微小孔を有するNi基一方向凝固超合金の高温低サイクル疲労き裂発生に対する結晶特性の影響評価

抄録

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy with small holes subjected to transverse loading. To investigate the effect of the arrangement and the orientation of crystal 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 that developed along the hole edges. The results are summarized below. It was shown that the number of cycles to crack initiation tended to be small along the hole edges located on the grain boundaries by classifying the hole edges according to the position of the hole edge and the grain boundary. In addition, it was small along the hole edges located in the grain whose secondary axis was more inclined than the grain located in the opposite surface. The results of FEM analysis revealed that high stress developed along those hole edges. The location where high stress developed correlated with the number of crack initiation cycles not only among the hole edges in one specimen but also among the specimens subjected to different nominal strain range.