Mimicking Barkhausen noise measurement by in-situ transmission electron microscopy - effect of microstructural steel features on Barkhausen noise

A relationship between microstructural steel features and an outcome of the Barkhausen noise (BN) measurement was studied. Two different microstructures, martensite and pearlite-ferrite were used. Commonly, BN is linked directly to the sample hardness. A BN outcome from both martensite and pearlite-ferrite was, however, similar even though martensite has three times higher hardness. To reveal the connection between microstructural features and BN, a typical industrial BN measurement was mimicked by in-situ transmission electron microscopy (TEM). Martensite needed higher field strength to move domain walls (DWs) than pearlite-ferrite. In martensite, DWs gathered to areas with high dislocation density. Fe₃C lamellae in pearlite were strong pinning sites. DWs perpendicular and parallel to martensite laths started to move with the same field strength value. In pearlite, DWs perpendicular to lamellae started to move before the parallel ones. The RMS envelope of ferrite-pearlite starts earlier than that of martensite due to soft ferrite. Magnetically harder pearlite probably caused “a tail” and the envelope ends almost at the same time with martensite. . Nevertheless, similar peak width values were found for both samples. Martensite and pearlite have a lot of strong pinning sites, dislocations and Fe₃C, respectively. Fe₃C density is not as high as dislocation density. Ferrite has strong pinning sites only at low incidence, but as known, huge BN information volume compared to martensite and pearlite. This resulted in the similar pulse count from martensite and ferrite-pearlite.