Small Creep Crack Growth Interacting with Microstructural and Mechanical Factors in a Polycrystalline Ni-Based Superalloy

Abstract

Behavior of small crack propagation during creep was studied, in comparison with that of physically long crack propagation behavior, based on non-linear fracture mechanics approach. Through the work special attention is paid to understand: (1) microstructural aspect on small creep crack propagation interacting with grain boundaries and dendrite boundaries, (2) difference between small and macroscopic long creep cracks, (3) role(s) of stress concentration and relaxation with creep time in the crack propagation behavior emanating from a notch introduced as a simulation of cooling holes in blades and vane applications. From the experimental results, a specific crack growth behavior was found at the beginning of early crack growth stage (small crack), where the crack growth rate decreased with increasing Jc,st under both the isothermal creep (ITC) and thermal gradient creep (TGC). Also, crack under the TGC showed different crack growth rate than that under the ITC condition, where the higher and lower rates were achieved at lower and higher temperature side, respectively, under a given value of Jc,st.