We use a new variant of Molecular Static method for simulation of the atomic structure near nanovoids. In our model an iterative procedure is employed, in which the atomic structure in the void vicinity and the parameter determining the displacement of atoms embedded into an elastic continuum are obtained in a self-consistent manner. Results show that the atom displacements near nanovoids are significantly different for varies crystallographic directions in bcc metals. Not long ago we have obtained an equation of vacancy diffusion under strain. Now we use this equation and the atom displacements near nanovoids to evaluate shift of the void surface for varies crystallographic directions. We find the equation for the normal component of the vacancy flux to the surface of the sphere for some crystallographic directions and then calculate shifting rate of the void surface element in mentioned directions for the different supersaturations of the vacancies as a function of temperature.
The challenges facing the creation of brazed joints of 12Kh18N10T (AISI 321) austenitic stainless steel with high unbrazing temperatures for extreme working conditions in aerospace appliances are considered in this study. An amorphous-nanocrystalline nickel-based foil, Ni-7Cr-4.5Si-3.5Fe-2.6B, wt.%, is used for brazing the steel. Experiments on brazing regimes with various temperatures (1070-1160°С) and times of exposition (15-80 min) are carried out. The formation of Ni-based solid solutions with different Cr, Fe, Mn, Si, and Ti contents in the brazed seam is detected for all brazing regimes. Using Thermo-Calc software, the liquidus temperature located in a range between 1268 and 1388°С is calculated. To verify the calculated values, the unbrazing temperature is experimentally determined for specimens, with a composition of the solid solutions, formed in the center of the brazed seam during brazing at 1160°С/15 min, 1070°С/15, and 1160°С/80 min. The experimental results deviate from values computed by Thermo-Calc by no more than 54°С. The experimental samples obtained using the 1070°C/40 min and 1100°C/ 80 min regimes demonstrate high unbrazing temperatures equal to 1303°C and > 1330°C, respectively, which corresponds to the calculations with an accuracy of 2.5%. Taking into account the combination of properties (strength and unbrazing temperature), brazing regimes of heating to 1070-1100°C and an exposure time for 15-40 min can be recommended for the production of high-strength joints with high unbrazing temperatures.
We study the influence of the atomic structure in the vicinity of voids on their growth rate anisotropy. In the first part, we model the atomic structure in the vicinity of nanovoids in α-Fe and W using the advanced Molecular Statics method. In the second part, we use the earlier obtained equations that taking the influence of elastic fields into account to calculate the shifting rate of the void surface elements, and to evaluate the components of the strain tensor we use the atomic structure modeling results from the first part. The calculations are performed for voids of several sizes at certain oversaturation’s in a wide temperature range. The simulation results for the mentioned metals with a bcc structure show that displacements of atoms located along the crystallographic directions of the <100>, <110>, <111> types in the vicinity of the voids are significantly different, and this anisotropy of atom displacements leads to a reduction of spherical symmetry for the shifting rate of the surface elements. As the result, the initially spherical void shape becomes faceted.
Dependence of specific electrical resistance on temperature (20 -1600 ∘ C) and processing method in an aluminum-graphene (up to 2wt.%) composite is investigated.It is established that spark plasma sintering (SPS) under pressure 40 MPа does not influence on electrical resistance, whereas SPS at low pressure (<10 MPa) reduces electrical resistance at a room temperature on 6 orders. Lower values of electrical resistance (up to 90 Ω *mm) received at sintering in hot pressing set at radiating heating. It is supposed that the reason of sharp decrease in electrical resistance at the lowered pressure is presence of current pulsations during SPS. They induces magnetic fields in graphene flake which lead to their moving and forming of particles to electroconductive chains or their capture in arched cells at applied pressure.
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