A review of the microstructural contributions to the acoustic nonlinearity parameter measured with longitudinal and Rayleigh wave second harmonic generation in metals

Due to their sensitivities to a specimen's microstructural evolution during processes including heat treatment and plastic deformation, ultrasonic measurements of the acoustic nonlinearity parameter (β) are useful for monitoring microstructural changes and detecting incipient damage in metals. Despite its diagnostic capabilities, the entanglement of different microstructural contributions to the cumulative ultrasonic response renders the interpretation of β a nontrivial task, particularly in the absence of first-hand microstructural characterization. Here, a review of longitudinal and Rayleigh wave second harmonic generation research connecting microstructural features of metals to β is presented, organized by experiments and models of specific features such as dislocations and precipitates. Substantial attention is directed to any available microstructural characterization as it pertains to trends in β. Connections across the literature are discussed extensively, contradictions and anomalous results are highlighted, and commentary on the future directions and challenges of modern second harmonic generation research is dispersed throughout.