Magnetic structure and nanomechanical properties of USC-20L Nd–Dy–Fe–B sintered permanent magnets

Abstract

Scanning electron microscopy (EDS analysis), magnetic force microscopy (MFM) and nanoindentation were used in the metallographic study of the magnetic structure and nanomechanical properties of sintered USC-20L Nd–Dy–Fe–B rare-earth magnets (Ural Strip Casting Technology). The microstructure of the sintered USC-20L Nd–Dy–Fe–B magnet includes the Nd 2 Fe 14 B phase grains separated by lamellas of neodymium-rich phases. Nd–29.1%Fe–6.2%C–2.2%O–1.4%Dy inclusions are located at the triple junctions of Nd 2 Fe 14 B grains. Nd–4.5%Fe–9.1%O–6.7%C–4.5%Fe–2%Dy inclusions are located along grain boundaries and contain neodymium and dysprosium oxides. The chemical composition of grains: Fe–25%Nd–6.9%C–1.6%Dy–1.4%B. It was found that due to irregular grain growth, the neodymium-rich phase interlayers are connected to each other in the region of grain junctions causing internal stress concentration and cracking. A crack propagates along the grain boundary from one wetted grain joint to another due to the occurring mechanical stresses. A phenomenon of intergranular wetting by the neodymium-rich phase is observed at Nd 2 Fe 14 B/Nd 2 Fe 14 B grain boundaries. It was found that Nd-riched phases can pseudo-incompletely (or pseudo-partially) wet such grain boundaries, i.e. form a non-zero contact angle along the grain boundaries and in triple joints. Based on the magnetic force microscopy results, a conclusion was made about the presence of a one-dimensional domain structure; domains cross grain boundaries. The presence of pores and inclusions of Nd and Dy oxides localized along grain boundaries is noteworthy. The average transverse size of the banded-structure domain is ~1 μm, domain wall energy is γ ~ 14 kJ/m 2 , domain wall width is δ ~0.6·10 –9 m. The nanoindentation method was used to measure the values of nanohardness ( H , GPa), elastic modulus ( E , GPa), contact stiffness ( S , N/m), elastic strain energy ( W e , nJ) and plastic strain energy ( W p , nJ) in submicrovolumes of Nd 2 Fe 14 B grains. According to the measurement results, the minimum value of Nd 2 Fe 14 B grain adhesion was estimated: K int = 0.539 MPa·m 0.5 .