Magnetic Structure and Nanomechanical Properties of Sintered Permanent Magnets Nd–Dy–Fe–B USC-20L

Abstract

Using scanning electron microscopy (EDS analysis), magnetic force microscopy, and nanoindentation, a metallographic study of the magnetic structure and nanomechanical properties of sintered rare-earth magnets Nd–Dy–Fe–B of the USC-20L brand has been carried out (Ural Strip Casting Technology). The microstructure of the sintered Nd–Dy–Fe–B magnet of the USC-20L grade includes the following components: grains of the Nd2Fe14B phase are separated by lamellas of the phases enriched in neodymium. Inclusions Nd–29.1% Fe–6.2% C–2.2% O–1.4% Dy are located at the triple junctions of Nd2Fe14B grains. Inclusions Nd–4.5% Fe–9.1% O–6.7% C–4.5% Fe–2% Dy are along grain boundaries containing Nd and Dy oxides. The chemical composition of grains is Fe–25% Nd–6.9% C–1.6% Dy–1.4% B. It is found that, due to irregular grain growth, the interlayers of the enriched Nd phase are connected to each other in the region of grain junctions, causing a concentration of internal stresses and the appearance of a crack. The crack propagates along the grain boundary from one wetted joint of the grains to another, due to mechanical stresses. The phenomenon of intergranular wetting by a phase of Nd-enriched Nd2Fe14B/Nd2Fe14B grain boundaries is observed. It has been established that phases enriched with Nd can pseudo-incompletely (or pseudo-partially) wet such grain boundaries, i.e., form a nonzero contact angle along the grain boundaries and in triple joints. Based on the results of Magnetic Force Microscopy (MFM), a conclusion is 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 domain size of the banded structure is ~1 μm, the energy of the domain wall γ ~ 14 kJ/m2, and the width of the domain wall δ ~ 0.6 × 10–9 m. The nanoindentation method is used to measure the values of nanohardness (H, GPa), elastic modulus (E, GPa), contact stiffness (S, N/m), elastic work (We, nJ) and plastic strain (Wp, nJ) in submicrovolumes of Nd2Fe14B grains. According to the measurement results, the minimum value of Nd2Fe14B grain adhesion Kint = 0.539 MPa m0.5 is estimated.