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Mechanism of phase transformation in 2:17 type SmCo magnets investigated by phase stabilization
Scripta Materialia 113 (2016) 226–230
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Mechanism of phase transformation in 2:17 type SmCo magnets investigated by phase stabilization Z.Q. Xue a,b, L. Liu a, Z. Liu a,⁎, M. Li a, Don Lee a,c, R.J. Chen a, Y.Q. Guo b, A.R. Yan a,⁎ a b c
Laboratory of Rare-Earth Magnetic Functional Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China University of Dayton, Dayton, OH 45469, USA
a r t i c l e
i n f o
Article history: Received 21 August 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online 21 November 2015 Keywords: 2:17 type SmCo magnet 2:17R microtwin Lamella phase Phase transformation
a b s t r a c t Two kinds of lamella phases were observed in the phase transformations during aging of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets, which formed before and after the disappearance of microtwins respectively. Microtwins appeared with the precipitation of disordered 2:17R phases and disappeared with 2:17R phase ordering. Dy was introduced to investigate these transformations by stabilizing the 2:17H phases. A simplified atomic model was given to describe the formation of microtwins and a reasonable assumption was proposed as well to clarify the mechanism of these transformations. © 2015 Elsevier Ltd. All rights reserved.
The 2:17 type SmCo permanent magnets are well known for their advanced magnetic properties and excellent thermal stability, which are widely applied in microwave tubes, gyroscopes and accelerometers, reaction and momentum wheels to control and stabilize satellites, magnetic bearings, sensors and actuators [1–3]. The typical cellular microstructure of the 2:17 type SmCo alloy is critical to its magnetic performances, whose formation during the series of heat-treatments including solid solution treatment, isothermal annealing and slow cooling have been investigated and discussed greatly [4,5]. Phases after solid solution treatment are TbCu7 type 1:7H with minor Th2Ni17 type 2:17H structures, while after isothermal annealing they consist of the cellular structure crossed by lamella phases [6–9]. The cellular structure is composed of cell interiors with a rhombohedral Th2Zn17 structure (2:17R) and cell boundaries mixed with the detective phases of 2:7 and 5:19 (marked with DP), which transform into the ordered 1:5H phases with a hexagonal CaCu5 structure (1:5H) after slow cooling [10–12]. As for the phase structure of the lamella phase, there still exists controversy. L. Rabenberg et al. [13] suggested a rhombohedral Be3Nb (1:3R) structure for the lamella phase, while J. Fidler et al. [14] suggested it should be the 2:17H phase. To date, many researches are still performed trying to identify the exact structure of the lamella phase. Owing to the different recognition of the lamella phase, researchers proposed different assumptions to explain how the cellular microstructure evolved from the phase transformation during isothermal annealing. According to L. Rabenberg et al. [13], the solid solution stabilized from ⁎ Corresponding authors.
http://dx.doi.org/10.1016/j.scriptamat.2015.10.041 1359-6462/© 2015 Elsevier Ltd. All rights reserved.
high temperature by quenching has the 2:17H structure, which is defined as the initial state. On aging, 2:17H phases decompose into 2:17R and 1:5H phases with the formation of the cellular microstructure. The lamella phase forms after the precipitation of the 1:5H phase. However, according to A. E. Ray [15], the initial state is assumed to be a partially disordered 2:17R phase. With the ordering of 2:17R phases during isothermal annealing, the lamella phases form before the precipitation of the 1:5H phases. The lamella phase referred in the former assumption corresponds to the 1:3R phase [16] while the latter one corresponds to the 2:17H phase [17]. In short, the two assumptions differed in the exact phase structure of the initial state and argued whether the lamella phase formed before the precipitation of the 1:5H phase or not. The 1:5H phase here stands for the cell boundaries at high temperature before slow cooling, which have been corrected to the mixture DP phases [10–12]. Furthermore, the formation of periodic microtwins during phase transformation makes things complex, which could generate additional spots on the diffraction pattern and simulate the pattern misindexed as 2:17H [18], while the microtwins are confirmed to be 2:17R phases [15,19,20]. The appearance of microtwins shows up when the disordered 1:7H phases transform into ordered 2:17R phases during isothermal annealing [21], and those of step aged magnets contain the lamella phase on the basal plane with the disappearance of the microtwins [22]. All in all, it is of great importance to figure out what the lamella phases are in the 2:17 type SmCo permanent magnets and how the lamella phases evolve in the phase transformations during the heat treatments, which will greatly help optimize the magnetic properties. Interestingly, both of the 1:3R and 2:17H phases have been indexed as
Z.Q. Xue et al. / Scripta Materialia 113 (2016) 226–230
two kinds of the lamella phase in this study. In this case, we propose a reasonable assumption to clarify the mechanism of the phase transformation during aging. Besides, a simplified atomic model of the formation of 2:17R microtwins has been given in this paper as well. Ingots with nominal composition of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (SmCo) and Dy(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (DyCo) were prepared by induction melting and crushed into powders with the particle size of about 3.8 μm by conventional powder metallurgical technique. In our experiments, Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets were sintered by adding different ratios of DyCo alloy powders into SmCo alloy powders (0 wt.%, 20 wt.%, 50 wt.%, 80 wt.%). Before sintered, powders were mixed, aligned, pressed in a magnetic field up to 3 T and further compacted by using cold isostatic pressing. The samples were sintered at 1216 °C for 0.5 h then homogenized at 1190 °C for 3 h later. Quenched from the high temperature, the subsequent heat treatment after solid solution was isothermal aging at 830 °C for 12 h, then slowly cooling to 400 °C at a rate of 0.7 °C/min, followed at 400 °C for 3 h and finally by water quenching. Phases of 2:17 type SmCo magnets in solid solution, isothermal annealing and after slow cooling are defined as the initial, transition and final states in this paper, respectively. Measured by X-ray diffraction (XRD) and selected area electron diffraction (SAED), the phases of the initial and transition states of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets (typical 2:17 type SmCo magnets marked as x = 0) are shown in Fig. 1(a). Generally, the (203) and (204) peaks are characteristic peaks of the 2:17H and 2:17R phases respectively, while the absence of both the peaks represents the 1:7H phase [23]. Based on the indexed results of XRD and SAED patterns, the main phase of the initial state is 1:7H with minor 2:17H phases. With isothermal annealing going, the 2:17H disappears with the decomposition of the 1:7H into DP and 2:17R phases quickly in 30 min. After isothermal annealing for 12 h, the main phase of the transition state changes to DP and 2:17R phases. For better recognition of the phases of 2:17H and 2:17R, DP phases are ignored when indexing in this paper. The 2:17H disappears so fast that the phase transformation is hardly captured during aging, which is accompanied with the appearance and disappearance of the 2:17R microtwins. For further investigation of the relationship between the 2:17H phase and 2:17R microtwins, one way to slow down this transformation was taken into consideration. According to the theoretical calculation of R2Co17 compounds [24], for R = heavy rare earth elements, the average energy of the 2:17H phase is lower than that of the 2:17R phase. Thus, Dy was introduced to Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets to stabilize the 2:17H phase when in the transition state. As shown in Fig. 1(b), when mixed with 50 wt.% DyCo (x = 0.5), the 2:17H phase is retained until isothermal aging for 2 h in the transition state which didn't change the main
227
phase of the initial state. When x = 0.8, the 2:17H is successfully obtained in the final state. Microstructures of samples with different x values in the final state have been investigated by transmission electron microscope (TEM). Taking samples with x = 0.5 for example, Fig. 2 gives the indexed results of the SAED and the related fast Fourier transformation (FFT) patterns. The TEM characterization presents the typical cellular microstructure as described in the literature [25]: rhombic cells of 2:17R, boundaries of coherent 1:5H and 1:3R lamella phases parallel to the basal plane of the cells. The 2:17R spots are common to the 1:5H and the superimposed streaks are attributed to the 1:3R lamella phase. The optimal orientation selected for the observation is h
i 1100
1:5H
h i ¼ 0110
2:17R
h i ¼ 2130
1:3R
:
However, long and short range 2:17R microtwins are observed in the samples doped with Dy, which are supposed to be formed in the transformation from the 1:7H phase to the 2:17R phase. Not only the 2:17H phase was successfully stabilized by Dy doping, but also the formation of 2:17R microtwins. Compared with the series of high resolution TEM (HRTEM) images of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets, Fig. 3(a) shows that the more Dy dopes, the more microtwins are retained. The microtwin boundaries are corresponding to the thin lamella phase in the TEM characterization, and account for the extra spots in SAED which are indexed to be 2:17H phases. The extra spots in the samples with x = 0.8 are consistent with those in the initial state of samples without being doped with Dy, which are indexed to be 2:17H phases by SAED (Fig. 3(b)). That is to say, both of the 1:3R and 2:17H lamella phases coexist during the phase transformation in the transition state. The orientation relationship between the 2:17R phase and the 1:3R phase is: (0001)2 : 17R// (0001)1 : 3R and ½01102:17R ==½21301:3R , while that between the 2:17R phase and 2:17H phase is: (0001)2 : 17R//(0001)2 : 17H and ½01102:17R == ½01102:17H . As we know, the 1:7H, 2:17H and 2:17R are all derived from SmCo5 by replacing Sm atoms with dumbbell pairs of Co–Co, and different from the amounts of substitution [18,26]. In the 2:17R structure, three types of the planes stack along the c axis in a sequence of ABCABC…, while in the disordered 1:7H or ordered 2:17H structures only two types of the atomic planes stack along the c axis in a sequence of ABAB…. Occasionally, the 2:17R phases are twinned because of the reverse in this stacking, …ABCABACBA… type is possible. As reported in the earlier researches [27,28], the middle “B” in the stack …ABCABACBA… was identified to be the twin boundary while the occasion of the “ABA” was ignored. The single “B” corresponds to one atomic plane while the
Fig. 1. (a) XRD patterns of the samples in the initial and transition states in samples of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0), and the indexed results of the 1:7H and 2:17H phases in the initial state by SAED. (b) XRD patterns of the samples in the initial and transition states in samples of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0.5).
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Fig. 2. TEM characterization of Sm0.5Dy0.5(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnet in the final state: indexed results of HRTEM images and SAED patterns. Inset: the FFT patterns corresponding to the 2:17R twinned region, cell boundaries 1:5H phase and the lamella phase.
group of the “ABA” might be identified as one phase of the disordered 1:7H or ordered 2:17H structures. One or two atomic planes is easily missed by different observers when indexing the HRTEM. In this study, the extra electron spots were observed in the samples with the highly microtwinned when x = 0.8. The simulated patterns are indexed to be 2:17H, which proves that the microtwin boundaries are 2:17H phases, presented as the thin lamella phase in the TEM characterization. The simplified atomic model of the formation of microtwins is given in Fig. 4.
From the perspective of phase evolution, the more microtwins retained, the more ordering transformation from the short-range 2:17R microtwins to the long-range 2:17R phases were inhibited and the worse ordered 2:17R phases are. In the highly microtwinned situation, the final state of the magnets doped with Dy equals that of the transition state of the magnets without being doped with Dy during aging. The difference is that the cell boundaries are 1:5H rather than DP phases after slow cooling in the final state, which wouldn't affect the discussion of the transformation for cell interiors. Therefore, we
Fig. 3. TEM analyses on the Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets: (a) HRTEM images of the different densities of the microtwins in the samples with various x values; (b) the SAED indexed results of the 2:17H phase in the final state of the sample with x = 0.8 and its corresponding thin lamella phase in the morphology of the microstructure.
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229
Fig. 4. Atomic models of the formation of 2:17R microtwins in the phase transformation during 2:17R ordering.
assume the final state of the samples doped with Dy as the transition state of the Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 samples. In this case, an optimized assumption to clarify the mechanism of the phase transformation in SmCo alloys during aging is proposed. The phase transformation is divided into three states, which are the initial, transition and final states. Based on the observation, the initial state has mixed phases of supersaturated 1:7H and minor 2:17H after being quenched from ~ 1190 °C. The phase transformation (1:7H → 2:17R + DP) first occurs on aging with the formation of the cellular structure. The 2:17R phase grows based on the 2:17H structure randomly along the two opposite directions of the c axis, which becomes the disordered 2:17R phases in the form of the short-range periodic microtwins. Correspondingly, the microtwin boundaries are 2:17H phases. Considering it is the very beginning of the heat treatment, the diffusion of Zr and Cu has not begun yet. It is agreed that a high Cu content favors the occurrence of microtwins instead of the formation of a lamella phase, while Zr stabilizes the 2:17H phase even in samples with high Cu content [29]. Later in the isothermal annealing, the shortrange microtwins evolve into long-range gradually during the 2:17R ordering, and then well scheduled 2:17R forms without microtwins at last [16]. Meanwhile, the 2:17H microtwin boundaries decompose into ordered 2:17R and 1:3R phases. 2:17R forms when more Sm are replaced by Co–Co dumb pairs in the 2:17H structure, and then the 1:3R lamella phase forms where the more Sm segregates. As the microtwins disappear, 1:3R firstly forms in the region of better ordered
2:17R (the long-range microtwins) while 2:17H microtwin boundaries were retained in the highly microtwinned region (the short-range microtwins). That's the case when the 2:17H and 1:3R lamella coexist. After the 1:3R lamella phase precipitated, Zr begins to segregate [21]. On the slow cooling, the 1:3R lamella phase enriched in Zr serves as the diffusion path for the rapid segregation of Cu to the cell boundaries [30], contributing to the ordering from DP phases into the highanisotropy 1:5H phase and responsible for the great increase of Hc [10–12,19]. Redistribution is accomplished by diffusion of the transition metals through the relatively open Co sites of the lattice [20]. Eventually, the final state of the magnets is 2:17R cells enriched in Fe depleted in Cu and Zr, 1:5 boundaries enriched in Cu depleted in Zr and Fe, the 1:3R lamella phase strongly enriched in Zr depleted in Cu, which is in agreement with ref. [12] where the finding has been also proven by a high-resolution electron transmission microscope. In summary, microstructures of the lamella phases and the related phase transformations during aging in the sintered Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets have been investigated and discussed in this paper. Dy is introduced to inhibit the 2:17R ordering from the short-range microtwins to the longrange ordered by stabilizing the 2:17H phase microtwinning boundary. A simplified atomic model has been given to describe the formation of 2:17R microtwins. Based on the discovery of two kinds of lamella phases in the phase transformation, a reasonable assumption has been proposed to clarify the mechanism of the phase transformation during
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aging in 2:17 type magnets, which will greatly help obtain higher magnetic properties and better thermal stability. Acknowledgments The work was supported by the following foundations: National Natural Science Foundation of China No.51301190, Zhejiang Provincial Natural Science Foundation for Youth of China No. LQ13E010004, Ningbo Science and Technology Project No. 2014B11009, and the Ningbo Innovative Research Team No. 2012B81001. References [1] A.K. Pathak, M. Khan, K.A. Gschnieidner Jr., R.W. McCallum, L. Zhou, K.W. Sun, K.W. Dennis, C. Zhou, F.E. Pinkerton, M.J. Kramer, V.K. Pecharsky, Adv. Mater. 27 (2015) 2663. [2] O. Gutfleisch, M.A. Willard, E. Bruck, C.H. Chen, S.G. Sankar, J.P. Liu, Adv. Mater. 23 (2011) 821. [3] L. Liu, Z. Liu, M. Li, D. Lee, R.J. Chen, J. Liu, W. Li, A.A.R. Yan, Appl. Phys. Lett. 106 (2015) 052408. [4] X.Y. Song, N.D. Lu, M. Seyring, M. Rettenmayr, W.W. Xu, Z.X. Zhang, J.X. Zhang, Appl. Phys. Lett. 94 (2009) 023102. [5] H. Kronmüller, D. Goll, Scr. Mater. 47 (2002) 545. [6] Z.X. Zhang, X.Y. Song, W.W. Xu, M. Seyring, M. Rettenmayr, Scr. Mater. 62 (2010) 594. [7] O. Gutfleisch, K.H. Müller, K. Khlopkov, M. Wolf, A. Yan, R. Schäfer, T. Gemming, L. Schultz, Acta Mater. 54 (2006) 997.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Mechanism of phase transformation in 2:17 type SmCo magnets investigated by phase stabilization Z.Q. Xue a,b, L. Liu a, Z. Liu a,⁎, M. Li a, Don Lee a,c, R.J. Chen a, Y.Q. Guo b, A.R. Yan a,⁎ a b c
Laboratory of Rare-Earth Magnetic Functional Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China University of Dayton, Dayton, OH 45469, USA
a r t i c l e
i n f o
Article history: Received 21 August 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online 21 November 2015 Keywords: 2:17 type SmCo magnet 2:17R microtwin Lamella phase Phase transformation
a b s t r a c t Two kinds of lamella phases were observed in the phase transformations during aging of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets, which formed before and after the disappearance of microtwins respectively. Microtwins appeared with the precipitation of disordered 2:17R phases and disappeared with 2:17R phase ordering. Dy was introduced to investigate these transformations by stabilizing the 2:17H phases. A simplified atomic model was given to describe the formation of microtwins and a reasonable assumption was proposed as well to clarify the mechanism of these transformations. © 2015 Elsevier Ltd. All rights reserved.
The 2:17 type SmCo permanent magnets are well known for their advanced magnetic properties and excellent thermal stability, which are widely applied in microwave tubes, gyroscopes and accelerometers, reaction and momentum wheels to control and stabilize satellites, magnetic bearings, sensors and actuators [1–3]. The typical cellular microstructure of the 2:17 type SmCo alloy is critical to its magnetic performances, whose formation during the series of heat-treatments including solid solution treatment, isothermal annealing and slow cooling have been investigated and discussed greatly [4,5]. Phases after solid solution treatment are TbCu7 type 1:7H with minor Th2Ni17 type 2:17H structures, while after isothermal annealing they consist of the cellular structure crossed by lamella phases [6–9]. The cellular structure is composed of cell interiors with a rhombohedral Th2Zn17 structure (2:17R) and cell boundaries mixed with the detective phases of 2:7 and 5:19 (marked with DP), which transform into the ordered 1:5H phases with a hexagonal CaCu5 structure (1:5H) after slow cooling [10–12]. As for the phase structure of the lamella phase, there still exists controversy. L. Rabenberg et al. [13] suggested a rhombohedral Be3Nb (1:3R) structure for the lamella phase, while J. Fidler et al. [14] suggested it should be the 2:17H phase. To date, many researches are still performed trying to identify the exact structure of the lamella phase. Owing to the different recognition of the lamella phase, researchers proposed different assumptions to explain how the cellular microstructure evolved from the phase transformation during isothermal annealing. According to L. Rabenberg et al. [13], the solid solution stabilized from ⁎ Corresponding authors.
http://dx.doi.org/10.1016/j.scriptamat.2015.10.041 1359-6462/© 2015 Elsevier Ltd. All rights reserved.
high temperature by quenching has the 2:17H structure, which is defined as the initial state. On aging, 2:17H phases decompose into 2:17R and 1:5H phases with the formation of the cellular microstructure. The lamella phase forms after the precipitation of the 1:5H phase. However, according to A. E. Ray [15], the initial state is assumed to be a partially disordered 2:17R phase. With the ordering of 2:17R phases during isothermal annealing, the lamella phases form before the precipitation of the 1:5H phases. The lamella phase referred in the former assumption corresponds to the 1:3R phase [16] while the latter one corresponds to the 2:17H phase [17]. In short, the two assumptions differed in the exact phase structure of the initial state and argued whether the lamella phase formed before the precipitation of the 1:5H phase or not. The 1:5H phase here stands for the cell boundaries at high temperature before slow cooling, which have been corrected to the mixture DP phases [10–12]. Furthermore, the formation of periodic microtwins during phase transformation makes things complex, which could generate additional spots on the diffraction pattern and simulate the pattern misindexed as 2:17H [18], while the microtwins are confirmed to be 2:17R phases [15,19,20]. The appearance of microtwins shows up when the disordered 1:7H phases transform into ordered 2:17R phases during isothermal annealing [21], and those of step aged magnets contain the lamella phase on the basal plane with the disappearance of the microtwins [22]. All in all, it is of great importance to figure out what the lamella phases are in the 2:17 type SmCo permanent magnets and how the lamella phases evolve in the phase transformations during the heat treatments, which will greatly help optimize the magnetic properties. Interestingly, both of the 1:3R and 2:17H phases have been indexed as
Z.Q. Xue et al. / Scripta Materialia 113 (2016) 226–230
two kinds of the lamella phase in this study. In this case, we propose a reasonable assumption to clarify the mechanism of the phase transformation during aging. Besides, a simplified atomic model of the formation of 2:17R microtwins has been given in this paper as well. Ingots with nominal composition of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (SmCo) and Dy(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (DyCo) were prepared by induction melting and crushed into powders with the particle size of about 3.8 μm by conventional powder metallurgical technique. In our experiments, Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets were sintered by adding different ratios of DyCo alloy powders into SmCo alloy powders (0 wt.%, 20 wt.%, 50 wt.%, 80 wt.%). Before sintered, powders were mixed, aligned, pressed in a magnetic field up to 3 T and further compacted by using cold isostatic pressing. The samples were sintered at 1216 °C for 0.5 h then homogenized at 1190 °C for 3 h later. Quenched from the high temperature, the subsequent heat treatment after solid solution was isothermal aging at 830 °C for 12 h, then slowly cooling to 400 °C at a rate of 0.7 °C/min, followed at 400 °C for 3 h and finally by water quenching. Phases of 2:17 type SmCo magnets in solid solution, isothermal annealing and after slow cooling are defined as the initial, transition and final states in this paper, respectively. Measured by X-ray diffraction (XRD) and selected area electron diffraction (SAED), the phases of the initial and transition states of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets (typical 2:17 type SmCo magnets marked as x = 0) are shown in Fig. 1(a). Generally, the (203) and (204) peaks are characteristic peaks of the 2:17H and 2:17R phases respectively, while the absence of both the peaks represents the 1:7H phase [23]. Based on the indexed results of XRD and SAED patterns, the main phase of the initial state is 1:7H with minor 2:17H phases. With isothermal annealing going, the 2:17H disappears with the decomposition of the 1:7H into DP and 2:17R phases quickly in 30 min. After isothermal annealing for 12 h, the main phase of the transition state changes to DP and 2:17R phases. For better recognition of the phases of 2:17H and 2:17R, DP phases are ignored when indexing in this paper. The 2:17H disappears so fast that the phase transformation is hardly captured during aging, which is accompanied with the appearance and disappearance of the 2:17R microtwins. For further investigation of the relationship between the 2:17H phase and 2:17R microtwins, one way to slow down this transformation was taken into consideration. According to the theoretical calculation of R2Co17 compounds [24], for R = heavy rare earth elements, the average energy of the 2:17H phase is lower than that of the 2:17R phase. Thus, Dy was introduced to Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets to stabilize the 2:17H phase when in the transition state. As shown in Fig. 1(b), when mixed with 50 wt.% DyCo (x = 0.5), the 2:17H phase is retained until isothermal aging for 2 h in the transition state which didn't change the main
227
phase of the initial state. When x = 0.8, the 2:17H is successfully obtained in the final state. Microstructures of samples with different x values in the final state have been investigated by transmission electron microscope (TEM). Taking samples with x = 0.5 for example, Fig. 2 gives the indexed results of the SAED and the related fast Fourier transformation (FFT) patterns. The TEM characterization presents the typical cellular microstructure as described in the literature [25]: rhombic cells of 2:17R, boundaries of coherent 1:5H and 1:3R lamella phases parallel to the basal plane of the cells. The 2:17R spots are common to the 1:5H and the superimposed streaks are attributed to the 1:3R lamella phase. The optimal orientation selected for the observation is h
i 1100
1:5H
h i ¼ 0110
2:17R
h i ¼ 2130
1:3R
:
However, long and short range 2:17R microtwins are observed in the samples doped with Dy, which are supposed to be formed in the transformation from the 1:7H phase to the 2:17R phase. Not only the 2:17H phase was successfully stabilized by Dy doping, but also the formation of 2:17R microtwins. Compared with the series of high resolution TEM (HRTEM) images of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets, Fig. 3(a) shows that the more Dy dopes, the more microtwins are retained. The microtwin boundaries are corresponding to the thin lamella phase in the TEM characterization, and account for the extra spots in SAED which are indexed to be 2:17H phases. The extra spots in the samples with x = 0.8 are consistent with those in the initial state of samples without being doped with Dy, which are indexed to be 2:17H phases by SAED (Fig. 3(b)). That is to say, both of the 1:3R and 2:17H lamella phases coexist during the phase transformation in the transition state. The orientation relationship between the 2:17R phase and the 1:3R phase is: (0001)2 : 17R// (0001)1 : 3R and ½01102:17R ==½21301:3R , while that between the 2:17R phase and 2:17H phase is: (0001)2 : 17R//(0001)2 : 17H and ½01102:17R == ½01102:17H . As we know, the 1:7H, 2:17H and 2:17R are all derived from SmCo5 by replacing Sm atoms with dumbbell pairs of Co–Co, and different from the amounts of substitution [18,26]. In the 2:17R structure, three types of the planes stack along the c axis in a sequence of ABCABC…, while in the disordered 1:7H or ordered 2:17H structures only two types of the atomic planes stack along the c axis in a sequence of ABAB…. Occasionally, the 2:17R phases are twinned because of the reverse in this stacking, …ABCABACBA… type is possible. As reported in the earlier researches [27,28], the middle “B” in the stack …ABCABACBA… was identified to be the twin boundary while the occasion of the “ABA” was ignored. The single “B” corresponds to one atomic plane while the
Fig. 1. (a) XRD patterns of the samples in the initial and transition states in samples of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0), and the indexed results of the 1:7H and 2:17H phases in the initial state by SAED. (b) XRD patterns of the samples in the initial and transition states in samples of Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0.5).
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Fig. 2. TEM characterization of Sm0.5Dy0.5(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnet in the final state: indexed results of HRTEM images and SAED patterns. Inset: the FFT patterns corresponding to the 2:17R twinned region, cell boundaries 1:5H phase and the lamella phase.
group of the “ABA” might be identified as one phase of the disordered 1:7H or ordered 2:17H structures. One or two atomic planes is easily missed by different observers when indexing the HRTEM. In this study, the extra electron spots were observed in the samples with the highly microtwinned when x = 0.8. The simulated patterns are indexed to be 2:17H, which proves that the microtwin boundaries are 2:17H phases, presented as the thin lamella phase in the TEM characterization. The simplified atomic model of the formation of microtwins is given in Fig. 4.
From the perspective of phase evolution, the more microtwins retained, the more ordering transformation from the short-range 2:17R microtwins to the long-range 2:17R phases were inhibited and the worse ordered 2:17R phases are. In the highly microtwinned situation, the final state of the magnets doped with Dy equals that of the transition state of the magnets without being doped with Dy during aging. The difference is that the cell boundaries are 1:5H rather than DP phases after slow cooling in the final state, which wouldn't affect the discussion of the transformation for cell interiors. Therefore, we
Fig. 3. TEM analyses on the Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets: (a) HRTEM images of the different densities of the microtwins in the samples with various x values; (b) the SAED indexed results of the 2:17H phase in the final state of the sample with x = 0.8 and its corresponding thin lamella phase in the morphology of the microstructure.
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Fig. 4. Atomic models of the formation of 2:17R microtwins in the phase transformation during 2:17R ordering.
assume the final state of the samples doped with Dy as the transition state of the Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 samples. In this case, an optimized assumption to clarify the mechanism of the phase transformation in SmCo alloys during aging is proposed. The phase transformation is divided into three states, which are the initial, transition and final states. Based on the observation, the initial state has mixed phases of supersaturated 1:7H and minor 2:17H after being quenched from ~ 1190 °C. The phase transformation (1:7H → 2:17R + DP) first occurs on aging with the formation of the cellular structure. The 2:17R phase grows based on the 2:17H structure randomly along the two opposite directions of the c axis, which becomes the disordered 2:17R phases in the form of the short-range periodic microtwins. Correspondingly, the microtwin boundaries are 2:17H phases. Considering it is the very beginning of the heat treatment, the diffusion of Zr and Cu has not begun yet. It is agreed that a high Cu content favors the occurrence of microtwins instead of the formation of a lamella phase, while Zr stabilizes the 2:17H phase even in samples with high Cu content [29]. Later in the isothermal annealing, the shortrange microtwins evolve into long-range gradually during the 2:17R ordering, and then well scheduled 2:17R forms without microtwins at last [16]. Meanwhile, the 2:17H microtwin boundaries decompose into ordered 2:17R and 1:3R phases. 2:17R forms when more Sm are replaced by Co–Co dumb pairs in the 2:17H structure, and then the 1:3R lamella phase forms where the more Sm segregates. As the microtwins disappear, 1:3R firstly forms in the region of better ordered
2:17R (the long-range microtwins) while 2:17H microtwin boundaries were retained in the highly microtwinned region (the short-range microtwins). That's the case when the 2:17H and 1:3R lamella coexist. After the 1:3R lamella phase precipitated, Zr begins to segregate [21]. On the slow cooling, the 1:3R lamella phase enriched in Zr serves as the diffusion path for the rapid segregation of Cu to the cell boundaries [30], contributing to the ordering from DP phases into the highanisotropy 1:5H phase and responsible for the great increase of Hc [10–12,19]. Redistribution is accomplished by diffusion of the transition metals through the relatively open Co sites of the lattice [20]. Eventually, the final state of the magnets is 2:17R cells enriched in Fe depleted in Cu and Zr, 1:5 boundaries enriched in Cu depleted in Zr and Fe, the 1:3R lamella phase strongly enriched in Zr depleted in Cu, which is in agreement with ref. [12] where the finding has been also proven by a high-resolution electron transmission microscope. In summary, microstructures of the lamella phases and the related phase transformations during aging in the sintered Sm1 − xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 (x = 0, 0.2, 0.5, 0.8) magnets have been investigated and discussed in this paper. Dy is introduced to inhibit the 2:17R ordering from the short-range microtwins to the longrange ordered by stabilizing the 2:17H phase microtwinning boundary. A simplified atomic model has been given to describe the formation of 2:17R microtwins. Based on the discovery of two kinds of lamella phases in the phase transformation, a reasonable assumption has been proposed to clarify the mechanism of the phase transformation during
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aging in 2:17 type magnets, which will greatly help obtain higher magnetic properties and better thermal stability. Acknowledgments The work was supported by the following foundations: National Natural Science Foundation of China No.51301190, Zhejiang Provincial Natural Science Foundation for Youth of China No. LQ13E010004, Ningbo Science and Technology Project No. 2014B11009, and the Ningbo Innovative Research Team No. 2012B81001. References [1] A.K. Pathak, M. Khan, K.A. Gschnieidner Jr., R.W. McCallum, L. Zhou, K.W. Sun, K.W. Dennis, C. Zhou, F.E. Pinkerton, M.J. Kramer, V.K. Pecharsky, Adv. Mater. 27 (2015) 2663. [2] O. Gutfleisch, M.A. Willard, E. Bruck, C.H. Chen, S.G. Sankar, J.P. Liu, Adv. Mater. 23 (2011) 821. [3] L. Liu, Z. Liu, M. Li, D. Lee, R.J. Chen, J. Liu, W. Li, A.A.R. Yan, Appl. Phys. Lett. 106 (2015) 052408. [4] X.Y. Song, N.D. Lu, M. Seyring, M. Rettenmayr, W.W. Xu, Z.X. Zhang, J.X. Zhang, Appl. Phys. Lett. 94 (2009) 023102. [5] H. Kronmüller, D. Goll, Scr. Mater. 47 (2002) 545. [6] Z.X. Zhang, X.Y. Song, W.W. Xu, M. Seyring, M. Rettenmayr, Scr. Mater. 62 (2010) 594. [7] O. Gutfleisch, K.H. Müller, K. Khlopkov, M. Wolf, A. Yan, R. Schäfer, T. Gemming, L. Schultz, Acta Mater. 54 (2006) 997.
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