Microstructure and magnetic properties of high-pressure torsion synthesized hard magnetic materials

Lukas Emanuel Weissitsch

Research output: ThesisDoctoral Thesis

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Abstract

The rising public awareness of green technologies manifests itself in the increasing demand for renewable energy and electric mobility, leading to an annually raising production of permanent magnets. Performance and efficiency of generators, motors and electric components, rely on the use of high-performance permanent magnets, which contain a significant amount of cobalt or rare-earth elements. However, their production is associated with questionable working conditions and environmental problems related to mining and the availability of these materials is subject to a monopolistic situation. Supply shortages, already recognized by international institutions, represent a critical bottleneck for modern economic and social development.
This work is dedicated to the search of alternatives regarding hard-magnetic materials. Herein, the focus is on two main approaches. First, the utilization of magnetic coupling effects, namely 'exchange bias' and 'exchange coupling', to reduce the amount of critical elements. Second, the formation of a rare-earth free ferro-magnetic phase, in particular $\upalpha$-MnBi. For material processing, severe plastic deformation by high-pressure torsion is conducted, which allows for the application of a high, tunable amount of strain. Using phases with different magnetic properties, this top-down technique results in both, microstructural refinement and strongly refined composite structures, all while maintaining bulk sized samples.
In the pursuit of achieving an exchange bias, extensive microstructural processing and characterization of the Fe-Cr material system are performed. An observed co-deformation of powders limits a sufficient refinement for exchange bias due to a significant hardness increase. The deformation behavior is improved by using arc-melted Fe-Cr ingots as starting material, leading to a refined solid solution. A remarkable temperature stability is reported, hindering a decomposition into finely dispersed Fe and Cr phases. This holds true for both increased annealing temperatures and prolonged annealing times. A multi-sector 2-stage high-pressure torsion process is developed to refine the phases and simultaneously prevent intermixing of Fe and Cr. Despite promising results, no exchange bias is achieved within this material system.



SmCo$_5$ is deformed by high-pressure torsion and serves as a hard magnetic phase for an exchange coupled spring magnet. The coupling is demonstrated across a broad variation of chemical compositions. A change in the operating temperature allows switching between a coupled spring magnet and the uncoupled state. The influence of a high-pressure torsion deformation induced textured microstructure results in the formation of an anisotropic permanent magnet.
In obtaining bulk sized samples, the successful processing of the rare-earth free $\upalpha$-MnBi phase is emphasized. The high microstructural defect density induced by high-pressure torsion enhances the $\upalpha$-MnBi phase formation during a subsequent annealing procedure. A deformation at 2~GPa is preferred over 5~GPa, as well as a higher amount of applied strain. The thermal treatment is improved by applying vacuum conditions and an external magnetic field during annealing. The formed $\upalpha$-MnBi phase is studied and an influence of the sample's shear texture with respect to the applied field direction during annealing is found. This allows to tune the magnetic properties between an isotropic and anisotropic behavior.
Translated title of the contributionMikrostruktur und magnetische Eigenschaften von unter Hochdrucktorsion synthetisierten hartmagnetischen Materialien
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Mayrhofer, Paul Heinz, Assessor B (external)
  • Würschum, Roland, Co-Supervisor (external), External person
  • Pippan, Reinhard, Supervisor (internal)
  • Clemens, Helmut, Assessor A (internal)
DOIs
Publication statusPublished - 2024

Bibliographical note

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Keywords

  • severe plastic deformation (SPD)
  • high-pressure torsion (HPT)
  • microstructure tuning
  • nanocrystalline material
  • permanent magnet
  • rare-earth free
  • hard magnetic material
  • magnetic coupling mechanism

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