EFFECT OF CRYSTALLOGRAPHIC PROPERTIES ON LOW CYCLE FATIGUE STRENGTH AT ELEVATED TEMPERATURE FOR NI-BASED DIRECTIONALLY SOLIDIFIED SUPERALLOY

摘要

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy subjected to transverse loading. To investigate the effect of the arrangement and crystal orientation of grains on the crack initiation, the electron back-scatter diffraction (EBSD) method was applied on the surface of the tested specimens. In addition, finite element analysis that considered the plastic behavior of crystal grains was performed to evaluate the relationship between the crack initiation and the local stress or strain that develops near the grain boundaries. The results are summarized below. As for the effect of crystallographic properties in the specimen surface, cracks generally initiated near the grain boundaries that neighbored the grain whose secondary axis inclined to loading direction by more than 20 degrees and Schmid factor to uniaxial loading was more than 0.48. The crack location was not confirmed to correlate with the grain boundary misorientation of the neighboring grains on the surface. However, most cracks initiated near the grain boundaries that neighbored the grain whose secondary axis met the loading direction at a larger angle than the grain located in the opposite surface. The results of FEM analysis revealed that the location where high cumulative equivalent plastic strain developed generally corresponded to the crack location and that the cumulative equivalent plastic strain correlated with the number of crack initiation cycles.