloys exhibit threshold stresses between 8 and 22 MPa. A recent threshold stress model based on elastic interactions between dislocations and precipitates predicts correctly that Li additions in the Al–Li–Sc alloy reduce the threshold stress, while Yb in the Al–Li–Sc–Yb alloy increases it. The model is also in agreement with the threshold stresses of all Al–Sc–X alloys published to date.

Slow Crystallization of Al-Sc Alloys: Growth of Spherical Intermetallic Particles

The solidification of binary Al-Sc melts containing small amounts of scandium (< 2 wt.%) have been investigated. We studied concentration profiles in long Al-Sc as cast ingots formed by slow cooling of the liquid alloys. It was found that minor addition of transition metals like Ti can lead to the formation of large (about 10-20 μm) spherical intermetallic particles (Al3Sc containing some amount of isomorphic Ti). These sphere-like particles are forming during the crystallization of liquid solution of Sc in Al (before the precipitation on nanosize spherical particles from the solid solutions). The results of SEM and EDX investigations of these alloys are also presented in this paper.

Effects of Yb and Zr microalloying additions on the microstructure and mechanical properties of dilute Al–Sc alloys

It is known that Zr and Yb partition to the Al3Sc precipitates created during aging when microalloyed separately in dilute binary Al–Sc alloys. Addition of Zr delays precipitate coarsening, thereby improving the coarsening resistance of the ternary Al-Sc-Zr alloys. Addition of Yb increases the resistance against dislocation climb, thereby improving the creep resistances of the ternary Al-Sc-Yb alloys. A combination of microhardness, creep, and atom probe tomography measurements provide evidence that these effects of Zr and Yb additions are cumulative in quaternary dilute Al–Sc–Yb–Zr alloys: Yb increases their creep resistance at 300°C compared with ternary Al–Sc–Zr alloys and Zr improves their coarsening resistance at 300°C compared with ternary Al–Sc–Yb alloys. Additionally, excellent coarsening resistance is observed at 350 and 375°C.

Study of decomposition of oversaturated solid solution in Al-Sc-Zr alloys at different ratio of scandium and zirconium

Using electrical resistivity and hardness measurement methods and also electron microscopy analysis, the aging processes were studied in the Al-rich alloys of the Al-Sc-Zr system for contents of 0.1–0.3% Sc and 0.04–0.28% Zr. A significant increase in the alloy hardness as compared with the quenched state was already established during aging at a temperature of 350°C after 1h. The character of the change in electrical resistivity and hardness during aging turned out to be similar for all studied alloys. However, the strengthening effect in the ternary Al-Sc-Zr alloys was higher than that in the binary Al-Sc alloys with the same Sc contents. Decomposition of the supersaturated Al solid solution was accompanied by precipitation of nanosize particles of the strengthening phases.

Precipitation Hardening Behaviour of Al-2Sc Micro Sheets

The continuous miniaturisation of products needed e.g. in the automotive or the micro electronic sector requires process chains which allow the manufacturing of microscopically small components in high quantities. The development of the required processes and technologies is the aim of the Collaborative Research Centre 747 “Micro Cold Forming” of the German Research Foundation. A necessary step in the manufacturing process chain is the heat treatment, which enables the adjustment of the semi-finished micro components to cold forming. Finally the usage properties have to be adjusted e.g. by precipitation hardening of aluminium alloys in order to increase the strength above the strain hardened level. To achieve a high strength aluminium alloy with elevated temperature stability in form of thin foils a new Al-Sc alloy with 2 mass-% scandium had to be developed within the Collaborative Research Centre. In comparison to conventional casting techniques it was possible to suppress Al3Sc precipitation and to ensure the supersaturated solid solution by using a Physical Vapour Deposition (PVD) Magnetron Sputtering process and low sputtering temperatures. Samples of the material are then artificially aged and characterised by ultra micro hardness measurements to evaluate the optimal ageing parameters. Besides this the microstructure was determined by light microscopy and Transmission Electron Microscopical (TEM) analysis.

Microstructural evolution and creep properties of precipitation-strengthened Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys

The aging behavior at 300 °C of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys is studied by local-electrode atom-probe tomography, transmission electron microscopy and microhardness measurements. The ternary alloys exhibit high number densities of coherent L1 2 precipitates ( N v ≅ 10 22 m - 3 ) at aging times up to 1536 h (64 days). In the Al–0.06Sc–0.02Gd alloy, the Al 3(Sc 1− x Gd x ) precipitates are always Sc-rich, displaying a small Gd concentration ( x < 0.12) in the precipitates. In the Al–0.06Sc–0.02Yb alloy, the precipitates are initially Yb-rich, Al 3(Yb 1− x Sc x ), with Sc diffusing subsequently to the precipitates, resulting in a core/shell structure and an overall Sc-rich composition, Al 3(Sc 1− x Yb x ). Gd and Yb, like other lanthanides but unlike the transition metals Zr and Ti, do not retard the coarsening kinetics compared with binary Al–Sc alloys. Additionally, the creep resistance of these alloys is greater than that of Al–Sc alloys. The coarsening kinetics and creep properties of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb alloys are compared with other Al–Sc-based alloys and with coarsening models for ternary alloys.

Thin Scandium Layer on Solid Aluminium: Thermodynamic Investigation

Thin scandium-containing layers fabricated by different methods are good systems to investigate some properties of Al-Sc intermetallic compounds (IMC). The electrochemical deposition of a thin scandium layer onto a solid metallic surface leads to complex diffusion processes, especially if the second metal forms a number of IMC with Sc. Here, we used the electromotive force (EMF) method to study thin Al-Sc films in dynamics. Also, the Al-Sc alloy layers were prepared by direct aluminium-scandium interaction. The results of SEM and EDX investigations of these alloys are also presented in this paper. Analogue coatings (like Sc on Ni, Y on Cu, Al on Ni etc.) were also considered in order to discuss the phase sequence formation.

The structural stabilities of Al3(Sc1−xMx) by first-principles calculations

The positive effects of Al3Sc phase on Al–Sc alloys can be promoted by adding to the third elements. The structure properties of Al3Sc when it dissolves the third elements are investigated by first-principles calculations. Special quasirandom structures are developed for the quasi-binary L12 structures. The calculations indicate that the elements of Ti, Zr, Y and Ta tend to substitute for Sc in Al3Sc, while Ni and Si prefer to substitute for Al. The present lattice parameters generally follow the Vegard’s law. The high solubility of Ti and Y in Al3Sc is revealed in the calculated quasi-binary phase diagram.

Preparation of Al-Sc Alloy by LiF-ScF&lt;sub&gt;3&lt;/sub&gt;-ScCl&lt;sub&gt;3&lt;/sub&gt; Molten Salt Electrolysis

The paper studied the preparation of Al-Sc alloy by molten salt electrolysis. LiF-ScF3-ScCl3 as the electrolyte, Sc2O3 as raw material, liquid Al as cathode and graphite as anode. The process condition of preparating Al-Sc alloy includes the influence of current density, electrolysis time, temperature of electrolysis, Back EMF, content of Sc in alloy and current efficiency.The content of Sc in alloy prepared by this method has reached to the maximum of 3.437%. The components of alloys showed by SEM were uniform, it is applicable for commercial purposes. Preparation of Al-Sc alloys with scandium oxide as raw materials, it not only reduce environment pollution, but also decease the cost of production. It is reported in many documents, preparing aluminum base alloy by the method of molten salt electrolysis[1-3], people could make many alloys such as Al-Ce、Al-La、Al-RE、Al-Ti、Al-Si-Ti in the aluminum cell, also the method to prepare some rare earth alloys such as Al-Sr、Nd-Fe、La-Ni was reported[4,5]. Use the method of electrolysis to produce some aluminum base alloy with high-melting-point , difficult to restore , high-priced element is a good method because it is technological process is brief, economy is rational, the technology is feasible, this viewpoint is broadly approved in the world[6]. It isn’t necessary to use high-priced Sc as raw material in preparing Al-Sc alloy with molten salt electrolysis, it could control the Sc amounts in the alloy through the different current efficiency and electrolysis time, production in this method could be continuous or semi-continuous, so it is easy to be automatic controlled. A new molten salt system is used in this paper, we use molten salt electrolysis to produce Al-Sc alloy, take scandium oxide as raw material, through the study of the effect of the current intensity，electrolysis time and the electrolysis temperature , Back EMF and cell voltage, the final production Al-Sc alloy contains Sc 2~6%.