Influence of microscopically distributed inhomogeneity and anisotropy of grains on high-temperature crack propagation properties of directionally solidified superalloy

The fluctuation of J-integral, during high-temperature fatigue crack propagation, due to the microscopic inclination of crack and elastic anisotropy of each grain, is investigated by means of a series of finite-element-analyses on a cracked body. The simulated material is a nickel-based directionally solidified (DS) superalloy, where the DS axis, load direction, and crack propagation axis are set to be perpendicular to each other. The magnitude of J is estimated using two-dimensional models simulated after an experimental test: (i) with the actual crack shape and grain arrangement, (ii) with the actual crack shape in a homogeneous body, and (iii) with a straight crack in a homogeneous body (averaged deformation behavior of the material). The microscopic inclination of crack propagation direction causes the sporadic drop of J at the point where the crack direction is largely inclined from the direction normal to the load axis. The anisotropy of grains causes the stepwise change in the a (crack length) vs. J relationship. Such changes in J due to the microscopic inhomogeneity directly relates to the change of the crack propagation rate in the transgranular cracking. Then, J, which takes into accounts the anisotropy of grains, correlates well with the crack propagation rate in the transgranular cracking. The grain-boundary cracking possesses fluctuated J, and shows weaker resistance to the propagation than the transgranular one.