Degree Of Short-range Order Research Articles

Order-disorder transformation in Au-Cu alloys studied by extended x-ray-absorption fine structure

We show that extended x-ray-absorption fine-structure studies can estimate the degree of shortrange order both above and below the transformation temperature of alloys displaying superlattices at low temperature. A substantial degree of short-range order was measured above the ordering temperature of 390\ifmmode^\circ\else\textdegree\fi{}C for Au${\mathrm{Cu}}_{3}$. The local Au-Cu pair separation and its fluctuation show discontinuities at the ordering temperature. Shifts in the absorption edge were also observed at the transformation.

Resistometric study of SRO-kinetics in α-AgAl

Short-range order (SRO) kinetics of Ag-(3.5, 7.5, 11.5, 15.5) at.% Al have been investigated by measurement of electrical resistivity during isochronal and isothermal annealing. Besides an increase of the atomic mobility with increasing Al-content the samples show a reduction of resistivity with increasing degree of SRO. After a small and sudden temperature change the new equilibrium state of SRO is adjusted by a simple relaxation process with an activation energy of 1.37 ± 0.1 eV. The microstructure of short-range ordered α-AgAl is therefore assumed to be homogeneous.

Short-range order in iron-base amorphous alloys

1. The short-range order in amorphous alloys of iron, determined from the distribution of the superthin fields on the iron nuclei, changes in replacement of boron atoms by other metalloids; the most ordered is the alloy with germanium, and the least ordered is the alloy with carbon. 2. The greatest magnetic moment corresponds to the alloy with the greatest degree of short-range order, and the thermal stability determined by the crystallization temperature is a maximum for the alloy with the greatest density of d electrons.

Scattering of atomic beams off stepped surfaces

Surface steps, because they are extended defects, give rise to strong diffuse scattering due to interference between the scattering off the different surface levels. We have calculated this diffuse scattering for a number of different configurations of random arrays of steps, using eikonal approximations which compare well with exact results in the case of a single step. This strong diffuse scattering is concentrated near the specular direction and its width is inversely proportional to the average separation of steps. Its shape gives information on the distribution and the degree of short-range order among the steps. The variation of the pattern with incident angle depends on the step height and also on the details of the step distribution.

Resistometric detection of short-range ordering in the metallic glasses Cu50Ti50and Ni35Ti65

The potentialities of combined electrical resistivity measurements and quench experiments to follow the evolution of local order and infer atom jump frequencies in short-range ordering alloys are recalled. Application to two amorphous alloys, Cu50Ti50 and Ni35Ti65, is presented. In either case, a reversible variation of the resistivity with quench temperature could be discriminated from an irreversible evolution linked to the metastability of these materials with respect to the 'ideal' glass. The reversible term is thought to be reflection of the thermodynamic equilibrium state of chemical short-range order. This effect is discussed in the light of recent neutron diffraction and EXAFS studies, which have produced clear evidence that the metal atoms have a preference for unlike neighbours in both alloy glasses considered. From the observed pattern of the resistivity ordering curves, the existence of ordering domains each characterised by a specific relaxation rate and evolution degree of short-range order, was inferred. Atom jump rates were also derived.

Field-dependent specific heat study of the spin-Peierls transition in TTF–AuS4C4(CF3)4

The compound TTF–AuS4C4(CF3)4 is one of the very few existing examples of the spin-Peierls transition, in which a system of 1D antiferromagnetic chains dimerizes spontaneously as a consequence of magnetoelastic coupling with the surrounding lattice. We have studied the spectific heat anomaly accompanying the SP transition for this compound as a function of an applied magnetic field. Our H=0 data differ considerably from those obtained earlier by Wei et al., and in fact remove the discrepancy found by them between their results for TTF-AuS4C4(CF3)4 and the isomorphous TTF–CuS4C4(CF3)4. The differences between the two experiments are probably due to thermal relaxation effects. The combination of the specific heat data now available confirms the conclusion drawn from our previous field-dependent differential susceptibility studies on the Au compound, namely that the SP transition involves a considerable degree of short-range order, starting to develop below T=1.2 Tsp. We have been able to follow the specific heat anomaly for fields up to the multicritical point in the H–T phase diagram.

On short-range order hardening in Cu-10 at.% Au

In Cu 0.9Au 0.1 polycrystals various degrees of short-range order have been produced by annealing between 150° and 450°C. The short range order parameter α 1 has been derived from measurements of diffuse X-ray scattering and of the electric resistance, α 1 ranged from −0.014 to −0.079. The critical resolved shear stress τ 0 was—independently of α 1—(32.7 ± 0.3) MPa. Thus an upper limit for short range order hardening can be given: 0.01 τ 0.

Elastic diffuse neutron scattering study of the defect structure of TiC 0.76 and NbC 0.73

The elastic diffuse neutron scattering of the non-stoichiometric carbides TiC 0.76 and NbC 0.73 has been measured at room temperature, in the (110) reciprocal lattice plane, for 0.2⩽q = 4π sin O/λ⩽4 A ̊ -1 . The Cowley-Waren short-range order parameters were obtained for the eight first shells of neighbours of the metalloid f.c.c. sublattice. The degree of short-range ordering was found much larger in NbC 0.76. Strong displacements of the metal atoms towards their first neighbour carbons occur in both cases.

Dependence of electrical resistivity on the degree of short range order in a nickel–copper alloy

Abstract The simultaneous measurement of the residual electrical resistivity and the diffuse neutron scattering supplies the experimental foundation for an analysis of the effects of atomic distribution in a binary alloy on the electrical resistivity. Using the Warren–Cowley short-range order (SRO) parameters of Ni–41·4 at.% Cu as derived from the diffuse neutron scattering for various SRO states corresponding to temperatures between 380 K and 730 K, the resistivity contribution of the alloy due to short-range order is calculated according to the theoretical formalism of Rossiter and Wells. By comparison with the experimental resistivity data it is shown that, for the investigated temperature range, the above formalism accounts quantitatively for the dependence of the electrical resistivity on the degree of short-range order. The main contribution to the resistivity was found to result from the atomic distribution in the first coordination shell. The present work confirms experimentally corresponding assumptions made earlier in interpreting resistivity measurements in terms of changes of the atomic distribution.

Elastic energy dissipation effects in α + β and β phase composition ranges of the palladium-hydrogen system

Elastic energy dissipation (internal friction) peaks, arising from both stress-induced, bulk lattice rearrangements of hydrogen interstitials (H(1) peak) and hydrogen dislocation interactions (H(2) peak), have been investigated over a wide range of the hydrogen content in alpha + beta and beta phase concentration ranges of the Pd-H system during both absorption and desorption of hydrogen. It has been confirmed that the H(1) peak is associated with a Zener type of relaxation process. In beta phase regions, asymmetries of the H(1) peak have been observed, which may be accounted for by a Curie-Weiss law type of temperature dependence of the relaxation strength. Differences in the degree of short-range order in beta phase regions have been suggested by the form of the dependence of relaxation strength upon hydrogen content. It has been found that the H(2) peak only appears significantly in alpha phase and alpha + beta phase concentration ranges.

Alpha-recoil damage in natural zirconolite and perovskite.

Zirconolite (CaZrTi2O7) and perovskite (CaTiO3) are key minerals in SYNROC, a ceramic material developed for the immobilization of high level nuclear reactor wastes. When these are incorporated in SYNROC, the long-lived radioactive actinide elements are preferentially partitioned into zirconolite and perovskite which are therefore subjected to the effects of alpha-recoil, resulting from the decay of these elements. These effects have been studied via X-ray and electron diffraction investigations of natural samples of zirconolite and perovskite of varying ages and varying uranium and thorium contents. The samples studied have received cumulative alpha doses ranging from 1.0 × 1018 to 1.1 × 1020α/g. The upper limit corresponds to the alpha irradiation which would be received by the zirconolite in SYNROC containing 10 percent of high level waste over a period of 5 × 108 years. These studies show that zirconolites remain crystalline up to and beyond alpha doses of 2 × 1019α/g. This dose would have accumulated in such a SYNROC zirconolite after a million years of storage. Electron microscopy revealed that the grains were composed of small crystalline domains which possessed the defect fluorite-type structure. After a dose exceeding that which would be received by SYNROC in 100 million years, zirconolites appeared metamict when studied by X-ray diffraction. However, the electron micrographs and diffraction patterns clearly demonstrate that the mineral continues to retain a large degree of short range order and in no way resembles a glass. The density changes produced in these zirconolites by irradiation are small and range from 0 to 3 % at saturation. Perovskite samples which have SYNROC ages up to 20, 000 years decrease in density by 1.8 ± 0.1 %. Their X-ray powder patterns are essentially unaffected. Comparative studies show that the perovskite lattice is even more resistant to the effects of alpha-recoil than the zirconolite lattice. The results demonstrate that zirconolite and perovskite are extremely resistant to the effects of nuclear radiation and will provide stable crystal structures for the containment of the radioactive waste elements during the time required for the radioactivity to decay to safe levels (typically 105-106 years).

GeSi alloys: A study of short-range order

A cluster Bethe lattice method has been employed for a study of the effects of short-range order on the electronic and optical properties of binary SiGe alloys. A realistic six-parameter tight-binding parameter Hamiltonian has been used for the study of chemically ordered and random sequences. In random alloys, one observes properties which are simply averages over the properties of the constituent atoms weighted with their concentrations. In chemically ordered alloys, the ionicity manifests itself as a dip in the valence-band electron density of states. The amount of ionicity is proportional to the strength of the dip which increases with the concentration of one constituent in minority to a maximum for a ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$ alloy. One observes an "ionic gap" of width \ensuremath{\sim} 1.2 eV in the ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$ alloy. This degree of short-range order may be measurable by photoemission experiments. The variation of the band gap with concentration is seen to be in good agreement with the experimental data available for the optical gap in crystalline SiGe alloys. We also observe a small impurity band in the band gap in both sequences except in the ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$ alloy which simulates a zinc-blende-type alloy.

Vacancy enhancement of diffusion after quenching and during irradiation in silver-zinc alloys

Quenching and annealing experiments were performed on silver-zinc alloys with 8.14 and 30 at.%Zn. From the changes of the electrical resistivity due to an increase of the degree of short-range order, the activation energy of self-diffusion was determined to be QSD=1.60 and QSD=1.38 eV for both alloys, respectively. For the migration energy of vacancies, a value EVM=0.64 eV was found for the alloy with 8.14 at.%Zn. Evidence is given that the vacancy migration energy EVM of the alloys with 30 at.%Zn is smaller than 0.60 eV in agreement with data given by Berry and Orehotsky (1968). The results of measurements of radiation enhanced diffusion obtained by a Russian and a French group, are reinterpreted. It follows that the increase of the degree of order during irradiation is obtained only by vacancy enhancement of diffusion and that the migration activation energy of self-interstitials is EIM approximately=0.46 eV and EIM approximately=0.41 eV for the alloys with 8.14 and 30 at.%Zn, respectively.

On the characteristics of amorphous UFe alloys formed by liquid quenching vs. irradiation techniques

Amorphous alloys based on a U-14.3 at. % Fe alloy have been prepared by a liquid quenching and/or irradiation technique. The resulting amorphous alloys, as characterized by x-ray diffraction and differential scanning calorimetry data, are similar but not identical. When compared to the liquid-quenched glass, the irradiated material shows a less intense, broader first peak in the interference function data and a larger, narrower exothermic peak upon crystallization. We conclude that the much higher “effective quench rates” inherent in fission fragment bombardment can produce an amorphous metal with a structure characterized by a reduced degree of short range order when compared to a liquid-quenched glass. The irradiated amorphous alloy is probably not a glass in the sense that the structure of a glass is closely related to that of the supercooled liquid which preceded it in its formation; it is more like a different amorphous solid requiring further structural characterization for a description. These results also indicate that the structure of liquid quenched glasses will be affected by subsequent fission fragment irradiation.

Hybridization, short range order, and entropy in metallic glass formation

The composition range favorable for melt quenching of metallic alloy glasses is that where a deep eutectic occurs. In the eutectic region the temperature of melt metastability can be comparable to the glass transition temperature. For alloy systems where deep eutectics occur, the melt has a high degree of short range order, and an attractive potential between unlike atoms. In the case of alloys between metalloids and transition or noble metals, both experimental and theoretical results show that the attractive potential is due to metal-d/metalloid-p hybridization. Analyzing the results of a recent experimental characterization of the temperature and composition dependence of the mixing free energy of liquid gold-silicon alloys shows that the attractive potential results in a large negative enthalpy with causes the deep eutectic, and also in a negative excess entropy which leads to a glass transition according to the ‘entropy crisis’ model.

Properties of vacancies and divacancies in copper-gold alloys

The activation energy for the increase in the degree of long-range order after quenching Cu3Au from 350 degrees C is determined to be 0.97 eV and is attributed to the migration activation energy of single vacancies E1VM. Two activation energies, 0.60 and 0-95 eV, are determined for the increase in the degree of short-range order in a copper-10 atomic per cent gold alloy after quenching from temperatures between 350 and 900 degrees C. These two activation energies are attributed to the migration activation energies of divacancies E2VM and single vacancies E1VM, respectively. The short-range order of the copper-gold alloy appears to be of the 'dispersed type'.

Magnetic dipole fields at interstitial sites in disordered binary alloys of ferromagnetic crystals

The dipole fields at interstitial sites occupied by positive particles (μ+,p) in f.c.c. and b.c.c. diluted binary alloys of ferromagnetic metals are investigated. The characteristic shapes of the distribution functions of the dipole fields is shown to depend on the interstitial site occupied by the positive particle, and they may give information on the formation of diatomic complexes of the light interstitials with substitutional impurity atoms as well as on the degree of short-range order of the alloy.

Electrical resistance of binary alloys with short-range order (the weak scattering approximation in the coherent potential approximation)

A simple version of the coherent-potential approximation is proposed for taking into account the short-range order in the first coordination sphere in a binary alloy. The residual electrical resistance is calculated in the limit of weak scattering. It is shown that the correlation contribution to the resistance depends very much on the position of the Fermi energy in the band, and that a nonmonotonic variation in the resistance is possible when short-range order develops in the alloy. The results are used for a qualitative interpretation of the different nature of the dependence of the resistance of alloys on the degree of short-range order.

Magneto-optical spectra of VI2

Magnetic circular dichroism (mcd) measurements were carried out on a singlecrystal VI2. The mcd spectra were interpreted in relation to the two magnetic structures of VI2 present at low temperatures. The temperature dependence of the intensity of the mcd band at 14 788 cm-1 indicates that the 120° magnetic structure observed above 15 K is not a fully ordered phase, but a paramagnetic phase with a high degree of short range order.

Investigation of short range ordering in polymers: 3. Studies of low molecular weight alkanes and molten polyethylene

Radial distribution function methods have been used to compare the short range ordering ( SRO) in molten polyethylene, with that in a number of n-alkanes. The SRO as monitored by the number and amplitude of intermolecular peaks increases with the number of carbon atoms in the alkane to C 16, which has a similar degree of SRO to hexatriacontane (C 36). The observed ordering decreases with increasing temperature, but the higher alkanes show less temperature dependence than the lower alkanes. For none of the measured systems do the intermolecular correlations persist over distances greater than ∼20 A ̊ , and the SRO observed in molten polyethylene is similar to that for the lower alkanes. The data does not support suggestions in the literature that the SRO should increase progressively with chain length to give a substantially parallel arrangement of chain segments in molten polyethylene over regions 50–100 Å in size.