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HYDROGEN-DISLOCATION INTERACTIONS AND INTRINSIC DISLOCATION RELAXATION IN
NIOBIUM
M. Maul, H. Schultz
To cite this version:
M. Maul, H. Schultz. HYDROGEN-DISLOCATION INTERACTIONS AND INTRINSIC DISLOCA- TION RELAXATION IN NIOBIUM. Journal de Physique Colloques, 1981, 42 (C5), pp.C5-73-C5-78.
�10.1051/jphyscol:1981510�. �jpa-00220967�
page C5-73
HYDROGEN-DISLOCATION INTERACTIONS AND INTRINSIC DISLOCATION RELAXATION
IN
NIOBIUMM. Maul and H. Schultz
Max-PZanck Institut fiir MetaZZforschung, Institut fiir Physik, Heisenbergstr.1, 0-7000 Stuttgart 80 and Institut fiir Theoretische und Angewandte Physik der Universittit Stuttgart, F. R. G.
Abstract.- The dislocation relaxation between 100 K and 200 K (IkHz) in plastically deformed niobium, formerly named "a-relax- ation" is a hydrogen Snoek-Koster relaxation (cold work relaxa- tion). This is shown by experiments on extremely degassed samples, which contained controlled amounts of hydrogen and/or oxygen.
The intrinsic, hydrogen-free dislocation relaxation appears bet- ween 30 K and 70 K (IkHz), the exact peak location depends on the interstitial (0,N) content. This relaxation is probably iden- tical with the "6 peak", observed earlier by Vazzolai and Nuovo.
1. Experiment.- In an ultra high vacuum system the internal friction of deformed Nb single crystals was measured in the temperature range from 15 K to 320 K by bending oscillations (1 kHz). All samples got a
"standard" degassing treatment by heating 3 hours at 2 3 0 0 ~ ~ and 10 -10 mbar to remove oxygen and nitrogen to a level of some atppm. It could be shown that the samples, following this standard treatment, still contained 5 atppm hydrogen, in contrast to thermodynamic equilibrium considerations, which let expect much lower H contents. After degassing, the sample surface was sealed with a surface oxyde in a low pressure oxygen atmosphere at 200°c, to protect the sample against hydrogen pick up. For extrem hydrogen degassing (<I atppm) long time degassing treatments (2 days, 1 week) at 1 0 0 0 ~ ~ and mbar were applied in connection with the "standard" treatment.
Some samples were doped with oxygen. Subsequently all samples were doped with hydrogen, usually by electrolytic charging in 10% H2S04.
The oxygen concentration was determined by residual resistance measure- ments. The hydrogen concentration was determined by a new method based on residual resistance measurements on attached niobium foils /I/.
All specimens were deformed in the apparatus by bending.
2. Results.- In specimens of low interstitial solute content a large internal friction peak appears after deformation. This peak was called in the past "a-relaxation" and is shown in fig.1 in a sample with 5 atppm residual hydrogen concentration. As we will show below this peak has to be considered as a hydrogen cold-work ~ e a k (Snoek-Koster Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981510
C5-74 JOURNAL DE PHYSIQUE
0 I I
0 100 200 300
T(K)
-
F i g . I : The a H - r e l a x a t i o n i n h i g h p u r i t y niobium a f t e r d e f o r m a t i o n a t 320 K .
CO< 5 atppm
F = 1.3 kHz 1 2 3 1 5 6 7
100 200 300
T(K)
-
F i g . 3 : I n f l u e n c e of hydrogen on t h e a H - r e l a x a t i o n a f t e r d e f o r m a t i o n a t 320 K i n h i g h p u r i t y Nb
1 cH = 7 atppm
2 26 atppm
3 34 atppm
4 75 atppm
5 1 3 6 atppm
6 > 200 atppm
7 > 200 atppm
F i g . 2 : I n f l u e n c e of oxygen on t h e a H - r e l a x a t i o n i n Nb s i n g l e c r y s t a l s a f t e r d e f o r m a t i o n a t 320 K
F i g . 4 : I n f l u e n c e of H on t h e a n - r e l a x a t i o n a f t e r d e f o r m a t i o n a t 320 K i n a 0-doped Nb-sample c = 204 atppm
c = 7 atppm
2 16 atppm
3 24 atppm
4 37 atppm
5 106 atppm
r e l a x a t i o n ) . T h e r e f o r e we c a l l t h i s peak h e r e a H - r e l a x a t i o n . I n f l u e n c e of oxygen
The a H - r e l a x a t i o n i s s u p p r e s s e d and s h i f t e d t o lower t e m p e r a t u r e s w i t h i n c r e a s i n g oxygen c o n c e n t r a t i o n . T h i s was a l s o found i n /2/ a f t e r n i t r o g e n - d o p i n q o f Ta. Sample Nb 67/1 was d e g a s s e d 2 days a t 1 0 0 0 ~ ~ and 10-lo mbar t o lower t h e r e s i d u a l H c o n c e n t r a t i o n . A s a consequence of t h i s t r e a t m e n t an a d d i t i o n a l s m a l l peak a t 30 K can be o b s e r v e d . I n f l u e n c e of hydrogen
The i n f l u e n c e of H-dopings on t h e a H - r e l a x a t i o n depends on t h e concen- t r a t i o n of i n t e r s t i t i a l s o l u t e s ( 0 , N ) . T h i s can be s e e n from f i g . 3 a n d 4 . I n samples w i t h low oxygen c o n t e n t and a w e l l developed a H - r e l a x a t i o n
( f i g . 3 ) no i n f l u e n c e of hydrogen on t h e peak h e i g h t c a n be o b s e r v e d .
s h i f t o c c u r s by H doping. F i g . 3 shows t h i s on a sample w i t h an i n t e r - m e d i a t e 0 c o n c e n t r a t i o n (204 atppm)
.
T h i s i n c r e a s e of t h e a H - r e l a x a t i o ni n samples w i t h h i g h e r 0 c o n c e n t r a t i o n s means, t h a t t h e r e l a x a t i o n s t r e n g t h and hence t h e m o b i l i t y o f d i s l o c a t i o n segments i s i n c r e a s e d by H-dopings. We s u g g e s t e d / 3 / , t h a t t h i s may be due t o a r e d u c t i o n of l o n g r a n g e f o r c e s between oxygen and d i s l o c a t i o n s .
F i g . 5 : Temperature of t h e a H - r e l a x a t i o n a s a f u n c t i o n o f t h e H c o n c e n t r a t i o n
+
e x p e r i m e n t a l p o i n t s--
c a l c u l a t e d by t h e model of t h e Snoek-Koster r e l a x a t i o n / 4 , 5 /The s h i f t of t h e r e l a x a t i o n t o h i g h e r t e m p e r a t u r e s a f t e r hydrogen dopings c a n b e w e l l d i s c r i b e d by t h e models of t h e Snoek-Koster r e l a x a - t i o n (cold-work peak) / 4 , 5 / . F i g . 5 f o r example show t h e peak tempera- t u r e a s a f u n c t i o n of hydrogen c o n c e n t r a t i o n i n t h e sample. The good agreement between t h e e x p e r i m e n t a l p o i n t s ( c r o s s e s ) and t h e model c a l - c u l a t i o n ( c u r v e ) i s a s t r o n g i n d i c a t i o n t h a t t h i s i s indeed a hydrogen c o l d work peak a s s u g g e s t e d e a r l i e r by Mazzolai e t a l . /6/ and F e r r o n e t a l . / 7 / .
The i n t r i n s i c d i s l o c a t i o n r e l a x a t i o n
To o b s e r v e t h e r e l a x a t i o n p r o c e s s e s c a u s e d by hydrogen-free d i s l o c a t i o n segments we t r i e d t o remove t h e hydrogen from t h e samples by a long t i m e d e g a s s i n g t r e a t m e n t ( 1 week a t 1 0 0 0 ~ ~ and 1 0 - l o mbar)
.
The e x p e r i m e n t a l r e s u l t s o b t a i n e d on such a sample a r e shown i n f i g . 6 and 7. A f t e r d e f o r m a t i o n a t 20 K and s u b s e q u e n t a n n e a l i n g a t 300 K t o g e t a s t a b l e s t a t e , two r e l a x a t i o n peaks a r e v i s i b l e ( c u r v e 1 , f i g . 6)
.
A f t e r measuring c u r v e 1 t h e UHV pump system was t u r n e d o f f , s o that t h e specimen took up some hydrogen from t h e r e s i d u a l g a s . A f t e r H con- t a m i n a t i o n t h e 70 K peak i s reduced and s h i f t e d t o lower t e m p e r a t u r e s whereas t h e a H - r e l a x a t i o n i s enhanced and s h i f t e d t o h i g h e r ternperatu- r e s . The i n c r e a s e of t h e a H - r e l a x a t i o n i s c o n n e c t e d w i t h t h e d i s a p p e a r - a n c e of t h e d i s l o c a t i o n r e l a x a t i o n p r o c e s s e s a t 30-70 K . S i m i l a r be- h a v i o u r was o b s e r v e d and d i s c u s s e d i n Fe / 8 / . No f u r t h e r i n c r e a s e ofC5-76 JOURNAL DE PHYSIQUE
t h e a H - r e l a x a t i o n w i t h i n c r e a s i n g H - c o n c e n t r a t i o n s h o u l d o c c u r i f a t 30-70 K a l l d i s l o c a t i o n s a r e p i n n e d by H and hence a r e immobile. T h i s i s i n d e e d t h e c a s e a s c a n b e s e e n from f i g . 6 c u r v e 3 and 4 , and f i g . 3 .
F i g . 6 : I n f l u e n c e of hydrogen o n t h e r e l a x a t i o n p r o c e s s e s i n a Nb s i n g l e c r y s t a l a f t e r e x t r e m hydrogen d e g a s s i n g (1 week) 1 c H < < 1 atppm
2 1 atppm
3
-
5-10 atppm4 -10-20 atppm
F i g . 7 : Frequency (modulus) c u r v e s c o r r e s p o n d i n g t o t h e damping c u r v e s i n f i g . 6 .
The d e s c r i b e d e x p e r i m e n t a l f a c t s s u g g e s t t h a t t h e 70 K peak i s due t o t h e movement of h y d r o g e n - f r e e d i s l o c a t i o n segments whereas t h e a - r e l a x a t i o n i s c a u s e d by t h e same d i s l o c a t i o n segments, b u t d e c o r a t e d H w i t h hydrogen. T h i s i n t e r p r e t a t i o n i s f u r t h e r s u p p o r t e d by t h e modulus
( f r e q u e n c y ) c u r v e i n f i g . 7 . A comparison of c u r v e s 1 and 2 shows t h a t t h e r e l a x a t i o n s t r e n g h t of t h e 70 K peak i s s h i f t e d t o t h e a H - r e l a x a t i o n by hydrogen doping and hence t h a t t h e same d i s l o c a t i o n s a r e i n v o l v e d i n b o t h r e l a x a t i o n p r o c e s s e s .
Samples w i t h h i g h i n t e r s t i t i a l s o l u t e c o n t e n t s
I n a sample w i t h h i g h oxygen c o n c e n t r a t i o n (900 atppm) ( f i g . 8 , c u r v e 1 ) a second r e l a x a t i o n p r o c e s s i s v i s i b l e i n t h e v i c i n i t y of 30 K . A s a consequence of t h e h i g h 0 c o n c e n t r a t i o n t h e cxH-relaxation i s v e r y s m a l l and a p p e a r s a t a r e l a t i v e l y low t e m p e r a t u r e (150 K ) . Curve 3 i n f i g . 8 shows a measurement o f V a z z o l a i e t a l . / 6 / o b t a i n e d on a Nb sample w i t h h i g h i n t e r s t i t i a l s o l u t e c o n t e n t (0,N) a t a f r e q u e n c y of 45 kHz.
A s i n o u r measurement ( c u r v e 1 ) two r e l a x a t i o n peaks a r e o b s e r v a b l e ,
sample of Mazzolai et al., we believe that in both measurements the same relaxation peaks are observed.
After doping our sample with -50 atppm hydrogen the 30 K peak has disappeared and the aH-relaxation is enhanced by a factor of 10. This behaviour corresponds to that expected for an intrinsic dislocation peak (70 K peak, fiq.6), which is suppressed by hydrogen and the in- crease of the ~ ~ - ~ e a k corresponds to samples with high oxygen concen- tration.
I
Fig.8 : Comparison of our meas-?,
s urements (IkHz) on a Nb single
X
b crystal (-900 atppm oxygen) after
deformation (curve 1 ) and subse-
05- quent hydrogen doping (-50 atppm)
with one result of Mazzolai et al.
/6/(45kHz) obtained on a sample with high interstitial solute
content after deformation.
Curve 2: Nb
+
50 atppm HO o Curve 3: Mazzolai et al.
T(K)
- -
These experimental facts lead to the conclusion, that the 30 K peak is an intrinsic dislocation peak as proposed earlier by Mazzolai et al.. The microscopic mechanism of the 30 K and the 70 K peak should be the same. The difference of the peak temperatures in high purity specimen (peak at 70 K) and in specimen with high oxygen concentrations
(peak at 30 K) is caused by the reduction of the free dislocation lenght due to the high oxygen concentration.
To account for the fact that in samples with high interstitial solute contents the intrinsic dislocation relaxation can be observed even in the presence of some atppm hydrogen,we suggest that in such samples hydrogen atoms can be partly trapped at interstitial solute atoms. This has the effect that the dislocations are partly free of hydrogen even at low temperatures and the intrinsic dislocation peak may be observed. Due to the investigations reported in this paper on Nb, and regarding the similar properties of V and Ta in respect to hydrogen solubility and the work of other authors, cited above, it can be ex- pected that in all three metals the 150/200 K ( 1 kHz) dislocations relaxation effects are caused by hydrogen decorated dislocation seg- ments. This view gets support by /9/ and /lo/.
C5-78 JOURNAL DE PHYSIQUE
I n summary, d u e t o t h e d e t a i l e d e x p e r i m e n t a l r e s u l t s o n Nb, a s g i v e n i n t h i s p a p e r , we a r e c h a n g i n g o u r o p i n i o n c o n c e r n i n g t h e n a t u r e o f t h e 1 5 0 / 2 0 0 K d i s l o c a t i o n r e l a x a t i o n e f f e c t s i n g r o u p V t r a n s i t i o n m e t a l s , a s e x p r e s s e d i n / 2 , 3 / , / 1 2 - 1 7 / . T h i s d i s l o c a t i o n r e l a x a t i o n e f f e c t i s a h y d r o g e n S n o e k - K o e s t e r r e l a x a t i o n ( H c o l d - w o r k p e a k ) a s s u g g e s t e d b y o t h e r a u t h o r s c i t e d a b o v e . More d e t a i l s a r e g i v e n i n /18/.
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