This paper is a study of sintering mechanisms, structure, and mechanical properties of ultrafine-grained 95W-Ni-Fe tungsten heavy alloys. Powder particle sizes were controlled by mechanical ...activation (MA) of original coarse-grained components and by addition of ultrafine particles. W-Ni-Fe alloys were obtained by sintering in hydrogen and Spark Plasma Sintering (SPS) in a vacuum. The dependence of ultrafine-grained (UFG) alloy density on sintering temperatures has been found to be non-monotonic with a maximum corresponding to the optimal sintering temperature. It has been demonstrated that the sintering activation energy of UFG alloys is significantly lower than that of coarse-grained alloys. It has been shown that the optimal SPS temperature for mechanically activated nanopowders goes down by 350–400 °C in comparison with the optimal sintering temperature in hydrogen for coarse-grained 95W-Ni-Fe powder composition. The reason for a lower optimal sintering temperature lies in a decreased activation energy of grain-boundary diffusion and formation of a non-equilibrium solid solution of nickel and iron in the surface layer of tungsten α-W particles during high-energy MA. High-energy MA and SPS were used to obtain samples of UFG tungsten alloys with high mechanical properties: macro-elastic limit – up to 2250 MPa, yield stress – up to 2500 MPa.
•Obtained ultrafine-grained tungsten alloys with high macroelastic limit and yield stress.•Dependence between density of tungsten alloys and sintering temperatures is non-monotonic.•Sintering activation energy of nanopowders is lower than that of coarse-grained powders.•Optimal sintering temperature of nanopowders is lower than that of coarse-grained alloys.•SPS activation energy is lower compared to sintering of powders in hydrogen.
The strongest event of geomagnetically induced currents (GIC) detected by the North‐West Russian GIC network occurred during the main phase of the magnetic storm on 28 and 29 June 2013. Extremely ...high value, 120 A, was recorded in the 330 kV transformers on Kola Peninsula in the 04–07 magnetic local time (MLT) sector. The Defense Meteorological Satellite Program (DMSP) spacecraft took a sequence of ultraviolet (UV) auroral images in the southern hemisphere and observed multiple omega bands. The ionospheric equivalent electric currents based on the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network reveal a sequence of current vortex pairs moving eastward with the speed of 0.5–2.5 km/s that fits to the electrodynamics scheme of omega bands. Although the temporal variations of the associated current system are slow, the omega bands can be responsible for strong magnetic variations and GIC due to fast propagations of currents in the azimuthal direction.
Key Points
Auroral omega bands related to large GIC
Eastward drifting ionospheric current structures produce large GIC
Ionospheric current motion is as important as its intensity variation
•Width modes and shape anisotropy are essential for finite width magnetic waveguides.•Width modes help to bend the signal transmission direction in cross-like structures.•Shape anisotropy provides ...the transmission in the crosses in a wide frequency range.•The inhomogeneity of the waveguide width can filter width modes.•Using a resonator as a waveguides junction can help for effective signal transfer.
Microwaveguides for spin waves based on yttrium iron garnet films and cross-like structures built on such waveguides are experimentally studied. Effects of the waveguides geometry, location of the microantennas for excitation and detection of the spin waves, and geometry of the waveguides junctions on the spin wave excitation and propagation efficiency are discussed.
We demonstrate laser-induced nonreciprocity of spin waves in the ferromagnetic-semiconductor structure. Surface spin waves in yttrium iron garnet film grown at the top of n-type gallium arsenide ...substrate were studied by means of Brillouin light-scattering spectroscopy. It is shown that spin-wave dispersion can be modified in a controlled manner by laser radiation. We observe the difference of up to 225 MHz when comparing the frequencies of counterpropagating spin waves. We attribute this frequency shift to the mutual influence of nonreciprocal spin-wave modal profiles and differences in magnetic anisotropies at two film surfaces as the result of laser-induced conductivity variation in GaAs substrate. We propose a simple model based on analytical dipole theory to describe the induced spin-wave nonreciprocity. Our results show the possibility of integration of magnonics and semiconductor electronics on the base of YIG/GaAs structures.
We consider the problem of three-dimensional motion of a passively gravitating point in the potential created by a homogeneous thin fixed ring and a massive point located in the center of the ring. ...The motion of a passively gravitating point admits two first integrals. We first consider the integrable case of an invariant motion in the equatorial plane and then consider the general case of three-dimensional motion, where we classify the possible trajectories of a point depending on the values of the first integrals. Finally, some previous results for similar problems are compared.
This paper provides an overview of Spark Plasma Sintering (SPS), which is an advanced technology for high-speed sintering of powder materials by applying mechanical pressure to the powder compact and ...heating it using a pulsed direct current. Some examples of the successful application of the SPS technology in producing materials with high strength characteristics are shown. By optimizing the SPS regimes, the hardness of pure WC ceramics Hv=30÷31GPa (20% higher than that of conventional materials), hardness of pure Al2O3 ceramics Hv=23.5GPa (50% higher than that of conventional materials) and limit strength of the heavy alloy W–Ni–Fe σT=2500MPa (2 times higher than that of conventional materials) are achievable.
•Spark Plasma Sintering (SPS) is an advanced technology for high-speed sintering of powder.•By optimizing the SPS regimes, the hardness of pure WC ceramics increases by 20%•Hardness of pure Al2O3 ceramics turns 50% higher than that of usual material.•Limit strength of the heavy alloy W–Ni–Fe achieved in SPS is 2 times higher than for regular material.
The processes of precipitation of particles of Al
3
X (X = Zr, Yb, Er, Hf) in cast conductive aluminum alloys have been studied, including those alloyed with Mg and Si. The alloys have been produced ...using the induction casting technique. To study the precipitation kinetics of particles, methods for measuring specific electrical resistivity (SER) and microhardness have been used. It has been shown that the studied alloys can be divided into three groups. Group I contains the alloys in which an increase in the annealing temperature results in a decreases in SER due to the precipitation of particles. Group II contains the alloys in which the precipitation of particles occurs during the crystallization of an ingot. The SER value of these alloys is close to that of aluminum. The SER value of the alloys of Group III remains almost constant upon annealing and is 3.0–3.4 μΩ cm, which indicates the high stability of the solid solution. The Johnson–Mehl–Avrami–Kolmogorov equation is used to study the precipitation kinetics of particles in alloys of Group I. It has been established that the activation energy of the precipitation of particles in alloys of Group I is close to the activation energy of bulk diffusion, while the values of the decomposition rate coefficient (
n
= 0.5–0.8) in the Johnson–Mehl–Avrami–Kolmogorov equation are lower than the theoretical value
n
= 1.5 typical of the precipitation of particles in the bulk of grains. The observed contradiction is associated with the presence of large primary or eutectic Al
3
X particles in the structure of alloys. It has been shown that the Al–0.25% Zr–0.25% Er–0.15% Si alloy exhibits an appropriate set of properties; after annealing their characteristics meet the requirements for the designed alloys: SER is less than 2.95 μΩ cm and the microhardness is ~550 MPa.
Thomson scattering of the core edge and divertor plasma regions of a tokamak with reactor technologies is discussed. The rationale and choice of technical solutions are given, the composition of the ...Thomson scattering diagnostic complex is discussed, as well as an estimate of the accuracy of measuring both electron temperature and density. Particular attention is paid to ensuring the functionality of the proposed diagnostics in the reactor mode of the tokamak operation and the results of testing diagnostic equipment in the experiments on Globus-M2 tokamak.
A study is performed of geomagnetic pulsations with periods from unity to tens of minutes and associated geomagnetically induced currents. The relationship is considered between the efficiency of ...current excitation by geomagnetic pulsations and the parameters of the interplanetary magnetic field and solar wind plasma at different delays. Data from measuring the geomagnetic field and geomagnetically induced currents in the Russian North and Finland are used for analysis. It is shown that the efficiency of excitation of geomagnetically induced currents by pulsations is higher if the solar wind speed does not fall notably below 500 km s
−1
for several hours.
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