Al3 Sc Research Articles

Effect of annealing on morphology, optical reflectivity, and stress state of Al–(0.19–0.53) wt.% Sc thin films prepared by magnetron sputtering

Degradation caused by formation of hillocks is a problem when traditional aluminum thin film acts as the reflector of the novel automobile lamp in which light emitted diode (LED) serves as the light source. This degradation in lamp performance is attributed to the annealing effect caused by cyclic change of temperature. Thin Al–0.19–0.53 wt.% Sc alloy films prepared by magnetron sputtering are potential candidates for substituting pure aluminum films as the reflector due to their excellent optical reflectivity and corrosion resistance. The annealing effect on the morphology of Al–Sc thin films arisen from thermal cycle during the service of auto lamps is of interest. The films were heated from room temperature to 450 °C and cooled down to examine their surface morphology. Hillocks on the pure Al film could be dramatically reduced in Al–0.19–0.48 wt.% Sc films and their formation was completely inhibited in Al–0.53 wt.% Sc. Examination through scanning electron micrographs (FE-SEM) reveals that Al–0.53 wt.% Sc comprises nano grains that are much finer than those in other films. According to stress analysis on the film, we proposed a mechanism for interpreting the formation of hillocks on the Al film and their inhibition on Al-Sc films. Al–0.53 wt.% Sc thin film is suggested to be a candidate for reflectors in a LED auto lamp.

FIRST--PRINCIPLES STUDY OF THE PHASE STRACTURES OF Al--Sc ALLOYS

The formation enthalpies of the Al-Sc compounds and the Al-rich solid solution phase have been studied from first-principles based on plane-wave pseudopotential method. To calculate the electronic structure of the disordered Al-Sc alloys, the small-size special quasirandom structures (SQS), which exhibited similar structure environments as that of the disordered Al-Sc alloys, are employed. It is shown that the Sc atoms in supersaturated solid solution tend to segregation forming ordered Al3Sc (L12) and AlSc (L10) precipitations. The nucleation mechanism of Al3Sc precipitation phase in the supersaturated Al(Sc) solid solution is analyzed in terms of the calculated densities of states and distributions of electron densities.

Adiabatic shearing behaviour of unweldable Al–Sc alloy under extremely high shear loading

The present study applies a compressive type split Hopkinson pressure bar to investigate the adiabatic shear behaviour of unweldable Al–Sc alloy under high strain rates ranging from 3·0 × 105 to 6·3 × 105 s–1. The effects of the strain rate on the shear stress, adiabatic shear band characteristics, and fracture features of the unweldable Al–Sc alloy are systematically examined. The results show that both the shear stress and the strain rate sensitivity increase with increasing the strain rate. In addition, it is shown that an adiabatic shear band is formed within the deformed specimens for all values of the strain rate. As the strain rate is increased, the width of the shear band decreases, but the microhardness increases. Moreover, the distortion angle and the magnitude of the local shear strain near the shear band both increase with increasing the strain rate. At a strain rate of 3·0 × 105 s–1, the fracture surface is characterised by multiple transgranular clearage fractures. However, for strain rates greater than 4·5 × 105 s–1, the fracture surface has a transgranular dimple-like characteristic, and thus it is inferred that the ductility of the unweldable Al–Sc alloy improves with increasing the strain rate.

Modeling short-range ordering in liquids: The Mg–Al–Sn system

A liquid solution of components A and B may often exhibit a tendency towards short-range ordering (SRO). This may be modeled by the Modified Quasichemical Model (MQM) which attributes the SRO to the preferential formation of nearest-neighbor A–B pairs or, alternatively, by an associate model which attributes the ordering to the formation of A n B m associates or molecules. Although both models can often provide similar and equally good fits to experimental thermodynamic and phase equilibrium data in a binary system, the MQM provides significantly better predictions of the thermodynamic properties of ordered ternary liquid phases A–B–C solely from the optimized model parameters of the A–B, B–C and C–A binary sub-systems. This is illustrated through coupled thermodynamic/phase diagram optimization of the Mg–Al–Sn system. A similar example for the Mg–Al–Sc system is also presented.

Differences of grain-refining effect of Sc and Ti additions in aluminum by empirical electron theory analysis

The differences of grain-refining effect between Sc and Ti additions in aluminum, which cannot be substantially explained by traditional theories, were carefully studied. The empirical electron theory (EET) of solids and molecules was employed to calculate the valence electron structures (VES) of Al 3Ti and Al 3Sc. The conclusions can be drawn that, in the two alloys Al-Ti and Al-Sc, the different valence electron structures of Al 3Ti and Al 3Sc and the consequent differences of growth habit of the two particles, and the different interfacial electron density between particles and matrix fundamentally lead to the differences of grain-refining effect between Sc and Ti additions on aluminum and make Sr the better grain-refiner of aluminum.

Roles of impurities on precipitation kinetics of dilute Al–Sc alloys

High-purity (HP) aluminum and commercial purity (CP) aluminum (major impurities: ∼250 at. ppm Si and ∼130 at. ppm Fe) are alloyed with ∼250 to ∼1100 at. ppm Sc and ∼50 at. ppm RE (RE = La, Ce, Pr, or Nd). The alloys are homogenized at 640 °C and aged at 300 °C. The precipitation kinetics, basic mechanical properties, and microstructure are studied using AC electrical conductivity, microhardness measurements, scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy, and atom-probe tomography, respectively. The Fe and RE elements form micrometer-scale diameter Al ∼3(Fe,RE) primary precipitates, which have no effect on the mechanical properties. Silicon accelerates the precipitation kinetics of nanometer-scale diameter Al 3Sc precipitates, increasing their number density, thereby resulting in higher microhardness values for CP aluminum than the HP aluminum having the same Sc concentration. Additionally, the Sc equilibrium solubility in the α-Al matrix is estimated and Orowan's strengthening mechanism is confirmed for the Al 3Sc precipitates.

Comparison of residual microstructures associated with impact craters in Al–Sc and Al–Ti alloys

Al–Sc and Al–Ti semi-infinite targets were impacted by high-speed projectiles with velocities of 0.8, 2 and 4 km s −1, respectively. The results show that deep columned craters with hemispherical bottoms were formed in the Al–Ti target, while near-hemispheroidal or relatively shallower craters formed in the Al–Sc alloy. It is concluded that different microstructures of Al–Sc and Al–Ti alloys, including different grain sizes and secondary particles precipitated in the matrix, result in their having greatly different capabilities of resisting impact. Residual microstructures of different samples are further discussed. It is possible that, due to the very large amount of energy imported into the target by high-speed impact, secondary Al 3Sc lost its coherency and consequently recrystallization occurred.

Impact deformation behaviour and dislocation substructure of Al–Sc alloy

This paper employs a compressive split-Hopkinson pressure bar to investigate the impact deformation behaviour of Al–Sc alloy under high strain rates of 1.2 × 10 3 s −1, 3.2 × 10 3 s −1 and 5.8 × 10 3 s −1, respectively, and temperatures of −100 °C, 25 °C and 300 °C. It is shown that for a constant temperature, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, while the activation volume decreases. Conversely, for a constant strain rate, the flow stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the activation volume increases. It is found that the impact deformation behaviour of Al–Sc alloy can be accurately described using the Zerilli–Armstrong constitutive equation. Optical microscopy (OM) observations reveal that the specimens fail principally as the result of an adiabatic shearing mechanism. Furthermore, scanning electron microscopy (SEM) observations show that the fracture surfaces are characterised by a dimple-like structure, which indicates a ductile failure mode. Transmission electron microscopy (TEM) observations indicate that the dislocation density and cell size are related to the strain rate, flow stress and temperature. Finally, the TEM observations suggest that the strengthening effect observed in the deformed Al–Sc alloy is the result of Al 3Sc precipitates within the matrix and at the grain boundaries, which suppress dislocation motion and prompt an increase in the work hardening stress.

Comparing efficiencies of genetic and minima hopping algorithms for crystal structure prediction

In this work several crystal structure prediction problems which have been studied by first-principles evolutionary algorithms recently are revisited. We increased the system size to see how the search efficiency changes with respect to problem size. We find that the relative performance and underlying mechanism of genetic algorithms in crystal structure searches for Al(x)Sc(1-x) strongly depend on the system composition as well as the size of the problem. Because of this strong dependence, caution should be taken in generalizing performance comparison from one problem to another even though they may appear to be similar. We also investigate the performance of the search algorithm for crystal structure prediction of boron with and without a priori knowledge of the lattice vectors. The results show that the degree of difficulty increases dramatically if the lattice vectors of the crystal are allowed to vary during the search. Comparison of the minima hopping algorithm with the genetic algorithm at small (<10 atoms) to larger problem sizes is also carried out. At the small sizes we have tested, both methods show comparable efficiency. But at large sizes the genetic algorithm becomes advantageous over minima hopping.

The effect of initial microstructure and processing temperature on microstructure and texture in multilayered Al/Al(Sc) ARB sheets

AbstractA commercial purity Al alloy and an Al-0.3Sc (wt.%) alloy, the latter in either the supersaturated or artificially aged condition, were accumulative roll bonded at either 200 or 350°C to high strain to generate sheet materials consisting of 32 or 64 alternating layers of Al and Al(Sc). The microstructure and texture of the processed materials were investigated mainly using electron backscattered diffraction scanning electron microscopy and transmission electron microscopy. The deformation microstructure and texture of these two alloy combinations were strongly influenced by both the initial heat treatment condition of the Al(Sc) alloy whereby large-scale shear bands were generated during rolling when a dispersion of fine Al3Sc particles is present in the Al(Sc) layers. The effect of initial microstructure and processing temperature affected the subsequent recrystallization microstructure and texture of the Al/Al(Sc) composite during annealing at 350°C. Here, the Al(Sc) layers remain unrecrystallized in all materials with the Al layers undergoing continuous and discontinuous recrystallization after low and high temperature ARB, respectively. The lack of recrystallization in the Al(Sc) layers generated an alternating recrystallized/recovered microstructure in all materials.

Phase transformations in isochronally annealed mould-cast and cold-rolled Al–Sc–Zr-based alloy

Abstract The effect of small additions of scandium and zirconium on microstructure, thermodynamic and mechanical properties during the non-isothermal annealing of the mould-cast and cold-rolled aluminium was studied. Electrical resistometry together with the microhardness (HV0.3) measurements and transmission electron microscopy compared to differential scanning calorimetry results were used to characterize the effect. The Al–Sc–Zr-based alloy with a small addition of Ti in both states reaches the peak hardness by annealing to 330 °C due to precipitation of the Al 3 Sc particles. Relative resistivity changes (above 240 °C) of both alloys exhibit similar dependence on annealing temperature. Precipitation of particles (L1 2 structure) at 390 °C and 450 °C was observed by TEM. The apparent activation energy for precipitation of Al 3 Sc particles was calculated. The decomposition sequence of oversaturated solid solution of the studied Al–Sc–Zr-based alloy is compatible with the recently published decomposition sequence of Al–Sc–Zr system and the formation of layer rich in Ti, namely: Sc-rich clusters → Al 3 Sc phase → layer rich in Zr → layer rich in Ti → Al 3 (Sc,Zr,Ti) phase.

Thermodynamic analysis of Al–Sc, Cu–Sc and Al–Cu–Sc system

Encouraged by the promising application of the Al–Cu–Sc system in the development of high strength aluminum alloy, the Al-rich region of this system was assessed by the CALPHAD approach. In order to make the models used for the phases in the three binary sub-systems compatible with each other, Al–Sc and Cu–Sc system were remodeled. Substitutional solution model was used for solution phases and sublattice model for binary intermetallic phases and the ternary compound Al8−xCu4+xSc. Comparisons between the calculated and experimental results showed that most of the experimental information was satisfactorily described by the present thermodynamic description.

Valence electron structure analysis of morphologies of Al 3Ti and Al 3Sc in aluminum alloys

In present work, the valence electron structures (VESs) of Al 3Ti and Al 3Sc are studied by employing the empirical electron theory of solids and molecules (EET), giving theoretical explanation on the different morphologies of Al 3Ti and Al 3Sc in aluminum alloys. It is demonstrated by experiments that Al 3Ti grows to a needle-like shape because of its anisotropy of VES, while Al 3Sc has a symmetrical morphology for its high-degree symmetry of VES. Besides, Al 3Ti gets a larger size than Al 3Sc because of their different VESs.

The microstructure, optical, and electrical properties of sol–gel-derived Sc-doped and Al–Sc co-doped ZnO thin films

Transparent conductive ZnO:Al–Sc (1:0.5, 1:1, 1:1.5 at.% Al–Sc) thin films were prepared on glass substrates by sol–gel method. The microstructure, optical, and electrical properties of ZnO:Sc and ZnO:Al–Sc films were investigated. Results show that Sc-doping alone obviously decreases grain size and degrades the crystallinity; there is an amorphous phase on the surface of ZnO grains; the transmittance spectra fluctuate dramatically with a large absorption valley at about 430–600 nm. However, Al–Sc co-doping can stabilize grain size and improve the microstructure; an average visible transmittance of above 73% is obtained with no large absorption valley; the amorphous phase does not appear. The optical band gaps of ZnO:Sc and ZnO:Al–Sc films (3.30–3.32 eV) are blue-shifted relative to pure ZnO film (3.30 eV). Hall effects show that the lowest resistivity of 2.941 × 10 −2 Ω cm and the maximum Hall mobility of 24.04 cm 2/V s are obtained for ZnO:Al–Sc films while ZnO:Sc films do not exhibit any electrical conductivity. Moreover, there is an optimum atomic ratio with Al to Sc of 1:0.5–1 at.%. Although the resistivities are increased compared with that of ZnO:Al film, the Hall mobilities are raised by one order of magnitude.

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al 3Sc/Al 3(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al 3Sc/Al 3(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.

Comparative study of dynamic impact properties and microstructures of weldable and unweldable aluminium–scandium (Al–Sc) alloys

The dynamic deformation behaviours of two Al–Sc alloys, one weldable and one unweldable, are investigated at strain rates ranging from 1·2 × 103 to 5·9 × 103 s–1 and temperatures of –100, 25 and 300°C respectively, using a compressive split Hopkinson pressure bar. The fracture features and microstructures of the impacted specimens are examined using scanning electron microscopy and transmission electron microscopy respectively. The stress–strain relationships indicate that for both alloys, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Moreover, the flow stress, work hardening rate and strain rate sensitivity are higher in the unweldable Al–Sc alloy than in the weldable alloy. In both alloys, the activation volume is dominated by a thermally activated mechanism and increases as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. In describing the plastic deformation behaviour of the two Al–Sc alloys using the Zerilli–Armstrong fcc constitutive model, the error between the predicted flow stress and the measured stress is found to be <5%. The scanning electron microscopy observations reveal that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of a ductile fracture mode. The dimple like structures on the fracture surfaces of the unweldable Al–Sc alloy are shallower than those on the fracture surfaces of the weldable Al–Sc alloy, which indicates that the weldable Al–Sc alloy has a better ductility than the unweldable Al–Sc alloy. The transmission electron microscopy images show that the microstructures of the two alloys contain different types of precipitates. Specifically, the unweldable Al–Sc alloy contains both Al3Sc and Al2Cu particles, whereas the weldable Al–Sc alloy contains Al3Sc precipitates only. These particles prevent dislocation motion, and therefore prompt a significant strengthening effect. The transmission electron microscopy observations also reveal that in both alloys, the dislocation density increases with increasing strain rate, but decreases with increasing temperature. Furthermore, it is found that the dislocation density of the unweldable Al–Sc alloy is higher than that of the weldable Al–Sc alloy. In other words, the dislocation cells in the unweldable Al–Sc alloy are smaller than those in the weldable Al–Sc alloy. Thus, it is inferred that the unweldable Al–Sc alloy has a higher flow stress than the weldable Al–Sc alloy.

Precipitation of (Al,Si) 3Sc in an Al–Sc–Si alloy

Three-dimensional atom probe has been applied to investigate the solubility of Si in Al3Sc in an Al–0.16Sc–0.05Si alloy. It was found that L12-structured Si-containing Al3Sc precipitates formed after the alloy was aged at 300 °C. Results of first-principles calculation indicated that Si atoms tend to substitute Al sites in Al3Sc, and thus the precipitates can be represented as (Al,Si)3Sc. The (Al,Si)3Sc was found to have a dramatic precipitation-strengthening effect in the cast alloy.

Mechanical Properties of an Accumulative Roll Bonded Aluminium Alloy Composite

A multilayered sheet composite of commercial purity Al and Al-0.3%Sc alloys was produced by accumulative roll bonding. The final sheet material consisted of 64 ultra fine grained layers, each of ~7.8mm in thickness. The as-deformed material was annealed at temperatures ranging from 250 to 350°C to study the changes in microstructure and their associated influence on mechanical properties. The as-deformed structures largely comprised of high angle grain boundaries in the Al layers and low angle grain boundaries in the Al(Sc) layers. During annealing, the structures in the Al(Sc) layers remained unaltered, whereas the Al layers recrystallized rapidly to the full layer thickness. The mechanical properties of the Al-Al(Sc) composite were measured and found to be unique in strength and ductility with annealing temperature having a significant influence on these properties.

Strain rate and temperature effects on dynamic properties of high-strength weldable aluminum-scandium alloy

The dynamic mechanical behavior, fracture characteristics, and microstructural evolution of high-strength weldable aluminum-scandium (Al–Sc) alloy were investigated using a compressive split Hopkinson pressure bar at strain rates of 1.3 × 103, 3.6 × 103, and 5.9 × 103 s−1, respectively, and temperatures of −100, 25, and 300 °C. The results showed that the flow stress, work hardening rate, and strain rate sensitivity increase with increasing strain rate but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Moreover, the fracture strain decreases with increasing strain rate and decreasing temperature. It was shown that the Zerilli–Armstrong face-centered-cubic (fcc) constitutive equation provides accurate predictions of the mechanical response of the Al–Sc alloy under the considered strain rate and temperature conditions. Scanning electron microscopy observations revealed that the fracture surfaces of the impacted specimens are characterized by transgranular dimpled features, which are indicative of a ductile failure mode. Moreover, transmission electron microscopy observations indicated the presence of both fine and coarse randomly dispersed precipitates within the matrix and at the grain boundaries. It was found that an increasing strain rate reduces the size of the dislocation cells within the impacted Al–Sc microstructure and therefore increases the dislocation density. However, at higher temperatures, the dislocations are annihilated, leading to a reduction in the dislocation density and a corresponding increase in the dislocation cell size. The variations observed in the size and density of the dislocation cells were found to be consistent with the dynamic tendencies noted in the stress–strain response of the Al–Sc alloy.

Compounds with a `stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl–Klemm and coordination-defect concepts

The bixbyite structure (Mn(2)O(3)) (Ia3) is often described as a distorted face-centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M(4) tetrahedra are centred at 16c. In anti-bixbyite structures (Mg(3)N(2)), cation vacancies are centred in empty N(4) tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of gamma-Si. This means that anti-bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li(3)AlN(2) and Li(3)ScN(2)) fully satisfy this expectation. They are really anti-bixbyites 'stuffed' with Al(Sc). The presence of these atoms in 16c is illuminated in the light of the extended Zintl-Klemm concept (EZKC) [Vegas & García-Baonza (2007). Acta Cryst. B63, 339-345], from which a compound would be the result of 'multiple resonance' pseudo-structures, emerging from electron transfers between any species pair (like or unlike atoms, cations or anions). The coordination-defect (CD) concept [Bevan & Martin (2008). J. Solid State Chem. 181, 2250-2259] is also consistent with the EZKC description of the pseudo-structures. A more profound insight into crystal structures is gained if one is not restricted to the contemplation of classical anions and cations in their conventional oxidation states.