Magneto-acoustic and Barkhausen emission from domain-wall interactions with precipitates in Incoloy 904

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Abstract
Magneto-acoustic emission (MAE) and Barkhausen emission (BE) have been measured from single crystals and polycrystals of Incoloy 904 alloy, in which a uniform, random distribution of coherent spherical precipitates (non-magnetic inclusions) forms on isothermal ageing. The volume fraction of these inclusions remains constant but their diameter can be varied by the length of the heat treatment. BE activity and MAE activity profiles were measured as a function of the magnetizing field, and the number of Barkhausen events were counted as a function of their peak amplitude for each of the Incoloy specimens. At magnetic fields corresponding to the ‘knee’ regions of the hysteresis loops, there is a peak in both the MAE and BE activities. This is believed to be due to domain-wall creation and annihilation. At low fields the BE activity shows a peak while the MAE activity is at a minimum. This peak in BE activity is predominantly due to the thermodynamically irreversible 180° domain-wall motion and its intensity is found to be dependent on the microstructure, the activity being a maximum for an inclusion diameter of the same size as the domain-wall width. The small signal at low field in the MAE activity profiles is identified with irreversible 90° domain-wall movements, and again the intensity is dependent on the inclusion size. The variations in BE and MAE activities as a function of the ageing time are discussed in terms of the pinning of 90° and 180° domain walls by inclusions.