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The effect of Co addition on the fracture strength of NdFeB sintered magnets
Intermetallics 9 (2001) 269±272
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The eect of Co addition on the fracture strength of NdFeB sintered magnets Jianhua Jiang a,*, Zhenpeng Zeng a, Jian Yu a, Jiansheng Wu a, M. Tokunaga b a
Open Laboratory of State Education Commission for High Temperature Materials and Testing, Shanghai Jiao Tong University, Shanghai 200030, PR China b Magnetic and Electronic Materials Research Laboratory, Hitachi Metals, Ltd., 5200 Mikajikri, Kumagaya Saitama T360, Japan Received 14 January 2000; accepted 25 April 2000
Abstract The bending strength and fracture toughness of the ternary and Co-containing NdFeB sintered magnets have been studied. The microstructure and the fracture behavior were examined utilizing micro-analysis. It is shown that the bend fracture of the NdFeB magnets is mainly intergranular. The Co addition with proper content increases the bending strength of NdFeB sintered magnets by about 80% and this is mainly due to the strengthening of Nd-rich grain boundary phase. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Fracture toughness; D. Microstructure; E. Mechanical properties, theory
1. Introduction Since its development [1,2] in 1983, the production and application of NdFeB magnets have grown rapidly. It is well known that the NdFeB sintered magnets have received much attention not only for their excellent magnetic properties, but also for their simple preparation method. However, the strength and the toughness of the magnets are very low, which is disadvantageous to their applications. In order to enable the NdFeB magnets to be applied in new ®elds, such as electric vehicle drive motors in which high rpm and smaller gaps are necessary, the magnets need to be used as a type of structural material, so it is important to improve their strength. Rabinovich et al. [3] and Horton et al. [4] investigated the mechanical properties of NdFeB sintered magnets, including strength and fracture toughness. A wide range of NdFeB sintered magnets were used to measure the mechanical properties. The results revealed that the bending strength of ternary NdFeB magnets is a bit higher than 200 MPa and its fracture toughness is between 3.9 and 5.5 MPa m1/2. In this paper the in¯u* Corresponding author.
ence of Co addition on the strength and fracture behavior is investigated. 2. Experimental details The alloys with nominal compositions Nd15Fe78 x CoxB7 were used, where x=0, 3.5, 7.0 and were marked as alloy nos. 1, 2 and 3, respectively, in Table 1. Alloys were induction-melted in an aluminum crucible under an argon atmosphere. The cast ingots were pulverized to about 36 mesh by hydrogen decrepitation. The coarse powders were jet-milled to about 4.2 mm with nitrogen gas at a pressure of 0.7 MPa. Specimens were prepared by traditional powder metallurgical techniques. The pressed bodies were sintered under an argon atmosphere at 1080 C for 2 h and then water cooled, followed by tempering at 600 C for 2 h. The specimens for mechanical testing with the dimension of 7.014.063.0 mm were cut from the sintered blocks by spark erosion, and then the surfaces were mechanically polished before testing since the mechanical properties of materials are sensitive to their surface defects. The straight notch in the specimens for fracture toughness testing was made by spark erosion
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00063-7
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J. Jiang et al. / Intermetallics 9 (2001) 269±272
Table 1 Mechanical properties of testing alloysa Alloy no.
Bending strength sbb (MPa)
Fracture toughness KIC (MP am1/2)
Hardness Hv
1 2 3
148.1, 172.8 (160.5) 274.6, 293.7 (286.0) 241.2, 238.1 (239.2)
4.93, 4.66 (4.80) 3.07, 3.55 (3.31) 3.23, 3.49 (3.36)
536.0 568.0 527.0
a
Data in parentheses are average values.
with the width of about 0.14 mm and the depth of about 7.0 mm. The bending strength and the fracture toughness of the specimens were measured with a Shimadzu AG-100kN universal testing machine. The observation of microstructure and fracture surfaces was performed on a Philips XL-30 scanning microscope and the fracture surfaces were fresh without contamination. 3. Results 3.1. The bending strength and fracture toughness The mechanical properties of the specimens for three alloys are listed in Table 1. It is shown in Fig. 1 that the bending strength of the magnets is increased with Co addition from 0 to 3.5 at.%, while the bending strength decreases when Co content is increased from 3.5 to 7.0 at.%. However, the fracture toughness of the magnets decreases with Co addition. 3.2. Microstructure observation The SEM micrographs of alloy nos. 1 and 2 are shown in Fig. 2, which is on mechanically polished and slightly etched specimens, and the phase compositions of the alloys were further determined by EDX analysis, and the results are listed in Tables 2 and 3. For alloy no. 1 (see Fig. 2a and Table 2), in addition to the Nd2Fe14B matrix phase (marked as A in Fig. 2a), the following phases are identi®ed: Boron-rich phase NdFe4B4 (marked as B in Fig. 2a) and two types of Ndrich phases on dierent sites (marked as C and D in Fig. 2a). This result is similar to the conclusions of other researchers [5,6]. For alloy no. 2 (see Fig. 2b and Table 3), a similar microstructure is obtained with the only dierence in the Co content existing in some phases. It is also seen that oxygen appears just in the Nd-rich phase. 3.3. The fracture surfaces The morphologies of fracture surfaces for alloy nos. 1 and 2 after bend tests are shown in Fig. 3. The fracture surface of alloy no. 1 is almost all intergranular and only a little transgranular fracture is found. The surface
Fig. 1. Relationships between mechanical properties and Co content.
of alloy no. 2, however, is a combination of two kinds of fracture modes. The measurement shows that the ratios of transgranular fracture for alloy nos. 1 and 2 are 5 and 30%, respectively. EDX is used to analyze the composition of the surfaces in Fig. 3. The results are listed in Table 4. Experimental results reveal that: (1) the grain boundaries of alloy nos. 1 and 2 are Nd- and O-rich; (2) Co exists not only in grains (grain plane B) but also along grain boundaries (boundary A). 4. Discussion 4.1. The fracture mode of NdFeB sintered magnets Investigations show that the fracture mode of NdFeB sintered magnets is mainly intergranular, i.e. cracks propagate along the grain boundaries. The reason why the intergranular fracture appears in the magnets is the weakness of the boundaries, because there is a thin Ndrich phase containing oxygen along the boundaries and this phase has low strength. As we all know, a liquid
J. Jiang et al. / Intermetallics 9 (2001) 269±272
271
Fig. 2. SEM micrographs of the alloys.
Table 2 Phase composition of alloy no. 1 in Fig. 2a tested by the spot analysis of EDX Composition (at.%) Position
Nd
Fe
O
Phase
A B C (boundary) D (particle)
19 30 31 60
81 70 63 24
± ± 6 16
Nd2Fe14B NdFe4B4 Nd-rich Nd-rich
Table 3 Phase composition of alloy no. 2 in Fig. 2b tested by the spot analysis of EDX Composition (at.%) Position
Nd
Fe
Co
O
Phase
A B C D
31 19 74 75
66 76 10 11
3 5 14 ±
± ± 2 14
Nd(FeCo)4B4 Nd2(FeCo)14B Nd-rich Nd-rich
Nd-rich phase joins in the sintering process of NdFeB magnets and this phase has the lower melting point. When the magnets are heated up to 620 C, the Nd-rich phase starts to melt and thus form the liquid phase among the grains of matrix phase. Furthermore, when the magnets are cooled, this phase consolidates to be grain boundaries at last. Both the EDX results of our investigation in this paper and other studies before [7,8] indicate that the boundaries are Nd- and O-rich. The strength of the Nd-rich phase is low since its hardness is only Hv262 [9]. Therefore, when the magnets are loaded, the nucleation of cracks occur at the coarse matrix phase and they propagate along the boundaries, which results in intergranular fracture.
Table 4 EDX results of the fracture surfaces (at.%) Alloy no.
Position
Nd
Fe
1
Boundary A Plane B
66 21
28 79
2
Boundary A Plane B
55 20
34 76
Co
O 6
4 4
7
4.2. The eect of Co addition on the mechanical properties of NdFeB sintered magnets It is clear that when the strength of the Nd-rich phase is increased, the fracture strength of magnets can be improved since the growth of the cracks are prevented or delayed. The results of our study also indicate that the bending strength of sintered NdFeB is improved from 160 to 286 MPa after 3.5 at.% Co is added. But the fracture toughness decreases from 4.8 to 3.3 MPa m1/2. It is seen from the EDX results (see Table 3) of fracture surfaces that some for Co dissolves into the Nd-rich phase along boundaries and some Co substitutes for the Fe atoms in the matrix phase, forming Nd2(Fe,Co)14B. So the Nd-rich phase contains alloy element Co. Therefore, the strength of the Nd-rich phase is increased by adding Co, because it is dicult for cracks to grow along the boundaries. Normally, the growth directions of the cracks is changed and even transgranular fracture is formed in some areas. According to Fig. 3, the intergranular fracture ratios of alloy nos. 1 and 2 are about 95 and 70%, respectively. That means the addition of element Co declines the ratio of intergranular fracture by about 25%. This is the reason why the strength of the sintered NdFeB magnets is greatly improved by the addition of Co.
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J. Jiang et al. / Intermetallics 9 (2001) 269±272
Fig. 3. Fracture surfaces of the alloys.
5. Conclusions 1. When 3.5 at.% of Co is added, the bend strength of sintered NdFeB magnets is increased by about 80% compared with ternary NdFeB magnets, while the fracture toughness decreases about 30%. 2. The fracture mode of static bending for the NdFeB sintered magnets is intergranular. Due to the Nd-rich phase with O, whose strength is low, the intergranular fracture appears along the grain boundaries. This phase can be improved by adding Co addition with proper content, thus the intergranular fracture ratio on the fracture surface is decreased.
References [1] Croat JJ, Herbst JF, Lee RW, Pinkerton FE. J Appl Phys 1984;55:2078. [2] Sagawa M, Fujimura S, Togawa N, Yamamoto H, Matsuura Y. J Appl Phys 1984;55:2083. [3] Rabinovich YuM, Sergeev VV, Maystrenko AD, Kulakovsky V, Szymura S, Bala H. Intermetallics 1996;4:641. [4] Horton JA, Wright JL, Herchenroeder JW. IEEE Trans on Magn 1996;32(5):4374. [5] Tang WZ, Zhou SZ, Wang R. IEEE Trans on Magn 1988; 64(10):5516. [6] Ramesh R, Chen JK, Thomas G. J Appl Phys 1987;61(8):2993. [7] Zeng ZP. Rare Metal Materials and Engineering 1996;25(3):18 (in Chinese). [8] Li L, Graham CD. IEEE Trans on Magn 1993;29(6):2776. [9] Ohashi K et al. Proc. of 9th Workshop on REPM, 1987. p. 355.
www.elsevier.com/locate/intermet
The eect of Co addition on the fracture strength of NdFeB sintered magnets Jianhua Jiang a,*, Zhenpeng Zeng a, Jian Yu a, Jiansheng Wu a, M. Tokunaga b a
Open Laboratory of State Education Commission for High Temperature Materials and Testing, Shanghai Jiao Tong University, Shanghai 200030, PR China b Magnetic and Electronic Materials Research Laboratory, Hitachi Metals, Ltd., 5200 Mikajikri, Kumagaya Saitama T360, Japan Received 14 January 2000; accepted 25 April 2000
Abstract The bending strength and fracture toughness of the ternary and Co-containing NdFeB sintered magnets have been studied. The microstructure and the fracture behavior were examined utilizing micro-analysis. It is shown that the bend fracture of the NdFeB magnets is mainly intergranular. The Co addition with proper content increases the bending strength of NdFeB sintered magnets by about 80% and this is mainly due to the strengthening of Nd-rich grain boundary phase. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Fracture toughness; D. Microstructure; E. Mechanical properties, theory
1. Introduction Since its development [1,2] in 1983, the production and application of NdFeB magnets have grown rapidly. It is well known that the NdFeB sintered magnets have received much attention not only for their excellent magnetic properties, but also for their simple preparation method. However, the strength and the toughness of the magnets are very low, which is disadvantageous to their applications. In order to enable the NdFeB magnets to be applied in new ®elds, such as electric vehicle drive motors in which high rpm and smaller gaps are necessary, the magnets need to be used as a type of structural material, so it is important to improve their strength. Rabinovich et al. [3] and Horton et al. [4] investigated the mechanical properties of NdFeB sintered magnets, including strength and fracture toughness. A wide range of NdFeB sintered magnets were used to measure the mechanical properties. The results revealed that the bending strength of ternary NdFeB magnets is a bit higher than 200 MPa and its fracture toughness is between 3.9 and 5.5 MPa m1/2. In this paper the in¯u* Corresponding author.
ence of Co addition on the strength and fracture behavior is investigated. 2. Experimental details The alloys with nominal compositions Nd15Fe78 x CoxB7 were used, where x=0, 3.5, 7.0 and were marked as alloy nos. 1, 2 and 3, respectively, in Table 1. Alloys were induction-melted in an aluminum crucible under an argon atmosphere. The cast ingots were pulverized to about 36 mesh by hydrogen decrepitation. The coarse powders were jet-milled to about 4.2 mm with nitrogen gas at a pressure of 0.7 MPa. Specimens were prepared by traditional powder metallurgical techniques. The pressed bodies were sintered under an argon atmosphere at 1080 C for 2 h and then water cooled, followed by tempering at 600 C for 2 h. The specimens for mechanical testing with the dimension of 7.014.063.0 mm were cut from the sintered blocks by spark erosion, and then the surfaces were mechanically polished before testing since the mechanical properties of materials are sensitive to their surface defects. The straight notch in the specimens for fracture toughness testing was made by spark erosion
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00063-7
270
J. Jiang et al. / Intermetallics 9 (2001) 269±272
Table 1 Mechanical properties of testing alloysa Alloy no.
Bending strength sbb (MPa)
Fracture toughness KIC (MP am1/2)
Hardness Hv
1 2 3
148.1, 172.8 (160.5) 274.6, 293.7 (286.0) 241.2, 238.1 (239.2)
4.93, 4.66 (4.80) 3.07, 3.55 (3.31) 3.23, 3.49 (3.36)
536.0 568.0 527.0
a
Data in parentheses are average values.
with the width of about 0.14 mm and the depth of about 7.0 mm. The bending strength and the fracture toughness of the specimens were measured with a Shimadzu AG-100kN universal testing machine. The observation of microstructure and fracture surfaces was performed on a Philips XL-30 scanning microscope and the fracture surfaces were fresh without contamination. 3. Results 3.1. The bending strength and fracture toughness The mechanical properties of the specimens for three alloys are listed in Table 1. It is shown in Fig. 1 that the bending strength of the magnets is increased with Co addition from 0 to 3.5 at.%, while the bending strength decreases when Co content is increased from 3.5 to 7.0 at.%. However, the fracture toughness of the magnets decreases with Co addition. 3.2. Microstructure observation The SEM micrographs of alloy nos. 1 and 2 are shown in Fig. 2, which is on mechanically polished and slightly etched specimens, and the phase compositions of the alloys were further determined by EDX analysis, and the results are listed in Tables 2 and 3. For alloy no. 1 (see Fig. 2a and Table 2), in addition to the Nd2Fe14B matrix phase (marked as A in Fig. 2a), the following phases are identi®ed: Boron-rich phase NdFe4B4 (marked as B in Fig. 2a) and two types of Ndrich phases on dierent sites (marked as C and D in Fig. 2a). This result is similar to the conclusions of other researchers [5,6]. For alloy no. 2 (see Fig. 2b and Table 3), a similar microstructure is obtained with the only dierence in the Co content existing in some phases. It is also seen that oxygen appears just in the Nd-rich phase. 3.3. The fracture surfaces The morphologies of fracture surfaces for alloy nos. 1 and 2 after bend tests are shown in Fig. 3. The fracture surface of alloy no. 1 is almost all intergranular and only a little transgranular fracture is found. The surface
Fig. 1. Relationships between mechanical properties and Co content.
of alloy no. 2, however, is a combination of two kinds of fracture modes. The measurement shows that the ratios of transgranular fracture for alloy nos. 1 and 2 are 5 and 30%, respectively. EDX is used to analyze the composition of the surfaces in Fig. 3. The results are listed in Table 4. Experimental results reveal that: (1) the grain boundaries of alloy nos. 1 and 2 are Nd- and O-rich; (2) Co exists not only in grains (grain plane B) but also along grain boundaries (boundary A). 4. Discussion 4.1. The fracture mode of NdFeB sintered magnets Investigations show that the fracture mode of NdFeB sintered magnets is mainly intergranular, i.e. cracks propagate along the grain boundaries. The reason why the intergranular fracture appears in the magnets is the weakness of the boundaries, because there is a thin Ndrich phase containing oxygen along the boundaries and this phase has low strength. As we all know, a liquid
J. Jiang et al. / Intermetallics 9 (2001) 269±272
271
Fig. 2. SEM micrographs of the alloys.
Table 2 Phase composition of alloy no. 1 in Fig. 2a tested by the spot analysis of EDX Composition (at.%) Position
Nd
Fe
O
Phase
A B C (boundary) D (particle)
19 30 31 60
81 70 63 24
± ± 6 16
Nd2Fe14B NdFe4B4 Nd-rich Nd-rich
Table 3 Phase composition of alloy no. 2 in Fig. 2b tested by the spot analysis of EDX Composition (at.%) Position
Nd
Fe
Co
O
Phase
A B C D
31 19 74 75
66 76 10 11
3 5 14 ±
± ± 2 14
Nd(FeCo)4B4 Nd2(FeCo)14B Nd-rich Nd-rich
Nd-rich phase joins in the sintering process of NdFeB magnets and this phase has the lower melting point. When the magnets are heated up to 620 C, the Nd-rich phase starts to melt and thus form the liquid phase among the grains of matrix phase. Furthermore, when the magnets are cooled, this phase consolidates to be grain boundaries at last. Both the EDX results of our investigation in this paper and other studies before [7,8] indicate that the boundaries are Nd- and O-rich. The strength of the Nd-rich phase is low since its hardness is only Hv262 [9]. Therefore, when the magnets are loaded, the nucleation of cracks occur at the coarse matrix phase and they propagate along the boundaries, which results in intergranular fracture.
Table 4 EDX results of the fracture surfaces (at.%) Alloy no.
Position
Nd
Fe
1
Boundary A Plane B
66 21
28 79
2
Boundary A Plane B
55 20
34 76
Co
O 6
4 4
7
4.2. The eect of Co addition on the mechanical properties of NdFeB sintered magnets It is clear that when the strength of the Nd-rich phase is increased, the fracture strength of magnets can be improved since the growth of the cracks are prevented or delayed. The results of our study also indicate that the bending strength of sintered NdFeB is improved from 160 to 286 MPa after 3.5 at.% Co is added. But the fracture toughness decreases from 4.8 to 3.3 MPa m1/2. It is seen from the EDX results (see Table 3) of fracture surfaces that some for Co dissolves into the Nd-rich phase along boundaries and some Co substitutes for the Fe atoms in the matrix phase, forming Nd2(Fe,Co)14B. So the Nd-rich phase contains alloy element Co. Therefore, the strength of the Nd-rich phase is increased by adding Co, because it is dicult for cracks to grow along the boundaries. Normally, the growth directions of the cracks is changed and even transgranular fracture is formed in some areas. According to Fig. 3, the intergranular fracture ratios of alloy nos. 1 and 2 are about 95 and 70%, respectively. That means the addition of element Co declines the ratio of intergranular fracture by about 25%. This is the reason why the strength of the sintered NdFeB magnets is greatly improved by the addition of Co.
272
J. Jiang et al. / Intermetallics 9 (2001) 269±272
Fig. 3. Fracture surfaces of the alloys.
5. Conclusions 1. When 3.5 at.% of Co is added, the bend strength of sintered NdFeB magnets is increased by about 80% compared with ternary NdFeB magnets, while the fracture toughness decreases about 30%. 2. The fracture mode of static bending for the NdFeB sintered magnets is intergranular. Due to the Nd-rich phase with O, whose strength is low, the intergranular fracture appears along the grain boundaries. This phase can be improved by adding Co addition with proper content, thus the intergranular fracture ratio on the fracture surface is decreased.
References [1] Croat JJ, Herbst JF, Lee RW, Pinkerton FE. J Appl Phys 1984;55:2078. [2] Sagawa M, Fujimura S, Togawa N, Yamamoto H, Matsuura Y. J Appl Phys 1984;55:2083. [3] Rabinovich YuM, Sergeev VV, Maystrenko AD, Kulakovsky V, Szymura S, Bala H. Intermetallics 1996;4:641. [4] Horton JA, Wright JL, Herchenroeder JW. IEEE Trans on Magn 1996;32(5):4374. [5] Tang WZ, Zhou SZ, Wang R. IEEE Trans on Magn 1988; 64(10):5516. [6] Ramesh R, Chen JK, Thomas G. J Appl Phys 1987;61(8):2993. [7] Zeng ZP. Rare Metal Materials and Engineering 1996;25(3):18 (in Chinese). [8] Li L, Graham CD. IEEE Trans on Magn 1993;29(6):2776. [9] Ohashi K et al. Proc. of 9th Workshop on REPM, 1987. p. 355.