In the present work, we investigate the evolution of mosaicity during seeded Bridgman processing of technical Ni-based single crystal superalloys (SXs). For this purpose, we combine solidification experiments performed at different withdrawal rates between 45 and 720 mm/h with advanced optical microscopy and quantitative image analysis. The results obtained in the present work suggest that crystal mosaicity represents an inherent feature of SXs, which is related to elementary stochastic processes which govern dendritic solidification. In SXs, mosaicity is related to two factors: inherited mosaicity of the seed crystal and dendrite deformation. Individual SXs have unique mosaicity fingerprints. Most crystals differ in this respect, even when they were produced using identical processing conditions. Small differences in the orientation spread of the seed crystals and small stochastic orientation deviations continuously accumulate during dendritic solidification. Direct evidence for dendrite bending in a seeded Bridgman growth process is provided. It was observed that continuous or sudden bending affects the growth directions of dendrites. We provide evidence which shows that some dendrites continuously bend by 1.7° over a solidification distance of 25 mm.

Curves of thermal expansion parallel and perpendicular to the polar axis are shown for poled barium titanate and lead titanate zirconate ceramics. Anistropy is remarkably higher in the latter due to greater alignment by 90° switching during poling. The anisotropy disappears after heating through the Curie point, and is also virtually eliminated within a given temperature range below the Curie point after exposure to a temperature only slightly above this range. The contraction in the polar direction upon first heating is due largely to a decrease in the alignment of domains originally reoriented by 90° during poling. The reversible expansion on repeated heating is due to a combination of domain anisotropy and a reversible increase in nonpolar alignment during heating. The pyroelectric effect is also greatly reduced in subsequent heating, although piezoelectric constants are reduced only slightly. The pyroelectric effect is found to be largely primary in both barium titanate and lead titanate zirconate ceramics. The increase in nonpolar 90° domain alignment on second heating of the lead titanate zirconate causes the primary and secondary pyroelectricity to be of opposite sign near room temperature.

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Ni-based single crystal superalloys contain microstructural regions that are separated by low-angle grain boundaries. This gives rise to the phenomenon of mosaicity. In the literature, this type of defect has been associated with the deformation of dendrites during Bridgman solidification. The present study introduces a novel serial sectioning method that allows to rationalize mosaicity on the basis of spatial dendrite growth. Optical wide-field micrographs were taken from a series of cross sections and evaluated using quantitative image analysis. This allowed to explore the growth directions of close to 2500 dendrites in a large specimen volume of approximately 450 mm3. The application of tomography in combination with the rotation vector base-line electron back-scatter diffraction method allowed to analyze how small angular differences evolve in the early stages of solidification. It was found that the microstructure consists of dendrites with individual growth directions that deviate up to ≈4° from the average growth direction of all dendrites. Generally, individual dendrite growth directions coincide with crystallographic <001> directions. The quantitative evaluation of the rich data sets obtained with the present method aims at contributing to a better understanding of elementary processes that govern competitive dendrite growth and crystal mosaicity.

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