Stable Icosahedral Quasicrystals Research Articles

Precipitation of Stable Icosahedral Quasicrystal Phase in A Mg-Zn-Al Alloy

Precipitation of a stable quasicrystalline i-phase has been discovered in a Mg-6Zn-3Al (wt%, ZA63) magnesium alloy, possibly a first involving a stable i-phase. Dissolution of i-phase solidified at grain boundaries leads to precipitation in the Mg-matrix during cooling due to a solvus boundary observed by phase diagram calculation. Phase transformation, nucleation characteristics, morphology and interfaces with the matrix have been studied. Differential scanning calorimetry (DSC) and in-situ X-ray diffraction (XRD) showed dissolution and precipitation of i-phase during heating and cooling cycles, forming a hysteresis between about 250 and 325°C. The quasilattice parameter a R of i-phase was calculated to be 5.1704 A at room temperature, close to the c-parameter 5.1831A of the Mg-matrix. At dissolution/nucleation, the diffraction peaks of i-phase were sharper, (quasi)lattice constants a R and c Mg become equal to each other (5.225A) and the lattice strain in the matrix was minimized. Precipitation occurred with a fivefold plane parallel to a matrix pyramidal {0111} plane, which gave rise to two possible orientation relationships (OR): 5f||[0001] (OR3’) and 2f||[0001] (OR1). Presence of both ORs in the same precipitates generated icosahedral twinning. The precipitates were faceted sharply on fivefold, twofold and threefold planes which were on basal, prismatic and pyramidal planes of the Mg matrix. Results suggest the OR selection to be nucleation temperature dependent. Based on the crystallographic relationships, 3-dimensional tiling model of icosahedral lattice has been used to model lattice match with the matrix.

Computational prediction of an icosahedral quasicrystal

From the first quasicrystal discovered in the laboratory 30 years ago to the only known specimen of naturally occurring quasicrystals, quasicrystals with icosahedral symmetry have received great attention. There are more than one hundred stable icosahedral quasicrystals in metallic alloys; all are identified by their diffraction spectra. Despite this abundance, resolving the positions of the atoms within the solid has been possible only indirectly. Moreover, unlike dodecagonal and other axial quasicrystals, icosahedral quasicrystals have been observed neither in simulations nor in non-atomic (e.g. micellar or colloidal) systems, where real-space information would be available. Here we present an icosahedral quasicrystal discovered in computer simulation via self-assembly from the liquid phase. We provide a structure model by analyzing atomic surfaces and report the presence of phason flips. Our results constitute a direct microscopic confirmation of the higher-dimensional crystallographic description of icosahedral quasicrystals.

ChemInform Abstract: Discovery of Stable Icosahedral Quasicrystals: Progress in Understanding Structure and Properties

AbstractReview: 77 refs.

Discovery of stable icosahedral quasicrystals: progress in understanding structure and properties

The geometrical concept and structural framework of quasicrystals (QCs) were established shortly after the discovery of QCs. Specifically, the static and dynamic properties of the atoms and the electronic structures of quasiperiodic lattices were theoretically developed. Experimental studies lagged behind theoretical progress due to the lack of suitable samples. This situation changed with the discovery of several highly ordered stable QCs. Studies of these new QCs have provided new insights into the structure and properties of QCs. This article chronologically reviews the discoveries of various stable icosahedral QCs and subsequent studies that determined their structures, properties, and stabilization mechanisms.

Corrosion behavior of as-cast Mg 68Zn 28Y 4 alloy with I-phase

Mg 68Zn 28Y 4 alloys with stable icosahedral quasicrystals (Zn 60Mg 30Y 10) were prepared by cast method. By simulating the environment of ocean, the alloy was eroded in 3.5% (mass fraction) NaCl for 2, 4 and 30 h. The microstructures of the samples and eroded alloys were analyzed by OM and SEM. The compositions and the quasiperiodic structures were identified respectively by EDS and TEM. And the corrosion potential and corrosion current density before and after immersion were measured by potentiodynamic polarization measurements in 3.5% NaCl. The results show that I-phases grow in the mode of conglomeration, piling and transfixion. The Mg 7Zn 3 matrix and α(Mg) solid solution are eroded badly, while W-phase is eroded partially. At the same time, the I-phases exhibit excellent corrosion resistance property. The resistance to corrosion of Mg 68Zn 28Y 4 alloy is improved by increasing exposed I-phases. With adding element Y to Mg 68Zn 32 alloy, the corrosion current is decreased by one order of magnitude. And after the immersion of as-cast Mg 68Zn 28Y 4 alloy for 30 h, the corrosion current density is reduced by two orders of magnitude compared with that of uneroded Mg 68Zn 32 alloy.

Study on as-cast microstructures and solidification process of Mg–Zn–Y Alloys

The microstructures and phases of as-cast Mg–Zn–Y alloys were investigated by means of scanning electron microscopy (SEM), energy-dispersive spectrum (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) in this study. Stable icosahedral Mg30Zn60Y10 quasicrystals were obtained using common casting technique. With increasing Y content, the contents of quasicrystals increased, the distribution of quasicrystals was improved, and the morphologies of quasicrystal and other microstructures of Mg–Zn–Y alloys showed different characteristics. In addition, the solidification process of Mg–Zn–Y alloys was analyzed. Icosahedral Mg30Zn60Y10 quasicrystalline phase is easy to nucleate in slow cooling.

Mechanical alloying and self-propagating high-temperature synthesis of stable icosahedral quasicrystals

Abstract The phase evolution of the mechanically alloyed ternary 63%Al + 25%Cu + 12%Fe and 65%Al + 20%Cu + 15%Fe powder mixtures with milling time has been studied by X-ray diffraction method. It was found that an icosahedral quasicrystalline phase was formed directly during high-energy ball milling of the Al–Cu–Fe mixtures. The X-ray and scanning electron microscopic investigations demonstrated the possibility to use self-propagating high-temperature synthesis (SHS) in combination with preliminary mechanical activation for the synthesis of stable icosahedral quasicrystals. The typical morphology of the Al 63 Cu 25 Fe 12 icosahedral quasicrystals formed in the SHS process is a pentagonal dodecahedron with a size of 3–5 mm. The phase composition of the SHS products depends on the time of preliminary mechanical activation. The content of cubic intermetallic phase in SHS products increases with the time of preliminary mechanical activation of the 63%Al + 25%Cu + 12%Fe powder mixtures.

Insulator-like electrical transport in Al65−x Si x Cu20Ru15 (x=0.0, 0.5, 1.0, 1.5, 2.0) icosahedral quasicrystals

Low-temperature resistivity and magneto-resistance have been measured from 2.8 to 300 K for stable icosahedral quasicrystals Al65–x Si x Cu20Ru15 (x = 0.0, 0.5, 1.0, 1.5, 2.0). The low-temperature σ–T behavior, in the temperature range 2.8–15 K, for the Si-substituted samples are drastically different from undoped samples. The in-field σ–T for the Si-undoped sample has been found to follow ln σ vs. T −1/4 below 5 K, indicating that this sample lies in the insulating side of the metal–insulator (MI) transition. The substitution of silicon at the aluminum site drives the Al–Cu–Ru quasicrystals to the metallic side of MI transition.

Stable quasicrystals and approximants in Zn–Mg–Zr and Zn–Mg–Hf alloys

Stable icosahedral quasicrystals and an approximant crystal have been found in Zn–Mg–Zr and Zn–Mg–Hf alloys. The formation of primitive (P-type) icosahedral quasicrystals with compositions around Zn85Mg7Zr8 and Zn85Mg7Hf8 has been observed in both the as-cast and the annealed states. A face-centred (F-type) icosahedral phase with a composition around Zn76Mg17Hf7 and its corresponding approximant crystal having a composition around Zn75Mg19Hf6 with a space group of have been identified in a fully annealed Zn79Mg15Hf6 alloy. Chemical ordering between two distinctive icosahedral clusters, Zn54Mg12 and Zn54Hf12 (or Zn54Zr12) are proposed to explain the low content of Hf (or Zr) and the occurrence of superlattice diffraction peaks observed for the F-type icosahedral quasicrystal and its 1/1 approximant. The two icosahedral clusters are equivalent in number in order to satisfy the criterion of an electron-to-atom ratio e/a equal to 2.15. They are randomly arranged for the P-type icosahedral phase and regularly arranged respectively at even- and odd-parity twelvefold vertices in the three-dimensional Penrose tiling for the F-type icosahedral phase.

Excellent creep properties of Mg–Zn–Cu–Gd-based alloy strengthened by quasicrystals and Laves phases

A new type of Mg–Zn–Cu–Gd-based alloy strengthened by quasicrystal and Laves phase was developed. This alloy exhibits much better creep properties compared to AE42(Mg–4 wt% Al–2 wt% rare-earth) alloy, which is the benchmark creep-resistant magnesium die-casting alloy under the compressive creep condition of 180 °C and80 MPa. The new alloy also exhibits high room-temperature mechanical properties close to that of AZ91 alloy. The good mechanical properties are attributed to the special microstructure; the thermally stable icosahedral quasicrystals phase (i-phase) and Laves phase distributed along the grain boundary as a hard skeleton, and some fine β′1 precipitates distributed homogenously on the matrix. The dislocation morphology after the creep test was studied, and the strengthening mechanism was proposed.

Insulator-like electrical transport in structurally ordered Al 65Cu 20+ xRu 15− x ( x = 1.5, 1.0, 0.5, 0.0 and −0.5) icosahedral quasicrystals

Low-temperature resistivities, in zero-field and 8 T field, and magnetoresistance have been measured down to 1.4–300 K for stable icosahedral quasicrystals Al 65Cu 20+ x Ru 15− x ( x = 1.5, 1.0, 0.5, 0.0 and −0.5). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect ( τ so ∼ 10 −12 s). But the sample with x = −0.5 shows anomalous magnetoresistance and the anti weak-localization effect breaks down ( τ so to be 10 −15 s). The in-field σ– T between 5 K and 20 K, for x = 1.0, 0.5, 0.0 and −0.5 samples, and between 1.4 K and 40 K for x = 1.5 sample, follow a power-law behavior with an exponent of 0.5 and above ∼30 K the exponent ranges from 1.17 to 1.58. The observed power-laws basically characterize the presence of critical regime of the metal–insulator (MI) transition, dominated by electron–electron and electron–phonon inelastic scattering events respectively. In samples with x = 1.0, 0.5, 0.0 and −0.5 the in field σ– T has been found to follow ln σ-vs- T 1/4 below 5 K, which indicates the presence of variable range hopping. The observed transport features indicate the occurrence of proximity of metal–insulator transition in these Al–Cu–Ru quasicrystal samples.

Electrical Properties of a Stable Icosahedral Quasicrystal Zn–Fe–Sc

The electrical resistivity of a recently discovered stable icosahedral quasicrystal Zn 77 Fe 7 Sc 16 was found to have a T 1/2 dependence in low temperature region between 2 and 16 K, while it increased gradually by 6.8% with the decrease in temperature from 270 to 2 K. The increase is considered to arise from the weak localization of conduction electrons. Negative magnetoresistance was found below 32 K. The T 1/2 dependence of the resistivity and the negative magnetoresistance are interpreted on the basis of the short range magnetic order inferred from the spin-glass behavior.

Magnetic and electrical properties of a Cu–Ga–Mg–Sc icosahedral quasicrystal

We have investigated the magnetic susceptibility and the electrical resistivity of a new P-type icosahedral quasicrystal Cu 48.2Ga 33.8Mg 3Sc 15 and a cubic phase Cu 49Ga 36Sc 15, interpreted as a 1/1 approximant crystal of the quasicrystal. The magnetic susceptibility of the Cu 48.2Ga 33.8Mg 3Sc 15 quasicrystal showed an increase from −3.51 × 10 −4 to −2.78 × 10 −4 m 3 kg −1 with a rise in temperature from 120 to 300 K. The Cu 49Ga 36Sc 15 cubic phase showed also a rise from −6.18 × 10 −4 to −4.50 × 10 −4 m 3 kg −1 between 100 and 300 K. The increase in the susceptibility was accounted for by the temperature dependence of the Pauli paramagnetism. The experimental results strongly suggest the existence of a pseudogap in the electronic density of states at Fermi energy for the Cu 48.2Ga 33.8Mg 3Sc 15 quasicrystal and the Cu 49Ga 36Sc 15 approximant, which has been observed in various stable icosahedral quasicrystals. The electrical resistivity of the Cu 47.1Ga 32.9Mg 5Sc 15 quasicrystal showed a negative temperature coefficient in the temperature region between 2 and 270 K. The Cu 49Ga 35.8Sc 15.2 cubic phase showed a positive temperature coefficient in the temperature region. The tendency of the quasicrystal was qualitatively accounted for by the weak localization theory.

Magnetic and electrical properties of a new type Zn–Mg–Sc icosahedral quasicrystal

The magnetic susceptibility of the Zn–Mg–Sc icosahedral quasicrystals, which have a new type of atomic cluster structure, was found to increase with a rise in temperature from 100 to 300 K. The increase of the susceptibility was accounted for by the temperature dependent term of the Pauli paramagnetism, which indicated a pseudogap in the electronic density of states (DOS) near the Fermi energy. This new type quasicrystal may be stabilized by the pseudogap in the DOS, as in various stable icosahedral quasicrystals discovered to date. We measured the temperature dependence of the electrical resistivity. In the temperature range between 2 and 300 K, the resistivity increased by 0.08% with a decrease in temperature. These measurements were analyzed on the basis of the weak localization theory of electrons in quasicrystals.

New group of stable icosahedral quasicrystals: structural properties and formation conditions

Structural studies on the icosahedral quasicrystals in Zn–Mg–Sc, Cu–Ga–Mg–Sc, and Zn–Mg–Ti alloys as well as their corresponding 1/1 cubic approximants, have revealed that these quasicrystals belong to a new structural group similar to Cd-based quasicrystals. This group is characterized by a triple-shell icosahedral cluster different from both Mackay- and Bergman-types. The presence of the atomic cluster has been deduced from the structure model of the approximant crystal, Zn 17Sc 3, in which the clusters are embedded in a periodic network of so-called ‘glue atoms’. Density measurement suggested the presence of at least 2.7 Zn atoms in the first shell of the cluster in this approximant. The substitutional relationship in these three quasicrystals indicates the important role of Hume-Rothery rule for the formation of this type of quasicrystal. The occurrence of a P-type icosahedral quasicrystal in Zn–Mg–Yb alloy is also reported.

New stable icosahedral quasicrystalline phase in the Sc–Cu–Zn system

Stable icosahedral quasicrystals have been synthesized in the Sc–Cu–Zn system by quench-and-anneal or in-situ filtration methods. These have a composition near Sc16.2(3)Cu12.3(3)Zn71.5(6) at 480°C with a quasilattice constant of 4.995 6(7) Å determined by the Elser method from a single-phase Guinier pattern. An electron diffraction study confirms that the icosahedral quasicrystals have a primitive lattice. Cuboctahedral single crystals with dimensions up to 100 µm have been grown by melt-spinning techniques. A cubic approximant phase Sc14.2Cu10.3Zn75.5 (ScCu0.73Zn5.33) with a concordant lattice constant lies nearby.

Stable icosahedral quasicrystals in the Ag-In-Ca, Ag-In-Yb, Ag-In-Ca-Mg and Ag-In-Yb-Mg systems

On the basis of the idea that a quasicrystal is a compound in which the average number of valence electrons per atom is around 2.0 and that its formation is dominated by atom size, stable icosahedral quasicrystals have been successfully synthesized in Ag-In-Ca, Ag-In-Yb, Ag-In-Ca-Mg and Ag- In-Yb-Mg systems. The Ag-In-Ca and Ag-In-Yb icosahedral quasicrystals have stoichiometry around Ag 42 In 42 Ca 16 and Ag 42 In 42 Yb 16 . The Ag-In-Ca-Mg and Ag-In-Yb-Mg quasicrystals form in the composition ranges Ag42 x /2 In 42 x /2 Ca 16 Mg x ( x =0-7.5 at.%) and Ag 42 x /2 In 42 x /2 Yb 16 Mg x ( x =0-10 at.%). Electron diffraction studies confirmed that the icosahedral quasicrystals have a primitive lattice. The average number of valence electrons per atom in all these quasicrystals is 2.0.

Magnetic property of a Zn–Mg–Sc icosahedral quasicrystal

We have investigated the magnetism of a new Zn 80Mg 5Sc 15 P-type icosahedral quasicrystal and a Zn 85.5Sc 14.5 cubic phase, which is interpreted to be a 1/1 approximant of the new icosahedral quasicrystal, in the temperature region between 2 and 700 K. The magnetic susceptibility of the Zn 80Mg 5Sc 15 icosahedral quasicrystal shows an increase from −1.3×10 −7 to −0.8×10 −7 cgsemu/g with a rise in temperature from 90 to 700 K. The Zn 85.5Sc 14.5 cubic phase shows also a rise from −1.8×10 −7 to −1.0×10 −7 cgsemu/g between 120 and 700 K. The increase in the susceptibility with a rise in temperature is accounted for by a temperature dependence of the Pauli paramagnetism. The experimental results strongly suggest a pseudogap in the electronic density of states at Fermi energy for the Zn 80Mg 5Sc 15 quasicrystal and the Zn 85.5Sc 14.5 approximant, which has been observed in various stable icosahedral quasicrystals.

Stable Cd-Mg-Yb and Cd-Mg-Ca icosahedral quasicrystals with wide composition ranges

Stable icosahedral quasicrystals with wide composition ranges have been found in the systems Cd-Mg-Yb and Cd-Mg-Ca. The icosahedral quasicrystals form in the following composition ranges: 25-85at.%Cd, 0-60at.%Mg and 10-20at.%Yb for the Cd-Mg-Yb system; 33-85at.%Cd, 0-52at.%Mg and 10-20at.%Ca for the Cd-Mg-Ca system. An electron diffraction study confirmed that the icosahedral quasicrystals have a primitive lattice. Its quasilattice constant is proportional to a Goldschmidt atom diameter of the quasicrystals weighted by a factor of 1.75 in the range 0.5681- 0.5799nm for the Cd-Mg-Yb system and 0.5697-0.5778nm for the Cd-Mg-Ca system. The closely similar atom sizes of Cd and Mg, the valence electron ratio e / a =2.0 and the large atom sizes of Yb and Ca cause the wide composition ranges of the Cd-Mg-Yb and Cd-Mg-Ca quasicrystals. The metallurgical behaviour of the quasicrystals is discussed.

A new stable icosahedral quasicrystal in the Cd-Mg-Dy system

A new stable icosahedral quasicrystal has been found in annealed Cd-Mg-Dy alloys. The composition of the icosahedral phase was determined to be approximately Cd66Mg21Dy13. Powder X-ray and electron diffraction patterns revealed that the phase has a primitive icosahedral lattice with a quasilattice parameter aR = 0.5634 nm. The electron diffraction study confirmed that the phase has a well ordered primitive icosahedral structure.