On crystal mosaicity in single crystal Ni-based superalloys
P. Hallensleben, F. Scholz, P. Thome, H. J. Schaar, I. Steinbach, G. Eggeler, J. Frenzel.
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.
a.) Experimental setup of the seeded vertical Bridgman process, b.) Numerical results for the temperature field evolution during the Bridgman process with respect to varying withdrawal rates, c.) Experimentally determined liquidus isotherm for a withdrawal rate of 45 mm/h, d.) Comparison of experimentally determined and simulated liquidus isotherm for a withdrawalrate of 180mm/h