TY - BOOK

T1 - Microstructure and magnetic properties of high-pressure torsion synthesized hard magnetic materials

AU - Weissitsch, Lukas Emanuel

N1 - no embargo

PY - 2024

Y1 - 2024

N2 - 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.

AB - 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.

KW - Hochverformung

KW - Hochdrucktorsion

KW - Gefügestrukturabstimmung

KW - nanokristallines Material

KW - Dauermagnet

KW - Selten-Erd frei

KW - hartmagnetisches Material

KW - magnetische Kopplungsmechanismen

KW - severe plastic deformation (SPD)

KW - high-pressure torsion (HPT)

KW - microstructure tuning

KW - nanocrystalline material

KW - permanent magnet

KW - rare-earth free

KW - hard magnetic material

KW - magnetic coupling mechanism

U2 - 10.34901/mul.pub.2023.177

DO - 10.34901/mul.pub.2023.177

M3 - Doctoral Thesis

ER -