Swept frequency eddy current (SFEC) measurements of Inconel 718 as a function of microstructure and residual stress

The goal of this thesis was to determine the dependency of swept frequency eddy current (SFEC) measurements on the microstructure of the Ni-based alloy, Inconel 718 as a function of heat treatment and shot peening. This involved extensive characterization of the sample using SEM and TEM coupled with measurements and analysis of the eddy current response of the various sample conditions using SFEC data. Specific objectives included determining the eddy current response at varying depths within the sample, and this was accomplished by taking SFEC measurements in frequencies ranging from 100 kHz to 50 MHz. Conductivity profile fitting of the resulting SFEC signals was obtained by considering influencing factors (such as surface damage). The problems associated with surface roughness and near surface damage produced by shot peening were overcome by using an inversion model. Differences in signal were seen as a result of precipitation produced by heat treatment and by residual stresses induced due to the shot peening. Hardness of the material, which is related both to precipitation and shot peening, was seen to correlate with the measured SFEC signal. Surface stress measurement was carried out using XRD giving stress in the near surface regions, but not included in the calculations due to shallow depth information provided by the technique compared to SFEC. By comparing theoretical SFEC signal computed using the microstructural values (precipitate fraction) and experimental SFEC data, dependency of the SFEC signals on microstructure and residual stress was obtained