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SCIENCE D I R E C T a

JOURNAL OF RARE EARTHS 24 (2006) 85 - 88

Microstructural Characterization of Novel Ni- Containing Strip Casting Li Chengdong (+&#ji) * , Zhu Xuexin (&%%), Shi Likai ( A h A) (National Engineering Research Center for Non- Ferrous Metal Composites , General Ferrous Metals , Beijing 100088 , China )

Received 18 April 2005; received in revised from 23 June 2005

JOURNAL OF

MM EAIWWS

Nd-Fe-B Strips by

Research Institute for Non-

Abstract: The characteristics of novel Nd-Fe ( Ni , Co , Al )-B microstructure prepared by strip casting technique were studied. The novel microstructure was observed using scanning electron microscope (SEM) and transmission electron mi- croscope (TEM) . Along the direction of heat flow, there are two kinds of different microstructures. Close to the wheel side, there is a thick layer containing many polygonal NdzFel4B grains. Near the free surface side, however, there are rel- ative uniform platelike NdzFeldB grains whose growth direction is not completely the direction of the heat flow during solid- ification. ‘fie formation of the novel microstructure is presumed to be the contribution of the special temperature field and Ni component.

Key words: Nd-Fe (Ni, Co, A1)-B; strip casting; columnar crystals; novel microstructure; rare earths CLC number: TB3 Document code: A Article ID: 1002 - 0721 (2006)Ol - 0085 - 04

Strip casting technique, one of the new tech- niques, is widely used due to its particular advantag-

. In strip casting technique, homogenous and fine scaled microstructures ideal for producing high ( B H ) , , , magnets can be usually obtained. The rela- tive fast cooling rate during strip casting suppresses the formation of a-Fe dendrites and large Nd-rich pockets. Directional solidification causes the formation of co- lumnar grains containing a typical arrangement of hard magnetic Nd,Fe,,B regions and Nd-rich regions‘’’ .

Besides adopting new techniques like drop tubem or strip casting, it has been reported that reasonable adjustment of composition is also an effective route to obtain the ideal microstructure. For example, ele- ments Ni, Ti, Nb, Zr, V , Mo, W , A1 or Dy were added to Nd-Fe-B alloys to change their microstruc-

tures and corresponding propertiest3-’] . These two routes can also be taken together, such as Nd,, 5Fe79 76

Al, 24 Nb,, B, strips”’ , ( Nd, 93s Dy, 06s ( Fe, 9x5

A10 015 179 B6 I strips‘101, ( Nd, Dy > I 3 Fexo B6 strips‘”’, Nd,, Dy3BICol , Al, strips‘’’], Ndlo , Pr2 , Fe74 6c05 g B 6 ,Zr0 l G q , strips‘13’ and so on. But there is a paucity of literature about the Nd-Fe-B strips con- taining Ni. Could the ideal microstructure of Nd-Fe-B alloy containing element Ni be obtained by strip cast- ing technique? During the exploration process, an in- teresting case was found.

In this study, a novel microstructure of Nd-Fe-B

strips with a composition of NdXFe (Ni , Co , Al)71 B, (7’0, mass fraction) produced by strip casting tech- nique was reported.

* Corresponding author (E-mail : lichengdong2000 @ yahoo. corn. cn 1 Foundation item: Project supported by State ‘973’ Program (G2000067200); Postdoc Foundation of China, and Youth Foundation of GRINM

Biography: Li Chengdong ( 1973 - 1, Male, Doctor

Copyright @2OOx, the Chinese Society of Rare Earths. Published by Elsevier B . V . All rights reserved.

86 JOURNAL OF RARE EARTHS, Vol. 24 , No. 1 , Feb .2006

1 Experimental Ni-containing Nd-Fe( Ni , Co , A1)-B alloy ingots

were produced by induction melting in an argon atmo- sphere. About 3 kg ingots were placed in an alumina crucible with a nozzle of 1 mm x 3 mm at the bottom. Thermal properties of the sample taken from the ingot were studied in an argon atmosphere with a heating rate of 20 K - min- ' using DSC . The ingots were remelted and superheated to about 100 K above the equilibrium liquidus temperature, which was moni- tored by a thermocouple encased in a quartz glass sheath. After holding the temperature for a proper du- ration, the molten alloy flowed through the nozzle onto a rotating molybdenum wheel ( Ifs = 2.2 m s ~ ' ) in an argon atmosphere.

The phases and crystal growth orientation of the specimens were identified by XRD using Cu Ka radi- ation at room temperature. The microstructures were observed under SEM after polishing and etching in 3% HN03-alcohol solution, and thin foil TEM after ion beam thinning.

2 Results and Discussion Fig. 1 shows DSC curve of Ni-containing Nd-Fe

(Ni, Co, Al)-B sample. It is shown that the eutectic transformation temperature, peritectic transformation temperature and liquidus are 987.7 * 0.3 K , 1308.25 0 .3 K and 1576.85 0 .3 K , respectively. These data help to determine the process of remelting the ingot and holding the liquid metal to the pouring tempera- ture 1673 K .

The strips with straight fringes are uniform in shape. And the platelike strips have a typical thick- ness of 300 * 20 p n and a width of 4 mm.

Fig. 2 shows XRD pattern of the strip Ni-contain- ing Nd-Fe( Ni , Co , A1)-B alloys produced by strip ca- sting. XRD pattern of the specimen is well indexed to

600 700 800 900 1000 1 100 I Z O O 1300

'I emperaturc/%'

Fig. 1 DSC curve of Ni-containing Nd-Fe-B alloy at a heating rate of 20 K-min-'

be tetragonal Nd2Fe14 B phase. No clear diffraction peaks of a-Fe phase can be observed in the XRD pat- tern, which indicates that there is no a-Fe phase in the strip. It is valuable to notice that the (006) reflec- tion is the strongest one, but (410) and (41 1) reflec- tions are very weak or invisible. Therefore, the crystal growth orientation is along c axis, which is quite dif- ferent from conventional results of [ 4101 and [ 41 1 ] . The result is similar to that of Refs. [ 91 and [ 141 . It makes the grains suitable for being magnetized.

It is known that the magnetic properties of Nd-Fe- B alloys produced by rapid solidification processing technique are deeply dependent on grain size and shape of Nd2Fe14 B phase. So microstructures of the specimens are observed under SEM after polishing and etching in 3% HN03-alcohol solution. SEM micro- graph of the strip is shown in Fig. 3 ( a ) . It can be seen that along the direction of heat flow, there are two kinds of different characteristics of microstruc- tures. Close to the wheel side, there is a thick layer containing many polygonal Nd2FeI4 grains with the size from 5 to more than 40 pm and a very small quantity of Nd-rich phase. However, there is relative uniform appearance near the free surface side, not only for the platelike Nd2FeI4B grains with size of about 8 p n , but

2 0 i( - ) Fig.2 XRD pattern of Ni-containing Nd-Fe-B strips perpen-

dicular to cooling direction

Fig. 3 Micrographs of Nd-Fe-B strips showing different shapes of Nd2Fe14B phase

(a) Typical SEM micrograph of polished cross section of Ni- containing strips ; ( b ) Optical micrograph of Kd-Fe-B strips without Ni

Li C D et a l . Characterization of Novel Ni-Containing Nd-Fe-B Strips 87

Fig.4 Enlarged SEM micrograph of area in white inset in Fig. 3 ( a )

also for dispersed Nd-rich phase. For comparison, Fig. 3 ( b ) shows the optical microstructure of Nd-Fe-B strips without Ni element made by the same technology

as that of Nd-Fe( Ni , Co , A1)-B alloy. It can be seen that there are quite different characteristics in Figs. 3 ( a ) and (b ) . Fig.4 is the enlarged image of the part in white pane of Fig. 3 ( a ) . The growth direction of the grains is not the direction of heat flow during solidifi- cation. They are interlaced with each other in a disor- ganized form. Furthermore, the eroded imprint can be seen clearly, which proves that the corrosion resis- tance of Nd-rich phase is worse than that of Nd,Fe,,B.

In order to clarify the phase and the correspond- ing microstructure of the strips, thin-sliced specimens were prepared by ion milling and further microstructur- a1 investigations were performed by TEM .

Fig. 5 shows TEM micrograph of Ni-containing Nd-Fe-B strips taken parallel to the surface, which shows different shapes of Nd, Fe14 B phase and its two

Fig.5 TEM micrographs of Ni-containing Nd-Fe-B strips taken parallel to surface showing different shapes of Nd2Fe14B phase ( a - f ) ; two typical diffraction patterns of tetragonal Nd2FellB ( g , h ) ; diffraction pattern of the marked region in ( a ) , ( i )

88 JOURNAL OF RARE EARTHS, Vol. 24 , N o . d , Feb .2006

typical diffraction patterns. At the margin of NdzFe14B phase marked with 2 : 14 : 1 , there are shapes like semicircles as in Figs. 5 ( a ) and ( f ) , combination of straight lines and arc as in Figs. 5( a ) and (d) , circle in (b ) , snatchy ellipse as in Fig. 5 ( e ) , and polygon as in Figs. 5 ( b) , ( c ) , and ( d ) . So combining the micrographs of SEM with TEM, it can be concluded that the columnar grains of NdzFe14B have multiform appearances in 3D space. In Figs. 5( g) and (h) , the diffraction spots of NdzFe14B phase and the indexed re- sult are shown. Fig. 5( i ) shows the corresponding dif- fraction pattern of the marked region in Fig. 5 ( a ) , there are diffraction spots from the tetragonal NdzFe14B phase and Nd-rich phase. All the above results ob- tained by SEM and TEM indicate that NdzFe14B phase should be crystallized from the melt directly.

As to the novel microstructure in this study, there are too many nuclei formed closed to the wheel side, which makes polygonal NdzFe14B grains nucleated and grow up. And the appropriate temperature gradient can not start to build up until the bottom layer grew up. Although the temperature field is not ideal for NdzFe14B phase to grow up along the direction from bottom to the free surface, Nd2FeI4B phase could be crystallized along its [ 001 ] direction. So there are two layers formed with different microstructures.

Generally speaking, the microstructure of alloys is determined by their composition and preparation process. Without Ni addition, the strips are prepared by the same process as Nd-Fe ( Ni , Co , A1)-B alloy. However, no similar microstructure is found, which can be found in Fig. 3 . Therefore the formation of the novel microstructure is presumed to be the contribution of the special temperature field and Ni addition, and the parameters should be adjusted to obtain ideal mi- crostructure without any change of composition, which will be verified in further study.

3 Conclusions 1 . Without containing a-Fe phase, the NdzFe14B

crystal growth orientation is primarily along [ 001 ] , which is quite different from conventional results of [ 4101 and [ 41 1 ] . It makes the grains suitable for be- ing magnetized.

2. Along the direction of heat flow, there are two kinds of different microstructures. Close to the wheel side, there is a thick layer containing many polygonal NdzFe14B grains. Near the free surface side, however, there are relative uniform platelike NdzFe14 B grains whose growth direction is not completely the direction

of the heat flow during solidification. 3 . The formation of the novel microstructure is

presumed to be the contribution of special temperature field and Ni addition.

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