The role of oxygen-grain-boundary diffusion during intercrystalline oxidation and intergranular fatigue crack propagation in alloy 718

The design of new high-efficient gas turbines is closely associated with the need to increase the service temperature of its components. Today, the applicability of the polycrystalline Ni-base superalloy 718 is limited by its susceptibility to fast intergranular cracking during low-cycle fatigue in combination with hold times at maximum tensile stress and high temperatures, typically of about 650 degrees C. Static four-point-bending tests and fatigue tests with and without hold times on cylindrical specimens in a temperature range of 650 degrees C and in various atmospheres have revealed that this kind of intergranular cracking is not due to the formation of massive oxidation products along the grain boundaries. It can rather be attributed to the mechanism of "dynamic embrittlement" at a nanoscale, i.e., diffusion of elemental oxygen into highly stressed grain boundaries ahead of a growing crack, followed by decohesion. By microstructural evaluation of the mechanical tests and thermogravinietric oxidation experiments using analytical scanning electron microscopy (SEM) in combination with automated electron back-scatter diffraction (EBSD), it became evident that only a part of the grain boundaries is prone to oxygen-induced attack. This observation gave rise to applying a grain-boundary-engineering-type treatment to the as-received alloy 718 material, resulting in an increase in the fraction of low-Sigma CSL grain boundaries (coincidence site lattice). These special boundaries seem to have a high resistance to oxygen grain boundary diffusion, resulting in a decrease in the crack-propagation rate at the lower temperature of 650 degrees C and a less-pronounced intercrystalline oxidation attack at higher temperatures.