Crystal Structure Effects on Surface Fractures of Impacted Superalloy Cubes

Cubes (9.5 mm on edge) of polycrystalline and unidirectionally solidified forms of a nickel-based superalloy (62.5% Ni) were impacted in vacuum at 761 m/sec and 746 m/sec, respectively, by gas-gun-accelerated 3.18 mm-thick disks of 7075-T6 aluminum. Visible fractures on the cubes were primarily associated with the impacted surfaces. The polycrystalline cube showed an irregular pattern of many open cracks; the residual compression along the direction of impact is 3.3%. The unidirectionally solidified single crystal cube (impacted on a face normal to the direction) showed only minor cracks near the center of the cube face, extending to the centers of the face edges; the residual compression is 6.2%. The results are discussed in terms of the obstacle nature of grain boundaries and local stress buildup for cracking, along with the otherwise seemingly contrary result of lower polycrystal ductility relative to that observed for the undirectionally solidified single crystal material.