On the Influence of Cold Work on Eddy Current Characterization of Near-Surface Residual Stress in Shot-Peened Nickel-Base Superalloys

Shot-peened nickel-base superalloys exhibit 1–2% increase in apparent eddy current conductivity (AECC), which can be exploited for nondestructive residual stress assessment. Experimental evidence indicates that the excess AECC is due in part to elastic strains, i.e., residual stress, and in part to plastic strains, i.e., cold work. The very fact that the conductivity increases rather than decreases was originally thought to indicate that there was no significant cold work contribution to the observed AECC increase. This assumption was also supported by X-ray diffraction (XRD) results on fully relaxed specimens showing that the cold work induced widening of the diffraction beam only partially vanishes when both the residual stress and the AECC completely disappear due to thermal relaxation. However, we show in this paper that assuming that the conductivity change is entirely due to residual stress via the piezoresistivity of the material could result in an unacceptable overestimation of the magnitude of the compressive residual stress. Therefore, we investigated the different mechanisms through which cold work could influence the AECC in surface-treated nickel-base superalloys. It was found that neither the magnetic susceptibility nor the piezoresistivity of the material is affected significantly by cold work up to 50% plastic strain level, but the electrical conductivity does substantially increase due to microstructural changes. Based on these observations, we suggest that in future research the complex variations caused by cold work should be modeled by at least two main types of cold work parameters rather than by a single one in order to properly account for the otherwise contradictory effects of plastic deformation on eddy current conductivity and XRD measurements.