On the origin of mosaicity in directionally solidified Ni-base superalloys

Strickland, Joel; Nenchev, Bogdan; Tassenberg, Karl; Perry, Sam; Sheppard, Gareth; Dong, Hongbiao; Zhang, Ruiyao; Burca, Genoveva and D'Souza, Neil (2021). On the origin of mosaicity in directionally solidified Ni-base superalloys. Acta Materialia, article no. 117180.

DOI: https://doi.org/10.1016/j.actamat.2021.117180

Abstract

This article addresses the formation of low angle grain boundaries which give rise to mosaicity, a phenomenon which has only recently received attention in the single crystal Ni-base superalloy field. In this work, post-mortem advanced microscopy characterisation techniques are employed to deduce the dendrite tip growth kinetics from transverse sections of a single crystal turbine blade. 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 small angle grain boundaries which gives rise to mosaicity within single crystal Ni-base superalloys. Mosaicity demonstrates good correlation the local primary spacing, where faster growing dendrites demonstrate greater deviation of <001> from the growth axis, as compared with those growing at a slower rate. In light of these findings, the origin of mosaicity and its implication to secondary grain formation is discussed.

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