ROLE OF MICROSTRUCTURAL STATES ON THE LEVEL OF BARKHAUSEN NOISE IN PURE IRON AND LOW CARBON IRON BINARY ALLOYS

Abstract Barkhausen noise is generated by the irreversible and discontinuous motion of Bloch walls induced by an external magnetic field or a mechanical stress. These wall motions depend on the magnetic structure of the material (magnetocrystalline anisotropy for instance) and on the microstructural flaw density and distribution within the crystal. Barkhausen noise allows to characterize the microstructural state of ferromagnetic materials The aim of this work is to study the influence of various metallurgical and crystallographic parameters on the Barkhausen noise. High purity iron is studied as a reference material and is used to prepare carbon iron binary alloys (130 and 400 ppm) Characterization tests are carried out with an alternating magnetic field to reach one state of saturation to the other in the sample. The transducer used is a coil surrounding the sample. This transducer converts the variations of magnetic flow during magnetization in an electrical voltage. The signal is then amplified and filtered to record only the high frequency components. A quantitative analysis of the signal spectrum can be performed. RMS voltage is measured and provide characteristic parameters such as maximum amplitude, peak area The influence of different microstructural parameters is studied: — A pure iron sample is strained by monotonic tensile testing from 0 to 5% during loading or after unloading. Barkhausen noise is very sensitive to plastic strain an dinternal stress state, probably within the grains —Barkhausen noise depends on the carbon content as intersticial atoms and intra or intergranular iron carbides. The noise level increases slightly with the intersticial carbon content. Various aging treatments are performed to achieve £ carbide and Fe3C intragranular precipitates densities which are characterized by transmission electron microscopy observations and thermoelectric power measures. Barkhausen noise is also studied with samples in which all the carbon is precipitated as Fe3C at the grain boundaries. The Barkhausen noise level increases with the precipitation density for e carbides as well as Fe3C. Barkhausen noise also depends on precipitates coalescence. The level decreases with the degree of coalescence in the case of Fe3C precipitates The results are analyzed in terms of interaction between Bloch walls and different kinds of obstacles like crystal defects (grain boundaries, dislocation ) and precipitates (role of interfaces, mismatch stresses between precipitates and matrix, and magnetic nature of precipitates).