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The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys
Accepted Manuscript The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys Kuo Men, Kuoshe Li, Yang Luo, Dunbo Yu, Kun Zhang, Jinling Jin, Yongjun Mao PII: DOI: Reference:
S0925-8388(15)00472-7 http://dx.doi.org/10.1016/j.jallcom.2015.02.046 JALCOM 33420
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
13 October 2014 29 January 2015 6 February 2015
Please cite this article as: K. Men, K. Li, Y. Luo, D. Yu, K. Zhang, J. Jin, Y. Mao, The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/ j.jallcom.2015.02.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys Kuo Men
Kuoshe Li
Yang Luo
Dunbo Yu*
Kun Zhang
Jinling Jin
Yongjun
Mao
National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co., Ltd., Beijing 100088, China
Corresponding author: Dunbo Yu (Tel.: +86-10-82241180, E-mail: [email protected])
Abstract The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy was investigated in order to gain further understanding of the structural evolution of the α-Fe and metastable phase, and then verified it by TEM. The result shows that the crystallization can be viewed as a process consisted of the following two steps. Firstly, the TbCu7-type metastable phase and a small amount of α-Fe precipitate from the matrix. Then, the TbCu7-type metastable phase is decomposed into the α-Fe and Nd2Fe14B phase with suitable sizes. The decomposition of metastable phase can promote the crystallization of Nd2Fe14B phase, so it could be considered as an ‘autocatalysis’ process. In addition, the crystallization kinetic parameters of the Nd2Fe14B phase have also been obtained base on the resolved differential scanning 1
calorimetry peaks. The results provide a physical and dynamic insight into the crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys. Keyword: crystallization; metastable phase; autocatalysis kinetic equation 1. Introduction Nanocomposite two-phases magnets [1] composed of hard and soft magnetic phases with nanoscale grains have been studied extensively, since they have larger saturation magnetization, larger maximum energy product and better corrosion resistance. The strong exchange coupling interaction between two phases can make the magnet exhibit a single-phase hard magnetic property [2-4]. In order to obtain the magnets with high performance, attempts have been made to prepare nanocomposite two-phases magnets consisted of suitable grain size and to obtain the excellent magnetic performances. Among this, one of the most effective method is nano-crystallization of metallic glasses [5]. However, the crystallization behavior in this system is so complex that the knowledge of the crystallization mechanism is rare, for the process is influenced by lots of factors, such as precipitation of metastable phase [6], elements doping [7-9], heating rate [10], and so on. X. H. Cui [9] found that Nb addition enhanced the glass forming ability. Y. Gao [6] reported that Nd2Fe23B3-type phase can precipitate in the initial crystallization stage of the similar matrix alloy. J. Gao [11] studied the solidification of similar super-cooled melt and found that the TbCu7-type metastable phase forms in this process firstly, and then both α-Fe and Nd2Fe14B-type phases precipitate out from the matrix. Gabay and coworkers [12] considered that the 2
TbCu7-type metastable phase can be viewed as a solid solution of boron. However, the effects of metastable phase on the process is not so clear. In order to solve this problem, the crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy and the effects of primary-precipitated metastable phase on the process have been studied by DSC and non-isothermal crystallization kinetic methods. 2. Experiment The Nd9Fe85Nb0.5B5.5 alloys had been prepared by arc-melting the pure elements Nd, Fe, Nb and Fe–B alloy, and the ingots were then rapidly quenched by melt-spinning onto a molybdenum roller with a surface velocity of 24m/s and a quartz crucible orifice of 0.8 mm. The as-spun ribbons were then sealed in a quartz tube and annealed at various temperatures. The specimens were subsequently quenched after annealing at 560-730℃ for 3 min. All these procedures were performed in a high-purity argon atmosphere. The crystallization temperatures of these as spun ribbons were determined by differential scanning calorimetry (DSC) (RT-700, Mettler Toledo). Phase identification and structure analysis were performed using an X-ray powder diffractometer (XRD) (Co Ka, 0:15406 nm, D/Max-RA, Rigaku) and TEM (JEM-2010, JEOL). 3. Results and discussion
3
Fig. 1. XRD patterns of as-quenched Nd9Fe85Nb0.5B5.5 alloys with a surface velocity of 24m/s annealed at different temperatures
The evidence of phase transformation of matrix alloy has been shown in XRD patterns. As shown in Fig.1, the diffraction peaks of three phases are exhibited obviously, which represent the TbCu 7-type, α-Fe and Nd2Fe14B phases. At 560 ℃, the TbCu7-type phase appears firstly. With the increase of the annealed temperature, the peaks of the TbCu7-type phase decreases, then vanishes. Meanwhile, the Nd2Fe14B phase appears at 670 ℃. For α-Fe，at the temperatures of 560-640℃，the peak first increases slowly, and then increases rapidly between 640℃ and 730℃. So we speculate that the TbCu7-type phase would decompose at a high annealed temperature.
4
Fig. 2. DSC curves of as-quenched Nd9Fe85Nb0.5B5.5 alloy with a velocity of 24m/s at different heating rates from 10℃/min to 70℃/min.
In fact, the XRD patterns could reveal the phase precipitation of the crystallization process in a certain extent. In order to verity our viewpoint，the DCS test is introduced. Fig. 2 shows DSC curves of as-quenched Nd9Fe85Nb0.5B5.5 alloys with the different heating rates from 10℃/min to 70℃/min at the same velocity of 24m/s. The exothermic peaks in the DSC pattern represent different phases precipitating from the matrix. It is clearly revealed that the multi-step crystallization begins at 560℃. The DSC curves also exhibit the overlapping peaks between 560℃ and 800℃, which is depending on the heating rates. The appearance of such overlapping peaks could be provoked by several stages of the crystallization of compounds involving different constituents, produced during a reaction, or by crystallization of compounds involving the same constituents in several different stoichiometries [13].
5
Fig. 3. TEM and SEAD micrographs of as-quenched Nd9Fe85Nb0.5B5.5 alloys with a surface velocity of 24m/s annealed at different temperatures (a)560℃, (b)590℃, (c)620℃, (d)640℃, 6
(e)670℃, (f)700℃, (g)730℃. In the micrographs, A, B, C represent TbCu7-type metastable phase, α-Fe, Nd2Fe14B, respectively.
In order to clarify the mechanism of such crystallization behavior, the microstructures of these phase is also evaluated by the TEM. Fig. 3 shows TEM and SEAD micrographs of as-quenched ribbons with different annealed temperatures at a velocity of 24m/s. These micrographs can be viewed as the true evidence of the above-mentioned crystallization behavior, since they could reflect the state of phase precipitation and the size of grains in nanoscale. It is noted that the matrix has some degree of amorphous structure before annealing. However, some of TbCu7-type metastable phase precipitate out from the matrix at 560℃ firstly, then a small amount of α-Fe could be observed at 620℃. It is also noted that the dispersion of TbCu7-type phase and α-Fe grains tends to be uniform with temperature increasing from 560℃ to 640℃. After that, with the further increase of the annealed temperatures, the TbCu7-type phase vanishes with the appearance of the Nd2Fe14B phase. Subsequently, the grains of Nd2Fe14B phase and α-Fe grow up at the same time. And at last, the phase of Nd2Fe14B and α-Fe are distributed at the size of 20-40 nm equally. From the above, the crystallization behavior can be regarded as three phase precipitating from the matrix successively. So that the DSC curve can be resolved into three peaks. [13]
7
Fig. 4. The resolved DSC peaks for heating rate 30℃/min
The result of the resolved peaks is shown in Fig. 4, yielding three well separated symmetrical peaks, each corresponding to an individual step of the crystallization process. The three peaks are denoted as Tp1, Tp2 and Tp3 at temperatures of 609.6℃, 655.3℃ and 713.3℃, respectively. The dash line represents the fitted curve which fits the initial DSC curves (black line) well [13]. The crystallization process of the as-quenched alloy undergoes three steps for three phase precipitation. Tx1, Tx2, Tx3 present the onset precipitation temperatures of three phase. As is shown in Fig. 4, it could reasonably be inferred that the TbCu7-type metastable phase precipitates out from the matrix alloy firstly, and then it is decomposed into α-Fe and Nd2Fe14B phase. So the process of crystallization could be interpreted by two steps: (Ⅰ) TbCu7-type metastable phase and a small amount of α-Fe precipitate out from the matrix. (Fig.3 (a)-(d)) (Ⅱ) the TbCu7-type metastable phase vanishes for it is decomposed into α-Fe and Nd2Fe14B phase. (Fig.3 (e)-(g))
8
Two equations most commonly used for description of crystallization of amorphous alloys are Johnson–Mehl–Avrami(JMA) and the general Sestak-Berggren equation[14,16] The JMA model was firstly used to describe isothermal crystallization of glasses involving both nucleation and growth. However, it has been shown by Henderson[17] and also by De Bruijn[18] that this model could be extended to non-isothermal applications assuming that the crystals of a new phase grow from a constant number of nuclei and all nucleation is completed prior to crystal growth. Generally, the crystallization activation energy increases with crystallized volume fraction α in the this model. The crystallized volume fraction α at a certain point-in-time could be obtained by the ratio of the integral area at this time and the integral area of the whole exothermic peak, e.g. the crystallized volume fraction α at 700℃ was shown in Fig. 4, which could be obtained by the ratio of magenta area and cyan area. The obvious overlapping peaks shown in Fig. 4 reflect that the growth of TbCu7-type phase and the nucleation of α-Fe synchronize possibly [13], which is not suitable for the assumption of the JMA model that the growth of the grain occurs only after the nucleation has been finished. [14] In order to figure out the crystallization process and the interaction among each phase, we further study the mechanism of phase formation, the kinetic parameters and the kinetic equation. The kinetic equation of non-isothermal process can be expressed: 9
β·
= K(T) · f(α)
K(T) = A · exp (−E(α)⁄RT)
(1) (2)
in which A, β, f(α), E(α), represent the pre-exponential factor, the heating rate, the kinetic mechanism function and the crystallization activation energy, respectively. Crystallization activation energy E(α) could be calculated by Flynn-Wall-Ozawa equation [15] with different heating rates. lnβ = ln (A · E(α)⁄R · G(α)) − 1.052 × E(α)⁄RT
(3)
in which G(α) represents integral kinetic model function.
Fig.5. (a) Flynn-Wall-Ozawa equation plot for different crystallized volume fraction α, (b) the variation of crystallization activation energy E(α) with crystallized volume fraction α of Nd2Fe14B phase, (c) the normalized functions y(α) and z(α) for the crystallization process of the Nd 2Fe14B phase 10
As E(α) and G(α) are functions of crystallized volume fraction α, independent of the temperature, the E(α) at different α are calculated by taking the slopes of ln(β) vs. 1/T curves. The variation of E(α) of Nd2Fe14B phase is shown in Fig. 5(b), the crystallization activation energy of Nd2Fe14B phase is trending downward with α increasing which demonstrates that the crystallization is easy to take place kinetically. In order to verify the correct form of the crystallization kinetic model, a crucial criteria is applied. We test our experimental data using two special functions, y(α) and z(α). The maximum values being designated as α*y and α*z, respectively, defined as follows: y(α) =
z(α) ≈
· exp (E(α)⁄RT) ·T
(4) (5)
The characteristic of JMA model is that the value of α*z ≈ 0.632 and the value of α*y is always lower than the value of α*z. In addition, the curves of y(α) and z(α) functions have a convex shape. [14] The normalized functions y(α) and z(α) for the peak of Nd2Fe14B phase are shown in Fig. 5(c). It is observed that the value of α*z and α*y and the shape of y(α) is not suitable for the JMA model. Sestak-Berggren equation, an autocatalytic model, is useful for general description of the reaction in the solid state [16]: f(α) = α (1 − α)
(6)
The parameters of kinetic equation are obtained by fitting experiment data. This was calculated by Equation (1), (2), (3), (6). For M=0.95, N=0.89. 11
So the ‘autocatalysis’ crystallization kinetic equation of the Nd2Fe14B phase could be expressed: f(α) = α
.
(1 − α)
.
(7)
The crystallization process of Nd2Fe14B phase fits the autocatalytic model well. This is attributed to the internal transformation of crystal form. The TbCu 7-type metastable phase is supposed to be Nd2Fe17Bx (x=0-1) [19]. In this process, the TbCu7-type metastable phase precipitate firstly and then it is decomposed into α-Fe and Nd2Fe14B phase [20]. In the lattice of Nd2Fe14B phase, boron atoms are fixed to defined positions [21], i.e. 4g, and are generally considered to play a significant role in maintaining the stability of its lattice. In contrast, boron atoms lie in the interstitial sites of the TbCu7-type phase lattice very likely [12]. Therefore, boron atoms in TbCu7-type metastable phase always trend to lie relatively stable positions, which are the defined positions, spontaneously. Meanwhile, iron atoms are expelled to the boundary of phase, which become α-Fe finally. In addition, the iron-lacked Nd2Fe17Bx (x=0-1) transforms into Nd2Fe14B phase. This is why the process could also be considered as an ‘autocatalysis’ one reasonably. The magnetic energy is shown in Fig. 6. Magnetic energy of samples increases with crystallization temperature from 560℃ to 700℃, and then decreases at 730℃. The highest magnetic energy is about 10.96MGOe at the crystallization temperature of 700℃.
12
Fig. 6. magnetic energy of samples annealed at different temperatures
The magnetic energy is very low at 560℃ and 590℃, for there is large amorphous content at these crystallization temperatures. The microstructures are shown in the TEM pictures obviously that the grain size distribution is uniform at about 20~40nm at the temperature of 670℃ and 700℃. At about 730℃，some grain size would grow up larger than 50nm, this may result in the decrease of magnetic energy.
4. Conclusions The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy has two steps, TbCu7-type metastable phase and a small amount of α-Fe precipitate firstly, then the TbCu7-type metastable phase is decomposed into α-Fe and Nd2Fe14B phase. And the grains of Nd2Fe14B phase and α-Fe are distributed at the size of 20-40 nm equally. The decomposition of TbCu7-type metastable phase promotes the Nd2Fe14B phase formation, so it could be considered as an ‘autocatalysis’ process, which is suitable for the Sestak-Berggren equation. The crystallization kinetic equation of the Nd2Fe14B phase can be expressed by the function f(α) = α
.
(1 − α)
.
. 13
Acknowledgements The author would like to acknowledge the support by the National Natural Science Foundation of China (Grant Nos. 51401028) and the Science and Technology Project of Xicheng District, Beijing (Grant Nos. XCKJJH2013-33). The authors also wish to express their thanks to Dr. Du for help in performing the TEM analysis.
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Reference [1]Coehoorn R, de Mooij D B, de Waard C. Melts-pun permanent magnet materials containing Fe 3B as the main phase[J]. Journal of Magnetism and Magnetic Materials, 1989, 80(1):101-104. [2]Cho Y S, Kim Y B, Kim C S, et al. Magnetic properties of (Fe,Co)-B-Al-Nb alloys with ultrafine grain structure[J]. Magnetics, IEEE Transactions on, 1994, 30:4869 - 4871. [3]Schrefl T, Fischer R, Fidler J, et al. Two‐ and three‐dimensional calculation of remanence enhancement of rare‐earth based composite magnets (invited)[J]. Journal of Applied Physics, 1994, 76. [4]R. Skomcki, J. M. D. Coey. Giant energy product in nanostructured two-phase magnets [J]. Physical Review B, 1993, 48(6): 15812. [5]Lu K. Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties [J]. Materials Science and Engineering: R: Reports, 1996, 16(4): 161-221. [6]Gao Y, Zhang S, Liu B. Crystallization behavior of melt–spun Nd7Fe86Nb1B6 ribbons under different heating rates [J]. Journal of magnetism and magnetic materials, 2000, 208(3): 158-162. [7]Li J, Yang W, Zhang M, et al. Thermal stability and crystallization behavior of (Fe 0.75− x DyxB 0.2Si0.05)96Nb4
(x= 0–0.07) bulk metallic glasses [J]. Journal of Non-Crystalline Solids, 2013, 365:
42-46. [8]Chang H W, Lin W C, Shih C W, et al. Magnetic properties, phase evolution, and microstructure of directly cast Nd–Fe–Nb–Sn–B bulk magnets [J]. Journal of Alloys and Compounds, 2012, 545: 231-235. [9]Cui X H, Liu Z W, Zhong X C, et al. Melt spun and suction cast Nd-Fe-Co-B-Nb hard magnets with high Nd contents [J]. Journal of Applied Physics, 2012, 111(7): 07B508. 15
[10]Kojima A, Makino A, Inoue A. Rapid-annealing effect on the microstructure and magnetic properties of the Fe-rich nanocomposite magnets [J]. Journal of Applied Physics, 2000, 87(9): 6576-6578. [11]Gao, J., et al. Phase formation in undercooled NdFeB alloy droplets [J]. Journal of magnetism and magnetic materials 234.2 (2001): 313-319. [12]Gabay A M, Popov A G, Gaviko V S, et al. Investigation of phase composition and remanence enhancement in rapidly quenched Nd9(Fe, Co)85B6 alloys[J]. Journal of alloys and compounds, 1996, 237(1): 101-107. [13]Minic D G, Blagojevic V A, Mihajlovic L E, et al. Kinetics and mechanism of structural transformations of Fe 75Ni2Si8B13C2 amorphous alloy induced by thermal treatment[J]. Thermochimica acta, 2011, 519(1-2): 83-89. [14]Málek J. The applicability of Johnson-Mehl-Avrami model in the thermal analysis of the crystallization kinetics of glasses [J]. Thermochimica Acta, 1995, 267: 61-73. [15]Ozawa T. A new method of analyzing thermogravimetric data [J]. Bulletin of the chemical society of Japan, 1965, 38(11): 1881-1886. [16]ˇSesták J, Berggren G. Study of the kinetics of the mechanism of solid-state reactions at increasing temperatures [J]. Thermochim Acta, 1971, 3(1):1-12. [17]Henderson D W. Thermal analysis of non-isothermal crystallization kinetics in glass forming liquids [J]. Journal of Non-Crystalline Solids, 1979, 30(3): 301-315. [18]De Bruijn T J W, De Jong W A, Van Den Berg P J. Kinetic parameters in Avrami-Erofeev type reactions from isothermal and non-isothermal experiments[J]. Thermochimica Acta, 1981, 45(3): 315-325. 16
[19]Gao J, Volkmann T, Herlach D M. A metastable phase crystallized from undercooled NdFeCoZrGaB alloy droplets [J]. Journal of alloys and compounds, 2000, 308(1): 296-300. [20]Liu Y C, Li H W, et al. Magnetic properties optimization of nanocomposite Nd9Fe85B6 magnets by controlling microstructure of as-quenched ribbons [J]. Rare Metals, 2014, 33(3): 299-303. [21]Herbst J F, Croat J J, Pinkerton F E, et al. Relationships between crystal structure and magnetic properties in Nd2Fe14B [J]. Physical Review B, 1984, 29(7): 4176-4178.
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Highlights · The crystallization of Nd2Fe14B phase can be considered as an autocatalysis process. · The decomposition of metastable phase promotes the crystallization of Nd2Fe14B. · We draw the reasonable kinetic equation of this process and interpret it. · The kinetic parameters are obtained base on peak resolving of the DSC curves.
18
S0925-8388(15)00472-7 http://dx.doi.org/10.1016/j.jallcom.2015.02.046 JALCOM 33420
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
13 October 2014 29 January 2015 6 February 2015
Please cite this article as: K. Men, K. Li, Y. Luo, D. Yu, K. Zhang, J. Jin, Y. Mao, The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/ j.jallcom.2015.02.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys Kuo Men
Kuoshe Li
Yang Luo
Dunbo Yu*
Kun Zhang
Jinling Jin
Yongjun
Mao
National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co., Ltd., Beijing 100088, China
Corresponding author: Dunbo Yu (Tel.: +86-10-82241180, E-mail: [email protected])
Abstract The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy was investigated in order to gain further understanding of the structural evolution of the α-Fe and metastable phase, and then verified it by TEM. The result shows that the crystallization can be viewed as a process consisted of the following two steps. Firstly, the TbCu7-type metastable phase and a small amount of α-Fe precipitate from the matrix. Then, the TbCu7-type metastable phase is decomposed into the α-Fe and Nd2Fe14B phase with suitable sizes. The decomposition of metastable phase can promote the crystallization of Nd2Fe14B phase, so it could be considered as an ‘autocatalysis’ process. In addition, the crystallization kinetic parameters of the Nd2Fe14B phase have also been obtained base on the resolved differential scanning 1
calorimetry peaks. The results provide a physical and dynamic insight into the crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloys. Keyword: crystallization; metastable phase; autocatalysis kinetic equation 1. Introduction Nanocomposite two-phases magnets [1] composed of hard and soft magnetic phases with nanoscale grains have been studied extensively, since they have larger saturation magnetization, larger maximum energy product and better corrosion resistance. The strong exchange coupling interaction between two phases can make the magnet exhibit a single-phase hard magnetic property [2-4]. In order to obtain the magnets with high performance, attempts have been made to prepare nanocomposite two-phases magnets consisted of suitable grain size and to obtain the excellent magnetic performances. Among this, one of the most effective method is nano-crystallization of metallic glasses [5]. However, the crystallization behavior in this system is so complex that the knowledge of the crystallization mechanism is rare, for the process is influenced by lots of factors, such as precipitation of metastable phase [6], elements doping [7-9], heating rate [10], and so on. X. H. Cui [9] found that Nb addition enhanced the glass forming ability. Y. Gao [6] reported that Nd2Fe23B3-type phase can precipitate in the initial crystallization stage of the similar matrix alloy. J. Gao [11] studied the solidification of similar super-cooled melt and found that the TbCu7-type metastable phase forms in this process firstly, and then both α-Fe and Nd2Fe14B-type phases precipitate out from the matrix. Gabay and coworkers [12] considered that the 2
TbCu7-type metastable phase can be viewed as a solid solution of boron. However, the effects of metastable phase on the process is not so clear. In order to solve this problem, the crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy and the effects of primary-precipitated metastable phase on the process have been studied by DSC and non-isothermal crystallization kinetic methods. 2. Experiment The Nd9Fe85Nb0.5B5.5 alloys had been prepared by arc-melting the pure elements Nd, Fe, Nb and Fe–B alloy, and the ingots were then rapidly quenched by melt-spinning onto a molybdenum roller with a surface velocity of 24m/s and a quartz crucible orifice of 0.8 mm. The as-spun ribbons were then sealed in a quartz tube and annealed at various temperatures. The specimens were subsequently quenched after annealing at 560-730℃ for 3 min. All these procedures were performed in a high-purity argon atmosphere. The crystallization temperatures of these as spun ribbons were determined by differential scanning calorimetry (DSC) (RT-700, Mettler Toledo). Phase identification and structure analysis were performed using an X-ray powder diffractometer (XRD) (Co Ka, 0:15406 nm, D/Max-RA, Rigaku) and TEM (JEM-2010, JEOL). 3. Results and discussion
3
Fig. 1. XRD patterns of as-quenched Nd9Fe85Nb0.5B5.5 alloys with a surface velocity of 24m/s annealed at different temperatures
The evidence of phase transformation of matrix alloy has been shown in XRD patterns. As shown in Fig.1, the diffraction peaks of three phases are exhibited obviously, which represent the TbCu 7-type, α-Fe and Nd2Fe14B phases. At 560 ℃, the TbCu7-type phase appears firstly. With the increase of the annealed temperature, the peaks of the TbCu7-type phase decreases, then vanishes. Meanwhile, the Nd2Fe14B phase appears at 670 ℃. For α-Fe，at the temperatures of 560-640℃，the peak first increases slowly, and then increases rapidly between 640℃ and 730℃. So we speculate that the TbCu7-type phase would decompose at a high annealed temperature.
4
Fig. 2. DSC curves of as-quenched Nd9Fe85Nb0.5B5.5 alloy with a velocity of 24m/s at different heating rates from 10℃/min to 70℃/min.
In fact, the XRD patterns could reveal the phase precipitation of the crystallization process in a certain extent. In order to verity our viewpoint，the DCS test is introduced. Fig. 2 shows DSC curves of as-quenched Nd9Fe85Nb0.5B5.5 alloys with the different heating rates from 10℃/min to 70℃/min at the same velocity of 24m/s. The exothermic peaks in the DSC pattern represent different phases precipitating from the matrix. It is clearly revealed that the multi-step crystallization begins at 560℃. The DSC curves also exhibit the overlapping peaks between 560℃ and 800℃, which is depending on the heating rates. The appearance of such overlapping peaks could be provoked by several stages of the crystallization of compounds involving different constituents, produced during a reaction, or by crystallization of compounds involving the same constituents in several different stoichiometries [13].
5
Fig. 3. TEM and SEAD micrographs of as-quenched Nd9Fe85Nb0.5B5.5 alloys with a surface velocity of 24m/s annealed at different temperatures (a)560℃, (b)590℃, (c)620℃, (d)640℃, 6
(e)670℃, (f)700℃, (g)730℃. In the micrographs, A, B, C represent TbCu7-type metastable phase, α-Fe, Nd2Fe14B, respectively.
In order to clarify the mechanism of such crystallization behavior, the microstructures of these phase is also evaluated by the TEM. Fig. 3 shows TEM and SEAD micrographs of as-quenched ribbons with different annealed temperatures at a velocity of 24m/s. These micrographs can be viewed as the true evidence of the above-mentioned crystallization behavior, since they could reflect the state of phase precipitation and the size of grains in nanoscale. It is noted that the matrix has some degree of amorphous structure before annealing. However, some of TbCu7-type metastable phase precipitate out from the matrix at 560℃ firstly, then a small amount of α-Fe could be observed at 620℃. It is also noted that the dispersion of TbCu7-type phase and α-Fe grains tends to be uniform with temperature increasing from 560℃ to 640℃. After that, with the further increase of the annealed temperatures, the TbCu7-type phase vanishes with the appearance of the Nd2Fe14B phase. Subsequently, the grains of Nd2Fe14B phase and α-Fe grow up at the same time. And at last, the phase of Nd2Fe14B and α-Fe are distributed at the size of 20-40 nm equally. From the above, the crystallization behavior can be regarded as three phase precipitating from the matrix successively. So that the DSC curve can be resolved into three peaks. [13]
7
Fig. 4. The resolved DSC peaks for heating rate 30℃/min
The result of the resolved peaks is shown in Fig. 4, yielding three well separated symmetrical peaks, each corresponding to an individual step of the crystallization process. The three peaks are denoted as Tp1, Tp2 and Tp3 at temperatures of 609.6℃, 655.3℃ and 713.3℃, respectively. The dash line represents the fitted curve which fits the initial DSC curves (black line) well [13]. The crystallization process of the as-quenched alloy undergoes three steps for three phase precipitation. Tx1, Tx2, Tx3 present the onset precipitation temperatures of three phase. As is shown in Fig. 4, it could reasonably be inferred that the TbCu7-type metastable phase precipitates out from the matrix alloy firstly, and then it is decomposed into α-Fe and Nd2Fe14B phase. So the process of crystallization could be interpreted by two steps: (Ⅰ) TbCu7-type metastable phase and a small amount of α-Fe precipitate out from the matrix. (Fig.3 (a)-(d)) (Ⅱ) the TbCu7-type metastable phase vanishes for it is decomposed into α-Fe and Nd2Fe14B phase. (Fig.3 (e)-(g))
8
Two equations most commonly used for description of crystallization of amorphous alloys are Johnson–Mehl–Avrami(JMA) and the general Sestak-Berggren equation[14,16] The JMA model was firstly used to describe isothermal crystallization of glasses involving both nucleation and growth. However, it has been shown by Henderson[17] and also by De Bruijn[18] that this model could be extended to non-isothermal applications assuming that the crystals of a new phase grow from a constant number of nuclei and all nucleation is completed prior to crystal growth. Generally, the crystallization activation energy increases with crystallized volume fraction α in the this model. The crystallized volume fraction α at a certain point-in-time could be obtained by the ratio of the integral area at this time and the integral area of the whole exothermic peak, e.g. the crystallized volume fraction α at 700℃ was shown in Fig. 4, which could be obtained by the ratio of magenta area and cyan area. The obvious overlapping peaks shown in Fig. 4 reflect that the growth of TbCu7-type phase and the nucleation of α-Fe synchronize possibly [13], which is not suitable for the assumption of the JMA model that the growth of the grain occurs only after the nucleation has been finished. [14] In order to figure out the crystallization process and the interaction among each phase, we further study the mechanism of phase formation, the kinetic parameters and the kinetic equation. The kinetic equation of non-isothermal process can be expressed: 9
β·
= K(T) · f(α)
K(T) = A · exp (−E(α)⁄RT)
(1) (2)
in which A, β, f(α), E(α), represent the pre-exponential factor, the heating rate, the kinetic mechanism function and the crystallization activation energy, respectively. Crystallization activation energy E(α) could be calculated by Flynn-Wall-Ozawa equation [15] with different heating rates. lnβ = ln (A · E(α)⁄R · G(α)) − 1.052 × E(α)⁄RT
(3)
in which G(α) represents integral kinetic model function.
Fig.5. (a) Flynn-Wall-Ozawa equation plot for different crystallized volume fraction α, (b) the variation of crystallization activation energy E(α) with crystallized volume fraction α of Nd2Fe14B phase, (c) the normalized functions y(α) and z(α) for the crystallization process of the Nd 2Fe14B phase 10
As E(α) and G(α) are functions of crystallized volume fraction α, independent of the temperature, the E(α) at different α are calculated by taking the slopes of ln(β) vs. 1/T curves. The variation of E(α) of Nd2Fe14B phase is shown in Fig. 5(b), the crystallization activation energy of Nd2Fe14B phase is trending downward with α increasing which demonstrates that the crystallization is easy to take place kinetically. In order to verify the correct form of the crystallization kinetic model, a crucial criteria is applied. We test our experimental data using two special functions, y(α) and z(α). The maximum values being designated as α*y and α*z, respectively, defined as follows: y(α) =
z(α) ≈
· exp (E(α)⁄RT) ·T
(4) (5)
The characteristic of JMA model is that the value of α*z ≈ 0.632 and the value of α*y is always lower than the value of α*z. In addition, the curves of y(α) and z(α) functions have a convex shape. [14] The normalized functions y(α) and z(α) for the peak of Nd2Fe14B phase are shown in Fig. 5(c). It is observed that the value of α*z and α*y and the shape of y(α) is not suitable for the JMA model. Sestak-Berggren equation, an autocatalytic model, is useful for general description of the reaction in the solid state [16]: f(α) = α (1 − α)
(6)
The parameters of kinetic equation are obtained by fitting experiment data. This was calculated by Equation (1), (2), (3), (6). For M=0.95, N=0.89. 11
So the ‘autocatalysis’ crystallization kinetic equation of the Nd2Fe14B phase could be expressed: f(α) = α
.
(1 − α)
.
(7)
The crystallization process of Nd2Fe14B phase fits the autocatalytic model well. This is attributed to the internal transformation of crystal form. The TbCu 7-type metastable phase is supposed to be Nd2Fe17Bx (x=0-1) [19]. In this process, the TbCu7-type metastable phase precipitate firstly and then it is decomposed into α-Fe and Nd2Fe14B phase [20]. In the lattice of Nd2Fe14B phase, boron atoms are fixed to defined positions [21], i.e. 4g, and are generally considered to play a significant role in maintaining the stability of its lattice. In contrast, boron atoms lie in the interstitial sites of the TbCu7-type phase lattice very likely [12]. Therefore, boron atoms in TbCu7-type metastable phase always trend to lie relatively stable positions, which are the defined positions, spontaneously. Meanwhile, iron atoms are expelled to the boundary of phase, which become α-Fe finally. In addition, the iron-lacked Nd2Fe17Bx (x=0-1) transforms into Nd2Fe14B phase. This is why the process could also be considered as an ‘autocatalysis’ one reasonably. The magnetic energy is shown in Fig. 6. Magnetic energy of samples increases with crystallization temperature from 560℃ to 700℃, and then decreases at 730℃. The highest magnetic energy is about 10.96MGOe at the crystallization temperature of 700℃.
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Fig. 6. magnetic energy of samples annealed at different temperatures
The magnetic energy is very low at 560℃ and 590℃, for there is large amorphous content at these crystallization temperatures. The microstructures are shown in the TEM pictures obviously that the grain size distribution is uniform at about 20~40nm at the temperature of 670℃ and 700℃. At about 730℃，some grain size would grow up larger than 50nm, this may result in the decrease of magnetic energy.
4. Conclusions The crystallization behavior of as-quenched Nd9Fe85Nb0.5B5.5 alloy has two steps, TbCu7-type metastable phase and a small amount of α-Fe precipitate firstly, then the TbCu7-type metastable phase is decomposed into α-Fe and Nd2Fe14B phase. And the grains of Nd2Fe14B phase and α-Fe are distributed at the size of 20-40 nm equally. The decomposition of TbCu7-type metastable phase promotes the Nd2Fe14B phase formation, so it could be considered as an ‘autocatalysis’ process, which is suitable for the Sestak-Berggren equation. The crystallization kinetic equation of the Nd2Fe14B phase can be expressed by the function f(α) = α
.
(1 − α)
.
. 13
Acknowledgements The author would like to acknowledge the support by the National Natural Science Foundation of China (Grant Nos. 51401028) and the Science and Technology Project of Xicheng District, Beijing (Grant Nos. XCKJJH2013-33). The authors also wish to express their thanks to Dr. Du for help in performing the TEM analysis.
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Highlights · The crystallization of Nd2Fe14B phase can be considered as an autocatalysis process. · The decomposition of metastable phase promotes the crystallization of Nd2Fe14B. · We draw the reasonable kinetic equation of this process and interpret it. · The kinetic parameters are obtained base on peak resolving of the DSC curves.
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