MICROMECHANICS OF THE DAMAGE-INDUCED CELLULAR MICROSTRUCTURE IN SINGLE CRYSTAL Ni-BASED SUPERALLOYS

摘要

It is known that, when single crystal Ni-based superalloys have experienced a local damage associated with a plastic deformation during in-service and post-service periods of gas turbines, an undesirable mi-crostructure or cellular microstructure may be nucleated, resulting in a detrimental degradation of mechanical properties. In this work, an analytical method to investigate the morphological evolution of the cellular microstructure is explored and proposed. The method is essentially based on the Eshelby's micro-mechanics theory, and it is extended so as to be applied for a material system containing inclusions with high volume fraction, by employing the average stress field approximation by Mori and Tanaka. The proposed method enables us to discuss a stable shape of precipitate in the material system, which must be influenced by many factors: e.g. volume fraction of precipitate; 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 stable shape maps. The application of the method to predict the γ' rafting in superalloys during creep showed that the heterogeneity of plastic strain between matrix and precipitates may play a significant role in the shape stability of the precipitate. Furthermore, it was shown that the method was successfully applied to estimate the morphology of the cellular microstructure formed in CMSX-4 single crystal Ni-based superalloy.