Yikun Fang

Magnetic micro-structural uniformity of die-upset Nd-Fe-B magnets

Die-upset Nd13.62Fe75.70Co4.45B5.76Ga0.47 magnets have been prepared with height reduction (h) in the range of 60 to 88%. The energy product as high as 50.4 MGOe was obtained in the sample with h ∼ 70%. The magnetic domains of the samples are revealed by using magnetic force microscopy (MFM). The average domain widths of the die-upset samples with surface normal parallel (//) and perpendicular (⊥) to the loading direction are in the range of w//: 0.4-0.6 μm; w⊥: 0.9-3.8 μm, respectively. These interaction domains are formed due to the strong inter-granular exchange interaction and magnetostatic interaction between grains. It was found that the ratio of φrms// to φrms⊥ is a good indicator for the quality of the magnet, where the φrms// and φrms⊥ are defined as the root-mean-square values of phase shift for the MFM images. The microstructures have been investigated by scanning electron microscopy (SEM), MFM and SEM results indicate the magnetic and crystalline microstructures are uniform for the sample with...

Effect of Sm-rich liquid phase on magnetic properties and microstructures of sintered 2:17-type Sm-Co magnet

Abstract The effect of Sm-rich liquid phase (Sm 2 Co 3 ) on magnetic properties and microstructure of 2:17-type Sm-Co magnet was studied. Three phases existed in liquid phase ingot, and took on white, dark grey and grey, respectively. It was found that the composition of the grey area was similar to the nominal one. Our results indicated that the optimal composition was obtained at 3 wt.% liquid phase added, and the magnetic properties were B r =11.58 kGs, H ci >26 kOe, ( BH ) m =29.51 MGOe. B r increased by 3%, H ci was 13 times larger, and ( BH ) m was 6 times bigger than none liquid phase magnets. Moreover, with the aging time decreasing from 20 to 8 h, the squareness of the demagnetization curves got better, while the coercivity of the samples decreased. It revealed that Sm(Co, Cu) 5 phase also precipitated at particle boundaries. It maybe also enhanced the coercivity.

Revealing on metallurgical behavior of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 sintered magnets

The sintered magnets with the nominal composition of Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 were prepared by standard metallurgical method. The evolution of phase transformation of the specimens during heat treatments was investigated in detail. After isothermal aging at 1103 K for 20 hrs and step cooling to 673 K and keeping for 10hrs, the remanence Br of the specimens almost keeps constant (∼11.5 kGs), while the intrinsic coercivity Hcj increases from 7.9 to 31.5 kOe. The maximum energy product of the final magnet is close to 32 MGOe. The phase is single phase with 1:7H structure in the specimen A only annealed at 1453 K for 4hrs. It is found that a cellular microstructure with a platelet Z-phase have appeared in the sample after the isothermal aging in 1103 K for 20h, which contain 2:17R, 2:7R and 5:19H phases. Furthermore, the 1:5H phase has appeared after the step cooling to 873 K, together with some 2:17R, 2:7R and 5:19H phases. Interestingly, it is found that the phase transformation has completed after t...

Microstructures and magnetic properties of Ce32.15Co49.36Cu9.84Fe9.65 magnet sintered at different temperatures

The microstructures and magnetic properties of Ce32.15Co49.36Cu9.84Fe9.65 magnet sintered at the temperatures ranging from 1005 to 1105 °C were investigated. The results on scanning electron microscopy and X-ray diffraction analysis indicate that the remanence Br of the magnets is mainly influenced by the degree of the easy-axis orientation when sintering temperature is less than 1085 °C, the rapidly increasing amount of the secondly phase (5:19 phase) gives rise to the deterioration of the magnetic properties of the magnet above 1085 °C. Moreover, it is found that intrinsic coercivity Hci is strongly related to the content of copper in the matrix of the sintered magnets. The optimal sintering temperature is located in 1025∼1055 °C, the corresponding magnetic properties of the magnets are Br=0.685 T, Hci=350 kA·m−1, and maximum energy product (BH)m=85.6 kJ·m−3.

Microstructural Analysis During the Step-Cooling Annealing of Iron-Rich Sm(Co 0.65 Fe 0.26 Cu 0.07 Zr 0.02 ) 7.8 Anisotropic Sintered Magnets

Microstructural analysis during the step-cooling procedure of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 anisotropic sintered magnets was investigated systematically. The sintered magnet exhibits the maximum energy product of about 32 MGOe together with a remanence of about 11.5 kGs. It is found that the cellular structure has formed at the stage of isothermal annealing (heat treating at 1103 K for 20 h). Moreover, the average cell size of the 2:17 cell phase in magnets decreases from ~150 to ~105 nm, and the density of the lamella phase increases from ~0.03 to ~0.05 1/nm during the step-cooling procedure. It is also found that the Cu concentration is homogeneous in the specimen quenched at a temperature of 1103 K and then becomes nonuniform, especially enriched in the cell boundaries during the step cooling to 873 K. Interestingly, the typical twining structure along the [010] zone of the main phase exists in the specimen quenched at 873 K. Furthermore, it is inferred that the magnetization mechanism of the specimens transforms from the nucleation-dominated type to the coexisting type of pinning and nucleation effect during the step-cooling procedure.

Fractal study for the fractured surface of Nd-Fe-B permanent magnets

Fractal study was used to the analysis of the fractured surface of commercial N50-type Nd-Fe-B sintered magnets. The strain-stress properties of the specimens indicate that the elastic and plastic deformations occur simultaneously during intergranual cracking. The microfractography of the specimens exhibits typical brittle fracture pattern with some toughness dimples, indicating that plastic deformation has happened in some local areas. A “line-measuring dimension” Dline was selected to discuss the fracture behavior. The calculated Dline is about 1.28 and 1.29 for the specimens with c-axis parallel and perpendicular to the applied force direction, respectively. The line-measuring dimension analysis indicates that the fracture feature may be isotropic for our studied specimens, which is a bit inconsistent with previous report on the fracture characterization of Nd-Fe-B sintered magnets. More comprehensive fractal study is further needed to confirm the fracture behavior in detail in the future.

Analysis of NdFeB multi-pole ring by SEM and MFM

Sintered multi-pole ring that is made in America and China was analysed by SEM and MFM. The results show that the average size of American multi-pole ring is much smaller than the Chinese one and has much better orientation degree. The magnetic field of American specimen is much higher than that of the Chinese one and its average value is about 500 Gs, and more uniform distribution of the Nd-rich phase has been obtained.

Melt spinning process and its effect on the magnetic properties and structures of SM(CO, FE, CU, ZR) Z melt-spun ribbons

Sm-Co magnet was attracted attention due to its high Curie temperature and superior comprehensive magnetic properties. Different with powder metallurgy method, melt spinning is another efficient way to prepare Sm-Co magnet. The quenching speed in melt spinning is a critical factor to obtain ribbons with high magnetic properties. Recently, D.Y. Feng et al. reported that intrinsic coercivity of SmCo6.8Zr0.2 ribbons increases from 1.54kOe to 13.45 kOe with the wheel speed rising from 5 m/s to 60 m/s. And the crystallographic c-axis is parallel to the ribbon plane for the ribbons melt-spun at low speeds of 5 and 15 m/s, and this c-axis orientation is weakened at higher speeds. Tetsuji Saito et al. reported that Fe content has an effect on the phase composition. A small substitution of Fe for Co in the SmCo5 alloy leads to the formation of the Sm(Co, Fe)5 phase, but that the large substitution of Fe for Co results in the formation of other phases such as the Sm(Co, Fe)7 and Sm2(Co, Fe)7 phases. In this paper, the influence of the wheel surface speeds on the thickness of the as-spun Sm-Co ribbons was discussed firstly, and then the magnetic properties and microstructures of the Sm(Co0.93-x-yFexCu0.07Zry)7.8 ribbons with different Fe and Zr contents were investigated.

Magnetic Microstructures of High Performance Permanent Magnetic Materials

Magnetic domains are strong correlated to the macro-magnetic properties of magnetic materials. Among the methods to revealing magnetic domains, magnetic force microscopy (MFM) is a powerful tool with high lateral resolution and easy sample preparation. In this review, we summarize the recent magnetic domain investigations on three typical kinds of rare earth permanent magnets: high coercivity Nd-Fe-B sintered magnets, hot-deformed Nd-Fe-B magnets and 2:17-type Sm-Co sintered magnets. The influence of temperature, fabrication procedure parameters on these magnetic materials is mainly discussed. For Nd-Fe-B sintered magnets, the remanent state specimens show single domains and formation and grownup of reversal domains occur during thermal demagnetization process. The magnetic configurations of nano-scale anisotropic hot-deformed Nd-Fe-B magnets are typical interaction domains. For 2:17-type Sm-Co sintered magnets, the zig-zag domain walls distributed along the cell boundaries are revealed by MFM.