Determination of cleavage planes and fracture characterization of Ni-based single crystal superalloys

The room temperature fracture behavior of the Ge Rene N-4, CMSX-2, and CMSX-4C single crystal Ni-based superalloys was studied. All crystals were grown along the (001) direction and tensile tested in both helium and hydrogen atmospheres. A stereoscopic technique developed for use with a scanning electron microscope was applied to determine cleavage planes. Planar gamma(') morphologies also were examined to help determine cleavage planes. Helium charged specimens failed on a number of planes including the (111), (110), and (320). In most cases planes of the (111)-type initiated at the notch region and became smaller and smaller as they moved in radially. Tensile strengths in helium averaged 1000 MPa higher than that of the hydrogen charged specimens. Specimens tested in hydrogen generally failed on (100)-type planes originating from the notch region. This (100) region comprised 60 to 80 percent of the total fracture surface on most samples and appeared as large flat planes perpendicular to the growth direction of the crystal. The interior regions contained (100)-type planes as well as (321), (320), (210), and (111)-types. Hydrogen charged specimens also showed a high percentage of large cracks oriented at 90 deg to one another, indicative of the (100)-type fracture. The Ge Rene N4 and the CMSX-4C samples contained 3-5 percent gamma/gamma(') eutectic, while the CMSX-2 samples had little or no gamma/gamma(') eutectic. The relationship between gamma/gamma(') eutectic and the fracture surface has not been fully determined, but it is thought that the gamma/gamma(') eutectic may serve as a possible trapping site for hydrogen.