Fatigue-creep-environment Interactions in a Directionally-solidified Ni-base Superalloy

Directionally-solidified (DS) GTD-111 is a Ni-base superalloy designed to withstand damage occurring in the first and second stage blades of gas-powered turbines. Because of the distinctive microstructure of this material, the manner in which fatiguecreep-environment damage mechanisms interact to initiate cracks is sensitive to grain structure and chemical composition. Along with fatigue cycling, certain high temperature components such as blades are subjected to sustained dwell periods (i.e., creep) either in tension or compression. Experiments have been carried out to simulate a variety of thermal, mechanical, and environmental operating conditions endured by longitudinally (L) and transversely (T) oriented DS GTD-111. Tests in extreme environments/temperatures were needed to isolate one or at most two of the damage mechanisms. Microscopy along with strain-life data were then used to map the damage mechanisms occurring in both orientations of the as-cast, and in several cases pre-exposed material, subjected to creep-fatigue conditions.