Directionally solidified Ni-base superalloy is used for gas turbine blades for high efficiency thermal power plant. Since gas turbine blades are subject to high temperature creep condition due to the high speed rotation of rotor, it is important to evaluate the creep strength. There are many studies using the smooth specimen for directionally solidified Ni-base superalloy. However, these are not so many researches which concern the mechanical behavior of a notched specimen. Therefore, the researches of creep damage formation and the crack growth behavior around a notch tip have not yet been clarified. Recently, electron backscatter diffraction (EBSD) method has been conducted to evaluate the creep damage for Ni-base superalloy. However, most of the studies also use the smooth specimen. When the materials are practically used for the component of structures, creep damage or crack may be originated at the site of stress concentration of equipment such as cooling holes. Therefore, in order to evaluate the creep damage formation and the crack growth behavior, it is important to conduct the research using notched specimens from the view point of application to actual components.
In this study, creep damage formation and crack growth behavior of a notched specimen for directionally solidified Ni-base superalloy CM247LC under high temperature creep condition were investigated by conducting experiment and mechanical analysis. The interrupted observational test of creep crack growth was conducted to investigate the damage formation and the crack growth behavior around notches. In addition, the In-situ observation and the metallographical investigations were conducted for creep damaged specimens using SEM / EBSD analysis. Furthermore, in order to clarify the mechanism of creep damage formation behavior, the designed two-dimensional elastic-plastic creep finite element analysis was conducted for the model with various distributed grains obtained by EBSD analysis. And this analytical results were compared with experimental results.
As a result, the micro creep crack around a notch tip was found to be caused by accumulation of micro damage and voids. In addition, macro cracks were found to initiate just before final unstable fracture.
However, it is necessary to take into account for the variety of mechanical properties of each crystal orientation, the designed two-dimensional elastic-plastic creep finite element analysis was found to well represent the creep damage formation observed in experiments.