1394 IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5, SEPTEMBER 1990

MAGNETIC PROPERTIES OF MELT SPUN NdFe10Cr2-Nd0.6,Bo.33 PSEUDOBINARY ALLOYS

S.H. Huang, T.S. Chin, Y.S. Chen, and S.K. Chen Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30043, Taiwan, Rep. of China

C.H. Lin, C.-D. Wu, and W.C. Chang Materials Research Lab., ITRI, Hsinchu, 31000, Taiwan, Rep. of China

Abstract-The coercivity of the melt spun pseudobinary (NdFeloCS2!1-x(;di;)x alloys, in which Nd2B represents a compos1 ion o 67B0 3q,were systematlcally studi- ed. It was found that sigh icant coercivities are pos- sible with optimal additions of the Nd-B composition at x= 0.5 - 0.7. The as-spun (substrate velocity, VS = 10 m/s) coercivity increased from 0.1 kOe at x=O.O to ma- xima of at x= 0.5 and 0.7, due to'the Nd Fe B and a new Fe-Nd phase, respectively, as evi- deZcea4f rom thermomagnetic' analysis and x-ray dif frac- tion. Annealing the over-quenched amorphous x=0.7 alloy led to the crystallization of the Nd2Fe14B phase and a coercivity of 8.5 kOe was obtained.

7.0 and 7.5 kOe

INTRODUCTION

Single phase compounds RFelZexMx (R =rare earths, M = Ti, Cr, Si, etc.) with ThMn -type structure have been extensively studied [ 1,2]. '6owever, only a few studies were on melt-spun NdFe12-xM [3]. The coercivity of the stoichiometric melt spun rigbon is low due to the exis- tence of minor soft magnetic phases which can not be removed by the subsequent annealing processes.

to the Nd Fe14B phase by adding Ndo.67Bo 23 tomposition, and ma8e a systematic study on the resul an pseudobi- nary system by melt-spinning and post-annealing.

In this study, we attempted to turn the free iron

EXPERIMENTAL

The (NdFe10Cr2)1-x(Nd2B)x alloys were prepared by arc melting pure constituent elements of 99+% purity in a cold hearth under Ar protection. The alloys were melt at different substrate velocities ( V s ) , again under Ar atmosphere. The spun ribbons were then heat treated at 600, 700, BOO, and 900°C for a period of 10, 30, 60 and 90 min., respectively, at each temperature under a Ti gettered Ar atmosphere. X-ray diffraction(XRD) patterns were directly obtained from the ribbon surface with Cu Ka radiation. Magnetic properties were measured with a vibrating sample magnetometer (the maximun field being 17 kOe and with a thermomagnetic balance in an applied field of about 0.4 kOe. Curie temperatures were obtain- ed from the inflection points of thermomagnetic curves. Microstructure was studied with a scanning transmission electron microscope (STEM) and an electron probe niicro- analyzer (EPMA).

RESULTS AND DISCUSSION

The As-spun Nd(Fe,M)12 Alloys

In o u r preliminary work, the Nd(Fe,M) alloys (M=Ti, Cr, Al, Si, MO, V ) were studied by mel&2spinning. All as-spun ( V s = 10 m/s) alloys exhibit ThMn12-type (1-12) structure except M= V . However, various amounts of free iron coexist with the 1-12 phase in the x-ray diffrac- tion patterns, as shown in Fig. 1. Unidentified phases besides free iron were observed for M= Si and MO alloys which exhibit extremely low coercivity of less than 0.1 kOe. The alloy with M= Ti o r A1 o r Cr has as-spun coer-

*1:12 phase

1 I I 32 44 56

2e - Fig. 1 X-ray diffraction patterns Of aslndt spun (vS=

10 m/s) Nd(Fe,M)12 alloys

civity of, 0.40, 0.18, and 0.10 kOe, respectively, in the order of increasing free iron peak intensity. Hence coercivity might be correlated with the peak intensity of free iron in Fig. 1. Further annealing at various temperatures can not eliminate free iron nor improve the coercivity. In this work, Nd Fe Cr with the high- est content of free iron was.th;s tRosZn as the basic alloy, and doped with Nd2B (Ndo.67Bo.33) composition.

The As-spun (NdFe10Cr2)1-x(Nd2B)x Alloys

Fig.2 shows the effect of substrate velocity ( V s ) on intrinsic coercivity and magnetization of the as-spun x = 0.5 alloy. It is manifest that the best as-spun co- ercivity can be achieved at an optimal V s of 10 m/s. V s = 20 m/s led to amorphous and partial crystallization, while V s = 30 m/s led to totally amorphous structure.

7

G

n 0 4

9 E 2 U

0

0

X = 0 5 as-spm 70

- 60

- 20

1 I 0 4 10 Vs (m/sec) 20 30

Fig. 2 Effect of substrate velocity on magnetic proper- ties of as-spun (NdFeloCr2)o.5(Nd2B)o.5 alloy

0018-9464/90/0900-1394$01.00 0 1990 IEEE

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n

v 2 4

0 5

2

0 0.2 0.4 0.6 0.8 1

X

Fig . 3 E f f e c t of composi t ion on t h e magnet ic p r o p e r t i e s of as-spun ( V s = l O m/s) (NdFe10Cr2)1-x(Nd2B)x a l l o y

Composition dependence o f iHc and magnet iza t ion f o r r h e as-spun r ibbon a t t h e opt imal V s of 10 m/s is shown i n Fig. 3. The magnet iza t ion va lue measured a t a n app- l i e d f i e l d of 17 kOe, 6, 7, d e c r e a s e s monotonical ly w i t h t h e Nd-B c o n t e n t s . The decrease i n 0 17 may r e s u l t from t h e nonmagnetc alpha-Nd and a bor ide phase t o be d iscu- s s e d later i n t h e t e x t . The i n t r i n s i c c o e r c i v i t y i n c r e - ases s l i g h t l y from 0.1 kOe a t x = 0.0 t o 0.3 kOe a t x = 0.3, t h e n i n c r e a s e s d r a m a t i c a l l y t o 7.0 kOe a t x = 0.5. The r e a s o n of t h i s i n c r e a s e can be expla ined by t h e phases i n t h e a l l o y as g iven below.

XRD p a t t e r n s denote t h e c o e x i s t e n c e of t h e 1-12 phase and f r e e i r o n wi th x up t o 0.3. The i n t e n s i t y of i r o n phase d e c r e a s e s wi th x, and down t o a t r a c e amount a t x = 0.3, i n which t h e r e e x i s t s a n a l p h a Nd and a s m a l l amount of t h e Nd Fe B (2-14-1) phase. The 2-14-1 phase becomes t h e major2 p i k e i n t h e x = 0.5 a l l o y , i n which no 1-12 nor f r e e i r o n can be i d e n t i f i e d . While i n t h e x = 0.7 a l l o y t h e NdB6 becomes t h e major phase and no 2- 14-1 phase. The r e s u l t s of XRD a n a l y s e s on t h e as-spun ( V s = 10 m/s) a l l o y s are summarized i n Phase i d e n t i f i c a t i o n w a s f a c i l i t a t e d by thermomagnetic ana ly- ses t o be d iscussed i n t h e n e x t s e c t i o n .

x = 0.5 a l l o y is a t t r i b u t e d t o t h e e x i x t e n c e of t h e major 2-14-1 phase.

Table I.

The h igh c o e r c i v i t y of t h e as-spun

Table I. Phases i n t h e as-spun (Vs = 1 0 m/s) a l l o y s i n t h e o r d e r of decreased XRD i n t e n s i t y

x = 0.3 1-12 + a-Nd + a-Fe + (2-14-1) + (1-4)”

x = 0.5 2-14-1 + a-Nd + (1-4)* [l-4 = NdFe‘]

x = 0.7 NdB6 + a-Nd + 1-4”

( ) i n d i c a t i n g minor amount, * judging from Tc

Thermomagnetic and EPMA Analyses

Thermomagnetic c u r v e s f o r t h e as-spun x = 0 . 3 and 0.7 a l l o y s ( V s = 10 m/s) a r e shown i n Fig. 4 . Both o f them e x h i b i t t h e same Cur ie tempera ture (Tc) of 24OoC, which is approxirhately equal t o t h a t of melt spun Nd-Fe a l l o y r e p o r t e d by Croa t [ 5,6] . The r e p o r t e d c o e r c i v i t y of 7.45 kOe is a l s o c l o s e t o our d a t a of 7.5 kOe f o r x= 0.7 al- l o y , i n which t h e r e is no 2-14-1 phase. A new magnet ic phase of as-cast Nd-Fe a l l o y s was later proposed t o be a n oxygen s t a b i l i z e d NdFe40 phase [7] . We s u g g e s t t h a t i n t h e x=0.7 a l l o y , t h e 2&14-1 phase can no longer e x i s t due t o t h e d e p l e t i o n of B which forms t h e s t a b l e r NdB phase, hence t h e new NdFe4 phase h a s a chance t o c ry6s ta l l ize .

(b) x=0.3, ( c ) x=0.7,

10 d s , as-spun

0 100 200 300 400 Temperature OC )

Fib. 4 ‘Thermomagnetic c u r v e s of t h r e e a l l o y s shown

There is a n o t h e r Tc of 303°C f o r t h e x=0.3 a l l o y as shown i n Fig. 4. T h i s is t y p i c a l of t h e 2-14-1 phase, whose Tc i s 312OC [ 4 ] . is unders tandable c o n s i d e r i n g t h e doping of C r i n t o t h e 2- 14-1 phase. No 303OC d e f l e c t i o n is seen i n t h e spun x = 0.7 a l l o y , c o n s i s t e n t wi th t h e XRD d a t a of being no 2- 14-1 phase i n t h i s a l l o y .

The a l p h a Nd and b o r i d e phases were a l s o f r e q u e n t l y s e e n i n t h e as-spun x = 0.7 a l l o y dur ing EPMA a n a l y s e s , a s i l l u s t r a t e d i n Fig. 5. B r i g h t p a r t i c l e s , few microns i n s i z e , were abundant. Composition l i n e p r o f i l e s deno- t e t h a t t h e p a r t i c l e s a r e enr iched i n Nd and B whi le d e f i c i e n t i n Fe and C r . These p a r t i c l e s are be l ieved t o t o be t h e NdB6 phase, which is t h e most s t a b l e compound i n t h e b i n a r y Nd-B system. Both Nd and NdB6 phases may f a c i l i t a t e t h e format ion t h e NdFe4 phase. Dramatical decrease i n c o e r c i v i t y occurs due t o l a r g e amount of paramagnet ic Nd and NdB .

The decrease i n magnet iza t ion , 6 w i t f : i n c r e a s i n g Nd B, as previous ly shown i n Fig. $:’is due t o t h e mag- n e z i c d i l u t i o n of t h e nonmagnetic a-Nd and NdB6 phases .

The n i n e degree lower i n Tc

a t x = 0.9 and 1.0,

F ig . 5 EPMA l i n e p r o f i 1 . e ~ showi.ng ( a ) Nd and B, and ( b ) Fe and C r

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6 n 0 s W

0 3

0 0.2 0.4 0.G 0.8 X

Fig . 6 Composition de endence of t h e al.l.oys a f t e r an 8 nea3.ing a t 800 C f o r one hour

E f f e c t of Annealing

E f f e c t s of annea l ing on t h e over-quenched r ibbon were eva lua ted by varying t h e annea l ing t empera tu re and t i m e which i n d i c a t e d an optimum c o n d i t i o n of annea l ing a t 800'~ f o r 1 hour . F ig . 6 shows t h e composi t ion depen- dence-of iHc f o r t h e r ibbons prepared at Vs= 20 m / s and 30 m / s t h e n annea led a t 800°C f o r 1 hour . The tendency t h a t iHc i n c r e a s e s w i t h x up t o a maximum va lue a t x = 0.7 is c o n s i s t e n t w i th t h a t of as-spun a l l o y s , as shown i n F ig . 3. The maximum iHc va lue of t h e annealed a l l o y is 8.5 kOe compared t o 7 .5 kOe f o r t h e as-spun a l l o y i n which NdFe4 phase is though t to e x i s t . Table I1 shows t h e r e s u l t s of XRD anal-yses on t h e annea led x = 0 . 3 t o 0 .7 a l l o y s melt-spun a t Vs=20 and 30 m/s, r e s p e c t i v e l y .

Phases i n t h e annea led a l l o y s me1.t-spun a t shown V s , i n t h e o rde r of dec reas ing XRD i n t e n s i t y

Table 11.

x = 0.3 a-Fe + a-Nd + (1-12) + (2-14-1)

x = 0 .5 Vs = 20 m/s a-Fe + a-Nd f 2-14-1 + 1-12 Vs = 30 m/s a-Fe + a-Nd f 1-12 + 2-14-1

x = 0.7 Vs = 20 m/s a-Nd + 2-14-1 Vs = 30 m/s a-Nd f 2-14-1

Thermomagnetic cu rve of t h e annea led x=0.7 a l l o y (Vs= 20 m/s) i s shown i n Fig. 4. It e x h i b i t s a Tc of 3 0 3 O C , t y p i c a l of t h e 2-14-1 phase. It is appa ren t t h a t t h e 2-14-1 phase is t h e c r y s t a l l i z e d phase d u r i n g annea l ing of t h e amorphous x=O.7 a l l o y . The magnet ic hardening of t h e annealed a l l o y s is s t r o n g l y c o r r e l a t e d t o t h e e x i s - t e n c e of t h e 2-14-1 phase,

Summary

I n summary, f r e e i r o n i n t h e 1-12 Nd-Fe-Cr a l l o y s is successfu1l.y reduced w i t h t h e a d d i t i o n of a Nd 67B0.33 composi t ion. However, t h e i n c r e a s e o f iHc up ??I a pro- mising v a l u e above 7 kOe is a t t r i b u t a b l e t o t h e e x i s - t e n c e of t h e Nd Fe14B phase i n t h e as spun x=0.5 a l l o y and anneal.ed x=6.7 a l l o y , whereas t o t h e possib1.e e x i s - t e n c e of a NdFe phase i n t h e as-spun ~ ~ 0 . 7 a l l o y . Fur- t h e r i n v e s t i g a t t o n on t h e new magnet ic NdFe4 phase is i n p rogres s .

Acknowledgements

The a u t h o r s are indeb ted t o t h e Nat ional Sc ience Counci l of t h e Republ ic of China f o r sponsor ing t h i s work. The g r a n t number is NSC- 79-0416-E007-06.

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