Magnetic behavior of steel studied by in-situ Lorentz microscopy, magnetic force microscopy and micromagnetic simulations

Mari Honkanen, Suvi Santa-aho, Sami Kaappa, Lucio Azzari, Henri Lukinmaa, Jaakko Kajan, Samuli Savolainen, Mikko Palosaari, Lasse Laurson, Minnamari Vippola

Research output: Other conference contributionAbstractScientific

10 Downloads (Pure)

Abstract

Background
Many industrially relevant steels are ferromagnetic such as ferritic-pearlitic steel characterized in this study. Ferritic-pearlitic steels are commonly used, for example, in automotive components. Microstructural features of ferromagnetic materials influence their mechanical and micromagnetic properties. Traditionally, the microstructure, for example, cementite (Fe3C) content, has been studied with destructive methods. To save time and money, destructive methods should be replaced with non-destructive testing (NDT) techniques. One of the potential NDT methods is magnetic Barkhausen noise (MBN) testing. It is used in industry to detect for example localized microstructure and stress
variations. MBN testing is based on the motion of magnetic domain walls in ferromagnetic materials exposed to a time-varying external magnetic field. The motion of domain walls is hindered by the microstructural pinning sites such as carbides, grain boundaries, and dislocations. In the varying magnetic field, domain walls stop and finally jump over the pinning features, causing discontinuous and abrupt changes in the magnetization of the workpiece. These changes result in electromagnetic burst-like signal, i.e., Barkhausen noise, measured with an inductive coil. However, the applicability of the Barkhausen noise method is currently limited due to the stochastic nature of the phenomenon itself. Thus, more scientific knowledge is needed. Recently, we mimicked and visualized the Barkhausen noise measurement by in-situ Lorentz microscopy. This study gives general information about the behavior of domain walls in ferromagnetic steel. In addition, we have utilized magnetic force
microscopy and micromagnetic simulations to deepen our knowledge on the magnetic behavior of steels.

Methods
In this study, we used multi-instrumental and computational approach. Traditional microstructural characterization of ferritic-pearlitic steel was carried out by SEM-EBSD-TKD and (S)TEM-EDS. The
magnetic structure of the thin sample was studied by Lorentz microscopy (Fresnel mode), while the bulk sample was studied by magnetic force microscopy (MFM). The dynamics of domain walls were studied by in-situ Lorentz microscopy. A varying, external magnetic field was generated by a normal objective lens of TEM, and the images were collected in LOW MAG mode using objective mini lens.
The recorded frames of each sample were jointly post-processed as a single video using video denoising and frame alignment procedures. To measure a single point magnetic flux density generated with different excitation values of the normal objective lens inside the TEM, we used a custom-made
holder equipped with a Hall-effect sensor. We also run micromagnetic simulations to verify domain wall dynamics in certain magnitudes of magnetic fields.

Results
Our multi-instrumental characterization, dynamical in-situ Lorentz microscopy studies, and micromagnetic simulations with the complex ferritic-pearlitic structure revealed the interaction of different domain walls and pinning sites. Thus, we could visualize and verify hypotheses related to the origin of Barkhausen noise signal. Comparing Lorentz microscopy and MFM results, we indicated that thin and bulk samples studied have similar magnetic structure. So, TEM studies are also relevant from the industrial point of view, although usually bulk samples are used in industrial applications. To measure the magnetic field strength generated by the normal objective lens of TEM in dynamical in-situ studies, we built a custom-made Hall-effect sensor holder. It measures the flux density at the same location as the TEM sample. Based on the measurements, the objective lens of our TEM has almost linear response to the magnetic field strength, and when the objective lens is switched off, the
magnetic field in the sample area is close to 0 mT. Based on our studies, the carbides are very strong pinning sites for domain walls. In addition, larger
globular and thicker lamellar carbides can have their own magnetic structure. In the increasing magnetic field, domain walls in the ferritic matrix perpendicular to the lamellar cementite carbides begin to move first. Then, the domain walls inside the carbides start to disappear. Finally, domain walls parallel to the lamellar carbides move. However, some of them are very strongly pinned by carbides. When the magnetic field is decreased back to 0 mT, the domain walls appear in the opposite order. We simulated the magnetization dynamics where microstructural information is extracted from the SEM-TKD and (S)TEM results. To explain the domain wall behavior in certain magnitudes of the magnetic field as observed using in-situ Lorentz microscopy, we ran dynamical micromagnetic
simulations to reproduce the domain wall disappearance in the globular carbide. In general, the simulations supported very well the interpretation of the experimental findings, although the re-appearance of the domain walls with decreasing field could not be reproduced. In the next step, our multi-instrumental and computational approach will be extended by transport ofintensity equation (TIE) method and off-axis electron holography. These results will be reported in the near future. Imaging methods and micromagnetic simulations have their own limitations. Despite using multi-imaging techniques, it is probable that not all magnetic features can be visualized by microscopes and on the other hand, simulations cannot always reproduce all events observed experimentally. However, our combined multi-instrumental and computational approach gives novel knowledge on how iron-based carbides affect magnetic domain wall dynamics in ferromagnetic steel.

References
Honkanen, Santa-aho, Laurson, Eslahi, Foi, Vippola: Mimicking Barkhausen noise measurement by in-situ transmission electron microscopy – effect of microstructural steel features on Barkhausen noise, Acta Materialia 221 (2021) 117378.
Santa-aho, Honkanen, Kaappa, Azzari, Saren, Ullakko, Laurson, Vippola: Multi-instrumental approach to domain walls and their movement in ferromagnetic steels – Origin of Barkhausen noise studied by microscopy techniques, Materials & Design 234 (2023) 112308
Honkanen, Santa-aho, Vippola: Magnetic domain structure of ferromagnetic steels studied by Lorentz microscopy and Magnetic force microscopy, Abstract in Scandem 2023 conference, Uppsala, Sweden, 2023
Honkanen, Lukinmaa, Kaappa, Santa-aho, Kajan, Savolainen, Azzari, Laurson, Palosaari, Vippola: Magnetic domain wall dynamics studied by in-situ Lorentz microscopy with aid of custom-made Hall-effect sensor holder, submitted to Ultramicroscopy (2024).
Original languageEnglish
Publication statusPublished - 2024
Publication typeNot Eligible
EventEuropean Microscopy Congress - Copenhagen, Denmark
Duration: 25 Aug 202430 Aug 2024

Conference

ConferenceEuropean Microscopy Congress
Abbreviated titleEMC
Country/TerritoryDenmark
CityCopenhagen
Period25/08/2430/08/24

Fingerprint

Dive into the research topics of 'Magnetic behavior of steel studied by in-situ Lorentz microscopy, magnetic force microscopy and micromagnetic simulations'. Together they form a unique fingerprint.

Cite this