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DISLOCATION RELAXATION PEAKS IN HIGH PURITY TUNGSTEN SINGLE CRYSTALS
U. Ziebart, H. Schultz
To cite this version:
U. Ziebart, H. Schultz. DISLOCATION RELAXATION PEAKS IN HIGH PURITY TUNG- STEN SINGLE CRYSTALS. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-691-C9-696.
�10.1051/jphyscol:19839104�. �jpa-00223338�
Colloque C9, suppl6ment au n012, Tome 44, d6cembre 1983 page C9-691
DISLOCATION RELAXATION PEAKS IN HIGH PURITY TUNGSTEN SINGLE CRYSTALS
U. Ziebart and H. S c h u l t z
ax-~Zanck-Institut fur MetaZlforschung, I n s t i t u t fW Physik,
Heisenbergstrasse 1, 0-7000 S t u t t g a r t 80 and I n s t i t u t fur Theoretische und Angewandte Physik der Universittlt S t u t t g a r t , F.R.G.
Abstract
-
Experimental observations on t h e dislocation internal f r i c t i o n spectrum of high purity tungsten s i n g l e c r y s t a l s a r e reported f o r the tem- perature range 15K-800K by low frequency torsion pendulum technique. The observed relaxation e f f e c t s a r e c l a s s i f i e d as ( a l , a ) , and y relaxations, by comparison with t h e s i m i l a r dislocation damping spectrum of Mo. Relaxation e f f e c t s occur a t -1 Hz in the following temperature ranges: ( a 8 , a ) , 46K-170K;8 , 250K-450K; y , 600 K. The y peak disappears quickly a f t e r heating t o 800 K.
The r e s u l t s are compared with e a r l i e r observations of other authors.
As molybdenum tungsten belongs t o t h e b.c.c. t r a n s i t i o n metals, having a very low s o l u b i l i t y f o r i n t e r s t i t i a l impurity atoms. Hydrogen, oxygen and nitrogen are v o l a t i l e and a r e usually disregarded as possible i n t e r s t i t i a l s o l u t e atoms. Only carbon i s mostly present in small quantities. I t i s soluble in a noticeable manner only a t high temperatures ( 1 ) . The carbon Snoek Peak was observed a t 653K-683K (0.5-1 Hz) by several authors ( 2 ) - ( 4 ) . This peak i s unstable and disappears by pre- c i p i t a t i o n of carbon. The behaviour of carbon, quenched in s o l i d solution, was also investigated by r e s i s t i v i t y measurements following systematic annealing treatments ( 5 ) . Also aging and pinning of dislocations by mobile carbon atoms was studied ( 6 ) . The activation energy of migration f o r carbon in tungsten i s estimated from those studies t o 1.9-2.0 eV.
Due t o i t s high melting point (369510 and the low chemical a f f i n i t y t o gaseous im- p u r i t i e s , tungsten can be purified very e f f e c t i v e l y by electron-beam zone me1 t i n g , and t h i s procedure allows also t o prepare s i n g l e c r y s t a l s . Such high purity s i n g l e c r y s t a l s , having residual resistance r a t i o s between 32000 and 92000, measured on 2.2 mm PI c r y s t a l s without corrections of the s i z e e f f e c t , were used i n our i n v e s t i - gations. Special care was directed f o r an e f f e c t i v e decarburization, whic was carried out in an UHV system by high-frequency heating t o 1500 K a t 2x10-' mbar oxygen pressure. The internal f r i c t i o n measurements were carried out with a computer operated torsion pendulum. I t was t h e same pendulum as used e a r l i e r by Rodrian ( 7 ) and Grau ( 8 ) . However, the desk computer was replaced by a PDP 11/23 computer
(Digital Equipment). In the following we a r e presenting our f i r s t observations on the dislocation internal f r i c t i o n spectrum of tungsten, which will be continued.
Fig. 1 shows the flow s t r e s s vs. temperature curve of a high purity tungsten crystal (<110>). We can notice the knee temperature TKg600 K. Below t h i s temperature ther- mally activated motion of screw dislocations determines the macroscopic p l a s t i c flow.
The r a t e determining process i s believed t o be the formation of k i n k pairs in screw dislocations. This process should be responsible a l s o f o r the internal f r i c t i o n peak ( y ) ( 9 ) , which appears in our measurements a t 680 K ( 1 Hz), as will be shown below. Non-screw (710) dislocation can move e a s i e r than screws and enable microde- formation. Kink-pair formation in 71° dislocations i s related t o the internal f r i c - tion peak ( a ) , here near 180 K. Below the a peak, several subpeaks a ' e x i s t , over- lapping partly with ( a ) , which are believed t o belong t o kink diffusion of geometri- cal kinks i n screw dislocation segments. ( 8 ) ( 1 0 ) ( 1 1 )
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19839104
C9-692 JOURNAL DE PHYSIQUE
Fig. 1
C r i t i c a l flow s t r e s s ( 0 ) vs.
temperature ( T ) of a high purity tungsten single crystal (<110>), residual resistance r a t i o
(RRR) = 53000
Tensile deforma ion speed i- = 1 . 6 ~ 1 0 - 5 s - I
7 .
...
2.6 1 ' -.. ...
....
Fig. 2b
Modulus curve ( f 2 ) vs. T . Same specimen as f i g . 2a Fig. 2a
Damping ( Q - l ) vs. temperature ( T ) high purity W single crystal W 155 L 1, <110>, RRR=92000.
P l a s t i c deformation: 1% tension a t 600K+0.5% tension a t 450 K
+
2 ~ 0 . 1 % torsion a t 450 K.f = 1.6 Hz,
Amp1 i tude: 1 x 1 0 - ~ heating r a t e 1.8 K/min.
t 6 5-
m 0
- 4 -
X7a3-1
2 1
0 200 400 600 800
T [ K l -
I:
. . . . . . . . . ...
a'
,a . 7 2 3
I /
I
-
I 2 ..-... ..--...
.%.
I
.... .
.-
...: . ...
.I I I I I I I
f o l l o w i n g weak p l a s t i c deformation ( d e t a i l s see f i g u r e c a p t i o n ) . We designate t h e r e l a x a t i o n processes i n f i g . 1 as ( a 8 , a ) , 6 and y analogue t o s i m i l a r r e s u l t s i n molybdenum ( 8 ) ( 1 0 ) ( 1 1 ) . The subpeaks ( a ' ,a) have n o t been separated. A d i s t i n c t B peak i s n o t observed i n f i g . 1 . The y peak a t 680 K ( 1 Hz) i s unstable and disappears by h e a t i n g t o 800 K. This i s shown i n f i g . 3a, 3b.
F i g . 3a
Damping ( Q - ~ ) vs. temperature (T) Annealing behaviour o f t h e d i s - l o c a t i o n i n t e r n a l f r i c t i o n ~ e a k s . High p u r i t y W s i n g l e c r y s t a i W 155 L 1 <110>.
Curve 1: same as f i g . 1 , curve 2: f o l l o w i n g ( 1 ) a f t e r h e a t i n g t o 738 K,
f = 1.6 Hz, amplitude 1 ~ 1 0 - ~ , h e a t i n g r a t e : 1.8 K/min.
F i g . 3b
Modulus curve ( f 2 ) vs.
same as f i g . 3a.
The disappearance o f t h e y peak i s probably due t o a n n i h i l a t i o n and/or bending of screw d i s l o c a t i o n s ( = t r a n s f o r m a t i o n i n t o a s t r o n g l y kinked d i s l o c a t i o n ) ( 8 , l l ) and n o t due t o d i s l o c a t i o n p i n n i n g by mobile carbon atoms. I t should be noted t h a t the 680 K ( 1 Hz) peak, f i g . 2 , (here c a l l e d ( y ) ) cannot be explained as a carbon-Snoek peak, which has a s i m i l a r peak temperature (2-4). I n o r d e r t o be detectable, a carbon-Snoek peak r e q u i r e s a t l e a s t -10 atppm carbon. Such a r e l a t i v e l y l a r g e carbon c o n c e n t r a t i o n would l e a d t o complete p i n n i n g o f those d i s l o c a t i o n s , which are respon- s i b l e f o r t h e ( a l , a ) r e l a x a t i o n and would produce a s t r o n g r e d u c t i o n o f t h e back- ground damping. See f o r example such behaviour, as observed i n t h e system Ta-0 ( r e f , ( l 2 ) , f i g . 2 ) .
C9-694 JOURNAL DE PHYSIQUE
The (al,a) r e l a x a t i o n show a systematic change by annealing between 450 K and 750 K, see f i g . 4. Between 450 K and 550 K (curve 4, f i g . 4 ) subpeak 2 and subpeak 3 o f ( a ' , a ) increases. Annealing between 600 K and 750 K l e t a t f i r s t peak 3 decrease, whereas peak 2 s t i l l increases (curve 5, f i g . 4 ) , f i n a l l y a l s o peak 2 becomes smaller.
This behavious i s s i m i l a r t o Mo (8,11), and i s explained by t h e t r a n s i t i o n o f l o n g i n s t a b l e screw segments i n t o more s t a b l e k i n k e d p o s i t i o n s under t h e i n f l u e n c e o f i n t e r n a l stresses. This increases t h e number o f geometrical k i n k s . The analogues t o peak 1 and 2 are a t t r i b u t e d i n Mo t o geometrical k i n k m i g r a t i o n i n screw d i s l o c a - t i o n s . A more d e t a i l e d a n a l y s i s i n Mo r e v e a l e d a t l e a s t 3 subpeaks a ' below a ( 8 , l l ) .
F i g . 4a
Damping ( Q - l ) vs. temperature (T).
Annealing behaviour o f t h e ( a ' , a ) r e 1 a x a t i o n .
High p u r i t y W s i n g l e c r y s t a l W 155 L 4 <110>, RRR=34000.
P l a s t i c deformation: 1% t e n s i o n a t 600K+2x0.1% t o r s i o n a t 450 K.
Run 1: 15K-450K 2: 15K-500K - :
...
:
...
7: 15K-750K 8: 15K-800K
f = 18.5 Hz, amplitude 1 x 1 0 - ~ ~ h e a t i n g r a t e 1.8 K/min.
F i g . 4b
Modulus curve ( f 2 ) vs. ( T ) same as f i g . 4a.
Table Experimental r e s u l t s on t h e ( a ' ,a) d i s l o c a t i o n i n t e r n a l f r i c t i o n peaks, as observed i n t h i s work and by o t h e r authors. The subpeak number as given here i s n o t f i n a l . A more d e t a i l e d a n a l y s i s i s r e q u i r e d t o separate t h e subpeaks o f (al,a) compare ( 8 ) ( 1 1 ) .
( a 1 ,a) Subpeak No.
Ref.
t h i s work
I t ,I
Rieu (13) 45 kHz
I, I ,
12 kHz 1.2kHz
I
Chambers e t a1
.
2 1 kHz
I
Maul (16)
Table 2: Experimental r e s u l t s on t h e y d i s l o c a t i o n i n t e r n a l f r i c t i o n peak as ob- served i n t h i s work and by o t h e r authors.
Ref.
680 1.6 Hz
- -
571 1 Hz 1.5
-
800 45 kHz ( 1 . 5 ) * 536
*value o f r e f . (17)
t h i s work M a r t i n e t (17)
Rieu (13)
I n t a b l e 1 we summarize t h e peak temperatures o f t h e (al,a) peaks o f t h i s work and by o t h e r authors, and i n t a b l e 2 t h e r e s u l t s on t h e y peak. Concerning t h e (al,a) r e l a x a t i o n peaks, we should note t h a t a r e l a t i v e l y simple subpeak s t r u c t u r e i s ob- t a i n e d i n h i g h p u r i t y s i n g l e c r y s t a l s f o l l o w i n g o u r "standard p l a s t i c deformation1' 1-2% t e n s i o n
+
0.1-0.2% t o r s i o n .For t h e y peak we regard t h e d i f f e r e n c e s i n peak temperatures between t h e r e s u l t s o f M a r t i n e t (17) and ours as due t o t h e q u i t e d i f f e r e n t specimen s t a t e s . Much l a r g e r f r e e d i s l o c a t i o n l o o p l e n g t h are expected i n o u r s i n g l e c r y s t a l specimens and t h i s may e x p l a i n t h e d i f f e r e n c e s . We consider t h e y peak temperature o f Rieu (13) (800 K, 45 kHz) n o t as t h e r e a l peak temperature, b u t as i n f l u e n c e d by annealing. The r e a l peak temperature f o r h i g h p u r i t y s i n g l e c r y s t a l s , as used by Rieu, should be h i g h e r than 800 K f o r 45 kHz.
More work i s r e q u i r e d t o o b t a i n t h e a c t i v a t i o n parameters o f t h e observed r e l a x a t i o n peaks.
C9-696 JOURNAL DE PHYSIQUE References
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,
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