Magnetoacoustic and Barkhausen Emission in Ferromagnetic Materials

Magnetoacoustic emission (MAE) and Barkhausen emission (BE) have been studied in ferromagnetic materials placed in a magnetic field, varying at a few millihertz. Comparison of the two signals indicates the nature of the domain walls responsible for the activity at a given field strength. In order to characterize a specimen the strength of the emission around the hysteresis loop is measured. The technique has been used to measure non-destructively the effects of the following. (a) Precipitates: both MAE and BE are sensitive to the growth of precipitates in Incoloy 904, and can be used to monitor the precipitate size. (b) Disclocations: BE and MAE exhibit high sensitivity to plastic deformation, and this has been studied in α -Fe. (c) Radiation: neutron irradiation of an Fe-Cu alloy produces small changes in emission, although with sufficient sensitivity for useful characterization of radiation effects. (d) Tensile stress: MAE is sensitive to stress over the whole range, but particularly at low stresses. Such measurements lay the foundation for the use of MAE for residual-stress measurements.