LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS : III-COALESCENCE OF δ' PRECIPITATES IN FATIGUE : X-RAY LOW ANGLE SCATTERING INVESTIGATION

HAL Id: jpa-00226614

https://hal.archives-ouvertes.fr/jpa-00226614

Submitted on 1 Jan 1987

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS : III-COALESCENCE OF δ’ PRECIPITATES IN FATIGUE : X-RAY LOW ANGLE SCATTERING

INVESTIGATION

Y. Brechet, F. Livet

To cite this version:

Y. Brechet, F. Livet. LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS : III-COALESCENCE OF δ’ PRECIPITATES IN FATIGUE : X-RAY LOW ANGLE SCATTERING INVESTIGATION.

Journal de Physique Colloques, 1987, 48 (C3), pp.C3-717-C3-719. �10.1051/jphyscol:1987383�. �jpa- 00226614�

JOURNAL DE PHYSIQUE

C o l l o q u e C3, s u p p l 6 m e n t au n 0 9 , Tome 48, s e p t e m b r e 1987

LOW CYCLE FATIGUE OF BINARY A1-Li ALLOYS: 111-COALESCENCE OF

6' PRECIPITATES IN FATIGUE : X-RAY LOW ANGLE SCATTERING INVESTIGATION

Y. BRECHET and F. LIVET

Laboratoire de Thermodynamique et Physico-Chimie

Metallurgiques, ENSEEG, Domaine universitaire, B.P. 75, F-38402 Saint-Martin-d'HBres Cedex, France

ABSTRACT

A 1 2 . 5 w t % L i b i n a r y a l l o y s w i t h s m a l l 6 ' p r e c i p i t a t e s ( R m 1 0 1) ^were

c y c l i c a l l y d e f o r m ~ d i n t o r s i o n a t c o n s t a n t a p p l i e d s t r a i n a m p l i t u d e AE , w i t h t h e same s t r a i n r a t e " . The s i z e o f ' p r e c i p i t a t e s was measured b e f o r e and a f t e r f a t i g u e by X r a y low a n g l e s c a t t e r i n g . We have shown by t h i s method t h a t a n enhanced c o a l e s c e n c e t a k e s p l a c e d u r i n g f a t i g u e .

I t i s concluded t h a t t h e b e h a v i o u r o f 6 ' p r e c i p i t a t e s d u r i n g f a t i g u e is governed by two competing phenomena : t h e d i s s o l u t i o n induced by s h e a r i n g , and t h e c o a l e s c e n c e induced by s u r s a t u r a t i o n o f p o i n t d e f e c t s produced by f a t i g u e .

INTRODUCTION

The b e h a v i o u r o f 6 ' p r e c i p i t a t i o n r e l a t e d t o p l a s t i c l o c a l i z a t i o n h a s been s t u d i e d i n p a p e r 11. The p u r p o s e o f t h i s p a p e r is now t o i n v e s t i g a t e t h e a v e r a g e b e h a v i o u r o f t h i s second p h a s e i n a l l o y s s u b m i t t e d t o t o r s i o n c y c l i c l o a d i n g a t room t e m p e r a t u r e ( t h e macroscopic r e s u l t s o f t h i s mechanical t e s t i n g a r e t o b e found i n p a p e r I ) .

P l a s t i c s t r a i n i n g is known t o produce an e x c e s s i n p o i n t d e f e c t s ( 1 ) and t h e r e f o r e t o enhance d i f f u s i o n of s o l u t e atoms. The 6 c o a r s e n i n g is known t o f o l l o w ( 2 , 3 ) v e r y a c c u r a t e l y a L i f s c h i t z S l y o s o v Wagner k i n e t i c s ( 4 ) one e x p e c t s p l a s t i c f a t i g u e t o i n c r e a s e t h e r a t e o f p r e c i p i t a t e c o a r s e n i n g ( 5 , 6 ) .

MATERIALS and EXPERIMENTAL PROCEDURE

The e x p e r i m e n t s were c a r r i e d o u t on t h e same a l l o y s used f o r mechanical t e s t i n g ( p a p e r I ) b e f o r e and a f t e r f a t i g u e . The A12.5% w t L i a l l o y s were h e a t t r e a t e d a t 100°C d u r i n g l h (sample A ) l h 3 0 (sample B) 2h00 (sample C) 8h00 ( s a m p l e D ) . The f a t i g u e t e s t s were performed till r u p t u r e a t a p p l i e d s t r a i n a m p l i t u d e i n t o r s i o n of 0.3%, 0.5% and 0.8%. The s t r a i n r a t e was k e p t c o n s t a n t from an a m p l i t u d e t o a n o t h e r o n e , s o t h a t t h e p e r i o d had t o b e changed. F o r each t e s t t h e s t r e s s a m p l i t u d e and t h e number o f c y c l e t o r u p t u r e ( t a b l e 1 ) were measured.

The r a d i i of 6 ' p r e c i p i t a t e s were t o o s m a l l t o b e a c c u r a t e l y measured by T.E.M. b u t were i n v e s t i g a t e d by s m a l l a n g l e X Ray s c a t t e r i n g .

I n t h e s m a l l a n g l e X Ray a p p a r a t u s a copper s o u r c e ( h = 0.1542nm) h a s been u s e d , w i t h a N i f i l t e r t o e l i m i n a t e t h e K p a r a s i t i c r a d i a t i o n . Our beam had a p o i n t f o c u s geometry. S c a t t e r i n g i n t e n s i t y was c o l l e c t e d on a p o s i t i o n s e n s i t i v e d e t e c t o r I3 i n t h e q ( = 4 IT s i n fi ) r a n g e from 0.15 t o 6nm-I s p e c t r a were r e c o r d e d and t r e a t e d by a S o l a r minicomputer. C a l i b r a t i o n o f t h e e x p e r i m e n t h a s been done by c a r e f u l measurements o f t h e beam i n t e n s i t y w i t h N i f i l t e r s . C o r r e c t i o n s were a p p l i e d f o r background, b r e h m s t r a h l u n g , sample a b s o r b t i o n and g e o m e t r i c a l e f f e c t s .

RESULTS

The a v e r a g e r a d i u s was e s t i m a t e d from t h e maxima o f t h e c u r v y $ I ( ~ ) and o f q I ( q ) . The f a t i g u e t e s t s were performed a t a s t r a i n r a t e 6 = 10- 3 . ^{The mean}

a m p l i t u d e i n s t r e s s was t a k e n t o be e q u a l t o t h e a r i t h m e t i c mean between maxjmum s t r e s s a m p l i t u d e and s t r e s s t o f a i l u r e ( c f p a p e r I ) ( t o t a k e i n t o a c c o u n t t h e l i g h t s o f t e n i n g ) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987383

C3-718 JOURNAL DE PHYSIQUE

TABLE 1 gives the average radius before and after fatigue for each sample

I ^heat I strain ]mean stress1 Number of I Lasting of I average radius of I

1 treatment 1 amplitude I amplitude I cycles to I the test 1 6 'precipitates ( A ) I

1 ^{4 E /2} 1 A 0 /2 1 failure / I

A lbefore fatigue1 I R o = 8 . 5

0.3% 194.5MPa 1 1430 1 17160s I R =13.5

I

I A l I

R = 11

B before fatigue1 I I I R o = 8

1

/

510 1 10200 S R = 13

I

I ^B

1

C lbefore fatigue1 I 1 I ^{R o = 7}

( 38160 s 1 R = 16 I

C 0.3 % 1 90.5 MPa 1 3180

D before fatigue R = 10

I

1

/

/

/

1 0.5% 1116 MPa 1 290 1 5800s / R =13.5 I I 0.8% 1138 MPa 1 90 1 2880s I ^{R =10.5} I

I

DISCUSSION

One can first notice that the relevant parameter to characterize the coalescence under fatigue is the lasting of the test more than the amplitude of straining. The coalescence observed is realy a plastic deformation effec whith an

7' 2 -1 estimation of the diffusion coefficient at room temperature of D = 5.10-' 'm S (I), the maximum coalescence which would have been expected du 'ng a time t = 50000 s (the most lasting fatigue test) would have been of (3) 1.7 lo-'%, which is far under the observed coalescence during fati ne can have an order of magnitude of the enhanced coalescence by plotting F:? :R versus t (which would be a straight line in a LSW kinetics) (figure 1).

A:A FIGURE 1

0:.

C:o 0:.

the estimation of the slope gives low3' m3sd1 : the coalescence which has taken place during fatigue is of the same order fXt magnitude of the one which would have taken place with a diffusion coefficient D 6 orders of magnitude larger than the room temperature diffusion coefficient.

This order of magnitude of the ratio _of fatigue created vacancy concentration

< C 7 over thermal equilibrium concentration C can be estimated (6) from the Meckin

E s t r i n model of p o i n t d e f e c t s p r o d u c t i o n / I / / , assuming t h a t dominant s i n k s f o r v a c a n c i e s a r e d i s l o c a t i o n s /7/8/ one g e t s

< C 7 = -6 + O . I ( E . A , ~ . ~ ~ )/(2D,.ll )

b e i n g t h e d e n s i t y of d i s l o c a t i o n s , y t h e s h e a r modulus, Do t h e d i f f u s i o n c o e f f i c i e n t f o r v a c a n c i e s a t room t e m p e r a t u r e .

A u m e r i c a l e s t i m a t i o n 1 a d s t o $C>D 10-19 t o lo-*'. A t mom t e m p e r a t u r e

ED therefore<C)/C w10+' t o 10-

.

The r e p e a t e d s h e a r i n g o f p r e c i p i t a t e s by d i s l o c a t i o n s l e a d s t o a r e d i s s o l u t i o n o f p r e c i p i t a t e s i n s l i p bands, and t h e s m a l l e r t h e p r e c i p i t a t e s a r e , t h e sooner t h e y w i l l be s h e a r e d enough t o d i s a p p e a r ( / 7 / ) . T h e r e f o r e t h i s mechanism adds a new d r i v i n g f o r c e i n t h e c o a l e s c e n c e p r o c e s s which f a v o u r s t h e l a r g e p r e c i p i t a t e s a t t h e expense o f t h e s m a l l ones : t h i s d r i v i n g f o r c e , d i f f e r e n t from t h e i n t e r f a c i a l energy r e d u c t i o n which r u l e s t h e LSW k i n e t i c s c a n be expected t o change n o t o n l y t h e speed o f t h e c o a l e s c e n c e p r o c e s s b u t a l s o t h e power law i t s e l f .

CONCLUSION

The behaviour o f 6 ' p r e c i p i t a t e s under f a t i g u e a p p e a r s t o be c o n t r o l l e d by two c o m p e t i t i v e phenomena : t h e r e p e a t e d s h e a r i n g which l e a d s t o a d i s s o l u t i o n o f p r e c i p i t a t e s ( p a r t 11) and t h e f a t i g u e induced c o a l e s c e n c e which makes them grow. On t h e average t h e c o a l e s c e n c e p r o c e s s is dominant a s can be s e e n from X Ray d i f f r a c t i o n e x p e r i m e n t s , b u t t h e l o c a l i z a t i o n of p l a s t i c deformation i n i n t e n s e s l i p bands c r e a t e s r e g i o n s of t h e m a t e r i a l where t h e d i s s o l u t i o n p r o c e s s is dominant. I n t h e s e r e g i o n s , v a c a n c i e s c r e a t e d by f a t i g u e enhance t h e d i f f u s i o n o f l i t h i u m , and p r e v e n t t h e r e p r e c i p i t a t i o n i n t h e bands.

AKNOYLEDGMENTS

The a u t h o r s thank Aluminium Pechiney company f o r having provided t h e m a t e r i a l , and f o r f i n a n c i a l s u p p o r t o f one o f them (Y.B.).

REFERENCES

/1/ MECKING H., ESTRIN Y. S c r i p t a Met. 1 4 , 815 ( 1 9 8 0 ) . /2/ SAINFORT P. T h e s i s Grenoble U n i v e r s i t y (1984).

/3/ LIVET F., BLOCH D., S c r i p t a Met. 1 9 , (1985).

/4/ LIFSCHITZ I . , SLYOSOV V. J . Phys. CHem. S o l i d s , 1 9 , 35 (1961).

/5/ CALDERON H.A., WEERTMANN J.R., FINE M.E. S c r i p t a Met. 18, 587 (1984).

/6/ SOUAMI N., FAGOT M . , CHOMEL P., COTTU J.P., S c r i p t a Met. 20, 1673 ( 1 9 8 6 ) . /7/ BRECHET Y . , LOUCHET F., MARCHIONNI C., VERGER-GAUGRY J.L., P h i l . Mag. under

p r e s s .

/8/ WINTEMBERGER M., Acta Met. 7 , 549 ( 1 9 5 9 ) .