Theory of Decay of Superfluid Turbulence in the Low-Temperature Limit

We review the theory of relaxational kinetics of superfluid turbulence—a tangle of quantized vortex lines—in the limit of very low temperatures when the motion of vortices is conservative. While certain important aspects of the decay kinetics depend on whether the tangle is non-structured, like the one corresponding to the Kibble-Zurek picture, or essentially polarized, like the one that emulates the Richardson-Kolmogorov regime of classical turbulence, there are common fundamental features. In both cases, there exists an asymptotic range in the wavenumber space where the energy flux is supported by the cascade of Kelvin waves (kelvons)—precessing distortions propagating along the vortex filaments. At large enough wavenumbers, the Kelvin-wave cascade is supported by three-kelvon elastic scattering. At zero temperature, the dissipative cutoff of the Kelvin-wave cascade is due to the emission of phonons, in which an elementary process converts two kelvons with almost opposite momenta into one bulk phonon. Along with the standard set of conservation laws, a crucial role in the theory of low-temperature vortex dynamics is played by the fact of integrability of the local induction approximation (LIA) controlled by the parameter Λ=ln?(λ/a ₀), with λ the characteristic kelvon wavelength and a ₀ the vortex core radius. While excluding a straightforward onset of the pure three-kelvon cascade, the integrability of LIA does not plug the cascade because of the natural availability of the kinetic channels associated with vortex line reconnections. We argue that the crossover from Richardson-Kolmogorov to the Kelvin-wave cascade is due to eventual dominance of local induction of a single line over the collective induction of polarized eddies, which causes the breakdown of classical-fluid regime and gives rise to a reconnection-driven inertial range.     

Effect of cooling rate on phosphorus segregation behavior and the corresponding precipitation of γ″ and γ′ phases in IN718 alloy

Effect of segregation behaviors of P at different cooling rates on the precipitation of γ″ and γ′ phases and the corresponding strength are investigated. The precipitation of γ″ and γ′ phases during cooling is sensitive to P concentration. With increasing the concentration of P, the amount of γ″ and γ′ particles increases after air cooling. With decreasing the cooling rate, the accelerating effect of P on the precipitation of γ″ and γ′ phases decreased first and then increased, which demonstrates the concentration of P dissolved in the grain interior decreases first and then increases. The different effects of P on γ″ and γ′ phases with different cooling rates were analyzed by the kinetic characteristic of nonequilibrium grain-boundary segregation. The characteristic of nonequilibrium grain-boundary segregation of P in superalloy is further confirmed, and the phenomenon caused by critical cooling rate is captured.     

On the nature of chemical bonding in γ-boron

The chemical bonding in γ-boron has been discussed, based on the known crystal structures of the new recently discovered phase. The unexpected polarity of some B-B bonds and a sensible electron counting scheme are presented. The relationship between the chemical bonding and the material hardness is also discussed.     

Statistics of Polymer‐Chain Conformations in the Intrinsic Coordinate System

The statistics of polymerchain conformations in the intrinsic coordinate system, the directions of whose axes are determined by the anisotropy of a polymer coil at certain instantaneous distribution of the conformations, is considered. It is shown that even for an absolutely flexible polymer chain the shape of the coil in the intrinsic coordinate system is anisotropic. Full distribution functions of the chain links in space are constructed for both the absolutely flexible linear polymer and the linear polymer chain which possesses finite flexural rigidity, and degrees of anisotropy of these macromolecules are calculated.     

Variation of the optical energy gap with γ-radiation and thickness in Bi-thin films

The effect of -radiation and thickness on the optical energy gap of Bi-thin films has been investigated by measuring their optical absorbance. The measurements were carried out on thermally evaporated films having thicknesses in the range 5–20 nm. Different -radiation doses were used ranging from 0–300 Mrad. The optical energy gap as well as the absorption coefficient were found to be -dose dependent.     

Comparison of the cathodo- and γ-luminescence spectra of scintillation crystals

The spectra of pulsed cathodoluminescence (PCL) of CsI:Tl, YAG:Ge³⁺,Tb³⁺, and LFS-3 crystals excited by high-power (～10 MW) short-time (～2 ns) action of an electron beam with an average particle energy of ～150 keV have been compared to the spectra of steady-state luminescence of the same crystals under the action of γ-photons with an energy of 511 keV emitted from a radioactive ²²Na sample. It is established that the two spectra are identical, which allows the PCL method to be used for rapid analysis of scintillation crystals.     

The Decay of a Quantized Vortex Ring and the Influence of Tracer Particles

We capture the decay of a quantized vortex ring in superfluid helium-4 by imaging particles trapped on the vortex core. The ring shrinks in time, providing direct evidence for the dissipation of energy in the superfluid. The ring with trapped particles collapses more slowly than predicted by an available theory, but the collapse rate can be predicted correctly if the trapping of the particles on the core is taken into account. We theoretically explore the conditions under which particles may be considered passive tracers of quantized vortices and estimate, in particular, that their dynamics on the large-scale is largely unaffected by the burden of trapped particles if the latter are spaced by more than ten particle diameters along the vortex core, at temperatures between 1.5 K and 2.1 K.     

On the γ' precipitates of the normal and inverse Portevin-Le Chatelier effect in a wrought Ni-base superalloy

The Portevin-Le Chatelier(PLC) effects in a wrought Ni-base superalloy with different γ' precipitates contents have been investigated. Detailed analysis on the serration type of the tensile curves indicates that the γ' precipitates have a decisive influence on the transformation from normal to inverse PLC behavior, which is rarely proposed in other works. It is considered that the γ' precipitates play the same role in PLC effect as temperature and strain rate for the investigated wrought Ni-base superalloy.     

Comparison of multiple prompt γ-ray analysis and prompt γ-ray analysis for the elemental analysis of geological and cosmochemical samples

Multiple prompt γ-ray analysis (MPGA) and conventional neutron-induced prompt γ-ray analysis (PGA) are nondestructive analytical methods for bulk chemical compositions, and their analytical capabilities were compared for elemental analyses of geological and cosmochemical samples. Detection sensitivities of PGA are often restricted by poor signal-to-noise ratios and interferences from different origins. MPGA can substantially reduce the background level, especially for hydrogenous samples, relative to PGA, which opens up a possibility to use lower energy prompt γ-rays of some trace elements. Although it is one of the major constituent elements of rock samples, Mg is hard to be determined by PGA. With MPGA, Mg contents could be determined with reasonable consistency with their corresponding recommended values in geological and cosmochemical samples by carefully selecting suitable coincident prompt γ-ray energy pairs without interference correction. MPGA was applied to a hydrogenous meteorite, Ivuna, which contains H at 2% mass level. MPGA detection limits for most of the elements studied can be reduced up to 1 order of magnitude when compared with PGA detection limits under the present experimental conditions.     

Effect of γ-irradiation on the physical and mechanical properties of chitosan powder

In this study, locally produced chitosan powder was irradiated with pre-determined doses of γ-ray (Co-60) of 10 kGy, 25 kGy, 50 kGy and 100 kGy respectively. The properties of both chitosan powder and the chitosan film were examined and compared with unradiated chitosan. Physical characteristic of the irradiated powder and film was studied using stereo microscope. It was observed that the γ-ray induces a noticeable colour tone intensity change to the chitosan. Further investigation using Fourier Transformed Infrared Spectroscopy (FT-IR) analysis has confirmed that the chain scission reaction was occurred as a result of γ-ray exposure through the depolymerization mechanisms. Interestingly, the degree of deacetylation (DD) of chitosan measured using FT-IR showed a negligible effect due to the exposure of γ-ray radiation. Further investigation on the viscosity average molecular weight (M_v) showed a reduction of M_v from 577 kD of pure chitosan to 458 kD, 242 kD, 159 kD and 106 kD for 10 kGy, 25 kGy, 50 kGy and 100 kGy of γ-radiated chitosan respectively. In addition, the tensile strength and elongation at break showed a similar decreasing trend with increasing dosage of γ-ray.     

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

The high-cycle fatigue (HCF) properties of γ-TiAl (gamma titanium aluminide) alloys are reviewed, particularly with regards to the deformation mechanisms active in the near-threshold cyclic loading regime. By examining the influence of lamellar orientation and thickness on the HCF threshold, in addition to more conventional microstructural considerations such as the grain size or the volume fraction of lamellar colonies, factors to improve the γ-TiAl microstructure for HCF are assessed. Finally, experimental methods and loading strategies are surveyed to identify techniques for improving HCF testing of γ-TiAl alloys. In this, we consider both the conservativism of differing approaches and the possibility to measure with suitable resolution the local mechanical behaviour under HCF of the lamellar γ-TiAl microstructure.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Coarsening kinetics of γ′ precipitates in Ni-Al and Ni-Al-Mo alloys

The ordered L1₂ precipitate coarsening kinetics without the influence of an external stress were studied in a Ni-Al(13.5 at%) alloy at 1413 K and a Ni-Al(13.8 at%)-Mo(5.9 at%) alloy at 1443 K. The Ni-Al-Mo alloy has a lattice mismatch of about –0.3% at the ageing temperature while the Ni-Al has a positive lattice mismatch of about 0.25% at the ageing temperature. For both alloys, the precipitates were initially cuboidal. After ageing for 3–10 min, the precipitates in the Ni-Al-Mo alloy split mostly into two parallel plates (doublets) or eight sub-cubes (octets), but the initial cuboidal precipitates in the Ni-Al alloy only showed the tendency to split into doublets. After further ageing, the precipitates in both alloys eventually aligned and agglomerated into groups consisting of many particles separated by a small distance of 30 nm, and the distribution of the precipitates became inhomogeneous. There was no linear relationship between the cube of the average precipitate size and the ageing time as predicted by the classical Lifshitz-Slyozov-Wagner theory. Instead, a retardation of the coarsening process is found.     

Automated identification and characterisation of secondary and tertiary γ′ precipitates in nickel-based superalloys

Abstract

The use of different electron loss edges in energy-filtered transmission electron microscopy (EFTEM) has allowed researchers to capture images of the morphology and size of precipitates in nickel-based superalloys. In this work, the authors discuss a computational methodology for automated detection of secondary and tertiary γ′ precipitates in EFTEM images. The optimum parameters for the automated region growing technique were identified using a combination of visual inspection and intensity information from the EFTEM images. The microstructural statistics obtained from the segmented γ′ precipitates agreed with those of the manually segmented precipitates. Then, automated segmented precipitates are used to extract microstructural information about the distributions of equivalent diameters of 656 tertiary precipitates along with the distances to the nearest secondary precipitates. The significance of this technique is its ability to automate segmentation of precipitates in a reproducible manner for acquiring microstructural statistics that relate to both processing and properties.     

On the γ′ precipitates of the normal and inverse Portevin-Le Châtelier effect in a wrought Ni-base superalloy

The Portevin-Le Châtelier (PLC) effects in a wrought Ni-base superalloy with different γ? precipitates contents have been investigated. Detailed analysis on the serration type of the tensile curves indicates that the γ? precipitates have a decisive influence on the transformation from normal to inverse PLC behavior, which is rarely proposed in other works. It is considered that the γ? precipitates play the same role in PLC effect as temperature and strain rate for the investigated wrought Ni-base superalloy.     

Effect of (n, γ) and (n, α) reactions of framework atoms on the (n, γ) recoils in Ba and Cs cations in high-silica zeolites

The effects of (n, γ) and (n, α) reactions of framework atoms on the fate of (n, γ) recoil species ¹³¹Ba²⁺ and ¹³⁴Cs⁺ from Ba²⁺ and Cs⁺ loaded in high-silica aluminosilicate and borosilicate zeolites (boron-zeotypes) have been studied. Studies were made of the recoil from “open to locked-in” and “locked-in to open” sites. In open to locked-in recoil, target ions were eluted with efficiencies of 90% where only (n, γ) processes were in operation, but in the presence of (n, γ) and (n, α) reactions, the efficiency fell to 80%. For locked-in to open site recoil, the corresponding efficiencies were in the ranges 50–60% and 40–50%. The effect of calcination temperature on the exchange of cations present was also ascertained. The high-silica zeolites used were synthesized according to published methods and were characterized by XRD, i.r., SEM, and t.g.-d.t.g./d.s.c. techniques, and all were found to be crystalline. In boron-zeotype materials, the magnitude of (n, α) damage was increased by the use of ¹⁰B isotope.     

Calculation of the γ-TiAl Lattice Resistance

Strength of Materials - The dislocation width and lattice resistance (Peierls stress) of a γ-TiAl alloy are calculated by the density ratio method. The lattice resistance is shown to decrease...     

Synthesis and characterization of γ-zirconium arsenate

The hydrothermal synthesis of a novel layered zirconium arsenate, Zr( AsO₄) (H₂AsO₄) · 2H₂O, with γ-type structure and preliminary data on its characterization by chemical and thermal analysis, X-ray diffraction (XRD) and ion exchange are reported. This XRD powder pattern is indexed on a monoclinic cell: a = 5.5699(2) Å, b = 6.8175(4) Å, c= 12.1203(6) Å and β= 103.15(1)°.