Effects of Robotic Hammer Peening on Structural Properties of Ni-Based Single-Crystal Superalloy: Dislocation Slip Traces and Crystallographic Reorientations

We studied the morphological and crystallographic properties of Ni-based single-crystal superalloys, addressing their surface and near-surface structural evolutions upon robotic hammer peening (RHP) by employing surface profiling, X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction. By varying the diameter (D-HH) of the spherical hammer head and the driving pressure (P-RHP), we found that the RHP process with the increased D-HH and P-RHP tends to improve the surface smoothness and, meanwhile, reduce the RHP-induced step height at the boundary between the peened and unpeened areas, especially on the surface with a deformed layer caused by wire electrical discharge machining (EDM). The surface step height can also be reduced by removing the EDM-deformed layer before the RHP process. Straight and crosshatching DSTs were observed in the transition zones surrounding the peened area, where none or minor crystal reorientations occurred. In comparison, the RHP-induced crystal reorientation is much more significant in the peened area and the reorientation direction is associated with the surface normal direction, i.e., the compression loading axis of the RHP process. The RHP-induced crystal reorientations are discussed and attributed to the non-Schmid behavior associated with multiple cross-slips of superalloys under dynamic compressive loadings.