SUMMARY

Nonlinear Ultrasonic (NLU) inspection techniques show promise for monitoring crystallographic defects in materials that are too small to be detected with traditional nondestructive inspection techniques. A necessary next step for NLU to be used for practical inspections is developing a physics-based correlation between the microscopic damage and the acoustic nonlinearity measured in NLU inspections. Individual contributors to acoustic nonlinearity (precipitation, dislocations, etc.) must be isolated and quantified to develop these models.
Second-phase precipitates in metals are critical crystallographic discontinuities that affect the performance of alloy systems. This research focuses on modeling the change in the classical acoustic nonlinearity parameter, β, due to precipitation in four isothermally heat-treated iron-based alloys (304L, 316L, 9%Cr, Fe-1%Cu). Calculated Phase Diagram (CALPHAD) software is used to predict precipitation in these alloys with an independent confirmation of the presence and density of these precipitates using electron microscopy techniques. A model is developed in which misfit dislocations formed at the interface of the matrix and grain boundary precipitates are the main cause of harmonic generation in the material. The integration of both the CALPHAD modeling and the physics-based models of harmonic generation will be used to create a predictive model of the change in β due to precipitation in steels.

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