Fcc Zr Research Articles

Stability of Supercooled Liquid and Transformation Behavior in Zr-Based Glassy Alloys

We investigated the stability of the supercooled liquid in Zr-based glassy alloys by examination of their transformation behavior. The primary phase is an fcc Zr2Ni phase in a Zr65Al7.5Ni10Cu17.5 glassy alloy with high glass-forming ability (GFA). By substitution of Ag, Pd, Au or Pt for only 1 at%Cu, an icosahedral quasicrystalline phase (I-phase) precipitates with the fcc Zr 2Ni phase. The primary phase changes to the single I-phase at higher noble metal contents. It is further found that the I-phase precipitates by a small amount of substitution for Cu with other elements as well as the noble metals, which have a weak or positive chemical affinity with one of the constitutional elements in the Zr‐Al‐Ni‐Cu glassy alloy. Thus, the slight deviation from the three component rules for high GFA is effective for the I-phase formation. The I-phase is also formed as a primary phase in the Zr65+y Al7.5Ni10Cu17.5−y (y = 1‐7) glassy alloys. A slight change in the composition has the similar effect as the addition of element such as noble metals and so on. An icosahedron is contained as a structure unit in the fcc Zr 2Ni and I-phases and hence the glassy structure is correlated with the local icosahedral atomic configuration. The high-resolution transmission electron microscopy image of the Zr70Al7.5Ni10Cu12.5 glassy alloy reveals a possibility of the existence of the icosahedral ordered regions. It is therefore, concluded that the icosahedral short- or medium-range order exists in the supercooled liquid as well as in the glassy phase and it stabilizes the supercooled liquid state in the Zr-based alloys. An I-phase also precipitates as a primary phase by substituting Pd, Au or Pt for only 1 at%Cu in the Zr70Cu30 glassy alloy. From these results, the appearance of the supercooled liquid region is attributed to the existence of the icosahedral atomic configuration consisting of Zr and Cu.

Effect of Zn addition on the crystallization process in Zr 65Al 7.5Ni 10Cu 17.5 metallic glass

Zr 65Al 7.5Ni 10Cu 12.5Zn 5 and Zr 65Al 7.5Ni 5Cu 17.5Zn 5 metallic glasses, which were obtained by adding Zn to the well-known glass-former Zr 65Al 7.5Ni 10Cu 17.5, were prepared by a melt-spinning method and the crystallization process was studied by differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopy. Two exothermic peaks were observed in the DSC curves. The low temperature one corresponds to the simultaneous precipitation of an icosahedral quasicrystalline phase (I-phase) and the face centered cubic Zr 2Ni (FCC–Zr 2Ni) phase in the Zr 65Al 7.5Ni 10Cu 12.5Zn 5 alloy. The initial precipitate for the Zr 65Al 7.5Ni 5Cu 17.5Zn 5 alloy is a single I-phase. Since an I-phase does not occur in the crystallization process of Zr 65Al 7.5Ni 10Cu 17.5 metallic glass, the present results demonstrate that the addition of Zn is effective in promoting the precipitation of an I-phase, in addition to the previously reported noble metals. Further annealing leads to the precipitation of other stable crystalline phases, indicating the metastable character of the I-phase and the FCC–Zr 2Ni phase. The reason for the change in the crystallization process caused by the addition of Zn was discussed on the basis of the chemical affinity and atomic radius of the constituent elements.

Transformation of Nano Icosahedral Phase in Zr<SUB>65</SUB>Al<SUB>7.5</SUB>Ni<SUB>10</SUB>Cu<SUB>17.5-x</SUB>Pd<SUB>x</SUB> (x=0 to 4) Glassy Alloys

We investigated the transformation behavior from glassy to icosahedral and/or fcc Zr 2 Ni phases in the Zr 65 Al 7.5 Ni 10 Cu 17.5-x Pd x (x = 0 to 4) glassy alloys with low oxygen contents of less than 400 ppm mass%. The primary phase is a metastable single fcc Zr 2 Ni phase in the x = 0 alloy and a single icosahedral quasicrystalline phase in the 2 to 4at%Pd alloys. The mixture phase of icosahedral and fcc Zr 2 Ni precipitates at the initial crystallization stage in the x = 1 alloy. It is therefore confirmed that the icosahedral phase is formed by the addition of I at% Pd to the Zr-Al-Ni-Cu glassy alloy. The icosahedral and Zr 2 Ni particles have grain sizes in the diameter range of 500 to 1000 nm and are in an isolated state for the sample annealed at a temperature near the crystallization temperature. A significant redistribution leading to the enrichment of Zr in the icosahedral phase is confirmed. Moreover, it is recognized that Ni and Cu are rejected from the icosahedral phase. We also observed the enrichment of Zr and Ni and the rejection of Cu in the Zr 2 Ni phase. No significant difference in Al content is observed among the icosahedral, Zr 2 Ni and residual glassy phases. The Pd content in the icosahedral phase is the same as that in the Zr 2 Ni phase. The difference of Zr, Ni and Cu contents among the icosahedral, Zr 2 Ni and remaining glassy phases is in the range of 2 to 9 at%. No significant difference in the Pd content is observed between the icosahedral and Zr 2 Ni phases. Considering that the diameter of the primary phase decreases significantly with increasing Pd content, it is suggested that Pd plays a dominant role in the increase in the number of nucleation sites of the icosahedral phase. Moreover, it is strongly suggested that the icosahedral phase originates from the icosahedral short-range order with an increase of nucleation rate as a result of the addition of noble metal.

Multiple-scattering Green-function method for space-filling cell potentials.

It is shown that the equations of multiple-scattering theory (MST) originally derived for scattering off collections of muffin-tin (MT) potentials, i.e., potential cells bounded by non overlapping spheres, remain valid in the case of arbitrarily shaped, non overlapping, and particularly space-filling potential cells. Specifically, it is shown that in the angular momentum representation the total scattering (transition) matrix, the Greenfunction, the Bloch function for a translationally invariant material, and the Lloyd formula for the change in the integrated density of states have forms that are invariant with respect to the partition of a given potential into non overlapping cells, and with respect to the choice of the cell centers. Ananalytic proof is provided for the vanishing of near-field corrections (NFC's)long conjectured to arise when the spheres bounding individual cells over lap one another or adjacent potentials. Thus, the well-known MST expressions, originally derived for the case of MT potentials, for obtaining the solution of the Schroedinger equation and hence determining the band structure and the charge density of materials, ordered or disordered, are rigorously valid in the completely general case of arbitrarily shaped cells. The differences between this work and previous attempts to generalize MST to non-MT space-filling potentials aremore » discussed. It is pointed out that in calculations involving non-MT potential cells, particular attention must be paid to the question of convergence of expansions in angular momentum eigenstates. This convergence is tested numerically in terms of cluster calculations and through the calculation of the electronic structure of elemental bcc Nb and fcc Zr and Rh.« less

Electronic structure of zirconium hydride: A proton NMR study

The proton spin-lattice relaxation times (${T}_{1}$) and Knight shifts (${\ensuremath{\sigma}}_{K}$) have been measured as a function of temperature in fcc ($\ensuremath{\delta}$ phase) and fct ($\ensuremath{\epsilon}$ phase) $\mathrm{Zr}{\mathrm{H}}_{x}$ for hydrogen concentrations $1.5\ensuremath{\le}x\ensuremath{\le}2.0$. Interactions with the conduction electrons were found to be the only important ${T}_{1}$ relaxation processes below 320 K for the high-purity $\mathrm{Zr}{\mathrm{H}}_{x}$ samples, and no anomalous temperature effects were observed between 320 and 100 K. The dominant hyperfine interaction for the protons was the transferred core-polarization term from the Zr $d$ band. Both ${({T}_{1e}T)}^{\frac{\ensuremath{-}1}{2}}$ and ${\ensuremath{\sigma}}_{K}$ indicate that the density of electronic states $N({E}_{F})$ at the Fermi level is very dependent upon hydrogen content with a maximum occurring near Zr${\mathrm{H}}_{1.83}$. This behavior is ascribed to modifications in $N({E}_{F})$ through the fcc-fct distortion associated with a Jahn-Teller effect in the $d$ bands. The proton NMR results are consistent with a recent band-theory calculation of fcc Zr${\mathrm{H}}_{2}$ and photoemission spectroscopy studies of $\mathrm{Zr}{\mathrm{H}}_{x}$ when the changes in $d$ bands caused by the Jahn-Teller tetragonal distortion are included. The fcc-fct distortions and electronic structures of the $\mathrm{Zr}{\mathrm{H}}_{x}$ phases are compared with the corresponding properties of the $\mathrm{Ti}{\mathrm{H}}_{x}$ system.

Specific Heats of Delta-Phase Zr-H and Zr-D

The specific heats of fcc Zr${\mathrm{H}}_{1.58}$ and Zr${\mathrm{D}}_{1.58}$ have been measured in the temperature interval 30-500\ifmmode^\circ\else\textdegree\fi{}C. The data are interpreted in terms of a harmonic oscillator model for the hydrogen and deuterium specific heat contributions. This model is in accord with recent inelastic neutron scattering studies and predicts an isotopic depression of the deuteride Einstein temperature by a factor of $\frac{1}{\sqrt{2}}$ relative to the hydride value. Over the interval 30-200\ifmmode^\circ\else\textdegree\fi{}C, the data and model are quantitatively consistent. At higher temperatures, the deuteride specific heat is somewhat smaller than anticipated. Quantities of noncubic $\ensuremath{\gamma}$-phase material are known to be present in the samples used, particularly in the deuteride, and this is thought to be a likely cause for the deviations. The hydride Einstein temperature calculated from the difference between deuteride and hydride specific heats at 150\ifmmode^\circ\else\textdegree\fi{}C is 1500\ifmmode\pm\else\textpm\fi{}300\ifmmode^\circ\else\textdegree\fi{}K, as compared with the inelastic neutron scattering value of 1500\ifmmode\pm\else\textpm\fi{}60\ifmmode^\circ\else\textdegree\fi{}K.

Optical Mode Scattering Contribution to Electrical Resistivity in Zirconium Hydride

Electrical resistivity measurements on fcc Zr${\mathrm{H}}_{1.54}$ and fct Zr${\mathrm{H}}_{1.96}$ between 1.2\ifmmode^\circ\else\textdegree\fi{}K and 400\ifmmode^\circ\else\textdegree\fi{}K have revealed a sizeable optical mode scattering contribution. Excellent fits to the data are obtained using a simple additive combination of Gr\"uneisen and Howarth-Sondheimer functions for the respective acoustical and optical mode scattering contributions. The corresponding acoustical and optical mode characteristic temperatures are in good accord with values derived from earlier inelastic neutron scattering experiments.