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.
A new model is proposed that describes the effect of impurities on the primary recrystallization in pure metals. In this model, the change in the temperature for the onset of recrystallization and ...the activation energy of the process is determined by the change in the relative free volume of the boundaries, which, in turn, is due to the presence of impurities at the boundaries. An equation is obtained that relates the recrystallization activation energy to the impurity concentration. It is shown that the recrystallization activation energy depends exponentially on the impurity concentration in grain boundaries.
A new model that describes primary recrystallization in metals has been proposed. This model determines the rate of recrystallization nucleus growth in terms of the kinetics of the decreasing power ...of disclination dipoles distributed within nucleus boundaries. The equation relating the volume fraction of recrystallized material to the temperature and annealing time has been derived. This equation has the form of the Avrami equation at
n
= 1 and an activation energy of diffusion along nonequilibrium grain boundaries.
The engineering part of the GLOBSYS code is presented, and the parameters of the Globus-3 facility, which is a development of the Globus program, are analyzed. The facility is primarily designed to ...provide a long pulse, a large toroidal magnetic field and strong heating. The concepts of searching for Globus-3 parameters under physical and engineering limitations are described. Obviously that reliable confinement and a large part of noninductive current are necessary to ensure existence of a plasma for a long time. Engineering constraints are involved in the choice of parameters in a more complex way: in some cases, it is overheating of the coils, in other cases, it is the total power supply, or the limit on the flux provided by the ohmic solenoid, or the strength of the constructions. The parameters of the Globus-3 spherical tokamak were preliminarily selected for the cases of a “warm” copper EMS (Electromagnetic system) and the EMS precooled to liquid nitrogen temperature. The exceeding of the duration of the plasma current plateau Δ
t
plateau
over the characteristic settling time of the plasma profiles τ
L
/
R
was chosen as the key condition. At values of the toroidal magnetic field
B
t
0
= 3 T, the condition Δ
t
plateau
> τ
L
/
R
cannot be attained even for precooled EMS. At
B
t
0
= 2 T, only options with precooled EMS can be considered acceptable, but the facility dimensions are fairly large. For the field
B
t
0
= 1.5 T, the options with “warm” EMS correspond to the duration of the plasma current plateau ~3 s (Δ
t
plateau
/τ
L
/
R
~ 1–1.5). In the case of precooled EMS, the duration of the plateau can increase to 12–13 s (Δ
t
plateau
/τ
L
/
R
~ 5). In the latter case, as a basis for further development of the Globus-3 facility, options with the following geometric dimensions are reasonable:
R
0
~ 0.6–0.7 m,
a
~ 0.35–0.4 m,
А
≤ 1.7–1.8,
k
95
~ 1.7–1.8. The minimum allowable value of the plasma current
under the condition of effective absorption of the input power of neutral injection has been calculated. In the Globus-3 facility,
I
p
≈ 0.8 MA was chosen as the base value.
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
The effect of grain boundary retardation by annealing twins in pure copper has been experimentally and theoretically investigated. A model that describes the influence of annealing twins on the ...migration of grain boundaries in pure metals has been proposed. The retarding force from the twins has been demonstrated to be analogous to the Zener retarding force created by incoherent fine particles. An equation has been derived to calculate the retarding force induced by annealing twins. The force has been demonstrated to be inversely proportional to the size of the twins and directly proportional to their volume fraction. The simulation results have been compared with the experimental results. A satisfactory correlation between the theoretical and experimental results has been achieved.
An array of ten built-in divertor probes was installed on the lower dome of the Globus-M spherical tokamak. It was used to measure the profiles of the floating potential, electron temperature, ion ...saturation current, and heat flux density to the divertor. It is demonstrated that there is a sharp change in the floating potentials of the probes located near the outer separatrix leg at the development of the peripheral instability.
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.
The paper dwells on the research conducted into high-rate consolidation of pure tungsten carbide (WC) nanopowders using the Spark Plasma Sintering technology. Studies included the effect that the ...original size of WC nanoparticles and their preparation modes have on density, structure parameters, and mechanical properties of tungsten carbide. Samples of high-density nanostructured tungsten carbide characterized by high hardness (up to 31–34 GPa) and improved fracture toughness (4.3–5.2 MPa m1/2) were obtained. It has been found that materials that show abnormal grain growth during sintering have lower values of sintering activation energy as compared to materials the structure of which is more stable during high-rate heating. A qualitative model is proposed that explains this effect through the dependence of the grain boundary diffusion coefficient on the grain boundary migration rate.
•Samples of nanopowders tungsten carbide were made using DC arc plasma synthesis.•Samples of ceramic WC with high mechanical properties were made using the SPS.•Abnormal grain growth while SPS tungsten carbide stems from α-W and W2C particles.•Under abnormal grain growth, the values of the SPS activation energy are low.
The results of measuring the electron temperature and density spatial distributions in plasma of the Globus-M2 tokamak using the Thomson scattering diagnostics are presented. The diagnostics provides ...measurements throughout the entire tokamak discharge, starting from time of gas breakdown. The Thomson scattering data were analyzed in order to determine the positions of the last closed flux surface, the plasma magnetic axis, and the radius of inversion during the saw-tooth oscillations. The results of measurements performed during the internal reconnection of magnetic field lines are presents, as well as the dynamics of spatial distributions of electron temperature, density and pressure during the plasma transition to the H-mode. The results of measuring the electron temperature distribution in the scrape-off layer using the Thomson scattering diagnostics are also presented for distances up to 4 cm outside the last closed flux surface.
