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Structure and magnetic properties of (Nd,Dy)16(Fe,Co)76−xTixB8 powders prepared by mechanical alloying
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 301 (2006) 279–286 www.elsevier.com/locate/jmmm
Structure and magnetic properties of (Nd,Dy)16(Fe,Co)76xTixB8 powders prepared by mechanical alloying J. Jakubowicza,, J.-M. Le Bretonb a
Institute of Materials Science and Engineering, Poznan University of Technology, M. Sklodowska-Curie 5 Sq., 60-965 Poznan, Poland b Groupe de Physique des Mate´riaux, UMR CNRS 6634, Universite´ de Rouen, Avenue de l’Universite´, 76801 Saint Etienne du Rouvray Cedex, France Received 4 May 2005; received in revised form 23 June 2005 Available online 24 August 2005
Abstract Nanocrystalline (Nd,Dy)16(Fe,Co)76xTixB8 magnets were prepared by mechanical alloying and respective heat treatment at 973–1073 K/30–60 min. An addition of 0.5 at % of Ti results in an increase of coercivity from 796 to 1115 kA m1. Partial substitution of Nd by Dy results in an additional increase of coercivity up to 1234 kA m1. Mo¨ssbauer investigations shows that for xp1 the (Nd,Dy)16(Fe,Co)76xTixB8 powders are single phase. For higher Ti contents (x41) the mechanically alloyed powders heat treated at 973 K are no more single phase, and the coercivity decreases due to the presence of an amorphous phase. A heat treatment at a higher temperature (1073 K) for longer time (1 h) results in the full recrystallisation of powders. The mean hyperfine field of the Nd2Fe14B phase decreases for titanium contents of 0pxp1, and remains constant for x41. This indicates that the Ti content in the Nd2Fe14B phase reaches its maximum value. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 75.50.Kj; 75.50.Ww; 76.80.+y Keywords: Mechanical alloying; Magnets; Nanocrystals; Nd2Fe14B; Mo¨ssbauer
1. Introduction
Corresponding author. Tel./fax: +48 61 665 3576.
E-mail address: [email protected] (J. Jakubowicz).
Since 1984 Nd2Fe14B-type materials have been the most investigated type of permanent magnet applications [1–3]. With the application of new processing technologies, like melt spinning, mechanical alloying or high-energy ball milling, new
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.07.025
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classes of materials were made, which are based on nanostructures, having a grain size below 30 nm [4–9]. In nanocrystalline materials, exchange coupling appears between ferromagnetic grains, resulting in both remanence enhancement and high coercivity [4–9]. Nanocrystalline and nanocomposite (with excess value of soft phase) hot-pressed magnets show magnetic properties that are comparable with those of sintered anisotropic magnets. However, the main disadvantage of nanocrystalline magnets in comparison with sintered magnets is a lower coercivity and slightly lower remanence, which results in lower (BH)max. Typically sintered magnets have Jr41.2 T, J H c ¼ 110021500 kA m1 and (BH)max in the range of 200–400 kJ m3, whereas melt spun or mechanically alloyed powder has J r ¼ 0:720:8 T, J H c ¼ 100021300 kA m1 and (BH)max ¼ 80–135 kJ m3. For practical applications the powders needs to be hot pressed into bulk magnets or bonded with a polymer, but with significantly lower Jr and (BH)max. Demagnetization is spontaneous in nanostructures, because the nanocrystallines grains are not magnetically isolated by a paramagnetic phase [6]. In order to increase the coercivity of a nanocrystalline magnet, a fine homogeneous microstructure is required. It can be achieved by optimizing the alloy composition and the processing conditions [10–12]. An attractive and successful method of increasing JHc is to substitute Fe with a small amount of refractory elements, like Cr, Zr, W, Nb, Ti and V [11,12]. Refractory elements generally suppress grain growth during heat treatment and the obtained grain size distribution is narrow and homogeneous [12]. The squareness of the hysteresis loop is improved as well. In this work the influence of titanium and dysprosium additions on the structural and magnetic properties of Nd16zDyzFe76yx CoyTixB8 (x ¼ 0; 0:25; 0:5; 0:75; 1:0; 2:0; y ¼ 0; 11:6; z ¼ 0; 3:2— all in at%) powders is investigated.
2. Experimental details Magnetic powders were prepared from high purity elemental Nd, Dy, Fe, Co, B and Ti
powders, using a SPEX 8000 mill. Hardened steel containers including steel balls and powders were applied in the mechanical alloying process. Powder handling was done in a glove box with an automatically controlled atmosphere. Mechanical alloying was done under argon atmosphere, for 48 h. The as-milled powder was then heat treated under argon atmosphere in the 973–1073 K range of temperature, for 30 or 60 min. The morphology of the powders was observed by scanning electron microscopy (SEM) using a Tescan Vega 5135 microscope. Demagnetization curves were measured using a home-made vibrating sample magnetometer in an external field of 2 T. The structural analysis of the heat treated powders was made by transmission mo¨ssbauer spectrometry at room temperature, using a conventional 57Co source in a rhodium matrix. The isomer shift (relative to metallic a-Fe at room temperature), quadrupolar splitting and hyperfine field are denoted by d, D and B, respectively. Estimated errors for the hyperfine parameters originate from the statistical errors s given by the fitting program, taking 3s.
3. Results and discussion 3.1. Morphology of the powders Elemental powder mixture after 48 h of mechanical alloying consists of highly dispersed a-Fe regions dispersed in an amorphous matrix [9]. A heat treatment in the 973–1073 K range of temperature results in the crystallization of the tetragonal Nd2Fe14B phase. For the mechanical alloying process, the Nd2Fe14B phase is the major phase in the Nd16F76B8 powder, a weak amount of the Nd1.1Fe4B4 phase being detected (see Section 3.3). Nd16F76B8 powder after mechanical alloying has no excess of a-Fe. For a Nd content lower than 16%, a small amount of a-Fe is obtained. The phenomenon was not observed in high-energy ball-milled (HEBM) powders [9]. The reason for such behavior is probably connected with different start points of both processes. In the case of mechanical alloying and HEBM we start from the elemental powders and arc-melted alloy, respectively, and the milling behavior and formation
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Fig. 1. Morphology of the elemental powders Nd (a), Fe (b), Ti (c), B (d), after mechanical alloying (e) and after heat treatment at 973 K/30 min (f).
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mechanism of Nd2Fe14B structure is different in both processes. Fig. 1 shows SEM images of the Nd, Fe, Ti and B elemental powders used for mechanical alloying. The SEM images of the Nd16Fe75.50Ti0.50B8 powder after 48 h of mechanical alloying and after heat treatment at 973 K for 30 min are shown as well. In spite of both the large size of neodymium and boron particles (200 mm) and the irregular shape of titanium particles, a homogeneous and fine grained powder mixture composed of agglomerates is formed after mechanical alloying (Fig. 1e). After heat treatment at 973 K for 30 min, the morphology of the powder does not significantly change (Fig. 1f). The grain size of the mechanically alloyed refractory-doped nanocrystalline powders is about 20 nm [12]. This grain size is accurate to achieve a high value of coercivity.
(a)
3.2. Magnetic properties Fig. 2 presents the demagnetization curves of parent (x ¼ 0) and some Ti-doped (x ¼ 0:5 and 0.75) nanocrystalline Nd16Fe76xTixB8 powders. The coercivity JHc of the powders, deduced from the demagnetizing curves, is presented in Table 1. The variation of JHc with the Ti content x in the Nd16Fe76xTixB8 powders is shown in Fig. 3. The maximum coercivity (1115 kA m1) is obtained for the x ¼ 0:5 powder. A higher or a lower Ti content results in a lower coercivity. The lowest value of coercivity is obtained for the x ¼ 2 powder, and this value is lower than that of the x ¼ 0 powder. The remanence is in the range of 0.6–0.75 T (Table 1). Introduction of Ti as well as Dy results in a reduction of remanence from 0.75 to 0.64 T for the parent Nd16Fe76B8 and Nd12.8Dy3.2Fe75.5Ti0.50B8, respectively. These results are consistent with those of Yan et al. [13], who reported a maximum coercivity of 1150 kA m1 for 1.5 wt% of Ti, in a Nd-rich Nd22Fe71B7 parent alloy, and suggested that a higher Ti content would lead to an abnormal grain growth in the magnets, thus resulting in a lower coercivity. According to Fidler [11], the addition of refractory elements to a Nd–Fe–B alloy leads to the formation of borides both in intergranular regions and within the Nd2Fe14B phase [11],
(b) Fig. 2. Demagnetization curves for the parent Nd16Fe76B8 (a) and for some Ti doped Nd16Fe76xTixB8 (b) powders.
resulting in grain growth inhibition during a heat treatment, that leads to a refinement of the microstructure. Both structural refinement and formation of borides, which can act as domain wall pinning centers, result into an increase of the coercivity. The hysteresis loop is thus significantly improved in refractory doped magnets [14], as both coercivity and energy product increase. Introduction of refractory elements results in a
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Table 1 Coercivity of the Nd2Fe14B-type nanocrystalline magnets heat treated at 973 K/30 min (other conditions are indicated in brackets) Composition
Coercivity, JHc (kA m1)
Remanence, Jr (T)
Nd16Fe76B8 Nd16Fe75.75Ti0.25B8 Nd16Fe75.50Ti0.50B8 Nd16Fe75.25Ti0.75B8 Nd16Fe75.0Ti1.0B8 Nd16Fe74.0Ti2.0B8 Nd16Fe74.0Ti2.0B8 (heat treated at 1073 K/30 min) Nd16Fe74.0Ti2.0B8 (heat treated at 973 K/60 min) Nd16Fe63.9Co11.6Ti0.5B8 Nd12.8Dy3.2Fe76B8 Nd12.8Dy3.2Fe75.5Ti0.50B8 Nd12.8Dy3.2Fe63.9Co11.6Ti0.50B8
796 955 1115 1098 955 478 661 637 796 1234 1234 971
0.75 0.73 0.71 0.71 0.67 0.60 0.64 0.64 0.70 0.64 0.64 0.63
Co results in higher Curie temperature, saturation polarization [7] and corrosion resistance [17]. 3.3. Mo¨ssbauer investigation
Fig. 3. Coercivity as a function of Ti content in Nd16Fe76x TixB8 powders.
smooth demagnetization curve with improved squareness, which is connected with a very fine and narrow distribution of grain size [11,14]. Compared to the Nd16Fe76xTixB8 powders, a higher value of the coercivity is obtained with partial substitution of Nd by Dy, and JHc reaches a value of 1234 kA m1 (Table 1). This is related to the increase of the anisotropy field of the Nd2Fe14B phase due to the presence of Dy [15]. Unsurprisingly, the presence of Co significantly reduces the coercivity of the powders. This is correlated to a reduction of the anisotropy field of the Nd2Fe14B phase [16]. However, the presence of
All the Mo¨ssbauer spectra were recorded at room temperature. The magnetic contribution of the Nd2Fe14B phase (six sextets) is the main contribution in all the spectra. This indicates that the Nd2Fe14B phase is the main phase in all the samples. Extra paramagnetic or magnetic contributions are fitted as well, which are discussed hereafter. The influence of Ti content in Nd16Fe76xTixB8 is evidenced in Fig. 4. For xp1, the Mo¨ssbauer relative intensity of the Nd2Fe14B contribution is 98%. A paramagnetic contribution is also detected (this contribution is displayed in Fig. 4), with a relative intensity of about 2% for all the spectra. The hyperfine parameters of the paramagnetic contribution (d ¼ 0:05 0:04 mm=s, D ¼ 0:70 0:06 mm=s) indicate that the corresponding phase is very probably the Nd1.1Fe4B4 boride phase. For x ¼ 2 (Table 2), the contribution of Nd2Fe14B is the main contribution, but its relative intensity (61%) is lower than for the samples with xp1. The same paramagnetic contribution is fitted, with the same intensity. However, two extra magnetic contributions are detected, which correspond to a-Fe (17%) and to an amorphous component (20%). The presence of an amorphous component
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-10
Velocity (mm /s) 0
+10
1.00
X=0 0.97 1.00
X = 0.25
0.97 1.00
Absorption (%)
X = 0.50
0.97 1.00
X = 0.75
0.97 1.00
X = 1.0
0.98 1.00
X = 2.0 0.99 Fig. 4. Room temperature Mo¨ssbauer spectra of the Nd16Fe76xTixB8 powders heat treated at 973 K/30 min. The contribution of the paramagnetic Nd1.1Fe4B4 phase is displayed in all the spectra. The magnetic contributions of both a-Fe(Ti) and amorphous phases are displayed in the spectrum of the x ¼ 2:0 powder.
in this powder suggests that the recrystallization is incomplete. For the Nd16Fe74Ti2B8 sample heat treated at 973 K/30 min (a) and 973 K/60 min (b), two extra
magnetic contributions are detected, which correspond to a-Fe(Ti) and to an amorphous component (Fig. 5). The relative intensities of the two extra components decrease as both temperature and time increase (Table 2). In the spectrum of the sample heat treated at 1073 K/30 min, no extra magnetic contribution is detected. The Mo¨ssbauer relative intensity of the contribution of the Nd2Fe14B phase in the Nd16Fe74Ti2B8 powder increases with both time and temperature of the heat treatment. This indicates that the volume fraction of the Nd2Fe14B phase increases with the annealing temperature, or with the annealing time at a given temperature. The results obtained here indicate that the low value of the coercivity, obtained for the Nd16Fe74Ti2B8 powder heat treated at 973 K/ 30 min (Fig. 3), is due to the incomplete crystallization of the Nd2Fe14B phase, in relation to the formation of extra phases. As annealing temperature or time increases, the proportion of the Nd2Fe14B phase increases, leading to an increase of the coercivity. Finally, the effect of the substitution of neodymium by dysprosium and of iron by cobalt in the Nd16Fe75.5Ti0.5B8 alloy was studied. The room temperature Mo¨ssbauer spectra of the powders with Dy and Co are shown in Fig. 6. In all these spectra the magnetic contribution of Nd2Fe14B is the main contribution (97–98% relative intensity), and the paramagnetic contribution of Nd1.1Fe4B4 (3–2% relative intensity) is the only extra contribution detected. The spectra of the Co containing samples are modified compared to the spectra of Co free samples (Fig. 6). This modification is interpreted as due to the presence of Co atoms in the Fe sites of the Nd2Fe14B structure. Consequently, the mean hyperfine field of the Nd2Fe14B phase is changed, being lower compared to Co free samples with the same Ti content (Fig. 7). Unsurprisingly, the presence of Dy has no significant influence on the mean hyperfine field of the Nd2Fe14B phase, as shown in Fig. 7. These results show that the powders are almost single Nd2Fe14B phase for xp1. When Ti content increases up to x ¼ 2, the powders heat treated at 973 K are no more single phase. As x increases, the
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Table 2 Relative intensity of the different contributions used to fit the Mo¨ssbauer spectra of the Nd16Fe74Ti2B8 powder after different heat treatments Annealing parameters
Nd2Fe14B (%)
Nd1.1Fe4B4 (%)
a-Fe(Ti) (%)
Amorphous contribution (%)
973 K/30 min 973 K/60 min 1073 K/30 min
6172 8172 9871
271 271 271%
1772 371 —
2072 1472 —
Velocity (mm/s) -10 1.00
0
Velocity (mm/s) +10
-10
0
+10
1.00 Nd1.1Fe4B4
0.97 1.00 Absorption(%)
Absorption (%)
0.99 1.00 α-Fe(Ti)
0.99 1.00
0.97 1.00 amorphous contribution
0.99
(a)
0.97
(b)
Fig. 5. Room temperature Mo¨ssbauer spectra of Nd16Fe74Ti2B8 powder for heat treatment at 973 K/30 min (a) and 973 K/60 min (b). The contributions of the extra phases are displayed as a dotted line.
mean hyperfine field of the Nd2Fe14B phase decreases, as shown in Fig. 7. This can be related to the presence of an increasing proportion of Ti in the Nd2Fe14B phase. For x41, the mean hyperfine field remains constant, indicating that the Ti content in the Nd2Fe14B phase does not increase anymore. As Ti is soluble in the a-Fe phase, the Ti atoms that are not dissolved in the Nd2Fe14B phase could promote the formation of a-Fe(Ti) regions, in agreement with the presence of the a-Fe contribution in the x ¼ 2 spectrum.
4. Conclusion Mechanical alloying is a successful process for the preparation of nanocrystalline magnets from microcrystalline elemental powders. Refractory Ti and rare-earth Dy are very effective in coercivity enhancement. Coercivity increases from 796 kA m1 for the parent Nd16Fe76B8 alloy, up to 1115 and 1234 kA m1 for Nd16Fe75.5Ti0.5B8 and Nd12.8Dy3.2Fe75.5Ti0.50B8 alloys, respectively. These results show that the amount of Ti content has a strong influence on the phase
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Velocity (mm/s) -10
0
+10
1.00 z=3.2 y=0 x=0
structure of mechanically alloyed powders. For xp1, the powders are single Nd2Fe14B phase. For higher Ti content (for instance x ¼ 2), the powders are no more single phase as a-Fe is formed, and, moreover, the presence of an amorphous contribution suggests that the crystallization process is incomplete at 973 K.
0.98 1.00
Acknowledgements z=3.2 Absorption (%)
y=0 x=0.5 0.97 1.00
Nanomaterials Laboratory was renovated under financial support of the Foundation for Polish Science (FNP) under a MILAB 83/2004 project. References
z=0 y=11.6 x=0.5 0.98 1.00
z=3.2 y=11.6 x=0.5 0.98
Fig. 6. Room temperature Mo¨ssbauer spectra of Nd16zDyz Fe76yxCoyTixB8 powders.
Fig. 7. Mean hyperfine field of the Nd2Fe14B phase as a function of the Ti content. The full and open symbols refer to the Nd16(Fe,Co)76xTixB8 and Nd12.8Dy3.2(Fe,Co)76xTixB8 powders, respectively. The dashed line is a guide for the eye.
[1] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078. [2] D. Brown, B.M. Ma, Z. Chen, J. Magn. Magn. Mat. 248 (2002) 432. [3] B.E. Davies, R.S. Mottram, I.R. Harris, Mat. Chem. Phys. 67 (2001) 272. [4] T. Schrefl, J. Fidler, J. Magn. Magn. Mat. 177–181 (1998) 970. [5] R.W. McCallum, A.M. Kadin, G.B. Clemente, J.E. Keem, J. Appl. Phys. 61 (1987) 3577. [6] T. Schrefl, J. Fidler, H. Kronmu¨ller, Phys. Rev. B 49 (1994) 6100. [7] M. Jurczyk, S.J. Collocott, J.B. Dunlop, P.B. Gwan, J. Phys. D: Appl. Phys 29 (1996) 2284. [8] M. Jurczyk, J. Alloys Comp. 299 (2000) 283. [9] M. Jurczyk, J. Jakubowicz, J. Magn. Magn. Mat. 185 (1998) 66. [10] Z. Chen, H. Okumura, G.C. Hadjipanayis, Q. Chen, J. Alloys Comp. 327 (2001) 201. [11] J. Fidler, in: Proceedings of the Seventh International Symposium on Magnetic Anisotropy and Coercivity in Rare Earth Transition Metal Alloys, The University of Western Australia, 1992, p. 11. [12] J. Jakubowicz, J. Alloys Comp. 351 (2003) 196. [13] A. Yan, X. Song, Z. Chen, X. Wang, J. Magn. Magn. Mater. 185 (1998) 369. [14] Z. Chen, H. Okamura, G.C. Hadipanayis, Q. Chen, J. Alloys Comp. 327 (2001) 201. [15] K.H.J. Buschow, Mat. Sci. Rep., A Rev. J. 1 (1986) 1. [16] K.H.J. Buschow, Rep. Prog. Phys. 54 (1991) 1123. [17] J.M. Le Breton, S. Steayert, M. Jurczyk, J. Teillet, in: Proceedings of the 15th International Workshop on rareEarth Magnets and their Applications, Dresden, 1998, p. 887.
Journal of Magnetism and Magnetic Materials 301 (2006) 279–286 www.elsevier.com/locate/jmmm
Structure and magnetic properties of (Nd,Dy)16(Fe,Co)76xTixB8 powders prepared by mechanical alloying J. Jakubowicza,, J.-M. Le Bretonb a
Institute of Materials Science and Engineering, Poznan University of Technology, M. Sklodowska-Curie 5 Sq., 60-965 Poznan, Poland b Groupe de Physique des Mate´riaux, UMR CNRS 6634, Universite´ de Rouen, Avenue de l’Universite´, 76801 Saint Etienne du Rouvray Cedex, France Received 4 May 2005; received in revised form 23 June 2005 Available online 24 August 2005
Abstract Nanocrystalline (Nd,Dy)16(Fe,Co)76xTixB8 magnets were prepared by mechanical alloying and respective heat treatment at 973–1073 K/30–60 min. An addition of 0.5 at % of Ti results in an increase of coercivity from 796 to 1115 kA m1. Partial substitution of Nd by Dy results in an additional increase of coercivity up to 1234 kA m1. Mo¨ssbauer investigations shows that for xp1 the (Nd,Dy)16(Fe,Co)76xTixB8 powders are single phase. For higher Ti contents (x41) the mechanically alloyed powders heat treated at 973 K are no more single phase, and the coercivity decreases due to the presence of an amorphous phase. A heat treatment at a higher temperature (1073 K) for longer time (1 h) results in the full recrystallisation of powders. The mean hyperfine field of the Nd2Fe14B phase decreases for titanium contents of 0pxp1, and remains constant for x41. This indicates that the Ti content in the Nd2Fe14B phase reaches its maximum value. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 75.50.Kj; 75.50.Ww; 76.80.+y Keywords: Mechanical alloying; Magnets; Nanocrystals; Nd2Fe14B; Mo¨ssbauer
1. Introduction
Corresponding author. Tel./fax: +48 61 665 3576.
E-mail address: [email protected] (J. Jakubowicz).
Since 1984 Nd2Fe14B-type materials have been the most investigated type of permanent magnet applications [1–3]. With the application of new processing technologies, like melt spinning, mechanical alloying or high-energy ball milling, new
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.07.025
ARTICLE IN PRESS 280
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classes of materials were made, which are based on nanostructures, having a grain size below 30 nm [4–9]. In nanocrystalline materials, exchange coupling appears between ferromagnetic grains, resulting in both remanence enhancement and high coercivity [4–9]. Nanocrystalline and nanocomposite (with excess value of soft phase) hot-pressed magnets show magnetic properties that are comparable with those of sintered anisotropic magnets. However, the main disadvantage of nanocrystalline magnets in comparison with sintered magnets is a lower coercivity and slightly lower remanence, which results in lower (BH)max. Typically sintered magnets have Jr41.2 T, J H c ¼ 110021500 kA m1 and (BH)max in the range of 200–400 kJ m3, whereas melt spun or mechanically alloyed powder has J r ¼ 0:720:8 T, J H c ¼ 100021300 kA m1 and (BH)max ¼ 80–135 kJ m3. For practical applications the powders needs to be hot pressed into bulk magnets or bonded with a polymer, but with significantly lower Jr and (BH)max. Demagnetization is spontaneous in nanostructures, because the nanocrystallines grains are not magnetically isolated by a paramagnetic phase [6]. In order to increase the coercivity of a nanocrystalline magnet, a fine homogeneous microstructure is required. It can be achieved by optimizing the alloy composition and the processing conditions [10–12]. An attractive and successful method of increasing JHc is to substitute Fe with a small amount of refractory elements, like Cr, Zr, W, Nb, Ti and V [11,12]. Refractory elements generally suppress grain growth during heat treatment and the obtained grain size distribution is narrow and homogeneous [12]. The squareness of the hysteresis loop is improved as well. In this work the influence of titanium and dysprosium additions on the structural and magnetic properties of Nd16zDyzFe76yx CoyTixB8 (x ¼ 0; 0:25; 0:5; 0:75; 1:0; 2:0; y ¼ 0; 11:6; z ¼ 0; 3:2— all in at%) powders is investigated.
2. Experimental details Magnetic powders were prepared from high purity elemental Nd, Dy, Fe, Co, B and Ti
powders, using a SPEX 8000 mill. Hardened steel containers including steel balls and powders were applied in the mechanical alloying process. Powder handling was done in a glove box with an automatically controlled atmosphere. Mechanical alloying was done under argon atmosphere, for 48 h. The as-milled powder was then heat treated under argon atmosphere in the 973–1073 K range of temperature, for 30 or 60 min. The morphology of the powders was observed by scanning electron microscopy (SEM) using a Tescan Vega 5135 microscope. Demagnetization curves were measured using a home-made vibrating sample magnetometer in an external field of 2 T. The structural analysis of the heat treated powders was made by transmission mo¨ssbauer spectrometry at room temperature, using a conventional 57Co source in a rhodium matrix. The isomer shift (relative to metallic a-Fe at room temperature), quadrupolar splitting and hyperfine field are denoted by d, D and B, respectively. Estimated errors for the hyperfine parameters originate from the statistical errors s given by the fitting program, taking 3s.
3. Results and discussion 3.1. Morphology of the powders Elemental powder mixture after 48 h of mechanical alloying consists of highly dispersed a-Fe regions dispersed in an amorphous matrix [9]. A heat treatment in the 973–1073 K range of temperature results in the crystallization of the tetragonal Nd2Fe14B phase. For the mechanical alloying process, the Nd2Fe14B phase is the major phase in the Nd16F76B8 powder, a weak amount of the Nd1.1Fe4B4 phase being detected (see Section 3.3). Nd16F76B8 powder after mechanical alloying has no excess of a-Fe. For a Nd content lower than 16%, a small amount of a-Fe is obtained. The phenomenon was not observed in high-energy ball-milled (HEBM) powders [9]. The reason for such behavior is probably connected with different start points of both processes. In the case of mechanical alloying and HEBM we start from the elemental powders and arc-melted alloy, respectively, and the milling behavior and formation
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Fig. 1. Morphology of the elemental powders Nd (a), Fe (b), Ti (c), B (d), after mechanical alloying (e) and after heat treatment at 973 K/30 min (f).
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mechanism of Nd2Fe14B structure is different in both processes. Fig. 1 shows SEM images of the Nd, Fe, Ti and B elemental powders used for mechanical alloying. The SEM images of the Nd16Fe75.50Ti0.50B8 powder after 48 h of mechanical alloying and after heat treatment at 973 K for 30 min are shown as well. In spite of both the large size of neodymium and boron particles (200 mm) and the irregular shape of titanium particles, a homogeneous and fine grained powder mixture composed of agglomerates is formed after mechanical alloying (Fig. 1e). After heat treatment at 973 K for 30 min, the morphology of the powder does not significantly change (Fig. 1f). The grain size of the mechanically alloyed refractory-doped nanocrystalline powders is about 20 nm [12]. This grain size is accurate to achieve a high value of coercivity.
(a)
3.2. Magnetic properties Fig. 2 presents the demagnetization curves of parent (x ¼ 0) and some Ti-doped (x ¼ 0:5 and 0.75) nanocrystalline Nd16Fe76xTixB8 powders. The coercivity JHc of the powders, deduced from the demagnetizing curves, is presented in Table 1. The variation of JHc with the Ti content x in the Nd16Fe76xTixB8 powders is shown in Fig. 3. The maximum coercivity (1115 kA m1) is obtained for the x ¼ 0:5 powder. A higher or a lower Ti content results in a lower coercivity. The lowest value of coercivity is obtained for the x ¼ 2 powder, and this value is lower than that of the x ¼ 0 powder. The remanence is in the range of 0.6–0.75 T (Table 1). Introduction of Ti as well as Dy results in a reduction of remanence from 0.75 to 0.64 T for the parent Nd16Fe76B8 and Nd12.8Dy3.2Fe75.5Ti0.50B8, respectively. These results are consistent with those of Yan et al. [13], who reported a maximum coercivity of 1150 kA m1 for 1.5 wt% of Ti, in a Nd-rich Nd22Fe71B7 parent alloy, and suggested that a higher Ti content would lead to an abnormal grain growth in the magnets, thus resulting in a lower coercivity. According to Fidler [11], the addition of refractory elements to a Nd–Fe–B alloy leads to the formation of borides both in intergranular regions and within the Nd2Fe14B phase [11],
(b) Fig. 2. Demagnetization curves for the parent Nd16Fe76B8 (a) and for some Ti doped Nd16Fe76xTixB8 (b) powders.
resulting in grain growth inhibition during a heat treatment, that leads to a refinement of the microstructure. Both structural refinement and formation of borides, which can act as domain wall pinning centers, result into an increase of the coercivity. The hysteresis loop is thus significantly improved in refractory doped magnets [14], as both coercivity and energy product increase. Introduction of refractory elements results in a
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Table 1 Coercivity of the Nd2Fe14B-type nanocrystalline magnets heat treated at 973 K/30 min (other conditions are indicated in brackets) Composition
Coercivity, JHc (kA m1)
Remanence, Jr (T)
Nd16Fe76B8 Nd16Fe75.75Ti0.25B8 Nd16Fe75.50Ti0.50B8 Nd16Fe75.25Ti0.75B8 Nd16Fe75.0Ti1.0B8 Nd16Fe74.0Ti2.0B8 Nd16Fe74.0Ti2.0B8 (heat treated at 1073 K/30 min) Nd16Fe74.0Ti2.0B8 (heat treated at 973 K/60 min) Nd16Fe63.9Co11.6Ti0.5B8 Nd12.8Dy3.2Fe76B8 Nd12.8Dy3.2Fe75.5Ti0.50B8 Nd12.8Dy3.2Fe63.9Co11.6Ti0.50B8
796 955 1115 1098 955 478 661 637 796 1234 1234 971
0.75 0.73 0.71 0.71 0.67 0.60 0.64 0.64 0.70 0.64 0.64 0.63
Co results in higher Curie temperature, saturation polarization [7] and corrosion resistance [17]. 3.3. Mo¨ssbauer investigation
Fig. 3. Coercivity as a function of Ti content in Nd16Fe76x TixB8 powders.
smooth demagnetization curve with improved squareness, which is connected with a very fine and narrow distribution of grain size [11,14]. Compared to the Nd16Fe76xTixB8 powders, a higher value of the coercivity is obtained with partial substitution of Nd by Dy, and JHc reaches a value of 1234 kA m1 (Table 1). This is related to the increase of the anisotropy field of the Nd2Fe14B phase due to the presence of Dy [15]. Unsurprisingly, the presence of Co significantly reduces the coercivity of the powders. This is correlated to a reduction of the anisotropy field of the Nd2Fe14B phase [16]. However, the presence of
All the Mo¨ssbauer spectra were recorded at room temperature. The magnetic contribution of the Nd2Fe14B phase (six sextets) is the main contribution in all the spectra. This indicates that the Nd2Fe14B phase is the main phase in all the samples. Extra paramagnetic or magnetic contributions are fitted as well, which are discussed hereafter. The influence of Ti content in Nd16Fe76xTixB8 is evidenced in Fig. 4. For xp1, the Mo¨ssbauer relative intensity of the Nd2Fe14B contribution is 98%. A paramagnetic contribution is also detected (this contribution is displayed in Fig. 4), with a relative intensity of about 2% for all the spectra. The hyperfine parameters of the paramagnetic contribution (d ¼ 0:05 0:04 mm=s, D ¼ 0:70 0:06 mm=s) indicate that the corresponding phase is very probably the Nd1.1Fe4B4 boride phase. For x ¼ 2 (Table 2), the contribution of Nd2Fe14B is the main contribution, but its relative intensity (61%) is lower than for the samples with xp1. The same paramagnetic contribution is fitted, with the same intensity. However, two extra magnetic contributions are detected, which correspond to a-Fe (17%) and to an amorphous component (20%). The presence of an amorphous component
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-10
Velocity (mm /s) 0
+10
1.00
X=0 0.97 1.00
X = 0.25
0.97 1.00
Absorption (%)
X = 0.50
0.97 1.00
X = 0.75
0.97 1.00
X = 1.0
0.98 1.00
X = 2.0 0.99 Fig. 4. Room temperature Mo¨ssbauer spectra of the Nd16Fe76xTixB8 powders heat treated at 973 K/30 min. The contribution of the paramagnetic Nd1.1Fe4B4 phase is displayed in all the spectra. The magnetic contributions of both a-Fe(Ti) and amorphous phases are displayed in the spectrum of the x ¼ 2:0 powder.
in this powder suggests that the recrystallization is incomplete. For the Nd16Fe74Ti2B8 sample heat treated at 973 K/30 min (a) and 973 K/60 min (b), two extra
magnetic contributions are detected, which correspond to a-Fe(Ti) and to an amorphous component (Fig. 5). The relative intensities of the two extra components decrease as both temperature and time increase (Table 2). In the spectrum of the sample heat treated at 1073 K/30 min, no extra magnetic contribution is detected. The Mo¨ssbauer relative intensity of the contribution of the Nd2Fe14B phase in the Nd16Fe74Ti2B8 powder increases with both time and temperature of the heat treatment. This indicates that the volume fraction of the Nd2Fe14B phase increases with the annealing temperature, or with the annealing time at a given temperature. The results obtained here indicate that the low value of the coercivity, obtained for the Nd16Fe74Ti2B8 powder heat treated at 973 K/ 30 min (Fig. 3), is due to the incomplete crystallization of the Nd2Fe14B phase, in relation to the formation of extra phases. As annealing temperature or time increases, the proportion of the Nd2Fe14B phase increases, leading to an increase of the coercivity. Finally, the effect of the substitution of neodymium by dysprosium and of iron by cobalt in the Nd16Fe75.5Ti0.5B8 alloy was studied. The room temperature Mo¨ssbauer spectra of the powders with Dy and Co are shown in Fig. 6. In all these spectra the magnetic contribution of Nd2Fe14B is the main contribution (97–98% relative intensity), and the paramagnetic contribution of Nd1.1Fe4B4 (3–2% relative intensity) is the only extra contribution detected. The spectra of the Co containing samples are modified compared to the spectra of Co free samples (Fig. 6). This modification is interpreted as due to the presence of Co atoms in the Fe sites of the Nd2Fe14B structure. Consequently, the mean hyperfine field of the Nd2Fe14B phase is changed, being lower compared to Co free samples with the same Ti content (Fig. 7). Unsurprisingly, the presence of Dy has no significant influence on the mean hyperfine field of the Nd2Fe14B phase, as shown in Fig. 7. These results show that the powders are almost single Nd2Fe14B phase for xp1. When Ti content increases up to x ¼ 2, the powders heat treated at 973 K are no more single phase. As x increases, the
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Table 2 Relative intensity of the different contributions used to fit the Mo¨ssbauer spectra of the Nd16Fe74Ti2B8 powder after different heat treatments Annealing parameters
Nd2Fe14B (%)
Nd1.1Fe4B4 (%)
a-Fe(Ti) (%)
Amorphous contribution (%)
973 K/30 min 973 K/60 min 1073 K/30 min
6172 8172 9871
271 271 271%
1772 371 —
2072 1472 —
Velocity (mm/s) -10 1.00
0
Velocity (mm/s) +10
-10
0
+10
1.00 Nd1.1Fe4B4
0.97 1.00 Absorption(%)
Absorption (%)
0.99 1.00 α-Fe(Ti)
0.99 1.00
0.97 1.00 amorphous contribution
0.99
(a)
0.97
(b)
Fig. 5. Room temperature Mo¨ssbauer spectra of Nd16Fe74Ti2B8 powder for heat treatment at 973 K/30 min (a) and 973 K/60 min (b). The contributions of the extra phases are displayed as a dotted line.
mean hyperfine field of the Nd2Fe14B phase decreases, as shown in Fig. 7. This can be related to the presence of an increasing proportion of Ti in the Nd2Fe14B phase. For x41, the mean hyperfine field remains constant, indicating that the Ti content in the Nd2Fe14B phase does not increase anymore. As Ti is soluble in the a-Fe phase, the Ti atoms that are not dissolved in the Nd2Fe14B phase could promote the formation of a-Fe(Ti) regions, in agreement with the presence of the a-Fe contribution in the x ¼ 2 spectrum.
4. Conclusion Mechanical alloying is a successful process for the preparation of nanocrystalline magnets from microcrystalline elemental powders. Refractory Ti and rare-earth Dy are very effective in coercivity enhancement. Coercivity increases from 796 kA m1 for the parent Nd16Fe76B8 alloy, up to 1115 and 1234 kA m1 for Nd16Fe75.5Ti0.5B8 and Nd12.8Dy3.2Fe75.5Ti0.50B8 alloys, respectively. These results show that the amount of Ti content has a strong influence on the phase
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Velocity (mm/s) -10
0
+10
1.00 z=3.2 y=0 x=0
structure of mechanically alloyed powders. For xp1, the powders are single Nd2Fe14B phase. For higher Ti content (for instance x ¼ 2), the powders are no more single phase as a-Fe is formed, and, moreover, the presence of an amorphous contribution suggests that the crystallization process is incomplete at 973 K.
0.98 1.00
Acknowledgements z=3.2 Absorption (%)
y=0 x=0.5 0.97 1.00
Nanomaterials Laboratory was renovated under financial support of the Foundation for Polish Science (FNP) under a MILAB 83/2004 project. References
z=0 y=11.6 x=0.5 0.98 1.00
z=3.2 y=11.6 x=0.5 0.98
Fig. 6. Room temperature Mo¨ssbauer spectra of Nd16zDyz Fe76yxCoyTixB8 powders.
Fig. 7. Mean hyperfine field of the Nd2Fe14B phase as a function of the Ti content. The full and open symbols refer to the Nd16(Fe,Co)76xTixB8 and Nd12.8Dy3.2(Fe,Co)76xTixB8 powders, respectively. The dashed line is a guide for the eye.
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