Nonlinear surface wave measurements on aged and laser shock surface-treated Inconel 718 superalloy

Abstract

Because of their ability to probe the material properties at different penetration depths, surface acoustic waves (SAW) are considered for nondestructively characterizing the near-surface properties of a component that has been previously modified. During long-term service, microstructural changes are unavoidable, which in many cases promote the formation of deleterious phases in a material that operates at elevated temperatures impacting the mechanical response of the component. This research investigates the behavior of the nonlinearity parameter (βR) measured using the second harmonic generation of Rayleigh surface waves propagating in the free surface of aged Inconel 718 (IN 718) superalloy specimens with and without laser shock processing (LSP). Also, microstructural and mechanical characterizations were conducted to track first the sole effect of aging time, and secondly, the combined effects of aging + LSP on microhardness, residual stress, and tensile properties of the samples. Our experimental results obtained from aged samples with and without LSP processing indicate that a superposition of different effects of aging and surface treatment in the material contribute to the βR parameter, including microstructure, compressive residual stress, and cold work effects. The experimental results also indicate that it is feasible to observe a perceivable change in the βR parameter of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The changes on the βR parameter in aged samples are due to the gradual precipitation of γ′ and γ" hardening phases and the further precipitation of δ-phase, and in aged samples with LSP, electron backscatter diffraction (EBSD) analysis showed that changes are due to microstructural, near-surface residual stresses and texture effects. These results indicate a distinctive trend that shows that the measured βR can be useful in tracking microstructural changes in this material during high-temperature operations.