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Heterogeneities in random structures and their effect on magnetic properties

Paweł Kozikowski

Abstract

The aim of the work is to quantitatively evaluate the heterogeneities in rapid-quenched soft magnetic materials and their effect on magnetic properties. Two types of rapid-quenched alloys were investigated, namely Fe and Co based amorphous and nanocrystalline Fe-Cu-Si-B alloys. The heterogeneity in amorphous alloys is understood as structural anisotropy in the amorphous phase defined as a shift in the halo top peak position in X-ray diffraction measurements, whereas in nanocrystalline alloys as a size and spatial distribution of the grains in amorphous matrix. To evaluate these heterogeneities, small and wide angle X-ray scattering has been used. The thesis was formulated in the following way: the degree of heterogeneity in amorphous and nanocrystalline soft magnetic materials has a direct effect on their properties. Samples of metallic glasses with various chemical compositions have been annealed under tensile stress below their crystallisation temperature. X-ray diffraction revealed structural anisotropy induced by annealing under tensile stress of around 300 MPa, which manifests by a shift in halo peak position. The value of this shift is considered as a measure of structural anisotropy and is directly responsible for the magnetic anisotropy. The origin of this anisotropy has been interpreted as “frozen” elastic strain remaining after tensile stress annealing. Post-annealing of samples without load (previously annealed under tensile stress) leads to homogenous structure. Nanocrystalline samples were solidified under different rates and subjected to annealing. Small angle X-ray scattering revealed that samples produces with the fastest quenching rate show the smallest grain size (several nm), whereas after annealing the relationship is inverse meaning that the samples produced with the fastest quenching rate have largest grains (40 nm in diameter). In addition sample reveal large difference in the size distribution which affects magnetic properties. As a result, the best soft magnetic properties were obtained for samples with moderate grain size (20 – 26 nm in diameter depending of the alloy chemical composition) and narrow grain size distribution. It was also revealed that the heterogeneity in the grain size is mainly caused by heterogeneous grain size through the sample thickness.
Record ID
WUTbb7b01151f684b6ebbb63575eb3d71d7
Diploma type
Doctor of Philosophy
Author
Paweł Kozikowski Paweł Kozikowski,, Undefined Affiliation
Title in Polish
xxx
Title in English
Heterogeneities in random structures and their effect on magnetic properties
Language
(en) English
Certifying Unit
Faculty of Materials Science and Engineering (FMSE)
Discipline
material sciences and engineering / (technology domain) / (technological sciences)
Status
Finished
Start date
28-09-2012
Defense Date
17-12-2014
Title date
17-12-2014
Supervisor
Internal reviewers
External reviewers
Lucjan Pająk Lucjan Pająk,, External affiliation of publication: University of Silesia
Pages
94
Keywords in English
xxx
Abstract in English
The aim of the work is to quantitatively evaluate the heterogeneities in rapid-quenched soft magnetic materials and their effect on magnetic properties. Two types of rapid-quenched alloys were investigated, namely Fe and Co based amorphous and nanocrystalline Fe-Cu-Si-B alloys. The heterogeneity in amorphous alloys is understood as structural anisotropy in the amorphous phase defined as a shift in the halo top peak position in X-ray diffraction measurements, whereas in nanocrystalline alloys as a size and spatial distribution of the grains in amorphous matrix. To evaluate these heterogeneities, small and wide angle X-ray scattering has been used. The thesis was formulated in the following way: the degree of heterogeneity in amorphous and nanocrystalline soft magnetic materials has a direct effect on their properties. Samples of metallic glasses with various chemical compositions have been annealed under tensile stress below their crystallisation temperature. X-ray diffraction revealed structural anisotropy induced by annealing under tensile stress of around 300 MPa, which manifests by a shift in halo peak position. The value of this shift is considered as a measure of structural anisotropy and is directly responsible for the magnetic anisotropy. The origin of this anisotropy has been interpreted as “frozen” elastic strain remaining after tensile stress annealing. Post-annealing of samples without load (previously annealed under tensile stress) leads to homogenous structure. Nanocrystalline samples were solidified under different rates and subjected to annealing. Small angle X-ray scattering revealed that samples produces with the fastest quenching rate show the smallest grain size (several nm), whereas after annealing the relationship is inverse meaning that the samples produced with the fastest quenching rate have largest grains (40 nm in diameter). In addition sample reveal large difference in the size distribution which affects magnetic properties. As a result, the best soft magnetic properties were obtained for samples with moderate grain size (20 – 26 nm in diameter depending of the alloy chemical composition) and narrow grain size distribution. It was also revealed that the heterogeneity in the grain size is mainly caused by heterogeneous grain size through the sample thickness.
Thesis file
  • File: 1
    Kozikowski.pdf
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Uniform Resource Identifier
https://repo.pw.edu.pl/info/phd/WUTbb7b01151f684b6ebbb63575eb3d71d7/
URN
urn:pw-repo:WUTbb7b01151f684b6ebbb63575eb3d71d7

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