Quasicrystalline Phase Research Articles

Effect of Scandium on the Structure and Corrosion Properties of Vapor-Deposited Nanostructured Quasicrystalline Al–Cu–Fe Films

The structure and corrosion properties of quasicrystalline Al–Cu–Fe and Al–Cu–Fe–Sc films 200–260 nm thick produced by three-electrode ion-plasma sputtering of assembled targets were studied. The structural and phase composition of the films was determined by transmission and scanning electron microscopy, electron microprobe analysis, and X-ray diffraction. The corrosion properties were examined in 3, 5, and 16% aqueous NaCl solutions (pH = 6.9) at room temperature. An icosahedral fine quasicrystalline phase was found to form in the films deposited. The sizes of coherent scattering domains indicate that the coatings are nanostructured. The addition of 0.5 at.% Sc to the quasicrystalline Al–Cu–Fe films increases their corrosion resistance as evidenced by measurements of stationary potentials in saline solutions of different concentrations.

Real-Time Atomic-Scale Visualization of Reversible Copper Surface Activation during the CO Oxidation Reaction.

By using in situ aberration-corrected environmental transmission electron microscopy, for the first time at atomic level, the dynamic evolution of the Cu surface is captured during CO oxidation. Under reaction conditions, the Cu surface is activated, typically involving 2-3 atomic layers with the formation of a reversible metastable phase that only exists during catalytic reactions. The distinctive role of CO and O2 in the surface activation is revealed, which features CO exposure to lead to surface roughening and consequently formation of low-coordinated Cu atoms, while O2 exposure induces a quasi-crystalline CuOx phase. Supported by DFT calculations, it is shown that crystalline CuOx reversibly transforms into the amorphous phase, acting as an active species to facilitate the interaction of gas reactants and catalyzing CO oxidation.

Correlated Unique Variation of Electrical Resistivity to Crystallization Behavior of the Zr52.5Cu17.9Ni14.6Al10Ti5 Metallic Glass

Due to the differences between the glass and crystalline phases, crystallization of metallic glass occurs with heat release, volume shrinkage, and electrical resistivity drastic changes. Electrical resistivity of the Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass during crystallization was investigated under both continuous heating and isothermal annealing. This amorphous alloy exhibits a continuous variation instead of sharp decline when reaches the onset crystallization temperature. This unique variation was found to be related to the formation of a few quasicrystalline phases. The slower phase transformation process of this metallic glass brings lots of grain boundaries, which results in increasing of resistivity at the last stage during isothermal annealing. These results imply that electrical resistivity measurement is a more intuitive approach to investigate structure evolution of metallic glasses.

Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase Transitions

The discovery of quasicrystals with crystallographically forbidden rotational symmetries has changed the notion of the ordering in materials, yet little is known about the dynamical emergence of such exotic forms of order. Here we theoretically study a nonequilibrium cavity-QED setup realizing a zero-temperature quantum phase transition from a homogeneous Bose-Einstein condensate to a quasicrystalline phase via collective superradiant light scattering. Across the superradiant phase transition, collective light scattering creates a dynamical, quasicrystalline optical potential for the atoms. Remarkably, the quasicrystalline potential is "emergent" as its eightfold rotational symmetry is not present in the Hamiltonian of the system, rather appears solely in the low-energy states. For sufficiently strong two-body contact interactions between atoms, a quasicrystalline order is stabilized in the system, while for weakly interacting atoms the condensate is localized in one or few of the deepest minima of the quasicrystalline potential.

Unveiling the vortex glass phase in the surface and volume of a type-II superconductor

Order-disorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the micro-scale. Vortex matter in type-II superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasi-crystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with in-plane positional displacements growing algebraically and short-range orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorder-driven phase transitions in glassy condensed matter.

THE ICOSAHEDRAL PHASE OF Al63Cu25Fe12 SYSTEM

The present work aimed to characterize the microstructure of the icosahedral phase (quasicrystalline phase-ϕ) of the system with stoichiometric composition of the quasicrystal Al63Cu25Fe12. The ternary alloy with nominal composition of Al63Cu25Fe12 was processed by mechanical alloying (MA) as a viable solid state processing method for producing various metastable and stable quasicrystalline phases. The structural characterization of the obtained samples was performed by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM), while the elemental composition of the chemical elements Al, Fe and Cu were determined by X-ray spectroscopy technique of dispersive energy (EDS). According to the results of XRD, the diffraction patterns of Al63Cu25Fe12 showed the presence of β-Al(Fe, Cu) and λ-Al13Fe4 phases coexist with the thermodynamic ϕ-phase quasicrystalline. Finally, elemental analysis indicates that during alloy synthesis there is little variation of the ideal composition. The results indicate that alloys with high percentage of icosahedral phase can be obtained by casting in the air.

New experimental studies on the phase diagram of the Al-Cu-Fe quasicrystal-forming system

Phase diagrams of quasicrystal-forming alloy systems, such as Al-Cu-Fe, are essential to the continued development and fundamental understanding of quasicrystalline alloys, especially for the synthesis of single quasicrystals. Isothermal sections of the Al-Cu-Fe system at 700, 800, and 1000 °C were constructed over the whole composition range by investigations of three Cu/Fe/Al5Fe2 diffusion triples and forty-seven equilibrated alloys using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Based on experimental measurements and reasonable assumptions, eleven, eight, and six three-phase equilibria were obtained at 700, 800, and 1000 °C, respectively. The existence of the ternary I-Al6Cu2Fe icosahedral quasicrystalline phase was confirmed at both 700 and 800 °C, and the ternary ω-Al7Cu2Fe phase was only found at 700 °C. The stable composition range of the I phase was measured to be Al67-56Cu24-31Fe9–13 and Al66-58Cu23-28Fe11–14 at 700 and 800 °C, respectively. A wide continuous solution region formed among the α-Fe, β-AlFe, and Cu3Al phases at 800 and 1000 °C. A sufficiently large amount of new experimental phase equilibrium data was collected for the Al-Cu-Fe system that will help improve future thermodynamic description of this ternary system within the CALPHAD (CALculation of PHAse Diagrams) framework for designing Al-Cu-Fe-based quasicrystals.

X-ray diffraction studies of rapid cooled Al-V and Al-Fe-V alloys

Purpose: of this paper is to deep and more complete knowledge about the features of phase and structure formation in Al-based alloys with transition metals (TM) Fe and V at rapid cooling from melt. It is known, that nonequilibrium synthesis conditions of such alloys lead to quasicrystalline, amorphous or metastable phases formation, which can significantly improve the physical-chemical properties and first of all the mechanical ones. But understanding of compositional dependences of structure features at formation under nonequilibrium conditions and the correlation of these dependences with physical properties of alloys is far to be clear. Design/methodology/approach: Structure of Al-enriched Al-V, Al-V-Fe rapid cooled alloys was studied by X-ray diffraction method. In order to estimate the influence of structural state of alloy on the mechanical properties the integral microhardness was studied by Vickers method. Findings: Two quasicrystalline icosaedral phases with different cell parameters are revealed in ternary alloys Al100-3xV2xFex (x=2-4). Increasing of transition metal content promotes the formation of phase with higher quasicell parameter embedded in amorphous matrix. With increasing of the transition elements total content from 6 up to 12 at. % the microhardness of alloys increased gradually from 867 to 3050 MPa. Research limitations/implications: Research of nonequilibrium alloys revealed crystalline structure of Al-V alloys and quasicrystalline embedded in amorphous matrix of Al-Fe-V ternary alloys. Obtained results suppose that further structure and physical properties studies of Al-Fe-V alloys will allows to find the conditions to control the producing of materials with desired properties. Practical implications: Using of rapid cooling method for synthesis of Al-enriched Al-Fe-V alloys give an opportunity to produce alloys with significantly improved mechanical properties. Originality/value: Nonequilibrium conditions of cooling allow significantly changes the structure and properties.

A unique CO-like micrometeorite hosting an exotic Al-Cu-Fe-bearing assemblage \u2013 close affinities with the Khatyrka meteorite

We report the discovery of a unique micrometeorite, containing an exotic Al-Cu-Fe alloy composed of two intermixed phases: khatyrkite (CuAl2) and stolperite (CuAl) and both containing minor Fe (<1.4 wt%). These phases are dendritic and rapidly co-crystallized at the binary system’s peritectic (~550 °C). The host micrometeorite is an otherwise typical S-type micro-porphyritic cosmic spherule containing relict olivine (Fo76–90, Cr2O3: 0.01–0.56 wt%, MnO: 0.03–0.32 wt% and CaO: 0.09–0.22 wt%) and a cumulate layered texture. These properties suggest the micrometeorite is derived from a carbonaceous chondrite (best matched to a CO chondrite) and entered the atmosphere a high speed (~16 kms−1), implying an origin from a highly eccentric orbit. This particle represents the second independent discovery of naturally occurring intermetallic Al-Cu-Fe alloys and is thus similar to the previously reported Khatyrka meteorite - a CV chondrite containing near-identical alloys and the only known natural quasicrystals. We did not observe quasicrystalline phases in this micrometeorite, likely due to the low amounts of Fe in the alloy, insufficient to stabilize quasicrystals. Our discovery confirms the existence of Al-Cu-Fe intermetallic alloys on chondritic parent bodies. These unusual phases require a currently unexplained formation process, we tentatively suggest this could represent the delivery of exotic interstellar material to the inner solar system via impact.

Development of an Al-Mn-Si-Based Alloy with an Improved Quasicrystalline-Forming Ability

This investigation deals with the influence of calcium, strontium and zinc on the formation of primary, metastable quasicrystals in Al-Mn-Si alloys as these can enhance the mechanical properties of alloys. The synthesized alloys were cast into a copper mould. The castings were characterized using standard tools for studying microstructure. The Al-Mn-Si-Zn-Ca-Sr alloy contained more of the primary icosahedral quasicrystalline (iQc) phase and less of the approximant α-Al-Mn-Si phase than the Al-Mn-Si alloy. The higher volume fraction of the primary iQc phase in the Al-Mn-Si-Zn-Ca-Sr alloy is explained by the reduction of the critical radius size for nucleation and by an increase in the number of nucleation sites for the iQc phase. Increased number of nucleation sites and higher volume fraction of the primary iQc are both related to a reduction of the surface tension for the Al-based melt caused by the presence of calcium and strontium. These two elements do not become incorporated into the iQc phase but have a large effect on the course of the solidification of the investigated alloys and their constitution at room temperature.

Fabrication of Al-matrix composite reinforced with quasicrystals using conventional metallurgical fabrication methods

Al-matrix composites reinforced with quasicrystals are mostly fabricated by methods that apply rapid solidification techniques or by means of powder metallurgy. In this work, we show a metallic system that forms such microstructure containing the quasicrystalline, Al7Cu2Fe and aluminum phases by casting an appropriate composition followed by a thermal treatment. An Al85Cu6.75Fe3.375Cr4.875 (%at.) alloy was arc-melted, annealed and subsequently characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. We show that it is possible to fabricate Al-matrix composites reinforced with quasicrystals using conventional metallurgical processes. This opens new perspectives for application of quasicrystalline alloys and composites.

Development of quasicrystal morphology in gas-atomized icosahedral-phase-strengthened aluminum alloy powders

The microstructures in gas-atomized powders and in blind-die-compacted material from three Al-Cr-Mn-Co-Zr alloys have been studied using X-ray diffraction and electron microscopy. The materials consist of a mixture of FCC Al and icosahedral quasicrystalline phases, but three different phase distributions are observed which depend on the powder particle size, and hence on the cooling rate. The finest particles contain a nano-composite mixture of equiaxed Al grains and fine (<200 nm) quasicrystalline dispersoids. Coarser particles exhibit large cellular dendritic Al grains with thin films of Co- and Cr/Mn-rich phases at the cell boundaries and coarser (up to 2 µm) quasicrystals at the particle surfaces. The largest particles contain coarse (up to 5 μm) radial quasicrystalline growths within the particle. The same microstructures were observed for each alloy, but the volume fractions of the microstructural types varied with the alloy content. These effects are explained on the basis of competing nucleation and growth phenomena during solidification of the atomized droplets. Since these microstructures are retained in the bulk material following blind-die-compaction, they could have a profound influence on the mechanical and other properties of materials made from such powders. The importance of such effects for alloy and process design are discussed.

Characterization in the Icosahedral Phase of Al63Cu25Fe12 System

The present work aimed to characterize the microstructure of the icosahedral phase (quasicrystalline phase-ϕ) of the system with stoichiometric composition of the quasicrystal Al 63 Cu 25 Fe 12 . The ternary alloy with nominal composition of Al 63 Cu 25 Fe 12 was processed by mechanical alloying (MA) as a viable solid state processing method for producing various metastable and stable quasicrystalline phases. The structural characterization of the obtained samples was performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the elemental composition was determined by dispersive energy spectroscopy (DES). The diffraction patterns of Al 63 Cu 25 Fe 12 showed the presence of quasicrystalline phase-ϕ, ω-Al 7 Cu 2 Fe, β-Al(Fe, Cu) and λ-Al 13 Fe 4 phases that coexist with the thermodynamic quasicrystalline phase-ϕ. In icosahedral phase oxidation of aluminum forms a dense layer on the passivating outer most surface of the quasicrystal which causes depletion in both copper and iron. It was observed not only the presence of alumina, γ-Al 2 O 3 , but also the formation of hematite and copper oxide in the dense layer. Finally, elemental analysis indicates that during alloy synthesis there is little variation of the ideal composition. The results indicate that alloys with high percentage of icosahedral phase can be obtained by casting in the air. DOI: http://dx.doi.org/10.17807/orbital.v11i3.1381

Quasicrystalline Phase Formation of the Al67Cu26Fe15 Alloy

O presente trabalho teve como objetivo caracterizar a microestrutura da fase icosaedral (f-fase quasicristalina) do sistema com composição estequiométrica do quasicristal Al67Cu26Fe15. A liga ternária com composição nominal de Al67Cu26Fe15 foi processada por Moagem Mecânica de Alta Energia, como um método viável de processamento no estado sólido para a produção de várias fases quasicristalinas metaestáveis e estáveis. A caracterização estrutural das amostras obtidas foi realizada por difração de raios-X (DRX) e Microscopia Eletrônica de Varredura (MEV), enquanto a composição elementar dos elementos químicos Al, Fe e Cu foram determinados pela técnica de Espectroscopia de raios X por energia dispersiva (EDS). Na superfície da liga quasicristalina, podemos observar no SEM a presença da alumina g-Al2O3, que contribui para as propriedades texturais de suporte catalítico deviso a sua atividade catalítica, visando maior desempenho em reações de oxidação do metanol. De acordo com os resultados de XRD, os padrões de difração da liga Al67Cu26Fe15 mostraram a presença das fases w-Al7Cu2Fe, b-Al(Fe;Cu) e l-Al13Fe4 coexistem com a f-fase quasicristalina em regime termodinâmico. Finalmente, a análise elementar indica que durante a síntese da liga existe pequena variação da composição ideal. Os resultados indicam que ligas com alto percentual de fase icosaedral podem ser obtidas por fundição ao ar.

Formation and stability of complex metallic phases including quasicrystals explored through combinatorial methods

Aluminum-based quasicrystals typically form across narrow composition ranges within binary to quaternary alloys, which makes their fabrication and characterization challenging. Here, we use combinatorial approaches together with fast characterization techniques to study a wide compositional range including known quasicrystal forming compositions. Specifically, we use magnetron co-sputtering to fabricate libraries of ~140 Al-Cu-Fe and ~300 Al-Cu-Fe-Cr alloys. The alloys compositions are measured through automated energy dispersive X-ray spectroscopy. Phase formation and thermal stability are investigated for different thermal processing conditions (as-sputtered and annealed at 400 °C, 520 °C and 600 °C for Al-Cu-Fe libraries; annealed at 600 °C for Al-Cu-Fe-Cr libraries) using automated X-ray diffraction and transmission electron microscopy. In both systems the compositional regions across which the quasicrystalline phase forms are identified. In particular, we demonstrate that the quasicrystalline phase forms across an unusually broad composition range in the Al-Cu-Fe-Cr system. Additionally, some of the considered alloys vitrify during sputtering, which also allows us to study their nucleation behavior. We find that phases with polytetrahedral symmetry, such as the icosahedral quasicrystal and the λ-Al13Fe4 phase, exhibit higher nucleation rates but lower growth rates, as compared to other phases with a lower degree of polytetrahedral order. Altogether, the here used combinatorial approach is powerful to identify compositional regions of quasicrystals.

Many-beam dynamical scattering simulations for scanning and transmission electron microscopy modalities for 2D and 3D quasicrystals

ABSTRACTWe present a general theory of electron scattering from both 2D and 3D quasicrystals, applicable to both transmission (forescatter) and scanning (backscatter) geometries. Specifically, simulations of conventional zone axis diffraction patterns, convergent beam diffraction patterns and electron backscatter diffraction for decagonal AlNiCo are presented. As expected, the geometry of the diffraction pattern is similar to kinematical patterns, but the intensity distributions show strong dynamical effects in all the modalities. Experimental electron backscatter diffraction patterns for AlNiCo are compared to simulated patterns for this 2D quasicrystalline structure as well as one of its crystalline monoclinic approximant phases, CoAl. While the simulated patterns from the quasicrystalline and approximant phases are very similar to the experimental patterns, direct comparison shows a better agreement with the decagonal phase.

Characterization of aluminium matrix compositesreinforced by Al–Cu–Fe-Cr quasicrystalline particles

The Al63Cu25Fe9Cr3 quasicrystalline was prepared by conventional casting method. The microstructures and phase composition of Al63Cu25Fe12 quasi-crystalline and Al65Cu20Fe10Cr5 phases were characterized by XRD and SEM. Aluminium based composites was fabricated by using the Al63Cu25Fe9Cr3 quasicrystalline particles as the reinforcing phase and ZL101 as matrix with mechanical stirring. The effects of quasicrystals content, on the microstructure and mechanical properties of the obtained composite, were investigated. The results show that the as-cast quasicrystalline material is composed of d-phase(Al65Cu20Fe10Cr5), I-phase(Al63Cu25Fe12), λ-Al3Fe, β-AlFe3 and a small amount of η-AlCu, θ-Al2Cu phases. After heat treatment, the quasicrystalline intermediate alloy composition tends to quasicrystal phase. In the composite material, the diffusion between the quasicrystalline and the matrix alloy caused the decomposition of the structure of quasicrystalline phase. With the increase of quasicrystalline content, the tensile strength of the composites increases. However, addition of the Cr element changes the acicular β-Fe(Al80-85Fe6-9Si10-15) phase into a man-like or bone-like α-Fe(Al80-85Fe4-8Si6-8Cr3-5) phase which increases the elongation of the composite. The best comprehensive mechanical properties corresponded to the 7 wt% quasicrystalline particles. Strengthening mechanism of quasicrystal reinforced aluminum matrix composites, a model that model considers the dislocation strengthening mechanism and the effect of load transfers mechanism on the tensile strength of composites. This theory calculates the tensile strength of quasicrystalline reinforced aluminum matrix composites and verifies the effectiveness of the model through experiments.

Microstructure and mechanical behavior of Al92Fe3Cr2X3 (X = Ce, Mn, Ti, and V) alloys processed by centrifugal force casting

Microstructural and mechanical characterization of Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys were performed. The alloys were processed by a method that uses centrifugal force to cast the samples into a rotating copper mold. Microstructural characterization was carried out by means of x-ray diffraction, scanning electron microscopy, and differential scanning calorimetry. Compressive tests at room and at 300°C were performed in selected samples to evaluate their mechanical properties. Microstructural characterization showed the formation of quasicrystalline phases as well as other intermetallic phases embedded within an Al-FCC matrix. The Ce-containing alloy exhibited promising results regarding quasicrystalline phase formation and stability as well as with respect to its mechanical properties at high temperatures. The quasicrystalline phase of this alloy appears to be stable up to 545°C when the DSC reveals an exothermic transformation. In addition, the presence of a eutectic structure surrounding the Al-FCC grains enhanced the mechanical strength of this alloy. At 300°C, the Ce-containing alloy showed yield strength and ultimate tensile strength of 180MPa and 360MPa, respectively. If compared to a commercial aluminum alloy 2024 at the T6 condition, close to 300°C, the alloy studied here showed an increase of more than 4 times in the yield strength, and almost 7 times in the ultimate tensile strength. The high thermal stability and mechanical properties at high temperatures of this alloy open interesting possibilities for further studies and future applications of this Al-Fe-Cr-Ce alloy.

Synthesis and Investigation of Quaternary Quasi-Crystalline Phase in Al – Cu – Fe – Cr Alloys

Formation of a quasi-crystalline phase under rapid solidification and heat treatment of alloys of the Al – Cu – Fe – Cr system is studied. The study is performed by x-ray diffractometry, optical, scanning and transmission electron microscopy and differential scanning calorimetry. It is shown that the quasi-crystalline Al65Cu20Fe10Cr5 phase is a mixture of icosahedral and decagonal phases. The substitution of iron with chromium destabilizes the icosahedral I-phase and promotes formation of a decagonal d-phase. After quenching from 880°C, the Al64Cu24Fe10Cr2 alloy acquires a pure I-phase, and the Al64Cu24Fe8Cr4 alloy acquires a d-phase.

The Effect of Composition on the Formation of a BCC Approximant in Ti–Cr–Al–Si–O Alloys

The microstructure, phase composition, and mechanical properties of Ti–Cr–Al–Si–O alloys in ascast state and after annealing at 800°C have been studied. The as-cast alloys with 50 and 60 at.% Ti have two phases: a 1/1 α(TiCrSi) cubic approximant of the quasicrystalline phase and an intermetallic Cr2Ti(C14) Laves phase in different ratios. The as-cast alloy with a high titanium amount (72 at.%) contains three phases: a 1/1α(TiCrSi) approximant and two α-Ti and β-Ti solid solutions. The effect of annealing on the phase composition of the alloys has been studied. After annealing, the alloys with the lowest (50 at.%) and highest (72 at.%) Ti contents change their phase composition with the formation of Ti5Si3 silicide, while the alloy with 60 at.% Ti exhibits high thermal stability—its phase composition remains unchanged. The changes in hardness of the starting and annealed alloys have been examined by automatic indentation. The highest hardness, 11.7 GPa, is shown by the as-cast Ti72Cr20Al2Si2(SiO2)4 alloy. The hardness of the annealed Ti50Cr30Al13Si2(SiO2)5 alloy increases to 14.8 GPa.