Micromechanics Approach to the Morphology of Cellular Microstructure in Single Crystal Ni-Based Superalloy
Motoki SAKAGUCHI and Masakazu OKAZAKI
Abstract:A cellular microstructure may be nucleated in single crystal Ni-based superalloys when they have experienced a local damage associated with a plastic deformation during their in-service and post-service periods of gas turbine. In this work, an analytical method to investigate the morphological evolution in the cellular microstructure is proposed. The method is essentially based on the Eshelby's micromechanics theory, and it has been extended so as to be applied for material system containing high volume fraction inclusions, by employing the average stress field approximation by Mori and Tanaka. This method enables us to estimate a stable shape of precipitate in the material system, which must be influenced by many factors: e.g. precipitate volume fraction; Young's modulus ratio and lattice misfit between matrix and precipitate; external stress field in multi-axial state; and heterogeneity of plastic strain between matrix and precipitate. A series of numerical calculations were summarized on the stable shape maps. The application of the method to predict the ƒÁ' rafting in superalloys during creep showed that the proposed method supplied reasonable explanations to all the experimental observations, and that the heterogeneity of plastic strain between matrix and precipitates may play a significant role in the shape stability of the precipitate. Furthermore, the method could successfully estimate the morphology in the cellular microstructure formed around the spherical indentation in CMSX-4 single crystal Ni-based superalloy. The present method will be a useful tool for failure analysis of superalloy components and for an estimation of in-service stress state. Key Words:Single crystal Ni-based superalloy, Cellular microstructure, Micromechanics, Average stress field approximation, Stable shape of precipitate, Heterogeneity of plastic strain, Rafting