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Title Page 1

Abstract 4

Contents 6

1. Introduction 11

2. Literature Review 14

2.1. Effects of Post-Sintering Annealing 16

2.2. Effects of Additive Elements Addition 20

2.2.1. Co Effects 20

2.2.2. Dy Effects 20

2.2.3. Cu, Al Effects 21

2.3. Effects of Dy-diffusion Process 25

3. Experimental Procedure 27

4. Results and Discussions 29

4.1. Sintered Magnet Study 29

4.1.1. Effects of PSA on the Magnetic Properties 29

4.1.2. Effects of PSA on the Microstructure of Cu-rich TJP and GBP 31

4.2. Strip-cast Study 36

4.2.1. Effects of Dy Content on the Magnetic Properties 36

4.2.2. Effects of Dy Content on the Microstructure 38

5. Conclusions 52

5.1. Relationship between Microstructure and Magnetic Property Changes of the Nd-Fe-B Sintered Magnet during PSA 52

5.2. Relationship between Microstructure and Magnetic Property Changes of the Nd-Fe-B Strip-Cast by Dy Addition 52

6. References 54

7. Accomplishments 57

List of Tables 10

Table 2-1. Variation of the magnetic properties, curie temperature, and... 22

List of Figures 7

Figure 2-1. Two way for the coercivity enhancement at operating temperature of... 15

Figure 2-2. SEM images of (a) As-sintered and (b) optimally annealed Nd-Fe-B... 18

Figure 2-3. Mechanism for the formation of metastable C-Nd₂O₃ phase 19

Figure 2-4. The Nd-O binary phase diagram and corresponding schematic image... 23

Figure 2-4. Variation of wetting angle (θ) as a function of Al content 24

Figure 2-5. Schematic illustration of the grain boundary diffusion process 26

Figure 4-1. Magnetic property changes of the Nd-Fe-B sintered magnet as a... 30

Figure 4-2. HRTEM micrographs and selected area diffraction patterns of the Cu-... 34

Figure 4-3. HRTEM micrographs and selected area diffraction patterns of the GBP... 35

Figure 4-4. Magnetic properties of the strip-cast (Nd₃₂.₅₇₋ₓ,Dyₓ)Febₐₗ.B₀.₉₇ as a...[이미지참조] 37

Figure 4-5. Back scattered electron micrographs of the surface and wheel side of... 40

Figure 4-6. (a) average inter-lamellar spacing and (b) difference between max. and... 41

Figure 4-7. Cross-sectional back scattered electron micrographs of the strip-cast... 42

Figure 4-8. (a) average grain alignment and (b) difference between grain alignment... 43

Figure 4-9. XRD patterns of (a) surface and (b) wheel side of the strip-... 46

Figure 4-10. Variation of texture coefficient of (006) as a function of Dy content 47

Figure 4-11. Intensity of α-Fe changes as a function of Dy content 48

Figure 4-12. (a) SEM image of the DyNdO₃ precipitate which is formed along the... 50

Figure 4-13. TEM image and SAD pattern of DyNdO₃ precipitate 51

초록보기

 The Dy addition to the Nd-Fe-B sintered magnet is effective way for the coercivity enhancement owing to large anisotropy field of Dy₂Fe₁₄B phase, but the problems related to the cost and the natural abundance are still remain. It is well known that the microstructural modification of the Nd-rich phases and refinement of the main phase improve the coercivity of the Nd-Fe-B sintered magnet. Here wereport that the microstructural evolution of Nd-rich grain boundary phases (GBP) in connection with triple junction phases (TJP) during post-sintering annealing (PSA). The Cu-rich TJP in the as-sintered sample was mostly a fcc-NdO phase but the GBP were a mixture of h-Nd₂O₃ and Nd phases. The fcc-NdO of the TJP in the as-sintered state gradually transformed to h-Nd₂O₃ during the 1st and the 2nd PSA steps. However, it transformed to a C-Nd₂O₃ phase as both a massive form, such as TJP, and a thin GBP after the modified 2nd PSA step. This suggests that the mechanism for the formation of metastable C-Nd₂O₃ may not be solely the interface energy. In contrast, the mixture of h-Nd₂O₃ and Nd of the GBP in the as-sintered state gradually transformed to C-Nd₂O₃, which is embedded in the amorphous matrix, as the PSA goes from the 1st to 2nd or modified 2nd PSA step. The formation of the C-Nd₂O₃ GBP with an amorphous phase is the main factor for increasing the coercivity (from 21.8 to 30.4 kOe) after the 2nd or modified 2nd PSA step. Because the microstructure of the sintered magnet is determined by the distribution of grains in the starting alloy, we also investigated the dependence of the microstructural and magnetic properties of an (Nd₃₂.₅₇﹘ₓ,Dyₓ)Febal.B₀.₉₇ (wt%), (x＝4.89, 6.51, 8.14, 9.77) strip-cast on the Dy content. The average inter-lamellar spacing is reduced and the grain uniformity and (001) grain alignment with the direction perpendicular to the plane of the strip cast are enhanced as the Dy content is increased. Also, the formation of α-Fe is increasingly suppressed as the Dy content is increased. These microstructural changes can be explained by the increase in the effective cooling rate of the melted alloy caused by the increase in the solidus temperature of the (Nd,Dy)₂Fe₁₄B phase resulting from the substitution of Dy with Nd. Also, the appearance of DyNdO₃ precipitates in the grain boundaries contributes to the grain refinement. Therefore, the improvement in the magnetic properties afforded by the addition of Dy depends not only on the large anisotropy field of Dy₂F₁₄B, but also on the grain refinement and alignment caused by the increased effective cooling rate and DyNdO₃ precipitates which formed along the Nd-rich grain boundaries.

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