Crystalline Approximant Phases Research Articles

Phase Transformation in ξ-Al-Ni-Rh Approximant Phase Induced by Electron Irradiation by In-situ TEM

Electron beam irradiation induced phase transition processes were studied on crystalline approximant phase ξ-Al-Ni-Rh in Al75Ni15Rh10 alloys. Approximant phase to amorphous transition phase occurred under different density of electron beam irradiation. Phase transition and separation occurred in a comparatively lower density of electron beam due to defects modulation and defects play an important role in this process. Higher density electron beam induces hollow holes with amorphous phase in the rim of the hole in the sample and face centered structure transition phase with parameter of 2.2 Å appears between the amorphous phase and ξ-phase. The appearance of the face centered structure indicates that it is the most possible shortest-range order in amorphous phase.

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.

Metastable quasicrystal-induced nucleation in a bulk glass-forming liquid.

This study presents a unique Mg-based alloy composition in the Mg-Zn-Yb system which exhibits bulk metallic glass, metastable icosahedral quasicrystals (iQCs), and crystalline approximant phases in the as-cast condition. Microscopy revealed a smooth gradual transition from glass to QC. We also report the complete melting of a metastable eutectic phase mixture (including a QC phase), generated via suppression of the metastable-to-stable phase transition at high heating rates using fast differential scanning calorimetry (FDSC). The melting temperature and enthalpy of fusion of this phase mixture could be measured directly, which unambiguously proves its metastability in any temperature range. The kinetic pathway from liquid state to stable solid state (an approximant phase) minimizes the free-energy barrier for nucleation through an intermediate state (metastable QC phase) because of its low solid-liquid interfacial energy. At high undercooling of the liquid, where diffusion is limited, another approximant phase with near-liquid composition forms just above the glass-transition temperature. These experimental results shed light on the competition between metastable and stable crystals, and on glass formation via system frustration associated with the presence of several free-energy minima.

An Icosahedral Quasicrystal and Its 1/0 Crystalline Approximant in the Ca-Au-Al System.

A new icosahedral quasicrystalline phase, CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu4.4Al1.6, aQC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)-9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca-Au-Al system. In the crystalline approximant, eight [Au3-xAl1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca4/4[Au3-xAl1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au-Al interactions over Au-Au and Al-Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au3-xAl1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.

Crystal–quasicrystal transition depending on cooling rates in directionally solidified Al–3Mn–7Be (at.%) alloy

The transition of primary phases from the crystalline approximant phase to icosahedral quasicrystal phase (I-phase) is observed in the directional solidified Al–3Mn–7Be (at.%) alloy. The structures of I-phase and approximant phases are characterized by SEM, TEM and HRTEM. The addition of Be significantly enhances the formation ability of I-phase, combining with the directional solidification method where the growth rates and temperature gradients can be separately controlled, giving rise to a promising way to prepare quasicrystal-reinforced Al-matrix composite.

Structural defects in Al–Ni–Rh and Al–Pd–Fe crystalline approximant phases

Abstract Structural defects of high density have been observed to exist in the as-cast crystalline approximants Al–Ni–Rh ξ and Al–Pd–Fe ε 16 which contain columnar atom clusters as the most fundamental building blocks. By means of high-resolution electron microscopy (HREM), the structural and distributional features of these defects have been revealed. It is clearly shown that, accommodating structural defects of high density in the Al–Ni–Rh ξ-phase can lead to the formation of a unique mixed structure in which nearly regular arrangement of structural defects can be established. On the basis of HREM observations, long-range structural features of the defective ξ- and ε 16-structures have been discussed in terms of the characteristic arrays of columnar atom clusters.

A new orthorhombic approximant of the icosahedral Al–Cu–Fe quasicrystal

We report on the formation of a new crystalline approximant phase of the icosahedral (i-)Al–Cu–Fe quasicrystal. This phase is formed during sintering of Al-based composites reinforced with i-AlCuFeB quasicrystalline particles. The structure of this phase has been characterized by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). TEM revealed that it is a B-centred orthorhombic phase with lattice parameters a = 1.166 nm, b = 1.195 nm and c = 3.44 nm. Its chemical composition, as determined by electron energy loss spectroscopy (EELS), is close to Al76.9Cu2.7Fe20.4, with an average number of valence electrons per atom e/a of 1.92, similar to the value in all other approximants of the i-phase discovered thus far. Initial results on local atomic arrangements along one of its pseudo-5-fold axes are also presented.

Defect Structures in Al-Ni-Rh Crystalline Approximants Studied by HREM and HAADF Techniques

By means of a combination of high-resolution electron microscopy (HREM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques, we have directly revealed that periodic arrangements in different manners for flattened hexagons constructed with atom columnar clusters can form two Al-Ni-Rh crystalline approximant phases. In contrast to periodic arrangements of flattened hexagons, configurations and distributions of various defects in these structurally-complicated alloy phases have been examined and their structural characteristics discussed. HREM observations clearly show that structural defects in Al-Ni-Rh crystalline approximants are of phason type and they are correlated with incorrect arrangements of atom columnar clusters. The distribution of high density planar defects can destroy the long-range periodicity in at least one direction in the pseudo decagonal symmetry plane. By means of the HAADF-STEM imaging technique, the existence of ill-formed atom columnar clusters in the core area of a linear defect, which is usually not visible in HREM observations, has been clearly revealed.

Electronic Tuning of Mg2Cu6Ga5. A Route to Crystalline Approximant and Quasicrystalline Phases

Studies of Mg2Cu6Ga5 reveal that this compound contains incomplete Bergman clusters in its structure and shows a pseudogap and empty bonding states just above the Fermi energy according to band calculations. Under a rigid band assumption, such a compound may be tuned to approximant and quasicrystal phases in which the required number of electrons are attained. Here, we replace part of Mg in the isotypic Mg2Cu6Ga5 with Sc, and both 1/1 approximant and icosahedral quasicrystal phases are obtained after some fine-tuning. This method closely correlates the pseudogap and bonding with Hume-Rothery concepts, thus giving useful directions for future quasicrystal searches, especially when approximants are not known.

Modeling the atomic structure of amorphous steels using crystalline approximants

Using the pair density function analysis of pulsed neutron diffraction data, the local topology of Fe-based metallic glasses with good glass forming ability was investigated upon alloying with transition metal ions of Y, Zr, or Mo. Distinct short and medium range atomic order with common characteristics in all the glasses is observed. The local order is well described by a geometrical model constructed from superposition of ``crystalline approximant'' phases which is different from Frank-Kasper polyhedra clustering or dense random packing models. The mechanism responsible for the structure in the glass phase might involve a frustration-induced structural disorder of the crystalline approximant phases as they are driven away from stable stoichiometric compositions.

Arrangement of atomic clusters in a 2/1 cubic approximant in the Al-Zn-Mg alloy system

An arrangement of the so-called Bergman clusters of atoms in a 2/1 cubic crystalline approximant phase, which is closely related to Frank-Kasper-type icosahedral quasicrystals, in an Al-Zn-Mg alloy system is discussed in detail. The 2/1 cubic structure has eight Bergman clusters in its unit cell and the positions of the clusters can be understood as twelvefold sites in the 2/1 cubic approximant of the three-dimensional Penrose lattice. The atomic clusters are located at vertices of two types of rhombohedron, which are formed by six rhombic planes with an edge length of 3 1/2 a 2 /( 2 + 1) 1/2 and a shorter diagonal of 2 a 2 /( 2 + 1) 1/2 ( is the golden ratio and a is a quasilattice constant), which correspond to half a body diagonal and an edge length respectively of the 1/1 cubic approximant unit cell. The present results provide an important key for understanding the arrangements of atomic clusters in icosahedral quasicrystals.

Precipitation of icosahedral quasicrystalline and crystalline approximant phases in Zr–Cu–(Co, Rh or Ir) metallic glasses

Zr 70Cu 30, Zr 70Cu 20Co 10, Zr 70Cu 20Rh 10 and Zr 70Cu 20Ir 10 glassy alloys were prepared by the single roller melt-spinning method and the crystallization process was studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy, with an emphasis on the initial stage. The Zr 70Cu 30 metallic glass crystallizes through the direct precipitation of the stable crystalline phase from the matrix. The addition of Co, Rh and Ir to the Zr 70Cu 30 metallic glass induces the precipitation of metastable phases prior to the formation of the stable ones. The metastable phases are a Ti 2Ni-type compound for Zr 70Cu 20Co 10, a mixture of the Ti 2Ni-type compound and an icosahedral quasicrystalline phase (I-phase) for Zr 70Cu 20Rh 10, and the I-phase for Zr 70Cu 20Ir 10 metallic glasses. The different effects of Co, Rh and Ir addition are explained based on their difference in atomic radius.

Precipitations of icosahedral quasicrystalline and crystalline approximant phases in Zr-Al-Ni-Cu-Ir metallic glasses

Metallic ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{5}{\mathrm{Cu}}_{17.5}{\mathrm{Ir}}_{5}$ and ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{12.5}{\mathrm{Ir}}_{5}$ glasses have been prepared and their crystallization process has been studied with emphasis on the initial stage. The initial precipitation phase is an icosahedral quasicrystalline phase (I phase) for the former alloy and a face-centered cubic ${\mathrm{Zr}}_{2}\mathrm{Ni}$ $({\mathrm{f}\mathrm{c}\mathrm{c}\ensuremath{-}\mathrm{Z}\mathrm{r}}_{2}\mathrm{Ni})$ phase for the latter one. Structural analysis revealed that the crystal structure of ${\mathrm{f}\mathrm{c}\mathrm{c}\ensuremath{-}\mathrm{Z}\mathrm{r}}_{2}\mathrm{Ni}$ contains icosahedral atomic clusters, implying that ${\mathrm{f}\mathrm{c}\mathrm{c}\ensuremath{-}\mathrm{Z}\mathrm{r}}_{2}\mathrm{Ni}$ can be regarded as the crystalline approximant for the Zr-based I phase. The preferential precipitation of quasicrystalline/approximant phases in the initial crystallization stage implies the existence of an icosahedral atomic cluster in the glassy states. The compositions of the two alloys were obtained by adding Ir to the ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{17.5}$ alloy, which is well known for the formation of a bulk amorphous alloy. The probable existence of icosahedral atomic clusters provided an explanation for the stability of the metallic ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{17.5}$ glass.

Structural Characteristics of Al-Co-Ni Decagonal Quasicrystals and Crystalline Approximants

The structural characteristics of several modulations of Al-Co-Ni decagonal quasicrystals and crystalline approximants, all of which are found in and around an Al-Co-Ni decagonal phase with a wide compositional range, are discussed systematically. The structures of a Ni-rich basic structure and the other modulations are characterized as quasiperiodic arrangements of columnar clusters of atoms with decagonal sections of 3.2 and 2.0 nm in diameter, respectively. Various types of electron diffraction patterns including so-called superlattice reflections are interpreted by tiling features of the atomic clusters as well as ordered arrangements of the atomic clusters with pentagonal symmetry in the tilings. From the structure of a crystalline approximant (W-(AlCoNi) phase), precise models of the atomic clusters are proposed, and also the origin of diffuse reflections often observed in the Al-Co-Ni decagonal quasicrystals may be interpreted.

Sharp Feature in the Pseudogap of Quasicrystals Detected by NMR

The {sup 27}Al and {sup 63}Cu spin-lattice relaxation rates were found to contain a large T{sup 2} term over a wide temperature range below 400K in several thermodynamically stable quasicrystalline alloys, including i -AlCuRu, i -AlPdRe, i -AlCuFe, and the crystalline approximant phase {alpha}-AlMnSi. The relaxation mechanism is proven to be electronic in origin. Such nonlinear temperature dependence is shown to be a clear signature of sharp features in the density of states at the Fermi level. The estimated width of this sharp feature is on the order of 20meV. {copyright} {ital 1997} {ital The American Physical Society}

Triacontahedral growth morphology in icosahedral ZnMg-Y alloy

We report here the first observation of triacontahedral growth morphology in a Zn-Mg-Y face-centred icosahedral alloy. Some considerations on the local order in this phase are given by a structural study of several crystalline phases in the binary Zn-Mg, Zn-Y and Mg-Y systems. Our experimental work shows that the icosahedral phase can coexist with both Mackay-type and Bergman-type (or Frank-Kasper-type) crystalline approximant phases of the Zn-Mg phase diagram. However, the occurrence of triacontahedral growth morphology allows us to classify it among the Bergman-type icosahedral alloys.

Hydrogenation of titanium-based quasicrystals.

Measurements of hydrogen loading and unloading in a quasicrystal are reported. The icosahedral phase (i phase) in a Ti-Zr-Ni alloy demonstrates hydrogen absorption from the gas phase at a temperature of 260 \ifmmode^\circ\else\textdegree\fi{}C and a pressure of 40 atmospheres. The incorporation of hydrogen causes the quasilattice to expand, with an increase in quasilattice constant of almost 7%, from ${\mathit{a}}_{\mathit{i}}$=5.18 to 5.52 \AA{}. Similar abilities for storing hydrogen are found in high-order crystalline approximant and amorphous phases of like composition. Of the phases examined, the i phase absorbs the most hydrogen, giving a hydrogen atom to metal atom ratio ([H]/[M]) of 1.6. Initial studies indicate that hydrogen desorption is hindered by phase transitions to stable hydride phases. Hydrogen loading at lower temperatures with higher ${\mathrm{H}}_{2}$ pressures favors higher [H]/[M] ratios for the i phase and the crystal approximant, and also minimizes the growth of more stable crystalline hydride phases.

Anomalous electronic behaviour of crystalline phases close to quasicrystals

The crystalline approximant phases α-AlMnSi and R-AlCuFe present for alloys of metallic elements anomalous electronic transport properties, with low conductivity values and strong temperature dependence of both the conductivity and the Hall coefficient. In both the α- and R-phases, the high field magnetoconductivity can be analysed by quantum interference effects. The transport behaviours are very similar to those of the high structural quality icosahedral phases. Moreover, the R-AlCuFe phase properties are identical to those of the AlCuFe icosahedral phases of the same composition. Surprisingly, the α-AlMnSi phase, of a much simpler structure than R-AlCuFe, presents the same anomalous behaviours. Approximants are very different from the icosahedral AlMnSi counterparts which behave like amorphous metals. The results indicate that the relevant length scale for understanding the transport properties is about the size of these crystalline unit cells, i.e. one or two nanometers.

Structural Relationships between Crystalline Approximant Phases and the Decagonal Mn-Al Phase

Structural Relationships between Crystalline Approximant Phases and the Decagonal Mn-Al Phase

Localized Magnetic Moments and Spin Freezing in Al-Based Amorphous, Quasicrystalline and Crystalline Approximant Phases

Al-based amorphous and quasicrystalline alloys have been prepared by high-rate sputtering and melt-quenching, respectively. The Al 67 Cu 20 Mn 13 crystalline approximant phase also has been made by annealing the quasicrystalline counterpart. The formation of the localized magnetic moments of 3d metals in the Al matrix is difficult except for Mn. The ratio of the magnetic Mn atoms estimated from the saturation magnetization and the Curie constant in the quasicrystalline phase is smaller than that in the amorphous phase. In the crystalline approximant phase, the ratio is comparable to that in the amorphous phase, but the average magnetic moment of magnetic Mn atoms is much smaller than that in the amorphous and quasicrystalline phases