Barkhausen and magneto-acoustic emission from ferromagnetic materials

Barkhausen emission (B.E.) and Magneto-acoustic emission (M.A.E.) can be detected from specimens in a magnetic field varying at a few millihertz. Comparison of the two signals can indicate the nature of the domain walls responsible for the activity at any particular field. In order to characterize a specimen the strength of the emissions around the hysteresis loop are measured together with the distribution of Barkhausen event sizes. This technique has been used to measure the effects of: (A) Microstructure. Both B.E. and M.A.E. are sensitive to dislocations, and the effects of cold-working and its removal by isochronal annealing has been studied in alpha-iron. A simple model of domain wall pinning is presented which enables the dislocation density to be estimated. M.A.E. and B.E. are also sensitive to the growth of precipitates in Incoloy 904 alloy and, for a certain regime of sizes, can potentially be used to monitor the precipitate diameter. B.E. is sensitive to smaller precipitates (-100 nm) than M.A.E. but, unlike M.A.E., its dependence on precipitate size is not monotonic. An understanding of the signal dependence is obtained from Lorentz microscopy. (B) Radiation damage. The sensitivity of B.E. and M.A.E. to radiation damage is quite small by virtue of the small size of defects present. Nevertheless measurements on neutron irradiated alpha-iron specimens in several microstructural states indicate: (a) an accelerated recovery from the cold-worked condition on isochronal annealing and (b) dissolution of nitrides and carbides which formed in preparatory heat treatments. Measurements on a neutron irradiated iron-copper alloy which was subsequently isochronally annealed indicated effects which were consistent with: (a) removal of dislocation loops formed during irradiation at 550°C and (b) growth of precipitates (probably copper) at 600°C which presumably formed during the irradiation, (i.e. the effect was smaller in unirradiated control specimens). These results suggest that B.E. and M.A.E. might be useful tools for the characterization of radiation effects. (C) Tensile stress. Both B.E. and M.A.E. are sensitive to applied tensile stress and measurements on a number of different materials indicate that the dependence of M.A.E. is monotonic (except in nickel) whereas that of B.E. is generally quite complex. Since the microstructural and stress dependences are often interrelated it would be difficult to use the technique to measure say residual stress in a practical material unless the exact condition of the microstructure could be determined. Consequently B.E. and M.A.E. were measured from mild steel specimens (4360 steel) which had recieved a number of different heat treatments. The effects of applied tensile stress on the amplitude and shape of the B.E. and M.A.E. profiles were investigated with a view to be able to use the M.A.E. to measure stresses without prior knowledge of the microstructure. It was found that certain parameters in the signal profile were much more strongly dependent upon the stress than on the microstructure for many of the material conditions. Therefore M.A.E. is potentially useful for residual stress measurements.