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ANALYSIS OF SNOEK-KOSTER (H) RELAXATION
IN IRON
J. San Juan, Gilbert Fantozzi, M. No, C. Esnouf, F. Vanoni
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C10, supplbment au n012, Tome 46, dbcembre 1985 page C10-127
ANALYSIS OF SNOEK-KOSTER (H) RELAXATION IN IRON
J. SAN JUAN, G. FANTOZZI*, M.L. NO' , C. ESNOUF' AND F. VANONI+*
D e p t o F i s i c a d e l E s t a d o S o l i d o , F a c u l t a d d e C i e n c i a s . U.P.V.,
Aptdo 6 4 4 , B i l b a o , E s p a n a
+G.E.M.P.P.M. I N S A d e L y o n , B 8 t . 502, 6 9 6 2 1 V i l l e u r b a n n e C e d e x , F r a n c e
" C E N G , DRF, 85 X , 38401 G r e n o b l e , F r a n c e
R6sum6
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La relaxation Snoek-Koster due & l'hydrogkne (S-K(H,)) dans le fer est constitue de deux composantes & environ 110 K et 155 K (1 Hz). Nous avons 6tudi6 les effets non lin6aires de cette relaxation S-K(H) et pr6cis6 ainsi les m6canismes responsables de chacun des processus.Abstract
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The Snoek-KGster relaxation due to hydrogen (S-K(H)) in iron shows two components at about 110 K and 155 K. The non- linear effects of this relaxation are studied and the mechanisms reponsible for each component are specified.I
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INTRODUCTIONThe S-K(H) relaxation is constituted of two components : SK-1 around 110 K and SK-2 at about 155 K (for a frequency of 1 Hz). We have previously studied the general features of these two components, particularly their behaviour during annealing
I 1 C) I
A complementarity between the S
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K(H) relaxation and the a peak is observed /3,4/and the ,(S
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K)H relaxation and the y peak can coexist in pure iron /2,5/. From these preceding results, the S-K(H) relaxation in iron has been attributed to the interaction of non-screw dislocations responsible for the a peak with hydrogen atoms.The different models of the S-K relaxation did not allow to explain all the charac- teristics of this relaxation and we have proposed a modified model which attributes the S-K(H) relaxation to the thermally activated formation of double kinks in the presence of hydrogen atoms (SK-1) and of di-atomic hydrogen clusters (SK-2).
Nevertheless, to explicit the interpretation, we must specify the S-K relaxation behaviour, particularly the correlation between the heights of a and S-K(H) peaks, the broadening factors, the longitudinal or transversal drag of hydrogen atoms. I 1
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EXPERIMENTAL PROCEDUREThe experimental procedure is described elsewhere /2/. The specimens of CENG pure iron /6/ undergo the following treatments :
a) deformation by tension of 2 % and by torson of 2 % at room temperature (Ta) ; hy- drogen was charged by electrolytic method (5% Hp SO4 + CS2) at Ta ; mounting in the pendul um
b) deformation by tension of 5 % and by torsion of 5 1 at Ta and hydrogen charging
at 250 K and mounting i n the pendulum at this temperature.
JOURNAL DE P H Y S I Q U E
111
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RESULTS AND DISCUSSIONFirstly, we have studied the dependence of the height of the two components of the S-K relaxation with the vibrating stress amplitude, by decomposing the internal friction spectra. For the decomposition, we suppose the two components are asym- metric, the high temperature part being broader than the low temperature part. Thus, no fictitious peak appears. Furthermore, we assume that the broadening factor and the asymmetry factor 0 are constant. The values which give the best decomposition are indicated in table 1, as also the values of the activation parameters for the two components /4/.
Table 1
Fig. 1 shows the variation of the peak height as a function of the vibrating strain amplitude. The SK-1 component increases slightly up to 2 .x then decreases slowly. This behaviour is not linked to the Par6 condition, the internal stresses being important but is due to the dragging of the hydrogen atoms. Indeed, when the stress increases, the dislocation speed must increase to sweep a higher area. For a given temperature, the dislocation speed is limited by the hydrogen diffusion and cannot increase ; so the area swept by the dislocation increases more slowly than the strain amplitude and the internal friction decreases.
The SK-2 component is very sensitive to the strain amplitude because the active dislocation length is higher than for the SK-1 component /2/. Furthermore, the pos- sibility of a breakaway process cannot be excluded.
Secondly, we have studied the effect of a bias stcess on the internal friction spectrum. The procedure is the followin : i) the IF spectrum is measured with a vibration strain amplitude E = 5 x 10-8 (fi-g. 2a) ; ii) at 4 K, a bias stress 0
s
= ii is applied and the IF is measured with the same strain amplitude c m (fig. 2b) ; i i i ) from 240 K to 4 K, the bias stress is maintened and the IF is measured during heating with
0s
(fig. 2c) ; i i i i ) 0 s is removed at 240 K and the IFspectrum is determined.
For the two treatments, we observe that the SK-1 component increases slightly and shifts weakly towards low temperature and there is no difference between the curves b and c of fig. 2. This behaviour is probably due to the participation of shorter segments of dislocations when the bias stress is applied.
The SK-2 component is strongly developped by the bias stress during the first appli- cation (fig. 2b) then decreases during the second run with the stress. Furthermore, the peak temperature is increased perceptibly.
f u s i o n o f hydrogen along t h e d i s l o c a t i o n ) cannot be r e t a i n e d , t h e two components being always present w i t h t h e b i a s s t r e s s ( f i g . 2b and 2c).
F i n a l l y , t h e problem o f t h e complementarity o f t h e S-K r e l a x a t i o n and t h e a peak must be resolved. Schoeck /8/ notes t h a t f o r t h e k i n k model t h e S-K(H) r e l a x a t i o n s t r e n g t h must equal the i n i t i a l a peak h e i g h t . This a s s e r t i o n i s n o t c o r r e c t . Indeed, t h e m i c r o c r e e p e x p e r i m e n t s show t h a t t h e m i c r o d e f o r m a t i o n s t a g e i s e q u i v a l e n t t o t h e S-K(H) one. For t h e I F measurements, we must t a k e i n t o account t h e broadening f a c t o r o f t h e peak : we can consider t h a t t h e product o f t h e peak h e i g h t
6 b y t h e broadening f a c t o r 3 i s constant f o r t h e two peaks, t h e two processes b e i n g s i m i l a r /9/.
Our r e s u l t s agree w i t h t h i s hypothesis i n t h e two cases (two components SK-1 and SK-2 o r o n l y t h e SK-2 component) as shown by t a b l e 2 and c o n f i r m t h e k i n k model and t h e complementarity between t h e a and S-K(H) r e 1 axation.
Table 2
: 6 (I.F.) : 8.5 6.1 3.7
REFERENCES
/1/ San Juan, J., Fantozzi, G., Esnouf, C., Vanoni, F., Proc; "HydrogPne e t Ma- t 6 r i a u x H-3" (ed. P. Azou) P a r i s , v o l . 1, (1982) 6-8.
/2/ San Juan, J., Fantozzi, G., Esnouf, C., Vanoni, F., Bernalte. A., J. Phys.,
44
(1983) C9-633./3/ i a k i t a ; K., Sakamoto, K., S c r i p t a Met.,
10
(1976) 339./4/ San Juan, J., These de d o c t o r a t , I n s t i t u t N a t i o n a l des Sciences Appliqu6es, Lyon (1984).
/5/ Matsui, H., Schultz, H., J. Phys., (1981) C5-115.
/6/ Vanoni, F., These de d o c t o r a t , U n i v e r s i t 6 de Grenoble (1973). /7/ R i t c h i e , J.G., S c r i p t . Met., 16 (1982) 249.
/8/ Schoeck, G., S c r i p t a Met., E 7 1 9 8 2 ) 233.
JOURNAL DE PHYSIQUE Fig.1.- E v o l u t i o n o f t h e r e l a x a t i o n s t r e n g t h f o r t h e SK-1 ( 0 ) and SK-2 ( * ) components o f t h e S-K r e l a x a t i o n as a f u n c t i o n o f t h e v i b r a t i n g s t r e s s a m p l i t u d e . 0 50 100 150 200 250 TEMP. K
Fig.2.- I . F . s p e c t r a f o r an hydrogen charged specimen w i t h a v i b r a t i n g s t r a i n a m p l i t u d e = 5 x 1 0 - ' ~ :
