Nanostructured magnets have been the focus of research since two decades. One of the remaining key challenges is to synthesize bulk nanostructured magnets of a reasonable size. Heavy plastic deformation, usually called severe plastic deformation (SPD), is one innovative, large scale processing route to synthesize bulk nanostructured materials in large dimensions and novel SPD setups for samples sizes up to a diameter of 60 mm and thickness of 12 mm have become available recently.
In this project, the potential to fabricate bulk nanostructured magnets by SPD as novel processing route was investigated. The aim of the project was to synthesize different nanostructured magnets by SPD as well as to tailor their microstructure to attain the desired magnetic properties. The SPD process has the advantage that bulk nanostructured magnets with a well-defined composition can be manufactured and the starting materials used can be manifold. By using powder blends, almost unlimited materials combinations beyond the equilibrium phase diagram were realized. Using the latter, the magnetic properties of SPD processed nanostructures materials were modified in wide range by subsequent thermal treatments. By using different material combinations and thermal treatments, unique nanostructures were obtained and the magnetic properties were optimized.
The increase of global energy consumption and growing demand for renewable energy necessitates hard magnetic materials (permanent magnets) with high energy products. Up-to-date hard magnets are mainly composed of rare earth transition metals and excellent magnetic properties close to theoretical predictions are already reached. However, the prize of rare earth elements is interconnected to politic instabilities in the countries of their origin, which often establish a monopoly market. Thus, an innovative field of research covers the partial or full replacement of rare earth metals in hard magnetic materials with the aim that they are still maintaining their high energy product and their excellent performance.
One promising research approach is to induce exchange coupling between hard and a soft magnetic phases in nanocomposite magnets. The required magnetic anisotropy is provided by the hard phase. It further stabilizes the exchange coupled soft magnetic phase against demagnetization, which leads to a high coercivity. The soft magnetic phase makes its high magnetization available, which is necessary for remanence enhancement. By selecting different magnetic starting materials, such as soft, hard and antiferromagnetic-ferromagnetic powders, different types of these nanocomposite magnets were synthesized by SPD in this project.
The project finally included fine tuning of the microstructure and resulting magnetic properties through adjustments in the composition of the magnetic phases, SPD processing parameters and annealing treatments. To achieve these goals, in-depth microstructural characterization was performed. Simultaneously, the magnetic properties were measured, which help to gain a deeper understanding to improve the performance of SPD processed nanostructured magnets.
