On the Origin of Mosaicity in Directionally Solidified Ni-Base Superalloys

Abstract

Understanding the formation of secondary grains within single crystal alloys is of central importance to the aerospace industry, where components are directionally solidified in an axial thermal gradient. Post-mortem advanced microscopy characterisation techniques are employed to deduce the dendrite tip kinetics from transverse sections of a single crystal turbine blade orthogonal to the growth direction. As a result, it has been possible to highlight the role of isotherm curvature in inducing lateral macro-segregation parallel to a growing solidification front. Using crystallographic data from time-of-flight energy-resolved neutron imaging and novel Bragg-dip post processing, it is established that lateral macro-segregation induces mosaicity within single crystals, where fastest growing dendrites demonstrate greater deviation of from the growth axis. Further, dendrites are found to grow in rows defined by the [100] and [010] crystallographic directions. In light of these findings, the origins of mosaicity are discussed.