- Email: [email protected]
The segregation of osmium to grain boundary dislocations in tungsten
Scripta
bIETALLURGICA
V o l . 17, pp. 1 0 4 3 - 1 0 4 6 , Printed in the U.S.A.
1983
Pergamon P r e s s Ltd. All rights reserved
THE SEGREGATION OF OS>II[JM TO GRAIN BOUNDARY DISLOCATIONS IN TUNGSTEN
H. C. Eaton Materials Science Program Louisiana State University Baton Rouge, Louisiana 70803 USA and Hans Norddn Department of Physics Chalmers University of Technology S-412 96 Goteborg, SWEDEN
(Received
June
6,
1983)
Introduction The progress made during the past decade towards an understanding of the structure of grain and interphase boundaries has been considerable. Attention is now being focussed on the application of these structural concepts towards an explanation of observed properties. Grain boundary chemistry problems, in particular, are being studied because of the major influence small composition differences have on the behavior of engineering metals. Although considerable information has been tabulated about the segregation of solute atoms to grain boundaries, little is known about the distribution of solute atoms within the grain boundary plane itself. It is of interest, for example, to discover whether or not impurity atoms preferentially segregate to grain boundary dislocations. It is certainly logical to assume that such processes occur since grain boundary dislocations exhibit so many of the properties that crystal lattice dislocations exhibit. For example, being localizatons of lattice misfit, they are also regions of high lattice strain. This lattice strain gives rise to characteristic contrast in the electron microscope and is quite likely to interact strongly with the strain field of impurity atoms. It remains, however, for experiment to prove or disprove this hypothesis. The nature of the solute distribution within the boundary plane is not understood due to experimental difficulties associated with doing very high spatial resolution microscopy and a chemical analysis from precisely the same volume of material. The atom probe and the field ion microscope, however, are capable of providing information of this type. Howell et al. (1,2) used the field ion microscope in a study of segregation of chromium atoms to grain boundaries in tungsten. Fortunately, chromium solute atoms image brightly in the field ion microscope compared to the tungsten solvent atoms. Therefore, by making bright spot counts, they observed that the chromium atoms did segregate to the grain boundary but that their distribution within the boundary plane was approximately random. In the present study, the field ion microscope and the atom probe are used in an analysis of grain boundary chemistry in tungsten doped with nickel and osmium. E~erimental Specimens were prepared from drawn tungsten wire which was commercially doped with small amounts of osmium. In the laboratory, the wire was sinter forged in nickel powder at 1100°C. The wire was prepared as a part of a larger study of tungsten wire reinforced composite materials. In those studies (3), and in others (4), it was established that nickel at the grain boundaries effectively reduced the recrystallization temperature of the tungsten. In the sinter
1043 0 0 3 6 - 9 7 4 8 / 8 3 $3.00 + .00 C o p y r i g h t (c) 1983 P e r g a m o n P r e s s
Ltd.
1044
SEGREGATION
forged material tungsten wire.
OF O S M I U M
IN T U N G S T E N
Vol.
17,
No.
8
of t h e p r e s e n t s t u d y t h e w i r e was composed of f i b r o u s g r a i n s t y p i c a l of drawn T h e s e f i b e r s were s u r r o u n d e d by a t h i n l a y e r of r e c r y s t a l l i z e d m a t e r i a l at the
s u r f a c e of t h e w i r e . The t u n g s t e n was b a c k p o l i s h e d in Agfa b r a n d p h o t o g r a p h i c d e v e l o p i n g s o l u t i o n and examined in t h e 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 s c o p e and t h e f i e l d ion m i c r o s c o p e p r i o r to c h e m i c a l a n a l y s i s i n t h e atom p r o b e . Results The apex o f t h e f i e l d e m i t t e r c o n t a i n i n g t h e g r a i n b o u n d a r y i s shown i n t h e b r i g h t f i e l d e l e c t r o n m i c r o g r a p h of F i g . 1. The g r a i n b o u n d a r y p l a n e i s a p p r o x i m a t e l y p a r a l l e l to t h e w i r e a x i s and t h e b o u n d a r y p l a n e c o n t a i n s s e v e r a l g r a i n b o u n d a r y d i s l o c a t i o n s which a r e a l s o a p p r o x i mately parallel to t h e s p e c i m e n a x i s . By t i l t i n g , i t was d e t e r m i n e d t h a t t h e d i s l o c a t i o n Burg e r s v e c t o r s had l a r g e [1101 c o m p o n e n t s . The [110] d i r e c t i o n was t h e t i l t a x i s o f t h e b o u n d a r y and t h e s e d e f e c t s were s i m i l a r to t h o s e formed by l o c a l i z a t i o n of t h e m i s m a t c h of t h e a x i a l p l a n e s and d i s c u s s e d by Loberg and Norden ( 5 ) . The same s p e c i m e n was imaged i n t h e f i e l d i o n m i c r o s c o p e and a t y p i c a l image i s shown i n F i g . 2. The g r a i n b o u n d a r y i s i n d i c a t e d by a r r o w s and a g r a i n b o u n d a r y d i s l o c a t i o n i n t e r s e c t i n g the central [110] p l a n e i s a l s o i n d i c a t e d by an a r r o w . The m i s o r i e n t a t i o n b e t w e e n t h e two g r a i n s was found by b o t h s t e r e o g r a p h i c a n a l y s i s of t h e f i e l d i o n m i c r o g r a p h and by 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 s c o p y t o be a p p r o x i m a t e l y 51 ° a b o u t t h e [110] d i r e c t o n . This is very near the Ell coincidence site lattice (CSL) m i s o r i e n t a t i o n . The CSL model p r e d i c t s t h a t g r a i n b o u n d a r y d i s l o c a t i o n s i n a ~11 b o u n d a r y w i l l have B u r g e r s v e c t o r s = ~2[i13], ~ = ~11332] and b = ~i[741], where a is the lattice parameter. The single set of dislocations observed to be in str~ng contrast are most likely then of the <741> type. This observation is consistent with the g.b criterion for the visibility of dislocations in the field ion microscope (6) and the observation of a single spiral in the field ion image. The same specimen was chemically analyzed in the atom probe. The analysis shown in the spectrum of Fig. 3 was obtained from a probe of the grain boundary direct]y over the dislocation indicated in Fig. 2. The spectrum of Fig. 4, however, was obtained after positioning the probe hole on the grain boundary but one probe hole diameter to one side of the probe hole position used to obtain the previous spectrum. In each case, approximately I}00 ions were collected. It is clear in both spectra that the four isotopes of tungsten in the 3 charge state are resolved. In both figures, the expected positions of the four strongest osmium peaks are indicated, but the peaks appear only in the spectrum obtained from probing the grain boundary dislocation. Although the spectra are not shown in the present note, nickel was also detected at the grain boundary but not preferentially at the grain boundary dislocation. Discussion It is clear from the spectra that the osmium is strongly segregated to the grain boundary dislocations. This is in contrast to the aforementioned observation of Howell et al. where the grain boundary segregant (chromium) was randomly distributed in the grain boundary plane and to the similar behavior of nickel in the present study. The differences in behavior can be explained by comparing the metallic radii of these three different solute atoms. An indication of the metallic radii can be obtained from the distance of closest approach of the atoms when they are in their pure metallic state. For nickel, chromium, and osmium the distances of closest approach are 0.2491 nm, 0.2498 n4n, and 0.2675 9m, ]espectively (7)~ ~erefore, the approximate atomic volumes become 8.09(10-°)nm ~, 8.16(10- ) n m , and 10.02(10 ) n m , respectively. Ignoring the complications due to crystal structure differences, a non-ideal c/a ratio in the osmium crystal lattice, etc., it can be estimated that each osmium atom can effectively occupy approximately 20% more volume in the tungsten lattice than either the nickel or the chromium atoms. Consequently,theosmiumis likely to be segregated to the slightly more open structure at the core of the grain boundary dislocation. Summary Field ion atom probe data is presented which shows that trace amounts of nickel and osmium segregate to grain boundaries in tungsten. The nickel atoms are randomly distributed in the
Vol.
17,
No.
8
g r a i n boundary p l a n e boundary d i s l o c a t i o n . atomic volumes.
SEGREGATION OF OS~IUM IN TUNGSTEN
l().~S
whereas t h e osmium i s s t r o n g l y s e g r e g a t e d to t h e core r e g i o n ot a g r a i n The d i f f e r e n c e s in b e h a v i o r o f the s o l u t e atoms i s e x p l a i n e d bv comparing Acknowledgements
The a u t h o r s would l i k e t o thank t h e Swedish N a t u r a l S c i e n c e R e s e a r c h C o u n c i l , the Natiom~l S c i e n c e F o u n d a t i o n , the Swedish I n s t i t u t e , and t h e L o u i s i a n a S t a t e U n i v e r s i t y Council ori Res e a r c h f o r s u p p o r t o f t h i s work. References 1.
P . R . Howell, D. g. F l e e t , T. F. Page and B. Ralph, Proc. 3rd I n t e r n a t i c , r , , l Conleren,.'e on t h e S t r e n g t h o f Metals and A l l o y s , Vol. 1, Cambridge, 1973.
2.
P.R.
3.
R. W. Warren, L. L a r s s o n and Konstanz, West Germany, 1980.
4.
L. Kozma, W. Huppmann, L. Bartha and P. Mezei, Powder M e t a l l u r g y 1, 7 (1981).
5.
B. Loberg and H. Norddn, Ark. Fys. 40, 413 (1970).
6.
S. Ranganathan, J . o f Appl. Phys. 37, 4346 (1966).
7.
B. D. C u l l i t y , 1956.
Howell, D. E. F l e e t ,
A. Hildon and B. Ralph, J. Micros. C-H. A n d e r s s o n ,
Elements o f X-Ray D i f f r a c t i o n ,
107,
155 (1976).
DGM Meeting on Cc~mposite M a t e r i a l s
p. 482, Addison-Wesley
'
Reading, Mass
'
FIG. 1 A b r i g h t f i e l d e l e c t r o n m i c r o g r a p h of the specimen (100,000x). The g r a i n boundary d i s l o c a t i o n s are i n d i c a t e d by an arrow.
FIG. 2 A f i e l d ion m i c r o g r a p h of the specimen (BIV=9kV, T=80°K). The g r a i n boundary i s i n d i c a t e d by two arrows on the edge o f t h e image and t h e g r a i n boundary d i s l o c a t i o n by t h e arrows in the v i c i n i t y o f t h e [110] p o l e .
1046
SEGREGATION
I00 03 z 0 u.. 0 ¢r uJ an
OF O S M I U M
WI84 wl83
80
Wl85 I
IN T U N G S T E N
Vol.
17,
No.
8
ON DISLOCATION
60 40
{ %11
. IIIIl~oslee los 190 I
20 Z 0
FIG. 3 An atom probe spectrum from the grain boundary dislocation. I00 03 Z 0 u_ 0 tr W an
8O-
182183 L84 wL85
OFF DISLOCATION
6040 20
Z 0
FIG. 4 An atom probe
spectrum
from a region on the grain boundary but not on the dislocation
bIETALLURGICA
V o l . 17, pp. 1 0 4 3 - 1 0 4 6 , Printed in the U.S.A.
1983
Pergamon P r e s s Ltd. All rights reserved
THE SEGREGATION OF OS>II[JM TO GRAIN BOUNDARY DISLOCATIONS IN TUNGSTEN
H. C. Eaton Materials Science Program Louisiana State University Baton Rouge, Louisiana 70803 USA and Hans Norddn Department of Physics Chalmers University of Technology S-412 96 Goteborg, SWEDEN
(Received
June
6,
1983)
Introduction The progress made during the past decade towards an understanding of the structure of grain and interphase boundaries has been considerable. Attention is now being focussed on the application of these structural concepts towards an explanation of observed properties. Grain boundary chemistry problems, in particular, are being studied because of the major influence small composition differences have on the behavior of engineering metals. Although considerable information has been tabulated about the segregation of solute atoms to grain boundaries, little is known about the distribution of solute atoms within the grain boundary plane itself. It is of interest, for example, to discover whether or not impurity atoms preferentially segregate to grain boundary dislocations. It is certainly logical to assume that such processes occur since grain boundary dislocations exhibit so many of the properties that crystal lattice dislocations exhibit. For example, being localizatons of lattice misfit, they are also regions of high lattice strain. This lattice strain gives rise to characteristic contrast in the electron microscope and is quite likely to interact strongly with the strain field of impurity atoms. It remains, however, for experiment to prove or disprove this hypothesis. The nature of the solute distribution within the boundary plane is not understood due to experimental difficulties associated with doing very high spatial resolution microscopy and a chemical analysis from precisely the same volume of material. The atom probe and the field ion microscope, however, are capable of providing information of this type. Howell et al. (1,2) used the field ion microscope in a study of segregation of chromium atoms to grain boundaries in tungsten. Fortunately, chromium solute atoms image brightly in the field ion microscope compared to the tungsten solvent atoms. Therefore, by making bright spot counts, they observed that the chromium atoms did segregate to the grain boundary but that their distribution within the boundary plane was approximately random. In the present study, the field ion microscope and the atom probe are used in an analysis of grain boundary chemistry in tungsten doped with nickel and osmium. E~erimental Specimens were prepared from drawn tungsten wire which was commercially doped with small amounts of osmium. In the laboratory, the wire was sinter forged in nickel powder at 1100°C. The wire was prepared as a part of a larger study of tungsten wire reinforced composite materials. In those studies (3), and in others (4), it was established that nickel at the grain boundaries effectively reduced the recrystallization temperature of the tungsten. In the sinter
1043 0 0 3 6 - 9 7 4 8 / 8 3 $3.00 + .00 C o p y r i g h t (c) 1983 P e r g a m o n P r e s s
Ltd.
1044
SEGREGATION
forged material tungsten wire.
OF O S M I U M
IN T U N G S T E N
Vol.
17,
No.
8
of t h e p r e s e n t s t u d y t h e w i r e was composed of f i b r o u s g r a i n s t y p i c a l of drawn T h e s e f i b e r s were s u r r o u n d e d by a t h i n l a y e r of r e c r y s t a l l i z e d m a t e r i a l at the
s u r f a c e of t h e w i r e . The t u n g s t e n was b a c k p o l i s h e d in Agfa b r a n d p h o t o g r a p h i c d e v e l o p i n g s o l u t i o n and examined in t h e 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 s c o p e and t h e f i e l d ion m i c r o s c o p e p r i o r to c h e m i c a l a n a l y s i s i n t h e atom p r o b e . Results The apex o f t h e f i e l d e m i t t e r c o n t a i n i n g t h e g r a i n b o u n d a r y i s shown i n t h e b r i g h t f i e l d e l e c t r o n m i c r o g r a p h of F i g . 1. The g r a i n b o u n d a r y p l a n e i s a p p r o x i m a t e l y p a r a l l e l to t h e w i r e a x i s and t h e b o u n d a r y p l a n e c o n t a i n s s e v e r a l g r a i n b o u n d a r y d i s l o c a t i o n s which a r e a l s o a p p r o x i mately parallel to t h e s p e c i m e n a x i s . By t i l t i n g , i t was d e t e r m i n e d t h a t t h e d i s l o c a t i o n Burg e r s v e c t o r s had l a r g e [1101 c o m p o n e n t s . The [110] d i r e c t i o n was t h e t i l t a x i s o f t h e b o u n d a r y and t h e s e d e f e c t s were s i m i l a r to t h o s e formed by l o c a l i z a t i o n of t h e m i s m a t c h of t h e a x i a l p l a n e s and d i s c u s s e d by Loberg and Norden ( 5 ) . The same s p e c i m e n was imaged i n t h e f i e l d i o n m i c r o s c o p e and a t y p i c a l image i s shown i n F i g . 2. The g r a i n b o u n d a r y i s i n d i c a t e d by a r r o w s and a g r a i n b o u n d a r y d i s l o c a t i o n i n t e r s e c t i n g the central [110] p l a n e i s a l s o i n d i c a t e d by an a r r o w . The m i s o r i e n t a t i o n b e t w e e n t h e two g r a i n s was found by b o t h s t e r e o g r a p h i c a n a l y s i s of t h e f i e l d i o n m i c r o g r a p h and by 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 s c o p y t o be a p p r o x i m a t e l y 51 ° a b o u t t h e [110] d i r e c t o n . This is very near the Ell coincidence site lattice (CSL) m i s o r i e n t a t i o n . The CSL model p r e d i c t s t h a t g r a i n b o u n d a r y d i s l o c a t i o n s i n a ~11 b o u n d a r y w i l l have B u r g e r s v e c t o r s = ~2[i13], ~ = ~11332] and b = ~i[741], where a is the lattice parameter. The single set of dislocations observed to be in str~ng contrast are most likely then of the <741> type. This observation is consistent with the g.b criterion for the visibility of dislocations in the field ion microscope (6) and the observation of a single spiral in the field ion image. The same specimen was chemically analyzed in the atom probe. The analysis shown in the spectrum of Fig. 3 was obtained from a probe of the grain boundary direct]y over the dislocation indicated in Fig. 2. The spectrum of Fig. 4, however, was obtained after positioning the probe hole on the grain boundary but one probe hole diameter to one side of the probe hole position used to obtain the previous spectrum. In each case, approximately I}00 ions were collected. It is clear in both spectra that the four isotopes of tungsten in the 3 charge state are resolved. In both figures, the expected positions of the four strongest osmium peaks are indicated, but the peaks appear only in the spectrum obtained from probing the grain boundary dislocation. Although the spectra are not shown in the present note, nickel was also detected at the grain boundary but not preferentially at the grain boundary dislocation. Discussion It is clear from the spectra that the osmium is strongly segregated to the grain boundary dislocations. This is in contrast to the aforementioned observation of Howell et al. where the grain boundary segregant (chromium) was randomly distributed in the grain boundary plane and to the similar behavior of nickel in the present study. The differences in behavior can be explained by comparing the metallic radii of these three different solute atoms. An indication of the metallic radii can be obtained from the distance of closest approach of the atoms when they are in their pure metallic state. For nickel, chromium, and osmium the distances of closest approach are 0.2491 nm, 0.2498 n4n, and 0.2675 9m, ]espectively (7)~ ~erefore, the approximate atomic volumes become 8.09(10-°)nm ~, 8.16(10- ) n m , and 10.02(10 ) n m , respectively. Ignoring the complications due to crystal structure differences, a non-ideal c/a ratio in the osmium crystal lattice, etc., it can be estimated that each osmium atom can effectively occupy approximately 20% more volume in the tungsten lattice than either the nickel or the chromium atoms. Consequently,theosmiumis likely to be segregated to the slightly more open structure at the core of the grain boundary dislocation. Summary Field ion atom probe data is presented which shows that trace amounts of nickel and osmium segregate to grain boundaries in tungsten. The nickel atoms are randomly distributed in the
Vol.
17,
No.
8
g r a i n boundary p l a n e boundary d i s l o c a t i o n . atomic volumes.
SEGREGATION OF OS~IUM IN TUNGSTEN
l().~S
whereas t h e osmium i s s t r o n g l y s e g r e g a t e d to t h e core r e g i o n ot a g r a i n The d i f f e r e n c e s in b e h a v i o r o f the s o l u t e atoms i s e x p l a i n e d bv comparing Acknowledgements
The a u t h o r s would l i k e t o thank t h e Swedish N a t u r a l S c i e n c e R e s e a r c h C o u n c i l , the Natiom~l S c i e n c e F o u n d a t i o n , the Swedish I n s t i t u t e , and t h e L o u i s i a n a S t a t e U n i v e r s i t y Council ori Res e a r c h f o r s u p p o r t o f t h i s work. References 1.
P . R . Howell, D. g. F l e e t , T. F. Page and B. Ralph, Proc. 3rd I n t e r n a t i c , r , , l Conleren,.'e on t h e S t r e n g t h o f Metals and A l l o y s , Vol. 1, Cambridge, 1973.
2.
P.R.
3.
R. W. Warren, L. L a r s s o n and Konstanz, West Germany, 1980.
4.
L. Kozma, W. Huppmann, L. Bartha and P. Mezei, Powder M e t a l l u r g y 1, 7 (1981).
5.
B. Loberg and H. Norddn, Ark. Fys. 40, 413 (1970).
6.
S. Ranganathan, J . o f Appl. Phys. 37, 4346 (1966).
7.
B. D. C u l l i t y , 1956.
Howell, D. E. F l e e t ,
A. Hildon and B. Ralph, J. Micros. C-H. A n d e r s s o n ,
Elements o f X-Ray D i f f r a c t i o n ,
107,
155 (1976).
DGM Meeting on Cc~mposite M a t e r i a l s
p. 482, Addison-Wesley
'
Reading, Mass
'
FIG. 1 A b r i g h t f i e l d e l e c t r o n m i c r o g r a p h of the specimen (100,000x). The g r a i n boundary d i s l o c a t i o n s are i n d i c a t e d by an arrow.
FIG. 2 A f i e l d ion m i c r o g r a p h of the specimen (BIV=9kV, T=80°K). The g r a i n boundary i s i n d i c a t e d by two arrows on the edge o f t h e image and t h e g r a i n boundary d i s l o c a t i o n by t h e arrows in the v i c i n i t y o f t h e [110] p o l e .
1046
SEGREGATION
I00 03 z 0 u.. 0 ¢r uJ an
OF O S M I U M
WI84 wl83
80
Wl85 I
IN T U N G S T E N
Vol.
17,
No.
8
ON DISLOCATION
60 40
{ %11
. IIIIl~oslee los 190 I
20 Z 0
FIG. 3 An atom probe spectrum from the grain boundary dislocation. I00 03 Z 0 u_ 0 tr W an
8O-
182183 L84 wL85
OFF DISLOCATION
6040 20
Z 0
FIG. 4 An atom probe
spectrum
from a region on the grain boundary but not on the dislocation