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Formation of nonequilibrium Cr3C carbide in CrC binary alloys quenched rapidly from the melt
Scripta METALLURGICA
Vol. 13, pp. 711-715, 1979 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved.
FORMATION OF NONEQUILIBRIUM Cr3C CARBIDE IN Cr-C BINARY ALLOYS QUENCHED RAPIDLY FROM THE MELT
A. Inoue and T. Masumoto The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal 980, Japan (Received May 15, 1979)
Introduction Splat quenching technique has been used in order to investigate the formation of nonequilibrium phases and the extension of solid solubilities in various kinds of alloys. For the last several years, the present authors also have p e r formed a series of investigations about the rapid quenching effect on the structure of X-C (X: V, Cr, Mn, Fe, Co, Ni, Me) binary alloys(l). And we have found that a new nonequilibrium carbide, Cr3C , is formed in the Cr-C binary alloys quenched rapidly from the melt. This paper deals mainly with the composition range in which the Cr3C carbide forms, its crystal structure, precipitation morphology and thermal stability. Experimental The specimens used were Cr70~88C12%30") binary alloys. Mixtures of electrolytic chromium and graphite were melted in argon atmosphere by an arc furnace. From these alloys, ribbon specimens of about 1 mm width and 0.03 mm thickness were prepared by using the rapid quenching apparatus designed for high melting alloys by the present authors. The outline of this apparatus and the procedures to prepare ribbon specimens have been described in a previous paper(2). The identification of phases in the as-quenched and tempered states was made by X-ray diffraction analysis using filtered Cr Ks radiation and transmission electron microscopy. Results and Discussion Figure 1 illustrates the equilibrium diagram of Cr-C binary system(3) and the constitution of as-quenched alloys. In this figure, the most interesting fact is that Cr3C carbide which has been unknown so far is formed in the range of about 13%22 at% carbon by rapid quenching. This carbide is a nonequilibrium phase as it is not present in the equilibrium state. As an example, Fig. 2 shows the transmission electron micrograph and the selected area electron diffraction patterns for the as-quenched Cr84C16 alloy. The structure is a very fine lamellar aggregate with a spacing of about 20 nm. The electron diffraction patterns shown in Figs. 2 (b) to (d) also indicate that two phases (Cr and a carbide) coexist in this microstructure. The large number of diffraction spots except those from chromium with a bcc structure show clearly that the carbide is not the equilibrium Cr23C 6 phase but a Fe3C-type carbide with an orthorhombic structure. From the structural analysis and the chemical composition of the alloy, this carbide is judged to be Cr3C. The lattice parameters of this carbide were determined to be a=0.458 nm, b=0.512 nm and c=0.680 nm by comparison with chromium phase(a = 0.2885 nm). These values are slightly larger than those of Fe3C(4 ) owing proba*)
In the present paper, all alloy compositions are represented by weighted values denoted by atomic percentage.
711 0036-9748/79/080711-05502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
712
NONEQUILIBRIUM Cr3C CARBIDE
Vol. 13, No. 8
bly to a dif£erence in the atomic size. It was also found that the relationship of crystal orientation between Cr3C and Cr agrees with that between Fe3C and eFe in the tempered carbon steels. That is, this relationship is shown as follows
(lO0)cr3C / /
(0il)cr
(010)Cr3 C / /
(III)c r
(001)Cr3C / /
(211)Cr
(Bagaryatskii's
relationship(5))
Below a b o u t 15 at% c a r b o n , t h e s t r u c t u r e o f t h e a s - q u e n c h e d a l l o y s i s a mixt u r e o f Cr23C 6 and Cr. One example o f t h e s t r u c t u r e i s shown i n F i g . 3. In the f i g u r e s , ( c ) i s t h e d a r k f i e l d image t a k e n from t h e 2~4 r e f l e c t i o n o f Cr25C6 c a r bide in (b). L a r g e p a r t i c l e s o f Cr23C 6 c a r b i d e a r e d i s p e r s e d i n chromium m a t r i x . For a l l o y s w i t h t h e c a r b o n c o n t e n t 5~tween 22 and 30 at%, t h e a s - q u e n c h e d s t r u c t u r e s c o n s i s t o f two t y p e s o f c a r b i d e s , n a m e l y , Cr23C 6 and Cr7C3, as shown i n F i g . 4. As i s c l e a r l y known from t h e s e r e s u l t s , the as-quenched structures of alloys w i t h c a r b o n c o n t e n t s e x c e p t from 13 t o 22 at% c o n s i s t o f t h e e q u i l i b r i u m p h a s e s i n s p i t e o f r a p i d q u e n c h i n g from t h e m e l t . And, t h e n o n e q u i l i b r i u m p h a s e o f Cr3C a p p e a r s o n l y i n t h e l i m i t e d c a r b o n r a n g e o f 13~22 at%. Although the reason is u n c e r t a i n a t p r e s e n t , i t may be e x p l a i n e d from t h e f o l l o w i n g t h r e e e v i d e n c e s : Firstly, t h e Cr3C f o r m i n g a l l o y s a r e r e s t r i c t e d in the lower m e l t i n g c o m p o s i t i o n r a n g e a r o u n d t h e e u t e c t i c p o i n t a t a b o u t 14 at% c a r b o n , as s e e n i n F i g . I . Seco n d l y , t h e r e i s a d i f f e r e n c e i n t h e c o m p l e x i t y o f c r y s t a l s t r u c t u r e among c a r bides. T a b l e I l i s t s t h e d a t a on t h e c r y s t a l s t r u c t u r e s and t h e u n i t c e l l v o l umes o f t h e chromium c a r b i d e s . Both Cr23C 6 and CrTC 5 have more complex s t r u c TABLE I Crystal structures, l a t t i c e p a r a m e t e r s and u n i t c e l l volumes o f chromium c a r b i d e s i n Cr-C b i n a r y s y s t e m Carbide
Crystal structure
Lattice
Cr3C
Ortho.
0.458
Cr23C 6
Cubic
1.064
Cr7C 3
Hex.
1.401
Cr3C 2
Ortho.
0.5533
parameters(nm) b c 0.512 0.680
0.2829
Unit cell volume(nm 5)
Reference
0.159
Present work
1.205
£6)
0.4532
2.511
(7)
1.147
0.180
(8)
tures and larger unit cell volumes compared with those of Cr3C carbide. Such differences may be related to the ease of nucleation and growth of their carbides from melts during rapid quenching(=105 K/s). Thirdly, the Cr-X-C [X: Fe, Co, Ni, Mo, Nb, Ta] ternary alloys around the eutectic trough easily form an amorphous phase by rapid quenching(2). This implies that the Cr-C alloys around the eutectic point also have a tendency for supercooling. From these three points, therefore, the long-range atomic rearrangement required for the nucleation of Cr23C 6 or CrTC 5 in the melt will be more difficult probably because of a strong tendency to supercool in the case of alloys having lower melting points. Hence, the rapidly quenched alloys around the eutectic point will acquire a nonequilibrium structure consisting of Cr3C and Cr. The CrBC carbide transforms into the equilibrium carbide upon tempering. This structural change is shown in Fig. 5. The Cr3C carbide remains unchanged at tempering temperatures below about 975 K. By successive tempering, however, the carbide reactions proceed from Cr3C to metastable Cr7C 5 and from Cr7C 5 to stable Cr23C6, as seen in Fig. 5 (b)~(d).
Vol. 13, No. 8
NONEQUILIBRIUM Cr3C CARBIDE
713
Conclusion A nonequilibrium chromium carbide of MsC type, Cr3C, was found in Cr78~87CI5%Z 2 binary alloys quenched rapidly from the melt. The Cr3C carbide has t h e same o r t h o r h o m b i c s t r u c t u r e as Fe3C and t h e l a t t i c e p a r a m e t e r s a r e a=0.458 nm, b=0.S12 nm and c = 0 . 6 8 0 nm. The l a t t i c e r e l a t i o n s h i p s between Cr3C and Cr a r e (100)Cr3 C / / (011)Cr , ( 0 1 0 ) C r , C / / (111)C r and (001)CrsC / / ( 2 1 1 ) C r . This c a r b i d e r e m a i n s u n c h a n g e d a t t e m p e r i n g t e m p e r a t u r e s below a b o u t 973 K, b u t by s u c c e s s i v e t e m p e r i n g t h e c a r b i d e r e a c t i o n s p r o c e e d from Cr3C t o m e t a s t a b l e Cr7C 3 and f i n a l l y t o s t a b l e Cr23C 6. References 1. A. I n o u e and T. Masumoto, The Res. I n s t . f o r I r o n , S t e e l and O t h e r M e t a l s , Tohoku U n i v . , S e n d a l , J a p a n , u n p u b l i s h e d r e s e a r c h ( 1 9 7 9 ) . 2. A. I n o u e , S. S a k a i , H. M. Kimura and T. Masumoto, T r a n s . JIM, 20, No.5 i n press(1979). 3. M e t a l s Handbook e i g h t h e d i t i o n , M e t a l l o g r a p h y S t r u c t u r e s and Phase Diagram, p.274, American S o c i e t y for M e t a l s ( 1 9 7 3 ) . 4. E.J. F a s i s k a and G. A. J e f f r e y , A c t a C r y s t . , 19, 463 ( 1 9 6 5 ) . S. Yu. A. B a g a r y a t s k i i , Dokl. Akad. Nauk SSSR,,,73~--1161 ( 1 9 5 0 ) . 6. D. M e i n h a r d t and O. K r i s e m e n t , A r c h . E i s e n h f i t - t e n w . , 33, 493 ( 1 9 6 2 ) . 7. A. W e s t g r e n , J e r n k o n t . A n n . , 119, 231 ( 1 9 3 5 ) . 8. S. R u n d q v i s t and G. R u n n s j ~ , ~ a Chem. S c a n d . , 2-3, 1191 ( 1 9 6 9 ) .
Rapidly quenched structure
~136 K
L÷C
21oo 2041K
't
L I
/
/
208K 6
/
II//*'S"~~K
/
!g
I
I
Cr3C2* C
iI
v
t I
I
0
I !
/
c3
I msox
I--
1700 I
Cr
I0
20 30 Corbon (ot%)
40
FIG. 1 Equilibrium phase diagram of Cr-C binary system and the structures in rapidly quenched state.
714
NONEQUILIBRIUM Cr3C CARBIDE
Vol.
13, No. 8
G
a
FIG. 2 Transmission electron micrograph(a) and the selected area diffraction patterns(b)~(d) for as-quenched Cr84C16 alloy.
@
0
O
FIG. 3 Transmission e l e c t r o n m i c r o g r a p h ( a ) , the s e l e c t e d a r e a d i f f r a c t i o n p a t t e r n ( b ) and t h e d a r k f i e l d i m a g e ( c ) u s i n g t h e 224 r e f l e c t i o n from Cr23C6, f o r a s - q u e n c h e d Cr88C12 a l l o y .
Vol. 13, No. 8
NONEQUILIBRIUM Cr3C CARBIDE
FIG. 4 Transmission electron micrograph(a) and the selected area diffraction pattern(b), for as-quenched Cr75C25 alloy.
FIG. 5 T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s and t h e s e l e c t e d a r e a d i f f r a c t i o n p a t t e r n s showing t h e s t r u c t u r e c h a n g e s o f r a p i d l y quenched Cr84C16 a l l o y upon t e m p e r i n g a t 973 K.
715
Vol. 13, pp. 711-715, 1979 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved.
FORMATION OF NONEQUILIBRIUM Cr3C CARBIDE IN Cr-C BINARY ALLOYS QUENCHED RAPIDLY FROM THE MELT
A. Inoue and T. Masumoto The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal 980, Japan (Received May 15, 1979)
Introduction Splat quenching technique has been used in order to investigate the formation of nonequilibrium phases and the extension of solid solubilities in various kinds of alloys. For the last several years, the present authors also have p e r formed a series of investigations about the rapid quenching effect on the structure of X-C (X: V, Cr, Mn, Fe, Co, Ni, Me) binary alloys(l). And we have found that a new nonequilibrium carbide, Cr3C , is formed in the Cr-C binary alloys quenched rapidly from the melt. This paper deals mainly with the composition range in which the Cr3C carbide forms, its crystal structure, precipitation morphology and thermal stability. Experimental The specimens used were Cr70~88C12%30") binary alloys. Mixtures of electrolytic chromium and graphite were melted in argon atmosphere by an arc furnace. From these alloys, ribbon specimens of about 1 mm width and 0.03 mm thickness were prepared by using the rapid quenching apparatus designed for high melting alloys by the present authors. The outline of this apparatus and the procedures to prepare ribbon specimens have been described in a previous paper(2). The identification of phases in the as-quenched and tempered states was made by X-ray diffraction analysis using filtered Cr Ks radiation and transmission electron microscopy. Results and Discussion Figure 1 illustrates the equilibrium diagram of Cr-C binary system(3) and the constitution of as-quenched alloys. In this figure, the most interesting fact is that Cr3C carbide which has been unknown so far is formed in the range of about 13%22 at% carbon by rapid quenching. This carbide is a nonequilibrium phase as it is not present in the equilibrium state. As an example, Fig. 2 shows the transmission electron micrograph and the selected area electron diffraction patterns for the as-quenched Cr84C16 alloy. The structure is a very fine lamellar aggregate with a spacing of about 20 nm. The electron diffraction patterns shown in Figs. 2 (b) to (d) also indicate that two phases (Cr and a carbide) coexist in this microstructure. The large number of diffraction spots except those from chromium with a bcc structure show clearly that the carbide is not the equilibrium Cr23C 6 phase but a Fe3C-type carbide with an orthorhombic structure. From the structural analysis and the chemical composition of the alloy, this carbide is judged to be Cr3C. The lattice parameters of this carbide were determined to be a=0.458 nm, b=0.512 nm and c=0.680 nm by comparison with chromium phase(a = 0.2885 nm). These values are slightly larger than those of Fe3C(4 ) owing proba*)
In the present paper, all alloy compositions are represented by weighted values denoted by atomic percentage.
711 0036-9748/79/080711-05502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
712
NONEQUILIBRIUM Cr3C CARBIDE
Vol. 13, No. 8
bly to a dif£erence in the atomic size. It was also found that the relationship of crystal orientation between Cr3C and Cr agrees with that between Fe3C and eFe in the tempered carbon steels. That is, this relationship is shown as follows
(lO0)cr3C / /
(0il)cr
(010)Cr3 C / /
(III)c r
(001)Cr3C / /
(211)Cr
(Bagaryatskii's
relationship(5))
Below a b o u t 15 at% c a r b o n , t h e s t r u c t u r e o f t h e a s - q u e n c h e d a l l o y s i s a mixt u r e o f Cr23C 6 and Cr. One example o f t h e s t r u c t u r e i s shown i n F i g . 3. In the f i g u r e s , ( c ) i s t h e d a r k f i e l d image t a k e n from t h e 2~4 r e f l e c t i o n o f Cr25C6 c a r bide in (b). L a r g e p a r t i c l e s o f Cr23C 6 c a r b i d e a r e d i s p e r s e d i n chromium m a t r i x . For a l l o y s w i t h t h e c a r b o n c o n t e n t 5~tween 22 and 30 at%, t h e a s - q u e n c h e d s t r u c t u r e s c o n s i s t o f two t y p e s o f c a r b i d e s , n a m e l y , Cr23C 6 and Cr7C3, as shown i n F i g . 4. As i s c l e a r l y known from t h e s e r e s u l t s , the as-quenched structures of alloys w i t h c a r b o n c o n t e n t s e x c e p t from 13 t o 22 at% c o n s i s t o f t h e e q u i l i b r i u m p h a s e s i n s p i t e o f r a p i d q u e n c h i n g from t h e m e l t . And, t h e n o n e q u i l i b r i u m p h a s e o f Cr3C a p p e a r s o n l y i n t h e l i m i t e d c a r b o n r a n g e o f 13~22 at%. Although the reason is u n c e r t a i n a t p r e s e n t , i t may be e x p l a i n e d from t h e f o l l o w i n g t h r e e e v i d e n c e s : Firstly, t h e Cr3C f o r m i n g a l l o y s a r e r e s t r i c t e d in the lower m e l t i n g c o m p o s i t i o n r a n g e a r o u n d t h e e u t e c t i c p o i n t a t a b o u t 14 at% c a r b o n , as s e e n i n F i g . I . Seco n d l y , t h e r e i s a d i f f e r e n c e i n t h e c o m p l e x i t y o f c r y s t a l s t r u c t u r e among c a r bides. T a b l e I l i s t s t h e d a t a on t h e c r y s t a l s t r u c t u r e s and t h e u n i t c e l l v o l umes o f t h e chromium c a r b i d e s . Both Cr23C 6 and CrTC 5 have more complex s t r u c TABLE I Crystal structures, l a t t i c e p a r a m e t e r s and u n i t c e l l volumes o f chromium c a r b i d e s i n Cr-C b i n a r y s y s t e m Carbide
Crystal structure
Lattice
Cr3C
Ortho.
0.458
Cr23C 6
Cubic
1.064
Cr7C 3
Hex.
1.401
Cr3C 2
Ortho.
0.5533
parameters(nm) b c 0.512 0.680
0.2829
Unit cell volume(nm 5)
Reference
0.159
Present work
1.205
£6)
0.4532
2.511
(7)
1.147
0.180
(8)
tures and larger unit cell volumes compared with those of Cr3C carbide. Such differences may be related to the ease of nucleation and growth of their carbides from melts during rapid quenching(=105 K/s). Thirdly, the Cr-X-C [X: Fe, Co, Ni, Mo, Nb, Ta] ternary alloys around the eutectic trough easily form an amorphous phase by rapid quenching(2). This implies that the Cr-C alloys around the eutectic point also have a tendency for supercooling. From these three points, therefore, the long-range atomic rearrangement required for the nucleation of Cr23C 6 or CrTC 5 in the melt will be more difficult probably because of a strong tendency to supercool in the case of alloys having lower melting points. Hence, the rapidly quenched alloys around the eutectic point will acquire a nonequilibrium structure consisting of Cr3C and Cr. The CrBC carbide transforms into the equilibrium carbide upon tempering. This structural change is shown in Fig. 5. The Cr3C carbide remains unchanged at tempering temperatures below about 975 K. By successive tempering, however, the carbide reactions proceed from Cr3C to metastable Cr7C 5 and from Cr7C 5 to stable Cr23C6, as seen in Fig. 5 (b)~(d).
Vol. 13, No. 8
NONEQUILIBRIUM Cr3C CARBIDE
713
Conclusion A nonequilibrium chromium carbide of MsC type, Cr3C, was found in Cr78~87CI5%Z 2 binary alloys quenched rapidly from the melt. The Cr3C carbide has t h e same o r t h o r h o m b i c s t r u c t u r e as Fe3C and t h e l a t t i c e p a r a m e t e r s a r e a=0.458 nm, b=0.S12 nm and c = 0 . 6 8 0 nm. The l a t t i c e r e l a t i o n s h i p s between Cr3C and Cr a r e (100)Cr3 C / / (011)Cr , ( 0 1 0 ) C r , C / / (111)C r and (001)CrsC / / ( 2 1 1 ) C r . This c a r b i d e r e m a i n s u n c h a n g e d a t t e m p e r i n g t e m p e r a t u r e s below a b o u t 973 K, b u t by s u c c e s s i v e t e m p e r i n g t h e c a r b i d e r e a c t i o n s p r o c e e d from Cr3C t o m e t a s t a b l e Cr7C 3 and f i n a l l y t o s t a b l e Cr23C 6. References 1. A. I n o u e and T. Masumoto, The Res. I n s t . f o r I r o n , S t e e l and O t h e r M e t a l s , Tohoku U n i v . , S e n d a l , J a p a n , u n p u b l i s h e d r e s e a r c h ( 1 9 7 9 ) . 2. A. I n o u e , S. S a k a i , H. M. Kimura and T. Masumoto, T r a n s . JIM, 20, No.5 i n press(1979). 3. M e t a l s Handbook e i g h t h e d i t i o n , M e t a l l o g r a p h y S t r u c t u r e s and Phase Diagram, p.274, American S o c i e t y for M e t a l s ( 1 9 7 3 ) . 4. E.J. F a s i s k a and G. A. J e f f r e y , A c t a C r y s t . , 19, 463 ( 1 9 6 5 ) . S. Yu. A. B a g a r y a t s k i i , Dokl. Akad. Nauk SSSR,,,73~--1161 ( 1 9 5 0 ) . 6. D. M e i n h a r d t and O. K r i s e m e n t , A r c h . E i s e n h f i t - t e n w . , 33, 493 ( 1 9 6 2 ) . 7. A. W e s t g r e n , J e r n k o n t . A n n . , 119, 231 ( 1 9 3 5 ) . 8. S. R u n d q v i s t and G. R u n n s j ~ , ~ a Chem. S c a n d . , 2-3, 1191 ( 1 9 6 9 ) .
Rapidly quenched structure
~136 K
L÷C
21oo 2041K
't
L I
/
/
208K 6
/
II//*'S"~~K
/
!g
I
I
Cr3C2* C
iI
v
t I
I
0
I !
/
c3
I msox
I--
1700 I
Cr
I0
20 30 Corbon (ot%)
40
FIG. 1 Equilibrium phase diagram of Cr-C binary system and the structures in rapidly quenched state.
714
NONEQUILIBRIUM Cr3C CARBIDE
Vol.
13, No. 8
G
a
FIG. 2 Transmission electron micrograph(a) and the selected area diffraction patterns(b)~(d) for as-quenched Cr84C16 alloy.
@
0
O
FIG. 3 Transmission e l e c t r o n m i c r o g r a p h ( a ) , the s e l e c t e d a r e a d i f f r a c t i o n p a t t e r n ( b ) and t h e d a r k f i e l d i m a g e ( c ) u s i n g t h e 224 r e f l e c t i o n from Cr23C6, f o r a s - q u e n c h e d Cr88C12 a l l o y .
Vol. 13, No. 8
NONEQUILIBRIUM Cr3C CARBIDE
FIG. 4 Transmission electron micrograph(a) and the selected area diffraction pattern(b), for as-quenched Cr75C25 alloy.
FIG. 5 T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s and t h e s e l e c t e d a r e a d i f f r a c t i o n p a t t e r n s showing t h e s t r u c t u r e c h a n g e s o f r a p i d l y quenched Cr84C16 a l l o y upon t e m p e r i n g a t 973 K.
715