Trans. Nonferrous Met. Soc. China 22(2012) 1118-1122
Effects of Cr3C2 content and wheel speed on amorphization behavior of melt-spun SmCo7-x(Cr3C2)x alloys
LI Li-ya, YI Jian-hong, LI Ai-kun, PENG Yuan-dong, XIA Qing-lin
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Received 18 May 2011; accepted 24 October 2011
Abstract: The effects of the Cr3C2 content and wheel speed on the amorphization behavior of the melt-spun SmCo7-x(Cr3C2)x (x=0.10-0.25) alloys were studied systematically by X-ray diffraction analysis (XRD), differential scanning calorimetry (DSC) and magnetic measurements. The ribbon melt-spun at lower wheel speed (20 m/s) has composite structure composed of mostly SmCo7 and a small amount of Sm2Co17R. The grain size of SmCo7 phase decreases with the increase of Cr3C2 content. With the increase of wheel speed, the XRD peaks become lower and accompanied with a broad increase in backgrounds, indicating a considerable decrease in the grain size of the SmCo7 phase. When the wheel speed increases to 40 m/s, SmCo7-x(Cr3C2)x alloys can be obtained in the amorphous state for 0.15≤x≤0.25 with intrinsic coercive Hci of 0.004-0.007 T. The DSC analysis reveals that SmCo7 phase firstly precipitates from the amorphous matrix at 650℃, followed by the crystallization of Sm2Co17 phase at 770℃.
Key words: SmCo7-type permanent magnets; Cr3C2; melt spinning; amorphization; hysteresis loops
1 Introduction
Permanent magnet materials capable of operating at elevated temperatures are needed for advanced power systems [1]. Most attention has been paid to the SmCo7-type magnets because of their large coercivity and high Curie temperature [2]. Powder metallurgy method has been used successfully to fabricate Sm(Co,Fe,Cu,Zr)7 bulk magnets with a coercivity of 1 T at 500℃ [3]. The microstructure of the sintered SmCo7-type magnets consists of Sm2Co17R phase as cells surrounded by SmCo5 boundary with Zr-rich platelet phases running across cells and cell boundaries. SmCo5 phase is responsible for enhancement of coercivity by domain wall pinning mechanism.
An alternative route to fabricate nanostructure, high-temperature magnets is mechanical alloying [4,5]. SmCo7-type nanophase hard magnets with high coercivity and enhanced remanent magnetization were synthesized using mechanically induced amorphization and the crystallization of nanoscaled grains during the subsequent annealing processes. Optimal coercivity of 2.1 T and remanent magnetization of 0.77 T have been obtained in Sm12.5Co85.5Zr2 magnet [6].
Besides mechanical alloying, melt spinning has been proved to be another effective route to fabricate nanocomposite permanent magnets, especially in the Nd-Fe-B system. To obtain nanocrystalline microstructure and high coercivity, it is necessary to make amorphous ribbons first and then crystallize them by annealing. Unfortunately, the amorphous formation ability of Sm-Co alloys is very poor [7]. Thus, the fine microstructure for high coercivity is difficult to realize in melt spun ribbons. However, it has been shown that a small amount of carbon addition is helpful for the grain refinement in the systems of Sm-Co-Hf-C [8], Sm-Co-Nb-C [9], and Sm-Co-Fe-C [10]. Recently, the effects of the addition of Cr3C2 on the magnetic properties and microstructure of SmCo7-type magnets have been investigated. It has been found that, even melt-spinning at a low wheel surface speed of 20 m/s, the grain size of Cr3C2-doped SmCo7 alloys is significantly reduced from 300-600 nm to below 80 nm [11]. In the present work, the effect of Cr3C2 content and wheel speed on the amorphization behavior of the melt-spun SmCo7-x(Cr3C2)x alloys was studied.
2 Experimental
Alloys with nominal compositions of SmCo7-x(Cr3C2)x (x=0.10-0.25) were prepared by arc melting under high purity argon atmosphere. Samples were remelted to ensure homogeneity and an excess of 7% Sm was added to compensate for the Sm loss during processing. The arc-melted ingots were cut into small pieces and then were melt-spun at 20, 30 and 40 m/s. The as-spun ribbons were sealed in quartz tube under vacuum and then annealed at 650-800℃ for 5 min to crystallize and develop a fine microstructure. The crystal structure of the ribbons was identified by Bruker D8 Advance/Discover X-ray diffraction (XRD) system with Phillips diffractometer using the Co Kα radiation. The phase transformation temperatures were determined by differential scanning calorimeter (DSC) at a heating rate of 40 K/min. Hard magnetic properties at room temperature were measured by a Lake Shore 7410 vibrating sample magnetometer (VSM) with the maximum field of 2.3 T. The magnetization of the ribbons could not be saturated using VSM, therefore the maximum magnetization M2T under 2 T was used to represent the saturation magnetization Ms.
3 Results and discussion
3.1 Effects of Cr3C2 content and wheel speed on structure of SmCo7-x(Cr3C2)x alloys
The progress of the amorphization process by melt spinning can be seen through the measurement of relative intensity of the XRD patterns of SmCo7-type phase. Figure 1 shows the XRD patterns for SmCo7-x(Cr3C2)x (x=0.10-0.25) ribbons spun at 20 m/s. It can be seen that SmCo7 main phase coexists with Sm2Co17R secondary phase, which is confirmed by the supperlattice reflection peak of (015), for the series of ribbons. Meanwhile, with increasing Cr3C2 content x from 0.10 to 0.25, the intensity of XRD peaks comes to be significantly weaker and the peak width becomes broader, indicating a dramatic decrease in the grain size of the SmCo7 phase. Interestingly, the formation of crystallographic texture is also observed from the XRD patterns. The intensity of diffraction (002) for the SmCo7 phase is strengthened significantly with Cr3C2 content x increased to higher than 0.1. This is similar to that observed in SmCo7Ti and SmCo5 alloys melt-spun at much lower wheel surface speed of 10-15 m/s. In the SmCo7- or SmCo5- type magnets, the intensity of (002) plane is considered measure of texture [12]. This texture is thought to be helpful for the fabrication of anisotropic permanent magnetic materials.
The XRD patterns for SmCo7-x(Cr3C2)x (x=0.10-0.25) ribbons melt-spun at 30 m/s are shown in Fig. 2. It is found that only the SmCo7 phase exists for the ribbon with x=0.10. Two phases, SmCo7 and Sm2Co17, are detected for ribbons with a higher Cr3C2 substitution. A similar dependence of the intensity of XRD peaks on Cr3C2 content is observed. It can be found that, with the increase of Cr3C2 content, the XRD peaks become significantly low and accompanied with a broad increase in backgrounds, indicating a considerable decrease in the grain size of the SmCo7 phase. The intensity of diffraction (002) for the SmCo7 phase is also gradually strengthened when Cr3C2 content x increases from 0.10 to 0.25. For the ribbon with x=0.20, the intensity ratio I(002)/I(111) is 4.04 which is much higher than 3.2 for SmCo7Ti and 2.9 for SmCo5 magnets [12]. This indicates that the addition of Cr3C2 may favor the alignment of SmCo7 crystalline grains during melt spinning. Further investigations are needed to understand this point.
Fig. 1 XRD patterns for SmCo7-x(Cr3C2)x (x=0.10–0.25) alloys melt-spun at 20 m/s
Fig. 2 XRD patterns for SmCo7-x(Cr3C2)x (x=0.10–0.25) alloys melt-spun at 30 m/s
Figure 3 shows the XRD patterns of SmCo7-x(Cr3C2)x ribbons melt-spun at 40 m/s as a function of Cr3C2 content. It can be seen that the peaks are found to be broadened and the intensities become significantly low with the increase of wheel surface speed to 40 m/s, indicating that the alloy is driven towards amorphous structure. For the alloys with x≥0.15, the crystalline structure disappears completely and an amorphous-type phase is developed progressively in the alloys.
Fig. 3 XRD patterns for SmCo7-x(Cr3C2)x (x=0.10–0.25) alloys melt-spun at 40 m/s
3.2 Magnetic properties
Hysteresis loops of the alloys melt-spun at 30 m/s are shown in Fig. 4. A systematic change in the shape of the loop with the addition of Cr3C2 can be seen and the magnetic properties evaluated from these loops are listed in Table 1. With increasing Cr3C2 content x, the remanence Mr of the alloys increases up to the maximum value of 0.61 T at x=0.20, beyond which it then decreases to 0.16 T at x=0.25. Meanwhile, the remanence ratio (Mr:M2T) of the alloy increases from 0.67 at x=0.10 to 0.76 at x=0.20, and then decreases to 0.34 at x=0.25. The Mr increases with increasing Cr3C2 content, which is likely attributed to the stronger inter-grain exchange coupling between SmCo7 phases due to the finer grain size as observed in the broadened XRD patterns. On the other hand, the coercivity Hci initially increases from 0.42 T at x=0.10 to 0.50 T at x=0.15 and thereafter decreases to 0.02 T at x=0.25. This behavior is attributed to size effect of coercivity in fine grain sizes, i.e from multi-domain configuration to superparamagnetic state through single domain size [13]. Another reason for the decrease of coercivity may be ascribed to the formation of minor amorphous phase [14,15]. In magnetization reversal, the amorphous phase can act as reverse domain wall nucleation site and will decrease the coercivity.
Fig. 4 Hysteresis loops of SmCo7-x(Cr3C2)x (x=0.10–0.25) melt-spun at 30 m/s
Table 1 Magnetic properties of SmCo7-x(Cr3C2)x (x=0.10–0.25) melt-spun at 30 m/s
Figure 5 corresponds to the hysteresis loops of the alloys melt spun at 40 m/s. Those alloys show soft magnetic behavior with narrow hysteresis loops. The coercivities of the as-spun ribbons with x≥0.15 are found to be very low, ranging from 0.004 T to 0.007 T, and decrease with the increasing of x. A reduction of the amount of SmCo7 crystal phase and the increase of the amorphous phase, as shown in Fig. 3, are responsible for this low coercivity.
Fig. 5 Hysteresis loops of SmCo7-x(Cr3C2)x (x=0.10–0.25) melt-spun at 40 m/s
3.3 DSC analysis of amorphous structure
Figure 6 presents the DSC curves for crystallization of amorphous SmCo6.75(Cr3C2)0.25 and SmCo6.80(Cr3C2)0.20 ribbons. There are two exothermic peaks in both crystallization curves. The first exothermic peak (650℃) can be attributed to the formation of SmCo7 phase initially from the amorphous phase, and the second one (770℃) is related to the formation of Sm2Co17 phase. Therefore, the crystallization behavior of this SmCo7 alloy doped with Cr3C2 is that SmCo7 phase first precipitates from the amorphous matrix at 650℃, followed by the crystallization of Sm2Co17 phase at 770℃. It should also be noticed that the crystallization behaviors of the two alloys with different Cr3C2 contents are distinctly similar.
Fig. 6 DSC curves of melt-spun SmCo7-x(Cr3C2)x (x=0.20, 0.25) glassy alloy ribbons
4 Conclusions
1) SmCo7-x(Cr3C2)x ribbons melt-spun at 20 m/s have composite structure composed of main SmCo7 and a small amount of Sm2Co17R phase. The grain size of SmCo7 phase decreases with increasing the Cr3C2 content.
2) With the increase of wheel speed, the XRD peaks of SmCo7-x(Cr3C2)x alloys become significantly low and accompanied with a broad increase in backgrounds, indicating a considerable decrease in the grain size of the SmCo7 phase. The ribbons melt-spun at 40 m/s exhibit amorphous structure in the range of 0.15≤x≤0.25.
3) In the amorphous state, SmCo7-x(Cr3C2)x (0.15≤x≤0.25) alloys are soft magnetic with intrinsic coercive of 0.004-0.007 T. The DSC analysis reveals that SmCo7 phase firstly precipitates from the amorphous matrix at 650℃, followed by the crystallization of Sm2Co17 phase at 770℃. It also can be drawn that the addition of Cr3C2 favors the high degree of alignment of SmCo7 crystalline grains during melt spinning.
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Cr3C2含量和快淬速度对SmCo7-x(Cr3C2)x合金非晶化行为的影响
李丽娅，易健宏，李爱坤，彭元东，夏庆林
中南大学 粉末冶金国家重点实验室，长沙 410083
摘 要：通过X射线衍射法(XRD)、差示扫描量热法(DSC)和磁性测量等方法系统研究Cr3C2含量和快淬速度对SmCo7-x(Cr3C2)x(x=0.10-0.25)非晶化行为的影响。结果表明，在低的快淬速度下(20 m/s)下，合金主要由SmCo7主相和少量Sm2Co17R相构成，且SmCo7相的晶粒尺寸随着Cr3C2含量x的增加而减小。随着快淬速度的增加，合金的XRD衍射峰强度变弱、衍射峰宽化，表明SmCo7主相的晶粒尺寸随着快淬速度的增加而减小。当快淬速度增加至40 m/s时，SmCo7-x(Cr3C2)x(0.15≤x≤0.25)合金均形成了非晶态结构，合金的磁滞回线表现为软磁性的窄回线，矫顽力为0.004~0.007 T。采用DSC对合金的晶化行为分析表明，在650℃时SmCo7相首先从非晶基体中析出，在770℃时Sm2Co17相析出。
关键词：SmCo7型永磁材料；Cr3C2；快淬；非晶化；磁滞回线
(Edited by LI Xiang-qun)
Foundation item: Project (51104188) supported by the National Natural Science Foundation for Young Scholars of China
Corresponding author: LI Li-ya; Tel: +86-731-88877328; E-mail: liliya@csu.edu.cn
DOI: 10.1016/S1003-6326(11)61292-2
