Fatigue crack propagation in single crystal CMSX-2 at elevated temperature

Fatigue Crack Propagation (FCP) tests were carried out on single crystal CMSX-2 as part of an investigation of the mechanisms of crack advance under conditions of fatigue loading in modern Ni-base superalloys intended for use in single crystal form. FCP tests were conducted at two temperatures, 25C and 700C, two environments, (laboratory air and high vacuum) and two crystallographic orientations. The morphology of the fracture surfaces was examined using scanning electron microscopy (SEM). Two primary modes of cracking were observed crystallographic crack propagation and gamma prime precipitate avoidance. Those areas of the crack surface associated with precipitate avoidance showed a 'wavy' morphology associated with the residual dendritic macrostructure. Increasing temperature was shown to decrease the amount of crystallographic cracking. Tests conducted at high temperature in laboratory air showed less crystallographic crack growth than those tested in vacuum. Transmission electron microscopy (TEM) was used to characterize the deformation mechanisms associated with each mode of cracking. Dislocations were found to travel in well-defined slip bands which passed through both the matrix and the precipitates in those areas associated with crystallographic crack propagation. Dislocations were found to segregate to the channels of gamma matrix between the precipitates for those regions associated with gamma prime avoidance crack morphologies.A three dimensional finite element analysis (FEA) was used to determine the stress fields ahead of the crack tips for several crack morphologies. Those regions on the crack surface which exhibited crystallographic cracking were found to have high values of the shear stress resolved in the direction of the Burgers vector and low normal stresses to the plane containing the Burgers vector. Those regions which exhibited gamma prime precipitate avoidance cracking morphologies were found to have low values of the shear stress resolved in the direction of the Burgers vector and high normal stresses to the faces of the gamma prime precipitates. The decrease in the amount of crystallographic cracking at high temperature appeared to result from the increasing hardness of the gamma prime precipitates with increasing temperature.