S. Kirchhoff, K. Maciejewski, H. Ghonem, University of Rhode Island, Kingston, RI
The influence of microstructure and environment on interganular crack growth in a P/M nickel base superalloy subjected to loading cycles with hold time has been examined. Research studies have shown that the sizes and spatial distributions of the γ’ precipitates have a measurable influence on the yield strength of Nickel based superalloys. This is due to the fact that these small precipitates make the lattice planes highly resistant to slip band formation at high temperature. The role of the strengthening phases of the microstructure on crack tip damage resistance is not well understood. Furthermore, dwell loading in air environment is known to promote crack tip damage acceleration due to the diffusion of air species, particularly oxygen. The role of microstructure on crack growth rate has been examined through variations in temperature, duration and cooling rates of the heat treatment stages; solutioning, stabilization and aging. Influence of long term exposure at 650°C, a typical service temperature, has also been examined. The changes in the size and volume fraction of the secondary γ’ as well as the formation and development of carbides associated with the modified microstructures were determined through SEM observations. A series of crack growth experiments with different hold times ranging from 100 seconds up to 2 hours has been carried out at 650oC and 700 oC in both air and vacuum environment. The microstructure of the test specimens included the as received conditions as well as microstructures which have seen modified heat treatment cycles. Results of these tests were compared and used to identify the roles pertaining to microstructure variations and environment on dwell crack growth damage mechanisms.
Summary: The influence of microstructure and environment on interganular crack growth in a P/M nickel base superalloy subjected to loading cycles with hold time has been examined. Research studies have shown that the sizes and spatial distributions of the ã’ precipitates have a measurable influence on the yield strength of Nickel based superalloys. This is due to the fact that these small precipitates make the lattice planes highly resistant to slip band formation at high temperature. The role of the strengthening phases of the microstructure on crack tip damage resistance is not well understood. Furthermore, dwell loading in air environment is known to promote crack tip damage acceleration due to the diffusion of air species, particularly oxygen. The role of microstructure on crack growth rate has been examined through variations in temperature, duration and cooling rates of the heat treatment stages; solutioning, stabilization and aging. Influence of long term exposure at 650°C, a typical service temperature, has also been examined. The changes in the size and volume fraction of the secondary ã’ as well as the formation and development of carbides associated with the modified microstructures were determined through SEM observations. A series of crack growth experiments with different hold times ranging from 100 seconds up to 2 hours has been carried out at 650oC and 700 oC in both air and vacuum environment. The microstructure of the test specimens included the as received conditions as well as microstructures which have seen modified heat treatment cycles. Results of these tests were compared and used to identify the roles pertaining to microstructure variations and environment on dwell crack growth damage mechanisms.
