Phase I-phase Research Articles

Internal friction of an Al–Cu–Fe icosahedral quasicrystal and its crystal approximant

The internal friction was measured for an Al–Cu–Fe icosahedral phase (i-phase) and an Al–Cu–Fe–Si 1/1 approximant phase (1/1-phase) by using a dynamical mechanical analyzer (DMA) in the frequency range 0.05–10 Hz and the temperature range 300–873 K. For the i-phase, four absorption peaks were observed at around 420, 580, 620 and 800 K, respectively. These were of the thermally activated relaxation type, and their activation parameters were evaluated. The parameters evaluated for the peak at 800 K indicate a relaxation process involving a collective atomic motion, which can be attributed to correlated phason jumps. This peak was absent for the 1/1-phase. These results are similar to the results of the internal friction measurements previously reported for an Al–Pd–Mn i-phase and its 1/1 approximant phase. Based on the phonon–phason dynamics model, the magnitudes of phason elastic constants were estimated for the Al–Cu–Fe i-phase.

Quasicrystalline phase formation during heat treatment in mechanically alloyed Al65Cu20Fe15 alloy

Quasicrystalline phase formation during heat treatment in the mechanically alloyed Al65Cu20Fe15 powders was studied by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The mechanical alloying was performed at speed of 300 rev min–1 for times up to 70 h. It was found that mechanical alloying of the Al65Cu20Fe15 powders did not result in the quasicrystalline (QC) icosahedral phase (i-phase) formation. The long time milling resulted in the formation of a cubic Al (Cu,Fe) solid solution phase (β-phase). The cubic Al (Cu,Fe) solid solution identified as β-phase was observed to be present as one of the major phases in the Al65Cu20Fe15 alloy. The formation of the quasicrystalline icosahedral phase (i-phase) was only observed for short time milled powders after additional annealing at temperature above 500°C. The present investigation also showed that a tetragonal Al2Cu phase (θ-phase) forms with short time milling. The tetragonal Al7Cu2Fe1 phase (w-phase) was observed after heat treatment of the short time milled and unmilled powders. The present investigation indicated that an effective process to prepare the quasicrystalline materials was using a combination of short time milling and subsequent annealing.

Effect of Titanium, Antimony, Cerium and Carbon Nanotubes on the Morphology and Microhardness of Mg-based Icosahedral Quasicrystal Phase

F or the flrst time, petal-like and spherical Mg-based icosahedral quasicrystal phase (I-phase) were obtained by introducing Ti, Sb, Ce and C nanotubes into Mg-Zn-Y alloy under normal casting conditions. The formation mechanism and stability criterion of spherical I-phase were discussed. The morphology and microhardness of I-phase and their determinants were studied in this paper. The results show that the difierent value of microhardness of I-phase could be attributed to the difierent kinds of the fourth component and its content, and its difierent innate characters. The flnal morphology of icosahedral quasicrystalline (IQC) is decided by the size of critical stable radius Rr, the content of the fourth component and degree of undercooling.

Effect of Mn and Cu on Spheroidized Process of Mg-Zn-Y-Based Icosahedral Quasicrystal Phase

In-situ spherical icosahedral quasicrystal phase (I-phase) was fabricated in Mg-Zn-Y-based alloys by metal mold casting route. The effects of Mn and/or Cu on spheroidized process, morphology and microhardness of primary I-phase were studied in this paper. The results showed that the solidified morphology of I-phase depended on the stability of spherical I-phase during the subsequent growth and critical radius Rr of the spherical I-phase. Only when actual radius smaller than critical radius Rr can spherical IQC be formed. The effects of the fourth component and degree of undercooling on Rr were studied for the first time. Amazing spherical minisize IQC had been produced by improving cooling rate using a water-cooled copper mold. Multicomponent spherical IQC is hopeful of being strengthening phase to novel metal composites.

Effect of Magnetic Field on Martensite to Intermediate Phase Transformation in Ni<sub>2</sub>MnGa

We have investigated the martensitic transformation behavior in a single crystal of Ni2MnGa under various magnetic field. The single crystal used in the present study exhibits an intermediate phase (I-phase) transformation at TI = 250 K and a martensitic transformation at TM = 202 K. Since the martensite phase (M-phase) of Ni2MnGa has a large magnetocrystalline anisotropy, the effect of magnetic field depends significantly on the direction of magnetic field. We have measured the reverse (i.e., M-phase to I-phase) transformation start temperature As from a single variant state to examine the effect of magnetic field because the forward (I-phase to M-phase) transformation usually forms a multivariant state of the M-phase. When the magnetic field is applied parallel to the easy axis, As increases linearly with increasing magnetic field. On the other hand, when the magnetic field direction is not parallel to the easy axis, As decreases in a low field region and then increases on further increasing the magnetic field. Such behavior of magnetic field dependencies of As are quantitatively explained by the Clausius-Clapeyron equation, where we have assumed that the magnetic field dependence of As agrees with the magnetic field dependence of the equilibrium temperature.

Microstructures and mechanical properties of quasicrystal reinforced Mg matrix composites

Abstract Mg-based composites reinforced with stable in situ icosahedral quasicrystal phase (I-phase) were fabricated in this study. The effects of Y content on the microstructures and mechanical properties of the composites were investigated. The results show that the composites comprised dendrites of α-Mg matrix separated by interdendritic I-phase and lamellar (I-phase + α-Mg) eutectic. With the increase of Y content, the content of I-phase increases; the content and size of (I-phase + α-Mg) eutectic cell increase; the lamellar space between I-phase and α-Mg in eutectic cell also increases; but the grain size of α-Mg matrix decreases. The failure mode of Mg–Zn–Y alloy matrix composites in tensile tests is quasi-cleavage fracture. With the increase of Y content, the tensile strengths of the composites increase, and the elongation rates increase firstly and then decrease.

Quasicrystallization of (Zr<sub>65</sub>Al<sub>7.5</sub>Cu<sub>27.5</sub>)<sub>95</sub>Ti<sub>5</sub> Glassy Alloy

Phase transformation in the melt-spun (Zr65Al7.5Cu27.5)95Ti5 glassy alloy was investigated by a combined use of X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was found that the crystallization mode of the Zr65Al7.5Cu27.5 glassy alloy was altered by the addition of Ti, and nanometer scaled icosahedral phase (I-phase) formation was observed in the primary crystallization stage of the (Zr65Al7.5Cu27.5)95Ti5 quaternary glass, which transformed to the stable Zr2Cu-type phase upon further annealing. The crystallization behavior of (Zr65Al7.5Cu27.5)95Ti5 glass was further discussed in terms of the activation energy as compared to that of Zr65Al7.5Cu27.5 alloy.

Microstructure and Mechanical Properties of Extruded Mg-Zn-Gd-Based Alloy Reinforced by Quasicrystals and Laves Phase

The microstructure and mechanical properties of Mg95.9Zn3.5Gd0.6 and Mg94.4Zn3.5Gd0.6Cu1.5 alloys reinforced by icosahedral quasicrystalline phase (I-phase) and Laves phase has been studied after extrusion at 573K. Extrusion can significantly refined the I-phase and Laves phase, and the strengthening effect of I-phase and Laves phase has been analyzed. Large volume of icosahedral phase in Mg95.9Zn3.5Gd0.6 has important role in its high UTS and elongation due to strong bonding effect at the I-phase/matrix interface for low interface energy. The Laves phase with cubic topological and close-packed structure in Mg94.4Zn3.5Gd0.6Cu1.5 alloy result in the higher heat resistance at elevated temperatures.

Phase transformations of icosahedral AlCuFe quasicrystals

Phase transformations with alloy compositions in the vicinity of the AlCuFe icosahedral quasicrystalline phase (i-phase) were studied. This phase forms via a ternary peritectic reaction at 882°C from liquid and two primary solid phases termed λ and β. With decreasing temperature, the icosahedral phase is a reaction partner with liquid, λ, β and tetragonal ω in two different transition reactions. Furthermore, two solid-state phase transformations, involving the i-phase (a quasi-binary eutectoid β → i + φ at 640°C and a ternary transition-type reaction i + β → ω + φ at 616°C), are reported. The conclusions are based on differential thermal analysis combined with powder X-ray diffraction investigations. A three-dimensional sketch of a wire model for the icosahedral phase was constructed, which demonstrates the shrinkage of the phase field of icosahedral phase with a decreasing temperature.

High Mechanical Properties of Cast Quasicrystal-Reinforced Mg-Alloys

Mg-8Zn-4Al-xY base alloys containing an icosahedral quasicrystal phase (i-phase) as a main strengthening phase were prepared by casting into a copper mould at moderate cooling rates. The Y addition was effective for decreasing the size of i-phase and the more homogeneousness of its dispersed state. The mechanical properties at room temperature were much superior to those of AZ91 alloy. The creep tests indicated a promising high temperature creep resistance of the quasicrystal-reinforced Mg-Zn-Al-Y cast alloy. The dislocation characteristic in crept specimens which containing i-phase and no i-phase was analyzed and strengthening mechanism was discussed.

Comparative study of local structure in Zr70Al10Ni20 and quasi-crystal-forming Zr70Al9Ni20Pd1 metallic glasses

The local structure of Zr70Al9Ni20Pd1 metallic glass, in which a nano-icosahedral quasi-crystalline phase (I-phase) is formed in the primary stage of crystallization, has been examined and compared with that of Zr70Al10Ni20, the supercooled liquid state of which has a high stability. Since the local environments around the Zr and Ni atoms do not change drastically by the addition of 1 at.% Pd to Zr70Al10Ni20, as evidenced by radial distribution function (RDF) and extended x-ray absorption fine structure (EXAFS) studies, we deduce that the icosahedral phase formed in the Zr70Al9Ni20Pd1 metallic glass has a local structure similar to that in Zr70Al10Ni20. Although a very slight rearrangement of Zr–Zr atomic pairs occurs during quasi-crystallization, the I-phase formation is achieved without disturbing the dominant local structure in the glassy state of the Zr70Al9Ni20Pd1. An icosahedral local structure is proposed for Zr–Al–Ni metallic glass system as well as for primary quasi-crystal (QC)-forming Zr-based metallic glasses.

Precipitation Behavior and Phase Stability of Intermetallic Phases in Fe-Cr-W-Co Ferritic Alloys

Precipitation behavior of intermetallic phases in ferrite matrix is investigated by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co (at%) alloys with and without 0.3at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873K. It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free alloy, the R-phase precipitates instead of I-phase at 873K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973K. The Laves phase formed at 973K transforms to the I-phase in the Si-added alloy but to the R-phase in the Si-free alloy during subsequent aging at 873K. The factors in controlling the phase stability of I-phase, R-phase and Laves phase precipitates in Fe-based alloys are discussed by the atomic size ratio and electron concentration factor (e/a).

Influence of the ingot solidification microstructure on the formation kinetics of icosahedral quasicrystalline phase during the crystallization of the corresponding amorphous alloy

Zr65Al7.5Cu12.5Ni10Ag5 ingots with different solidification microstructures are obtained by repeated arc melting of origin ingots. Under the same conditions, slices of amorphous alloys can be produced by suction casting in a copper mold. Differential scanning calorimetry (DSC) is employed to investigate the crystallization kinetics of the amorphous alloys. X-ray diffraction patterns (XRD) show that icosahedral quasicrystalline phase (I-phase) precipitates as primary phase during the crystallization of Zr65Al7.5Cu12.5Ni10Ag5 amorphous alloys. The results of Kissinger analyses suggest that the amorphous alloys made from the ingot with finer microstructure show better thermal stability, and that the I-phase precipitated becomes more stable against decomposition. Within the frame of structure heredity, the influence of ingot microstructure on the formation kinetics of I-phase during the crystallization of corresponding amorphous alloys is discussed.

Effect of the repeated melting of mother ingot on the stability of icosahedral phase precipitated from Zr-Al-Ni-Cu-Ag glass

Differential scanning calorimetry (DSC) shows that repeated melting of mother ingot improves the stability of icosahedral phase (I-phase) precipitated from Zr 65 Al 7.5 Ni 10 Cu 12.5 Ag 5 glass. The Kissinger analysis of crystallization implies that the effective activation energies for the precipitation of I-phase and its decomposition considerably increases due to the repeated melting of mother ingot. Repeated melting of mother ingot refines ingot microstructure. Because of structure heredity, the size of the short-range orders (SROs) in the glassy alloy produced from the mother ingot with finer microstructure becomes smaller, which makes the rearrangement of atoms more difficult, and the I-phase more stable.

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.

Undercooling And X-Ray Structural Studies Of Ti-Zr-Ni Liquids

ABSTRACTMaximum undercooling results for the icosahedral phase (i-phase), polytetrahedral C14 Laves phase, and solid solution phases are presented as a function of composition in Ti-Zr-Ni liquids. Containerless processing was achieved using the electrostatic levitation facility located at NASA/Marshall Space Flight Center. The maximum reduced undercooling decreases with increasing icosahedral short-range order in the ordered phase. The first synchrotron x-ray diffraction data from aerodynamically levitated liquids of Ti-Zr-Ni alloys suggest an icosahedral short-range order in the liquids, supporting Frank's hypothesis, correlating icosahedral order in the liquid with the nucleation barrier to the crystal phase. The strong negative heats of mixing between Ti/Zr and Ni and their relative atomic sizes likely favor the formation of this local icosahedral order.

Quasicrystalline phase formation in Al62Cu22.5Fe12.5 and Al55Cu22.5Fe12.5Be7 alloys

Formation of the icosahedral quasicrystalline phase in conventionally cast Al62Cu25.5Fe12.5 and Al55Cu25.5Fe12.5Be7 alloys were investigated. The icosahedral phase (I-phase) forming ability was greatly improved by partial replacement of Al by 7 at.% Be. The as-cast Al55Cu25.5Fe12.5Be7 alloy consisted of dendritic primary I-phase and interdendritic τ-phase, whereas that of an as-cast Al62Cu25.5Fe12.5 alloy consisted of various phases such as the β-, I-, and τ-phases, together with a small amount of the λ- and η-phases. The kinetic barrier for transformation into single I-phase by heat treatment was greatly reduced in an Al55Cu25.5Fe12.5Be7 alloy. The I-phase in an Al55Cu25.5Fe12.5Be7 alloy has the same face centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy and is of high quality without phason strain.

Formation of Icosahedral Quasicrystals in Mg-Al-X (X=Transition Metal) Ternary Systems

AbstractFormation of icosahedral quasicrystalline phase (i-phase) was investigated for Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary systems. A Mackay-icosahedron (MI) type i-phase was found to form in the Mg-Al-Rh system at the composition Mg8Al68Rh24. The i-phase has the F-type ordered structure and the quasilattice constant is 0.447nm. Differential thermal analysis (DTA) studies showed that the i-phase has a high stability. On the other hand, the formation of a Frank-Kasper (FK) type i-phase was found in the Mg-Al-Pt system at the composition Mg45Al42Pt13. The conditions necessary for the formation of the i-phases are discussed in terms of the atomic radius ratio and the electronegativity as well as the electron-per-atom ratio.

Icosahedral-phase formation and stability in Ti Zr Co alloys

We present the first report of icosahedral phase i-phase formation in rapidly quenched alloys of Ti Zr Co . Electron diffraction patterns of i TiZrCo 53 27 20 contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti 3d transition metal Si O and Ti Zr Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti Zr Ni i-phases, showing none of these features. The quasilattice constant for a i TiZrCo, 0 51 nm, is close to that of i TiZrNi . The i-phase in rapidly q quenched Ti Zr Co alloys is deeply metastable and transforms exothermically o to the hexagonal Laves phase about 500 C. The Laves phase transforms to the o T bcc solid solution phase b-Ti and the Ti Ni-type fcc structure for 630 C. A 2 reversible transformation between the b-Ti and the hexagonal solid solution phase a-Ti is observed on temperature cycling; the Ti Ni-type fcc phase is 2 stable over the entire temperature range.

Genetically engineered yeast cells and their applications

The first generation of yeast expression systems relies on inducible expression cassettes borne by multicopy plasmids for production of unmodified human P450s and on the endogenous NADPH-P450 reductase to support activities. A second generation of engineered yeast involved targeted genomic modifications allowing overexpression of the yeast reductase and coexpression of human cytochrome b 5 and of a phase II enzyme such as epoxide hydrolase. These features allow improved P450 turnover numbers and simulation of some phase I-phase II couplings. In the third generation, the human reductase was substituted for the yeast reductase by genome engineering. Simultaneously, induction procedures were optimized to reach high P450 specific contents. Dramatic improvements (1000-fold) of yeast-expressed P450 activities have thus been obtained. To get more insight into complex metabolic events, such as that of a typical pollutant: benzo[ a]pyrene, an approach was designed which involves a complementary use of yeast expression and computer simulations.