Papers by Keyword: Solid-State Diffusion

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: Bronze-processed Nb3Sn-based multifilamentary composites with external diameter of 0.8 and 0.5 mm and coupled Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy. After the two-staged annealing, 575°С, 150 h + 650°С, 200 h, commonly used for ITER conductors, a nanocrystalline layer of superconducting Nb3Sn compound is formed in every Nb filament as a result of solid-state reactive diffusion of Sn from the bronze matrix. It is demonstrated that in the wires of smaller external diameter the Nb filaments transformation into the Nb3Sn compound is more pronounced, that is the amount of the residual Nb is smaller. Besides, the nanocrystalline structure of the Nb3Sn diffusion layers is more perfect in 0.5 mm diameter wires, namely, the Nb3Sn grains are finer (their average size being 60 nm compared to 70 nm in 0.8 mm diameter wires) and are more uniform in sizes (the root mean square deviation of grain size distribution is correspondingly 15 and 17 nm).

Abstract: The analysis of physicochemical mechanism of the influence of mechanical activation (MA) of a charge mixture on the subsequent self-propagating high-temperature synthesis (SHS) of intermetallic compounds is performed. Numerical estimates have revealed an insignificant role of the energy stored in solid reactants due to cold work during MA. The characteristic time of relaxation of non-equilibrium vacancies, which were generated in metals by MA, during heating in the SHS wave is estimated, and their insignificant influence on the reaction kinetics at high temperatures is demonstrated. It is shown that a strong effect of preliminary MA on SHS can be attributed only to deformation-enhanced solid-state diffusion during MA, which can lead to the formation of a supersaturated solid solution and thus affect the conditions for nucleation of a product phase (intermetallic compound) upon heating.