Structure and Mechanical Properties in the Submicrovolumes of Sintered KS25 (Co–Sm) Permanent Magnets

The microstructure and the mechanical properties of the submicrovolumes in the rare-earth magnets KS25 sintered from a Co–25% Sm alloy is studied. The microstructure is studied by optical, electron, and atomic force microscopy. The microstructure is found to be characterized by the presence of SmCo 5 dendrites, the interdendritic space consisting of a mixture of the SmCo 5 and Sm 2 Co 17 phases, individual Sm 2 Co 17 -phase grains, Zr 5 Co 3 FeSm compound crystals (1–5 μm in size), and globular Sm 2 O 3 samarium oxide inclusions (2–10 μm in size). The hardness H (GPa) and Young’s modulus E (GPa) of the main Sm 2 Co 17 and SmCo 5 phases are determined by nanoindentation. The machinability of the magnets is estimated using the nanoindentation data; the additional pressure and adhesion at the boundary of the Sm 2 Co 17 and SmCo 5 phases are calculated. The calculation shows that the additional pressure exceeds the external pressure by a factor of 300, and the adhesion of the phases ( K int = 0.543 MPa m 0.5 ) is lower than that of the strengthening coatings by an order of magnitude. This finding can be the cause of magnet cracking along the boundary between the Sm 2 Co 17 and SmCo 5 phases during cutting and grinding.