ACOUSTIC HARMONIC-GENERATION DUE TO THERMAL EMBRITTLEMENT OF INCONEL-718

This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. I n this particular case, the damage was caused by ''thermal embrittleme nt'' during which the material, here the nickel-based alloy Inconel 71 8, loses a significant fraction of its fracture toughness. Harmonic ge neration was the experimental method used to characterize the microstr uctural changes in the material as a function of exposure time at elev ated temperatures. Tests were performed on two heats of Inconel 718 wi th slightly different chemistries, with one heat showing particular se nsitivity of the fracture toughness to the elevated temperature exposu re with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice o f material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the ha rmonic generation as well as the punch test data. An explanation for t he changes of the harmonic generation during the embrittlement process is speculative at the present time.