Diffusing Effects of Sintered NdFeB Magnets
The DyZn film on sintered NdFeB magnet surface was prepared through DC magnetron sputtering. The magnetic properties, thermal stability and microstructure of annealed and DyZn diffused magnets were compared. The coercivity enhances significantly from 1174.10kA/m to 1711.40kA/m by a striking increment of 45.7% with only a slight reduction in remanence from 1.20T to 1.19T after DyZn thermal diffusion treatment. The thermal stability of NdFeB magnets can be improved by grain boundary diffusion. The temperature coefficient of coercivity (β) and remanence (α) are improved from 0.506%/°C, 0.120%/°C to 0.481%/°C, 0.105%/°C in the range of 20°C–180°C, respectively. The irreversible magnetic flux losses of DyZn diffused magnet are reduced to 37.5% at 180°C for 2h and 29.8% in HAST for 288h, respectively. Dy elements diffusing along the grain boundary phase mainly aggregate between the grain boundary phase and the main grain epitaxial layer via SEM and EPMA observations. The optimization of microstructure and composition of grain boundary phases and the formed (Nd,Dy)2Fe14B .
Effect of Dy Addition on the Magnetic and Mechanical Properties of Sintered Nd–Fe–B Magnets Prepared by Double-Alloy Powder Mixed Method
The effect of Dy addition on the magnetic and mechanical properties of sintered Nd-Fe-B magnets prepared by the double-alloy powder mixed method was investigated. The results showed that the intrinsic coercivity of sintered Nd-Fe-B magnets increases gradually, and the squareness factor of sintered Nd-Fe-B magnets declines with increasing Dy content. The Vickers hardness of sintered Nd-Fe-B magnets increases with increasing Dy content, and the maximum fracture toughness of sintered Nd-Fe-B magnets is obtained when the Dy content is 2 wt%. The bending strength of sintered Nd-Fe-B magnets first declines, and then increases with increasing Dy content. The minimum bending strength of sintered Nd-Fe-B magnets is obtained at the Dy content of 2 wt%. The microstructure showed that the distribution of Dy content in main phase was not uniform because of the Dy diffusion of double-alloy mixed powder during the sintering process.
Effect of Dy2O3 intergranular addition on microstructure and magnetic properties of (Nd, Dy)–Fe–B sintered magnets
Dy2O3 powders with high melting point and low cost have been added into the (Nd, Dy)–Fe–B sintered magnets. Strengthened local magnetocrystalline anisotropy induced by intergranular addition of Dy-containing sources is effective to simultaneously enhance coercivity and minimize remanence loss for Nd2Fe14B-type sintered magnets. In this work, the addition of Dy2O3 promotes the formation of Dy-enriched shells surrounding the 2:14:1 phase grains and reduces the grain size effectively. As a result, the magnetocrystalline anisotropy field HA of the outer regions of the matrix phase is strengthened, accompanied with reduction of local demagnetization stray fields. Enhanced coercivity Hcj from 17.5 to 24.3 kOe has been achieved by the addition of 4.0 wt% Dy2O3. The existence of Dy2O3 is also beneficial to improve the grain alignment with the remanence Br maintained above 12.0 kGs. Optimized addition of 2 wt% Dy2O3 gives rise to maximized Hcj/kOe+(BH)max/MGOe as a result of balanced coercivity enhancement and magnetic dilution effect. Intergranular addition of Dy2O3 exhibits great potential in producing Nd2Fe14B-based magnets with high coercivity and reduced material cost.