Evolutions in Nonlinear Ultrasonics of Ni-Based Superalloys Due to Creep and Thermal Aging

We investigated the evolution of nonlinear acoustic characteristics, specifically the mixed-frequency response, with electromagnetic acoustic resonance (EMAR) in a nickel-based superalloy, Inconel718, under creep and thermal aging conditions. We clarified the relationship between these evolutions and the corresponding microstructural changes. EMAR is a combination of the resonant acoustic technique and a non-contact electromagnetic acoustic transducer (EMAT). We used bulk-shear-wave EMAT, which transmits and receives shear-waves propagating in the thickness direction of a plate specimen. Creep tests were conducted at 973 K, 310 MPa and 1033 K, 220 MPa, with several specimens interrupted at different time steps for analysis. The nonlinear ultrasonic properties during creep increased from the first stage to approximately 20% of the total life, after which they remained almost constant, and then increased rapidly from about 60% of the life until rupture. In contrast, the nonlinear properties of thermally aged material changed monotonically over time, with relatively small variations. These phenomena were interpreted in terms of dislocation recovery and microstructural change caused by the transformation of the precipitation-strengthened γ" phase, as supported by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) observations. This non-contact resonance EMAT measurement technique can effectively monitor the evolution of the bulk-shear-wave nonlinearity throughout the creep life, offering potential for assessing damage evolution and predicting the creep life of metallic metals.