EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Severe plastic deformation (SPD) processes like High pressure torsion (HPT) enable the production of nanocomposite materials. When applied to powder blends of permanent magnetic phases like SmCo$_5$ and binder metallic phase such as Cu, it opens up a new route for the production of novel magnetic materials. HPT of powder blends thus enables a nearly free selection of the magnetic phase and the grain boundary phase, beyond the limitation of conventional powder metallurgical sintering routes.
In this work, HPT is applied for the first time on a powder blend consisting of SmCo$_5$ as hard magnetic phase and copper as grain boundary phase. The ductile copper powder serves as a binder phase, carrying the plastic deformation and furthermore it decouples the hard magnetic SmCo$_5$ grains. For the production of nanocomposites, different mixing ratios between the two powders and a variation of the process parameters was investigated. Therefore, a two-step HPT process was applied. First the powder is precompressed with a pressure of 4.5 GPa and in the second step the actual deformation is done. After a process optimization, a consolidated disc with a nanosized microstructure and excellent magnetic properties with a coercivity of 1.4 T could be obtained in an as-HPT sample. After 20 HPT turns, a two phase microstructure, with strong refinement in particle size of SmCo$_5$ and Cu particles was achieved. The plastic deformation results therefore in a grain refinement as well as an increase of the defect density. The magnetic properties were further investigated by the analysis of the magnetic domain structure using magneto optical Kerr microscopy.
The results emphasize the wide freedom of microstructural design for magnetic materials using HPT of powder blends, which allows to tailor intrinsic properties via the material selection as well as extrinsic properties via the processing parameters.