Abnormal grain growth in YBa2Cu3O7−x high-Tc superconducting ceramics during high-temperature deformation

Abstract

Bimodal microstructure is formed in high-T c superconducting ceramics YBa 2 Cu 3 O 7−x deformed by torsion under pressure at T d = 1008°C. The appearance of second peak on the grain length distribution is associated with formation of abnormally coarse grains with length up to l = 300-500 μm. Cooling rate has almost no effect on the density of abnormal grains n and l, indicating that such grains are formed during deformation. The l value increases monotonically with increasing twist angle α, while n increases only up to α = 25°, and then drops. The value of α above which n decreases sharply corresponds to the formation of strong texture with a factor F ≥ 0.95. Formation of abnormal grains can be explained by strain-enhanced grain growth during deformation. Intergranular sliding produces damage zones, the healing of which takes place due to the local migration of grain boundaries. Apparently, abnormal grains appear in places of most intense intergranular sliding. As the texture increases the degree of freedom of movement of grains decreases, so the possibility of accommodation of sliding of neighboring grains decreases. This leads to increase in the number of damage zones and strain value in them that is accompanied by the growth of n and l. After it reaches a certain critical level of texture (F ≈ 0.95 at α ≥ 25°) the possibility of stress relaxation due to local migration of initial grains is exhausted, and dynamic recrystallization of abnormal grains begins. Therefore, when α ≥ 25° the n value is decreased sharply.

The Fabrication and Characterization of BaTiO3 Piezoceramics Using SLA 3D Printing at 465 nm Wavelength

“…However, the microstructure of the 3D-printed ceramics indicated that TiO 2 excess in the initial BT powder was necessary but not sufficient for AG growth (Figure 10c). Local strains occurring on green body pressing could be assumed as a supplementary factor for the formation of AG [29][30][31]. In a case of 3D printing, the powder particles underwent a slow packing process during the debinding, without any external mechanical pressure.…”