Structural evolution of topologically closed packed phase in a Ni-based single crystal superalloy

This work investigates the evolution of the crystal structure of the topologically closed packed orthorhombic P phase precipitate in a Re-containing Ni-based single crystal superalloy during thermal exposure. The P phase is formed with a thin needle morphology. The precipitate is formed from the matrix with an initial complex atomic arrangement which continues to evolve during the process of thermal exposure. Based on the experimental evidence of this study and the theoretically predicted structure reported in the literature, a mechanism of the structural transformation is proposed. The initial structure is composed of a parallelogram (P) atomic arrangement configuration, which gradually evolve into a rectangle (R) atomic arrangement configuration in the [100](P) projection. In the [010] projection, the initial structure is composed of alternating rows of a larger parallelogram (P') and a larger rectangle (R') configurations, which gradually evolve into an intricate structure of P'-R' along the length [001](P) direction and P'P'-R'R' along the transverse [100](P) direction. The initial structure is formed for its structural similarities to the gamma phase to minimize lattice mismatch. The final structure is evolved over time to conform to its thermodynamically more stable state. The structural evolution is achieved by collective atomic shuffling. These intricate atomic arrangements gives rise to the very faint and highly dense parallel striation lines along the transverse [010](P) direction in [100](P) view and along the length [010](P) direction in [010](P) view under transmission electron microscopy observation. Crown Copyright (C) 2019 Published by Elsevier Ltd on behalf of Acta Materialia Inc. All rights reserved.