Papers by Keyword: Nanocrystalline Structure

Abstract: Grain structures of Nb₃Sn layers, formed by solid-state diffusion, have been analyzed using statistical methods. To determine parameters of grain structure from grain size distributions, a statistical model with combination of lognormal and standard distributions was used. Histograms of grain size distributions in Nb₃Sn layers, formed by solid-state diffusion in different composites, appeared to have only one group of crystallites after various regimes of heat treatment. It has been established that there is strong correlation between average grain sizes and the standard deviations, and this statement is also fulfilled at the grain structure evolution under additional annealing.

Abstract: In the present study, the Nb₃Sn-based multifilamentary wires with coupled Nb filaments have been investigated by SEM and TEM after various regimes of intermediate annealing including short high-temperature heat treatments and after two-staged diffusion annealing. The formation of some amount of pre-reacted Nb₃Sn layers has been revealed in all the wires studied, and their amount depends on the wire diameter, temperature and duration of the intermediate heat treatment. The structure of final diffusion layers is also affected by the regimes of these preliminary treatments. This research enables the revealing of the optimal heat treatment schedules for the formation of most perfect nanocrystalline structure of superconducting layers ensuring the highest critical current densities.

Abstract: The effect of severe plastic deformation using MaxStrain (MS) device which is a part of the Gleeble thermo-mechanical simulator of rolling and forging processes on the structure and functional properties of Ti–50.0 at.% Ni shape memory alloy has been studied. The use of the MS module allows performing SPD of the material under isothermal conditions with precise control of the deformation parameters. The deformation temperature was lowered from 370 to 330 °C. The accumulated true strain varied from e=4.6 to 9.5. Structure features were studied by the transmission electron microscopy. The maximum completely recoverable strain was determined by a thermomechanical method using a bending mode for strain inducing. A mixed submicrocrystalline and nanosubgrained structure with average grain/subgrain size below 100 nm was formed using SPD at 330 °C. A very high completely reсoverable strain (9.3%) was obtained against a reference treatment (2.5%).

Abstract: The formation of nanocrystalline structures and mechanical properties were studied in a nitrogen-bearing 304-type stainless steel subjected to severe plastic deformation (SPD). The steel samples were processed at ambient temperature using three different methods, i.e., caliber rolling, multidirectional forging and high pressure torsion. All these techniques resulted in pronounced grain refinement. The microstructures consisting of austenite/ferrite crystallites with transverse dimensions of 50 and 30 nm evolved in the rolled and forged samples, respectively. The austenite fractions comprised approximately 0.4. In contrast, the microstructure consisted mainly of austenite with an average grain size of about 25 nm evolved after high pressure torsion. All samples of the stainless steel subjected to severe plastic deformation demonstrated significant strengthening. The ultimate tensile strengths of 2065 MPa and 1950 MPa, were obtained after rolling and high pressure torsion, respectively. The ultimate tensile strength of samples subjected to multidirectional forging was 1540 MPa.

Abstract: (α + γ) two phase stainless steel (Fe-21%Cr-4.8%Ni-1.5%Mo) powder was processed by high pressure torsion (HPT) and consolidation at room temperature. The received powder had fully α single phase due to the rapid cooling during gas atomizing process. Specimens after HPT process were heat treated at 1173K for 3.6ks. It was revealed that the decomposition of α phase to γ took place during the heat treatment. Detailed microstructure observation showed that an equiaxed (α + γ) micro-duplex structure was developed and its average grain size was approximately 3.2 micrometers. The same heat treatment given to the material without HPT resulted in a coarse two phase microstructure.Therefore, it is considered that an ultra fine grained microstructure was caused by increasing of nucleation sites for γ phase due to severe plastic deformation (SPD) of HPT process. Electron backscatter diffraction patterns (EBSD) analysis indicated that α phase has a {110}/ND strong texture, that is, the α phase seems to have single orientated coarse grain structure. The γ precipitates indicated a {111}/ND strong texture, and the crystallographic orientation relationship of Kurdjumov-Sachs was observed.

Abstract: The structure and physical properties of the Fe_72.5Cu₁Nb₂Mo_1.5Si₁₄B₉ nanocrystalline alloy have been studied both in terms of dynamics, using thermomagnetic analysis and statics, using specimens subjected to a complete course of heat treatment at the specified annealing temperature. In the course of nanocrystallization, there was a peak detected on the curve of permeability, that peak appeared several minutes later than the heat production peak. The permeability peak occurrence can be related to the formation of a sufficiently large amount of the crystalline ferromagnetic phase α-FeSi followed by saturating it with silicon due to diffusion.

Abstract: The structure and morphology of Nb₃Sn layers in superconducting Nb/Cu-Sn composites with ring (tubular) Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy after various regimes of diffusion annealing. It is demonstrated that the tubular geometry of Nb filaments, in which Sn diffuses from the bronze matrix both from inside and outside, ensures practically complete transformation of Nb into the superconducting Nb₃Sn phase. Besides, at certain regimes of the diffusion annealing this geometry enables the improvement of the superconducting layers morphology compared to that of wires with continuous filaments, namely, to obtain wide Nb₃Sn layers with fine equiaxed grains and to avoid the formation of columnar grains, which promotes enhanced current-carrying capacities of the wires.

Abstract: This work presents the results of hydrogen diffusion study in macro-and nanocrystalline titanium using the method of combining an electrolytic cell with vacuum chamber through the membrane. It has been stated that the formation of nanocrystalline structure, on the one hand, leads to the decrease in the effective hydrogen diffusion coefficient and, on the other hand, to the increase in the capability to accumulate hydrogen. This is observed due to more developed grain boundaries in nanocrystalline samples as compared with macrocrystalline.

Abstract: Technology for manufacturing products by magnetic pulse compaction from oxide powders of the (ZrO₂ – Y₂O₃) – Al₂O₃ system is presented in the paper. Diagram of the magnetic-pulse press with its operating principle being based on Ampere's law is given. Physical and mechanical properties of the obtained compacts are determined. The main feature of the designed technology is the reduced sintering temperature (200 °С) and the acquired fine-grained structure of the products. Another significant advantage achieved by applying the technology is the possibility for manufacturing fine-grained structure ceramic products with high mechanical properties.

Abstract: The article is devoted to the investigation of alumina-zirconia ceramics properties depending on its production technology. Analysis of the technological factors that allow acquiring fine-grained structure as well as high physical and mechanical properties of alumina-zirconia ceramics is presented in the paper. It is shown that when using magnetic pulse compaction due to the pulse impact and adiabaticity of the process the most compact structure is formed. It is possible to obtain pressings with the density of up to 68% of the theoretical value. The obtained compact structure secures high physical, mechanical, and operating characteristics of the finished product.