MICROSTRUCTURE DEGRADATION OF NICKEL BASED SINGLE CRYSTAL SUPERALLOY DURING CREEP

The effect of uniaxial and multiaxial stress conditions on microstructure degradation of single crystal nickel based superalloy CMSX-4 was studied during creep. Cylindrical specimens with multi gauge sections for tensile creep at uniaxial stress conditions and "U type" of notch for creep at multiaxial stress conditions were designed by elastic finite element method (FEM) calculations. The width of the gamma' (Ni-3(Al, Ti)) rafts decreases and length of the gamma' rafts and width of the gamma channels increase with increasing applied stress and creep time. The multiaxial stress conditions affect the degradation process of the gamma/gamma' microstructure when compared to that observed at uniaxial stress conditions. The observed degraded microstructure of creep specimens tested at laboratory conditions is related to that of ex-service turbine blades. External stress of about 50 MPa resulting from centrifugal forces during blade service is determined to be critical, beyond which the role of the internal stresses in formation of the rafted microstructure can be neglected. This stress is in an agreement with the creep experiments at a constant nominal stress of 60 MPa which showed classically rafted microstructure with minor fraction of spontaneous rafts. The width of the gamma' and gamma phases increases with increasing temperature and decreasing stress. Increased width of the gamma' phase in regions with lower stresses is in agreement with the previous laboratory experiments, wherein the widest gamma' rafts were measured in specimens exposed to the lowest experimental stresses in later stages of creep.