Effect of Grain Arrangement on Driving Force of Crack Propagation in High-Temperature Fatigue of Ni-Based Directionally Solidified Superalloy
Masato YAMAMOTO, Takashi OGATA and Takayuki KITAMURA
Abstract:Crack propagation test is carried out in high temperature fatigue of a Nickel-based directionally solidified (DS) superalloy, where the DS, load, and crack propagation axes are set to be perpendicular to each other. The magnitude of J-integral are estimated by the finite-element-method using 2-dimentional models; (I) with the actual crack shape and grain configuration, (ii) with the straight crack in homogeneous body, and (iii) with the actual crack shape in homogeneous body. The driving force (J-integral) of crack propagation is affected by two factors, the local crack propagation direction and anisotropy due to the grains. The former causes the sporadic drop of J-integral at the point where the crack direction is largely apart from the direction normal to the load axis. The latter causes the stepwise change in a (crack length) - J relationship which directly relates to the change of crack propagation rate in transgranular cracks. Then, the relationship between the J-integral which takes into accounts the factors and the crack propagation rate in transgranular crack shows a good correlation to a certain extent. The J-integral at the grain boundary cracking largely fluctuated and shows higher average magnitude than that in the other part. da /dN-J relation reveals that the intergranular crack has weaker resistance against propagation than that in the transgranular one, though it shows eminent fluctuation. Key Words:Directionally solidified superalloy, High-temperature fatigue, Crack propagation, J-integral, Microstructure