Phase and microstructure of TbCu7-type SmFe melt-spun powders

JOURNAL OF RARE EARTHS, Vol. 31, No. 4, Apr. 2013, P. 381

Phase and microstructure of TbCu7-type SmFe melt-spun powders LUO Yang (罗 阳), YU Dunbo (于敦波)*, LI Hongwei (李红卫), ZHUANG Weidong (庄卫东), LI Kuoshe (李扩社), LÜ Binbin (吕彬彬) (National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co., Ltd., Beijing 100088, China) Received 17 December 2012; revised 1 March 2013

Abstract: The SmxZr0.3Fe9.1–xCo0.6 (x=0.8, 0.9, 1.0) powders were prepared by melt-spun method with different quenching velocities. The phase and microstructure were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Th2Zn17-type structure of the as-cast state changed to TbCu7-type after quenching to a rotating molybdenum roll under certain velocity, and the formation of TbCu7-type phase was strictly depending on the Sm content and roll speed. The SEM morphology showed that the Fe-rich zone was typically fish-bone structure and TEM diffraction pattern indicated the nano-scale crystal size with TbCu7-structure when x=0.9, and FCC type γ-Fe on the basis of α-Fe formed in the non-equilibrium solidification could be detected by selected area electron diffraction (SAED) indexing in the x=0.8 samples. Keywords: TbCu7-type; melt-spun; phase; microstructure; rare earths

As a possible substitution for NdFeB bonded permanent magnetic powders, many studies have been focused on SmFeN compounds with Th2Zn17-type[1], ThMn12type[2,3] and TbCu7-type[4,5] in the past several years. The crystal structure and magnetic properties of nitrogenated interstitial compounds have been widely investigated. Due to the meta-stable phase of TbCu7-type structure, the formation of the binary Sm-Fe precursor compound has to be accomplished by non-equilibrium solidification methods as mechanical alloying[6] and melt spinning[3]. Earlier works about TbCu7-type compounds prepared by melt spinning suggested that Zr and Co have benefits to facilitate the formation of this structure and stabilize the iron-rich phases[7,8], and thus remarkable magnetic properties can be obtained. In this letter, the powders with the composition SmxZr0.3Fe9.1–xCo0.6 (x=0.8, 0.9, 1.0) was prepared by meltspun method. The effects of Sm content and cooling rate on the phase transition and microstructure were studied.

1 Experimental The powders with nominal composition SmxZr0.3Fe9.1–xCo0.6 (x=0.8, 0.9, 1.0) and SmFe9 were prepared by melt-spun method injected onto a Mo-disk (0.3 m diameter) under surface velocities up to 40 m/s in argon atmosphere, the precursors were melted under Argon using induction melting furnace starting from elemental Sm, Zr, Fe and Co (purity 99.9%), 10 wt.% Sm

was added to each sample to compensate the Sm evaporation. X-ray diffraction (XRD) analysis was performed with a Panalytical X'Pert PRO diffractometer using Cu Kα radiation. The samples for scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were pre-treated with 5 vol.% nitric acid alcohol and the samples for transmission electron microscope (TEM) and selected area electron diffraction (SAED) were prepared by metal encapsulation technique for cutting such thin films from fine particles[9].

2 Results and discussion 2.1 Phase formation TbCu7-type is the derivation of P6/mmm CaCu5-type structure with a certain pair of Fe atoms replacing Sm sites randomly, after the alloy suffered a rapid quenching process to the non-equilibrium state, the stable Th2Zn17type structure changes disorderly to the TbCu7-type[10,11]. The XRD pattern of Fig. 1 indicates a gradual transition from the rhombohedral Th2Zn17-type structure to hexagonal TbCu7-type structure, together with small amount of PuNi3-type SmFe3 and α-Fe in the samples of SmFe9. In the as-cast state, the peaks with high intensity of (220), (303) and (006) indicate the Th2Zn17-type, when the velocity increases to 32 m/s, the three peaks shift to a dominant peak combined with (200), (111) and

Foundation item: Project supported by the National High Technology Research and Development Program of China (863 Program) (2011AA03A402), the International Scientific and Technological Cooperation Projects (2010DFB53520) * Corresponding author: YU Dunbo (E-mail: [email protected]; Tel.: +86-10-82241180) DOI: 10.1016/S1002-0721(12)60290-7

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(002), (226) and (413) of Th2Zn17-type to a peak combined with (211) and (202), and also, the (024) disappeared, the phenomenon is consistent with Katter et al.[12] As to SmFeN magnetic powders with TbCu7-type structure, the Sm2Fe17 and α-Fe are two main inferior phases that can decrease the magnetic properties greatly, these two phases are mainly considered in this paper on the basis of principle TbCu7-type phase, the (006) and (024) peaks of Th2Zn17-type and (110) of α-Fe are appointed as the characteristic peaks, the appearance of which can indicate the existence of Th2Zn17-type in TbCu7-type structure. The XRD patterns of Sm0.8Zr0.3Fe8.3Co0.6 melt-spun powders are in Fig. 2, which shows that (006) peak of Th2Zn17-type at a lower speed of 12 m/s decreases to zero (at least to the XRD detection limit) when the quenching velocity is higher than 32 m/s, the cooling rate

JOURNAL OF RARE EARTHS, Vol. 31, No. 4, Apr. 2013

Fig. 1 XRD patterns of SmFe9 alloys under different quenching velocities

increases with the roll velocity, which brings the large degree of disorder for the Fe-Fe pair substitution for Sm in the as-made quenching material, the α-Fe phase appears at about 36 m/s and reaches high intensity compared to the main peak of TbCu7 at large undercooling of 42 m/s, the reason is partly due to the slightly greater loss of Sm in the process and also, the disappearance of Th2Zn17 structure. The least amount of Th2Zn17-type structure and α-Fe can be counted by intensity when x=0.9 (see Fig. 3) in this experiment. Even the velocity is 24 m/s, a little deviation from this composition (1 at.% of Sm) can lead to the appearance of other phases (see Fig. 2 when x=0.8 and Fig. 4 when x=1.0), the non equilibrium rapidly quench process disorders the crystal structure to form meta-stable TbCu7-type and α-Fe can inhabit the appearing of Th2Zn17-type structure to a large extent. The Sm1.0Zr0.3Fe8.1Co0.6 alloys in Fig. 4 show that (006) peak of Th2Zn17-type still exists even the velocity is higher than 32 m/s and a small amount of α-Fe phases appear in the pattern, as compared to the results in Fig. 3 and Fig. 2, the following conclusion can be got that the single TbCu7-type structure can only be formed under a certain Sm content and processing conditions, and the former is more important. The lattice parameters of a, c and c/a are calculated by JadeTM 5 software, it can be seen from Fig. 5 that under a certain Sm content x, the c/a value increases with the quenching velocity and the Sm0.9Zr0.3Fe8.2Co0.6 alloys have the highest c/a value at the same quenching speed. The value of c/a is the reflection of structure and size change of SmZrFeCo alloys, according to Sakurada et al.[13], when the c/a value of the melt-spun alloy dropped

Fig. 2 XRD patterns of Sm0.8Zr0.3Fe8.3Co0.6 alloys under different quenching velocities

Fig. 3 XRD patterns of Sm0.9Zr0.3Fe8.2Co0.6 alloys under different quenching velocities

LUO Yang et al., Phase and microstructure of TbCu7-type SmFe melt-spun powders

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Fig. 4 XRD patterns of Sm1.0Zr0.3Fe8.1Co0.6 alloys under different quenching velocities Fig. 6 Surface morphology of Sm0.9Zr0.3Fe8.2Co0.6 melt-spun flake alloys (a) Roller surface; (b) Free surface

Fig. 5 a, c and c/a value of SmZrFeCo melt-spun flake alloys

in the range of 0.84–0.88, the principle phase of TbCu7-type structure tends to form in the resultant material, and high velocity and Fe (Co) content can be beneficial to the improving of c/a value. 2.2 Microstructure The morphology of the melt-spun flake alloys was examined by SEM. Fig. 6 shows that the alloys crystallize differently on the two sides. The roller side has high temperature gradient that uniformizes the surface structure along the heat flux direction while the free surface with salient areas due to unstable thermal flow as contacting Ar atmosphere. The microstructure and composition EDS in Fig. 7 indicates that the Fe-rich structure (A) of rod-like or fish-bone-like is embedded in the SmFe matrix (B), the size of the Fe-rich structure is well-distributed with nearly 1–5 μm. What is more, Co content varies little in both Fe-rich and SmFe matrix due to its occupation of Fe

sites in TbCu7-type and α-Fe structure, and Zr content deviates a lot in the two parts due to Zr substitute for Sm sites in the powders[14]. The TEM images and SAED of SmZrFeCo melt-spun ribbons are shown in Figs. 8 and 9, from which it can be seen that there are highly mottled areas, scattering in the individual grain boundaries or accumulating as a cluster. For Sm0.9Zr0.3Fe8.2Co0.6 in Fig. 8, the main phase of the black mottled contrast in the Sm0.9Zr0.3Fe8.2Co0.6 meltspun alloys is indexed from SADE image as polycrystalline with TbCu7-type structure scattered homogeneously in the white matrix, with no second phase existing in the triangular grain boundary (“a” in Fig. 8), especially amorphous phases, which is well corresponding with the XRD results in Fig. 3. It can be seen from Fig. 2 that for the samples with x=0.8, a large amount of α-Fe appears when the velocity is up to 40 m/s. TEM image in Fig. 9 shows that crystal size is of nanoscale of less than 100 nm, and the single diffraction pattern of the selected cluster area is indexed as FCC (Face Central Cubic) γ-Fe rather than α-Fe. According to SmFe phase diagram[15], SmFe compounds are formed by peritectic reaction, γ-Fe precipitates at the temperature of 1280 ºC and changes to body central cubic α-Fe as the temperature decreasing gradually from 912 ºC, during rapid quenching process, the non-equilibrium solidification inhibits the transformation of γ-Fe to α-Fe, it is reasonable to find a small amount of γ-Fe in the melt-spun flakes. Meanwhile, a heat homogenization

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Fig. 8 TEM and SAED of Sm0.9Zr0.3Fe8.2Co0.6 melt-spun alloys prepared at a wheel speed of 32 m/s (the inset image is the polycrystalline diffraction of selected area, and “a” denotes the triangular grain boundary)

Fig. 7 Microstructure (a) and composition analysis of Sm0.9Zr0.3 Fe8.2Co0.6 melt-spun flake alloys on point A (b), B (c)

treatment process between 700–900 ºC can be introduced to initiate the transformation process of paramagnetic γ-Fe to ferromagnetic α-Fe, which will be of benefit to the improvement of magnetic properties. As the similar composition and rapid quenching process, the morphologies in Figs. 8 and 9 vary little, especially the crystal size in both samples. It can also be concluded that the TbCu7-type structure is closely related to the composition and process, especially the rare earth content, with 1 at.% variation leading to obvious transition of α-Fe, TbCu7- and Th2Zn17-type, while microstructure is not that sensitive to the composition, according to the literatures[16–19], addition of Ti and C can minimize the crystal size to a large extend.

Fig. 9 TEM and SAED of Sm0.8Zr0.3Fe8.3Co0.6 melt-spun alloys prepared at a wheel speed of 40 m/s (the inset image is the polycrystalline diffraction of selected area)

3 Conclusions The SmxZr0.3Fe9.1–xCo0.6 (x=0.8, 0.9, 1.0) powders were prepared by melt-spun method with different quenching velocities in this study and the as-prepared powders were examined by XRD, SEM and TEM. The results suggested that the three phases existed in the melt-spun al-

LUO Yang et al., Phase and microstructure of TbCu7-type SmFe melt-spun powders

loys. Th2Zn17-type, TbCu7-type and α-Fe were sensitive to Sm content and quenching velocity. x<0.9 led to the formation of α-Fe and Th2Zn17-type appeared when x>0.9 even the speed exceeded 32 m/s. The SEM morphology showed that the typical fishbone Fe-rich structure was embeded in SmFe matrix and TEM diffraction pattern indicated the nano-scale crystal size with TbCu7-structure when x=0.9, and FCC type γ-Fe formed on the basis of α-Fe in the non-equilibrium solidification could be detected by SAED indexing.

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