Air-coupled detection of Rayleigh surface waves to assess material nonlinearity due to precipitation in alloy steel
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Nonlinear ultrasonic waves have demonstrated high sensitivities to various microstructural changes in metal including coherent precipitates; these precipitates introduce a strain field in the lattice structure. The thermal aging of certain alloy steels leads to the formation of coherent precipitates, which pin dislocations and contribute to the generation of a higher harmonics in an initially monochromatic wave. The objective of this research is to develop a robust technique to perform nonlinear Rayleigh wave measurements in metals using a non-contact receiving transducer. In addition a discussion about the data processing based on the two-dimensional diffraction and attenuation model is provided in order to calculate the relative nonlinearity parameter. A precipitate hardenable material, 17-4 PH stainless steel, is used to obtain different precipitation stages by thermal treatment and the influence of precipitates on the ultrasonic nonlinearity is assessed. Conclusions about the microstrucutural changes in the material are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements of thermo-electric power, mircohardness and ultrasonic velocity. The results show that the nonlinearity parameter is sensitive to coherent precipitates in the material and moreover that precipitation characteristics can be characterized based on the obtained experimental data.