Magnetic structure of Co–25%Sm sintered magnets after electrical discharge machining

Abstract

Scanning electron microscopy and magnetic force microscopy were used to conduct the metallographic study of the surface microstructure of KS25 grade Co–25%Sm sintered rare-earth magnets after Electrical Discharge Machining (EDM). The chemical composition of the studied samples: Sm – 25 wt.%; Fe – 18 wt.%; Cu – 5 wt.%; Zr – 3 wt.%; Co – the rest. One of the sample surfaces was subjected to EDM in various ways with changes in such EDM parameters as the straight-line processing speed and offset. The microstructure of magnets contains four coexisting phases: SmCo5, Sm2Co17, Zr5Co3FeSm and Sm2O3. The grain size is 10–50 μm. Crystals of the Zr5Co3FeSm intermetallic compound are 1–5 μm in size, and globular inclusions of Sm2O3 samarium oxide are 2–10 μm. EDM affected the thickness and chemical composition of the defective layer. In general, the chemical composition varies slightly in the direction from the defective layer inward the sample: the content of Sm, Cu, O, and Zr decreases; the content of Fe and Co increases. At a distance of 500 μm from the defective layer inwards the sample, the grain size increases by 40–50 %, while the porosity decreases. At the same time, the size of Sm2O3 oxides slightly increases. The study of the magnetic structure on surfaces perpendicular to the axis of magnetization by means of magnetic force microscopy revealed the presence of a complex domain structure of grains in the form of a labyrinth with a domain size of ~3÷5 μm. Separate singledomain grains ~30÷50 μm in size were also found. Due to the material heating and oxidation, EDM promotes the domain structure of grains appearing in the form of a labyrinth instead of single-domain grains, and the SmCo5 → Sm2Co17 phase transition, which causes a decrease in coercive force.