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The purpose of this paper is to use SKPFM to characterize the gradient strengthened layer induced by the ultrasonic surface rolling process (USRP). A correlation between the dislocation, residual stresses, and SKPFM-derived Volta potential was obtained using scanning Kelvin probe force microscopy (SKPFM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM) and X-ray stress analysis. In the plastic deformation region, a localized fluctuation of Volta potential at structural interface is formed due to the decreased electron work function caused by dislocations pile-up, where the Volta potential at the interface is higher than the surrounding region. However, in the elastic deformation region, due to the slight influence of residual tensile stress, the potential value is still higher than the undeformed zone but much lower than the plastic deformation region, and the Volta potential map tends to be flat without localized potential fluctuations. [ABSTRACT FROM AUTHOR]