Observations of and model for insular grains and grain clusters formed during anomalous grain growth in N18 superalloy

Abstract

Anomalous grain growth has been studied in the solid state in an N18 superalloy. An unexpected observation was the presence of small grains, either isolated or clustered, embedded in the large grains. The misorientations of the grains and clusters with respect to the large grain have been measured by electron backscattering diffraction. It is found that a majority of the isolated grains exhibit boundaries of low interfacial energy, while in the clusters the misorientations mainly correspond to high energy random boundaries. A simple wetting model in terms of interfacial energies is proposed and discussed, which explains the occurrence of the low energy insular grains. A specific mechanism for the occurrence of the large energy grains is proposed, which qualitatively explains the experimental distributions of misorientations.