Designing strong and ductile soft-magnetic multicomponent alloys
Han, Liuliu; Raabe, Dierk (Thesis advisor); Gutfleisch, Oliver (Thesis advisor)
Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
Multicomponent alloy (MCAs) with almost unlimited composition space that places no boundaries on the magnitude of entropy and phase constitution offered by multi- principal elements have stimulated vast exploration regarding the definition, thermodynamic analysis, microstructure, mechanical property, deformation mechanism, functional property and potential future applications. A large territory of work on MCAs has been focused on mechanical behavior and deformation mechanisms. However, MCAs are hard to surpass the established materials when cost factors, sustainability, the price of alloying elements and the feasibility of scrap recovery and recycling are considered. Multiple promising targets for the future development of MCAs should focus on exploring the potential functional behaviour and combining mutually exclusive mechanical and functional properties. Magnetic MCAs have become one of the most promising topics among all these potential functional backgrounds. This is due to the most MCAs in face-centred cubic (fcc) and/or body-centred cubic crystal structures containing excessive ferromagnetic elements (Fe, Co and Ni), thus exhibiting ferromagnetic interactions. Yet meeting this target alone is not enough: magnets in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that sacrifice magnetic properties. To address the questions, a series of MCAs with the nominal compositions of Fe35-x/3Co30-x/3Ni30-x/3Ta5Alx (at.%) (x=3, 5, 7) were fabricated. Hot rolling and homogenization were performed at 1473 K. Further isothermal heat treatment was utilized at 1173 K for precipitation. The microstructural evolution, soft magnetic and mechanical properties of the as-cast, hot-rolled and homogenized (at 1473 K) Fe35Co30Ni30Ta5 (at.%) multicomponent alloy together with the corresponding aluminium-alloyed variants Fe35-x/3Co30-x/3Ni30-x/3Ta5Alx (at.%) (x= 3, 5, 7) and variants under different thermodynamic conditions were investigated. Multiple methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron, backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and atom probe tomography (APT) were probed for analyzing the microstructure of the alloys range from micro-scale to atomic-scale. The mechanical and magnetic performance was measured using uniaxial tensile tests and Physical Properties Measurement System (PPMS, Quantum Design).
- Division of Materials Science and Engineering 
- Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics