THE DISLOCATION-ENHANCED SNOEK PEAK IN Fe-C ALLOYS

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THE DISLOCATION-ENHANCED SNOEK PEAK IN Fe-C ALLOYS

L. Magalas, P. Moser, I. Ritchie

To cite this version:

L. Magalas, P. Moser, I. Ritchie. THE DISLOCATION-ENHANCED SNOEK PEAK IN Fe-C AL- LOYS. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-645-C9-649. �10.1051/jphyscol:1983997�.

�jpa-00223447�

JOURNAL DE PHYSIQUE

Colloque C9, supplkment au n012, Tome 44, d6cembre 1983 page C9-645

THE DISLOCATION-ENHANCED SNOEK PEAK I N Fe-C ALLOYS

L . B . Magalas, P. ~ o s e r * and I.G. Ritehie**

University o f Mining and MetaZZurgy, I n s t i t u t e of Metallurgy, AZ.

Mickiewicza 30, 30-059 Cracov, PoZand

*centre d f E t u d e s NucZe'aires de GrenobZe, De'partement de Recherche FondamentaZe, Section de Physique du SoZide, 85 X, 38041 Grenoble Cedex, France

ateria rials Science Branch, Atomic Energy of Canada, Research Company, WhitesheZZ NucZear Research Establishment, Pinawa, Manitoba,

ROE ZLO, Canada

A b s t r a c t : Some experimental c o n d i t i o n s which g e n e r a t e t h e d i s l o c a t i o n -

-- enhanced Snoek peak i n Fe-C a l l o y s a r e p r e s e n t e d . The r e s u l t s i n d i e a t e t h a t t h e movement of non-screw d i s l o c a t i o n segments i s involved i n t h e p e l a x a t i o n mechanism. The r e s u l t s a r e c o n s i s t e n t w i t h t h e movement of non-screw d i s - l o c a t i o n s i n a medium with a v i s c o s i t y p r o p o r t i o n a l t o t h e Snoek r e l a x a - t i o n t i m e , o r a l t e r n a t i v e l y , t o a l i m i t i n g c a s e of t h e g e n e r a t i o n of kink- p a i r s on non-screw d i s l o c a t i o n , i . e . a carbon Snoek-Kdster r e l a x a t i o n .

I - INTRODUCTION

Recent development of a s p e c i f i c model of t h e Snoek-KGster (S-K) r e l a x a t i o n ( 1 , 3 ) h a s l e d t o renewed i n t e r e s t i n t h e i n t e r n a l f r i c t i o n ( I F ) r e l a x a t i o r s a t t r i b u t e d t o t h e i n t e r a c t i o n s between i m p u r i t y i n t e r s t i t i a l atoms (IIA'S) and d i s l o c a t i o n s i n t h e bcc m e t a l s ( 3 ) . According t o Seeger and coworkers (4,5), t h e d i s l o c a t i o n - enhanced Snoek peak (DESP) can a l s o be e x p l a i n e d by a g e n e r a l i z e d t h e o r y of t h e i n t e r a c t i o m o f I I A ' s w i t h d i s l o c a t i o n k i n k s . I n t h e c a s e of t h e Fe-C a l l o y s s t u - d i e d h e r e , t h i s t h e o r y i m p l i e s t h a t t h e DESP i s due t o t h e s t r e s s induced move- ment of non-screw d i s l o c a t i o n s i n an atmosphere of i n t e r s t i t i a l C atoms whose e f f e c t i v e v i s c o s i t y i s p r o p o r t i o n a l t o t h e r e l a x a t i o n time rg of t h e normal Snoek peak.

The d e f i n i n g f e a t u r e of a DESP is t h a t a normal Snoek peak i s s u b s t a n t i a l l y ma- g n i f i e d i n h e i g h t by room temperature c o l d work ( C W ) , w h i l e t o a f i r s t approxi- mation t h e l i m i t i n g r e l a x a t i o n time T; and t h e r e l a x a t i o n e n t h a l p y H' remain un- changed, T h i s behaviour could be e x p l a i n e d i f room temperature CW produced r e - l a x a t i o n c e n t e r s with an i n c r e a s e d c o n t r i b u t i o n t o t h e r e l a x a t i o n s t r e n g t h p e r cen- t e r e.g. I I A p a i r s /6/. However, it i s d i f f i c u l t t o imagine t h a t t h e i n c r e a s e i n r e - l a x a t i o n s t r e n g t h p e r ? a i r outweighs t h e s m a l l e r number of r e l a x a t i o n c e n t e r s formed i n determining t h e r e l a x a t i o n s t r e n g t h . E s t i m a t e s of t h e r e l a x a t i o n parameters of t h e C-C r e l a x a t i o n from magnetic a f t e r e f f e c t s t u d i e s /7/ l e a d t o t h e e x p e c t a t i o n t h a t t h i s phenomenon would produce e x t r e m a t e l y s m a l l s a t e l l i t e I F peak, s l i g h t l y t o t h e h i g h temperature s i d e o f t h e normal Snoek peak.

I1 - MATERIALS AND TECHNIQUE

Wire samples (0.6 mm i n diameter and 100 mm long) of CEN-G p u r e Fe ( 8 ) p r e -

doped with 1000 appm o r 25 appm of C were annealed f o r 5h a t 823K i n p u r e He. These a l l o y s a r e r e f e r r e d t o a s ~ e - 1 0 0 0 and Fe-25, r e s _ o e c t i v e l y . I n each c a s e , a s a t u r a - t e d s o l i d s o l u t i o n of carbon was produced by f a s t c o o l i n g (%400K s - l ) from 823K i n a flow of He. The samples were immediately t r a n s f e r r e d t o a l i q u i d n i t r o q e n Dewar and s t o r e d a t 77K u n t i l r e q u i r e d f o r experimentation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983997

JOURNAL DE PHYSIQUE

I11 - EXPERIMENTAL RESULTS 3.1. The DESP in Fe-1000

The normal Snoek peak in Fe-1000 is shownln Fig. 1 (curve 1). It corresponds to _-- 120 appm of C in solid solution. The same curve was also obtained after annealing at 400K. Following torsional CW of 2.5% at 300K, the Snoek peak is transformed into a DESP (curve 2). Similar results have been reported recently in Fe-N (lo), Nb-0 and Ta-0 alloys (5) .

Peak heights of DESP'S following torsional CW at 77K and 300K are compared in Fig. 2 . Similar results for tensile deformations were also obtained (10). These results show that deformation at 300K is more efficient at enhancing the Snoek peak than defor- mation at 77K.

The activation parameters of the DESP in Fe-1000 are H~~~~ = 0.84 f 0.03 eV and ,DESP = 10-14.2 f 0.1 s which should be com ared with the parameters of the nor- mgl Snoek peak of as = 0.80 eV and r; = - ' s as reported by Diehl et a1 (11) .

3.2. The DESP in Fe-25 ---

The DESP and normal S-K peak obtained after 13% CW at 300K in Fe-25 are shown in Fig. 3 , curve 1, In contrast, on cooling the same sample, the S-K peak is stabi- lized (10,12,13) while the DESP is suppressed (curve 1'). A 5h strain ageing treat- Writ of a similarly pre-deformed sample also suppressed the DESP on heating. From these results it is concluded that the dislocations contributing to the DESP are not fully pinned. Further evidence of this for the Fe-1000 alloy is given in section 3.3.

The DESP in Fe-25 does not exhibit any appreciable amplitude dependence in the strain amplitude range investigated and is not generated or is negligibly small after CW at 77K.

3 -3

-

The IF spectra of Fe-1000 samples subjected to different deformation treatments at 300K are illustrated in Fig. 4. Curve 0 shows the normal Snoek peak which is sup- pressed after heating to 673K and cooling. Subsequent deformation of 1.5% in tor- sion at 300K generated the DESP on warm-up, curve 1 and it was suppressed on cool- down, curve 1'. This cyclic procedure was continued with increasing torsional de- formati'on up to 13%. Only the runs for 1.5%, 2.5% and 3.5% are shown in Fig. 5. On each successL~e run the DESP was generated with increasing height on warm-up and suppressed agarn on cool-down. In addition, it should be noted that the dynamic shear modulus (inset, Fig. 5) began to increase on the high temperature side of the DESP on warm-up.

From these results it is concluded that "fresh" dislocations are required to ge- nerate the DESP. By fresh dislocations we mean those dislocations that are sur- rounded by metastable atmosphere following deformation, or those that have been pulled from a stable fully-pinned configuration and have come to rest in a metas- table atmosphere during redeformation.

IV - DISCUSSION

Because of the direct evidence in the above results that the relaxation strength of the DESP increaseswith increasing amount of deformation /and, therefore, in- creasing dislocation density, A/, we assume that dislocation movement is involved in the mechanism and confine our discussion to such mechanisms. In particular, we consi'der the type of dislocations involved and the activation parameters of the relaxation.

Seeger and co-workers (4,5) attribute the DESP's to dislocation lines with small or negligible Peierls barriers (in the case of Fe this means non-screw disloca- tions) moving in an atmosphere of IIA's with low symmetry strain fields (in our case C interstitials). Under these circunstances it is shown that the effective vi'scosity of the atmosphere is proportional to -rS and therefore, the activation enthalpy of the process is H~ as observed experimentally (4). Although the li- miting relaxation time rDESP is proportional to Cd, the concentration of IIA's at the di'slocation line,Othe full expression has not been determined to date (5).

I t i s w e l l known t h a t i n r e l a t i v i t y pure samples of Fe deformation a t 77K produces a preponderance o f long, s t r a i g h t screw d i s l o c a t i o n s , whereas deformation a t 300K produces a t a n g l e of b o t h screw and non-screw components ( 1 4 ) . Consequently, compari-

son of t h e e f f e c t of deformation t e m p e r a t u r e s of 77K and 300K on t h e h e i g h t of t h e DESP (Fig. 2) r e p r e s z n t s s t r o n g evidence t h a t non-screw d i s l o c a t i o n s a r e involded.

More i m p o r t a n t l y , our i n a b i l i t y t o g e n e r a t e a DESP by deformation a t 77K i n t h e pure Fe-25 a l l o y s t r e n g t h e n s t h i s evidence. Thus, t h e experimental evidence of t h e d i s - l o c a t i o n s involved and t h e a v a i l a b l e d a t a on t h e a c t i v a t i o n parameters of t h e DESP i n Fe-C a l l o y s a r e i n agreement w i t h S e e g e r ' s model f o r t h e DESP's i n bcc m e t a l s . I t i s i n t e r e s t i n g t o n o t e t h a t t h e experimental c o n d i t i o n s which g e n e r a t e t h e DESP i n Fe-C a l l o y s a l s o f u l f i l l t h e requirements f o r one of t h e l i m i t i n g c o n d i t i o n s f o r t h e S-K r e l a x a t i o n on non-screw d i s l o c a t i o n s . These requirements a r e t h a t long range m i g r a t i o n of t h e I I A ' s can be d i s r e g a r d e d , i . e . Cd can be t r e a t e d a s a cons- t a n t , and p e q ~ >> l ( P e q i s t h e one dimensional d e n s i t y of k i n k s of one s i g n i n t h e r -

k k

ma1 e q u i l i b r i u m and L i s t h e s e p a r a t i o n of o b s t a c l e s insurmountable t o t h e k i n k s ) ( 4 ) . Under t h e s e c o n d i t i o n s , S e e g e r ' s (3) e x p r e s s i o n f o r t h e r e l a x a t i o n time may be r e w r i t t e n a s

p - K

=YLTCd e x p [ ( l / 2 H~ + H ' ) / ~ T ] ( 1 )

where Ha i s t h e r e l a x a t i o n e n t h a l p y of p e a k a ( a t t r i b u t e d t o k i n k - p a i r g e n e r a t i o n on non-screw d i s l o c a t i o n s ( 1 5 ) ) . For Fe H~ = 0.06 eV (15) and HS = 0.80 eV ( 1 1 ) . Thus, eqn [ I ] p r e d i c t s a r e l a x a t i o n e n t h a l p y of 0.83 eV i n e x c e l l e n t agreement w i t h t h e experimental v a l u e of 0.84 + 0.03 eV.

Although t h e p a r a m e t e r y i n eqn. [ I ] h a s n o t been determined r i g o r o u s l y , Seeger ( 1 ) a r g u e s on g e n e r a l grounds t h a t i t i s almost independent of T and Cd b u t p r o p o r t i o n a l t o L'. Consequently, T Z - ~ ^{( =} ykTCd) should show a s t r u c t u r e dependence. However most r e p o r t e d experimental s t u d i e s of t h e SfK peaks have given rzzK; 1 0 - ~ ~ s ( 5 , 1 6 ) . Thus, our r e s u l t o f .rBESP= 1 0 - ~ ~ . ~ ? ~ ~ s i s n o t i n c o n s i s t e n t with t h e i n t e r p r e t a t i o n o f t h e DESP a s t h e S-K f o r non-screw d i s l o c a t i o n s .

A s i s t h e c a s e w i t h most r e l a x a t i o n s a s s o c i a t e d w i t h t h e movement of d i s l o c a t i o n s , t h e r e l a x a t i o n s t r e n g h t s f o r b o t h of t h e mechanisms d i s c u s s e d above a r e c o n t r o l l e d by t h e p r o d u c t A L2. Again, t h e i n c r e a s e i n t h e peak h e i g h t of t h e e x p e r i m e n t a l l y measured DESP w i t h t h e amoun o f p l a s t i c deformation ( F i g s . 2,4) i s c o n s i s t e n t w i t h a dependence on ILL2. ( A i s t h e d i s l o c a t i o n d e n s i t y ) .

ACKNOWLEDGEMENT

One of t h e a u t h o r s (L.B. Magalas) wishes t o thank P r o f . G . Schoeck f o r h e l p f u l d i s - c u s s i o n s . T h i s work has been sponsored by Centre d l E t u d e s N u c l d a i r e s de Grenoble.

REFERENCES

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=,

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C9-648 JOURNAL DE PHYSIQUE

10. MAGALAS L.B., Thesis, University of Mining and Metallurgy, Cracov (1982) 11. DIEHL J., WELLER M., MENSCH W., V Conf. Point Defects and Defect Interactions

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Fig. 1 - Curve 1 is the normal Snoek peak in Fe-1000. Curve 2 is the DESP after 2.5% CW in torsion at 300 K. Curves

a

the corresponding changes in the elastic shear modulus.

Fig. 2 - The influence of CW in torsion at 77K curve b, at 300K curve c, on the peak heigh of the DESP in Fe-1000.

The level of the original Snoek peak height is indicated by a.

F i g . 3 - I F s p e c t r a a f t e r 13% CW a t 300K i n Fe-25. Curves 1 , a r e t h e I F and dynamic modulus changes, r e s p e c t i v e l y , d u r i n g warm-up. Ci~i-ves 1 ,

@ t h e r e s u l t s on cool-down.

F i g . 4 - The DESP and S-K peaks i n Fe-1000 a f t e r t o r s i o n a l d e f o r n a t i o n a t 300K. Curve 0 - t h e i n i t i a l Snoek peak. Curves 1 , @ ^{; 2,} @ ^; 3 ,

@ a r e t h e IF and dynamic modulus changes d u r i n warm-up a f t e r CW of 1.5%. 2.5% and 3.5%, r e s p e c t i v e l y . Cunves 1 1 , _; 2 ' . @ ^; 3 ' . @

a r e t h e corresponding r e s u l t s on cool-down.