Electron backscattered diffraction analysis of cold work in a shot peened single crystal nickel superalloy

Shot peening has traditionally been thought to improve the fatigue performance of polycrystalline nickel-based superalloys by inducing a compressive residual stress at the surface of the material. Jet engine turbine blades, manufactured from single crystal superalloys, are routinely exposed to high temperatures. This high temperature exposure can cause relaxation of residual stress arising from shot peening, while a surface layer of cold work is retained. The cold work layer therefore appears to be responsible for improved fatigue damage resistance in such components. The work reported in this study has built up a systematic understanding of how varying shot peening intensity, coverage and shot size affects the amount and depth of cold work by examining samples of a common superalloy, CMSX-4 (R), peened under eighteen different conditions. Local misorientation analysis in electron backscattered diffraction was used to investigate cold work structure. Intensity was seen to increase cold work depth, with a diminishing gradient, in mu m/Almen, at higher intensity. Coverage increased the amount of cold work at a given depth, the severity of slip bands and the regularity of cold work. Shot size had a weak effect on cold work, slightly increasing its depth with increasing shot size, but concurrently decreasing its magnitude. In addition, the orientation of the crystal matrix relative to the sample was found to influence the resultant cold work. Samples with shot peened face normals closer to a <110> orientation displayed lower cold work than those with normals closer to <100>.