LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS. II-MICROSTRUCTURAL CHARACTERIZATION AND IN SITU FATIGUE EXPERIMENTS

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LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS.

II-MICROSTRUCTURAL CHARACTERIZATION AND IN SITU FATIGUE EXPERIMENTS

Y. Brechet, F. Louchet, J.-L. Verger-Gaugry

To cite this version:

Y. Brechet, F. Louchet, J.-L. Verger-Gaugry. LOW CYCLE FATIGUE OF BINARY Al-Li ALLOYS.

II-MICROSTRUCTURAL CHARACTERIZATION AND IN SITU FATIGUE EXPERIMENTS.

Journal de Physique Colloques, 1987, 48 (C3), pp.C3-709-C3-716. �10.1051/jphyscol:1987382�. �jpa-

00226613�

JOURNAL DE PHYSIQUE

C o l l o q u e C 3 , s u p p l e m e n t a u n 0 9 , Tome 48, s e p t e m b r e 1987

LOW CYCLE FATIGUE OF BINARY A1-Li ALLOYS. 11-MICROSTRUCTURAL CHARACTERIZATION AND IN SITU FATIGUE EXPERIMENTS

Y. BRECHET, F. LOUCHET and J . L . VERGER-GAUGRY

L a b o r a t o i r e de T h e r r n o d y n a m i q u e

e t

P h y s i c o - C h i m i e

Metallurgiques, E N S E E G , D o m a i n e U n i v e r s i t a i r e , B.P. 75, F-38402 S a i n t - M a r t i n - d ' H e r e s C e d e x ,

France

ABSTRACT

The f a t i g u e b e h a v i o u r o f AlLi b i n a r y a l l o y s is i n v e s t i g a t e d a t t h e m i c r o s c o p i c s c a l e u s i n g T.E.M., STEM, and i n s i t u f a t i g u e i n T.E.M. on specimens p r e v i o u s l y f a t i g u e d i n t o r s i o n . I n t h e A10.7ILi s o l i d s o l u t i o n , a c e l l s t r u c t u r e is observed.

The A12.5%Li a l l o y w i t h s m a l l p r e c i p i t a t e s e x h i b i t p r e c i p i t a t e f r e e bands which r e f l e c t a s t r o n g l o c a l i z a t i o n . The A12,5%Li a l l o y s w i t h l a r g e p r e c i p i t a t e s show Orowan by p a s s i n g b u t a l s o s h e a r i n g by t h e Orowan Loops. The t h r e e t y p e s o f mechanisms were o b s e r v e d also by i n s i t u s t r a i n i n g e x p e r i m e n t s i n t h e T.E.M.

I- INTRODUCTION

Aluminium a l l o y s s t r e n g t h e n e d by A 1 L i p r e c i p i t a t e s a r e known t o have a s h o r t f a t i g u e l i f e t i m e . F a i l u r e o c c u r s r a t h e r by 3 t r a n s g r a n u l a r o r by i n t e r g r a n u l a r f r a c t u r e , depending on t h e a n n e a l i n g t i m e . The p u r p o s e o f t h e p r e s e n t work i s t o c l a r i f y t h e problem o f s t r a i n mechanisms and l o c a l i z a t i o n i n f a t i g u e i n t h e s i m p l e s t c a s e , t h a t o f a b i n a r y a l l o y , w i t h v a r i a b l e L i c o n t e n t and p r e c i p i t a t e s i z e .

The macroscopic a s p e c t s o f t h e f a t i g u e b e h a v i o u r o f t h r e e t y p e s o f b i n a r y A l - L i a l l o y s a r e d e s c r i b e d by S. Han and B. Wack i n t h e p r e s e n t c o n f e r e n c e ( 1 ) . The same a l l o y s a r e s t u d i e d h e r e under t h e m i c r o s c o p i c p o i n t o f view, u s i n g SEM, TEM and STEM t e c h n i q u e s , and i n s i t u TEM f a t i g u e t e s t s .

11- EXPERIMENTAL

-

Three t y p e s o f a l l o y s a r e u s e d , which have been p r o v i d e d by Cegedur P e c h i n e y , and cyded i t o r s i o n a t imposed t o t a l d e f o r m a t i o n a m p l ' t u d e

X

^{= 3.10-',} and 5 x 1 0 - ~ , w i t h a s t r a i n r a t e o f 10-3~-1 ( 1 )

-

A l l o y A is a s o l i d s o l u t i o n . A l l o y s B and C c o n t a i n o r d e r e d c o h e r e n t

'

p r e c i p i t a t e s (A1 L i ) , of.L12 s t r u c t u r e , which have been grown t o d i f f e r e n t s i z e s by a d j u s t i n g t h e a n i l e a l i n g t u n e and t e m p e r a t u r e ( 2 ) . a c c o r d i n g t o t h e f o l l o w i n g t a b l e :

The m i c r o s t r u c t u r e h a s been a n a l y z e d u s i n g a 200 CX JEOL TEM, and a VG HE501 STEM o p e r a t i n g a t 100 kV and equipped w i t h a T r a c o r N o r t h e r n TN 2000 a n a l y z e r . I n s i t u f a t i g u e t e s t s a r e performed a t room t e m p e r a t u r e , u s i n g a "home made" specimen h o l d e r which o p e r a t e s i n s h e a r , on t h i n f o i l s c u t p a r a l l e l t o t h e s u r f a c e o f macroscopic specimen, i n o r d e r t o keep t h e same l o c a l s t r e s s t e n s o r .

I

Type o f a l l o y

I

wt.%Li

1

0 . 7 ( 2 . 5

1

^{2 . 5}

1

l a v e r a g e r a d i u s of

I

s o l i d

I

1.5nm

I

20nm

I

A B _C

I

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987382

JOURNAL DE PHYSIQUE

111- ALLOY A

The s o l i d s o l u t i o n a f t e r f a t i g u e e x h i b i t s only a c e l l s t r u c t u r e , t h e o p e r a t i o n of which can be observed i n s i t u ( f i g . 1 ) . D i s l o c a t i o n s bend under s t r e s s o u t of s o u r c e s i n c e l l w a l l s , escape suddenly from them, and t r a v e l a c r o s s t h e c e l l . Some of them i n s e r t i n t o t h e o p p o s i t e w a l l . During t h e following h a l f - c y c l e , t h e i r c u r v a t u r e is r e v e r s e d . Most of them c r o s s - s l i p and t r a v e l a c r o s s t h e c e l l on d i f f e r e n t s l i p p l a n e s , a s i n d i c a t e d by t h e s l i p t r a c e s v i s i b l e on t h e f i g u r e , and demonstrating s l i p i r r e v e r s i b i l i t y .

Although p e r s i s t e n t s l i p bands a r e l i k e l y t o form a t lower s t r a i n amplitudes, a s shown by Dhers i n pure A 1 f c i g u r e d i n tension/compression (31, i n our c a s e , f o r s t r a i n amplitudes of some 10

.

t h e o p e r a t i o n of c e l l s t r u c t u r e s d e s c r i b e d above is probably dominant.

IV- ALLOY B

The f a t i g u e behaviour of a l l o y B with small s h e a r a b l e 6

'

p r e c i p i t a t e s (Ryl.5nm) has been discussed elsewhere ( 4 ) and w i l l be only summarized here. The m i c r o s t r u c t u r e of f a t i g u e d specimens does n o t e x h i b i t any c e l l s t r u c t u r e . I n s i t u f a t i g u e t e s t s show d i s l o c a t i o n avalanches l o c a l i z e d i n narrow s l i p bands ( f i g . 2 ) . and s u p e r l a t t i c e dark f i e l d images taken with t h e e l e c t r o n bean p a r a l l e l t o a [ll?]

d i r e c t i o n show narrow channels p a r a l l e l t o (111) s l i p p l a n e s , with a width of about 5nm, i n which 6 p r e c i p i t a t e s a r e n o t v i s i b l e ( f i g . 3 ) . On t h e o t h e r hand, g ' p r e c i p i t a t e s a r e c l e a r l y v i s i b l e between c h m n e l s , and e x h i b i t a s t r o n g c o a r s e n i n g on band s i d e s .

F i l t e r e d EELS images have been o b t a i n e d i n STEM, using two energy windows on both s i d e s o f t h e average plasmon peak ( a t 14.7 eV f o r t h e 2.5 w t % Li a l l o y (9.68 a t %)) t h e images obtained with t h e high energy window show b r i g h t bands on a dark background ( f i g . 4 1 , and t h o s e with t h e low energy window have a r e v e r s e d c o n t r a s t . S i n c e t h e energy of t h e plasmon peak is g i v e n by :

E(eV) : 15.09

-

3.97CLi(at%)

according t o ( 5 ) . t h e observed c o n t r a s t s agree with a s t r o n g d e c r e a s e of L i c o n c e n t r a t i o n i n s i d e t h e bands. The advantage of t h i s method is t o g e t r i d o f t h e e v e n t u a l i n f l u e n c e of t h i c k n e s s on t h e observed c o n t r a s t . Concentration p r o f i l e s of L i have a l s o been performed i n EELS, with a probe s i z e around lnm, and a s t e p of 5nm.

We o b t a i n CLi% 4 a t % i n t h e bands, whereas t h e average Li c o n c e n t r a t i o n measured by t h i s t e c h n i q u e is 9.8 a t % . A l o c a l enrichment i n Li(C % 1 2 a t % ) i s a l s o n o t i c e d on band s i d e s , i n agreement with t h e s u p e r l a t t i c e dark f i e k g o b s e r v a t i o n s .

-

The model we propose f o r t h e formation of channels deprived of L i during f a t i g u e is based on a m o d i f i c a t i o n of t h e s i z ~ dependence of t h e f r e e energy of p r e c i p i t a t e s when s t e p s and a n t i p h a s e boundaries a r e introduced by shear. I t can be e a s i l y shown _{A G(R)} (5) _{= -(4/3$} t h a t t h e - e n r g y of a _a

_y

_R

3

_{+ 4T 7} s _R

9

eared p r e c i p i t a t e can be w r i t t e n : _{( 1 )} where R is t h e p r e c i p i t a t z r a d i u s , %nd where

7

and 7 a r e e q u i v a l e n t volume and

s u r f a c e s p e c i f i c e n e r g i e s , which depend on t h e s h e a r s t r a i n

YV

^{= y V .} (1- yaE/b.yV)

5 s ⁼ y S . ( l + ~ 2 / 8 )

y a being t h e a n t i p h a s e energy, and 2 an e q u i v a l e n t s h e a r s t r a i n which t a k e s i n t o account t h e p o s s i b i l i t y f o r an a n t i p h a s e boundary t o be e r a s e d by a second d i s l o c a t i o n s h e a r i n g t h e p r e c i p i t a t e along t h e same plane.

From eq. (11, t h e c r i t i c a l r a d i u s f o r p r e c i p i t a t e d i s s o l u t i o n ( s a d d l e p o i n t of AG(R) ) d e f i n e d by ( 66 G/ b R) = 0 i s RC = 2

5

//P

.

^Since

⁷

^becomes

n e g a t i v e when E exceeds a criQ~P&i v a l u e E =b y

/

Y '(frsm e q . 2 ) , R: becomes i n f i n i t e a t T. = 2

,

and a l l p r e c i p i t a t e s i n C t h e \ a n 8 become u n s t a b l e before t h e cumulative equi:alen% s t r a i n E is reached l o c a l l y , which corresponds t o a c r i t i c a l time t = T. / ^E

,

and a l o c g l s t r a i n e s t i m a t e d t o about 20% i n t h e p r e s e n t case.

More p%ecise%y,

'9

has t o be compared t o t h e a c t u a l s i z e of p r e c i p i t a t e s , which slowly i n c r e a s e s ir? t 1 / 3 during f a t i g u e a ~ o r d i n g t o a lifschitz-Slyozov-Wagner law ( 2 ) , and which w i l l n e c e s s a r i l y c r o s s t h e R ( t ) curve a t a time t =tc, a t which the&' p r e c i p i t a t e s become u n s t a b l e and d i s s o l v e . a

V- ALLOY C

-

I n t h i s c a s e , p r e c i p i t a t e s a r e l a r g e , and a r e expected t o be by-passed by Orowan looping. I n s i t u experiments indeed show a t Ao =250 MPa by-passing e v e n t s ( f i g . 5 ) i n which loops produced by a d i s l o c a t i o n during a h a l f - c y c l e a r e o f t e n a n n i h i l a t e d by t h e same d i s l o c a t i o n d u r i n g t h e following h a l f - c y c l e , b u t wherr c r o s s s l i p e v e n t s during t h e t o and f r o movement of t h e d i s l o c a t i o n can accumulate o p p o s i t e l o o p s i n s t e a d of a n n i h i l a t i n g them, which is probably r e s p o n s i b l e f o r p a r t of t h e f a t i g u e i r r e v e r s i b i l i t y .

-

^However, s u p e r l a t t i c e dark f i e l d o b s e r v a t i o n s show t h a t a number of p r e c i p i t a t e s a r e sheared, i n s p i t e of t h e i r s i z e ( f i g . 6 ) . which was n o t expected i n Orowan Looping c o n d i t i o n s . I f we assume t h a t t h e y a r e sheared n o t by moving d i s l o c a t i o n s , but by Orowan loops under a d r i v i n g f o r c e due t o t h e l i n e t e n s i o n of t h e loop and t o t h e a p p l i e d s t r e s s , t h e i n c r e a s e of energy when t h e loop has p e n e t r a t e d a d i s t a n c e x i n t h e p r e c i p i t a t e i s :

where y is t h e APB energy,

r

t h e l i n e t e n s i o n , and R t h e p r e c i p i t a t e r a d i u s . This energy g8es through a maximum ( s a d d l e p o i n t ) :

9

-

^C)

wrn=nRL. (Ia-cb) (1- T*)r()L

f o r x = x = R (1-

~ r '

)

7 .

m :ya-u

I f t h e s a d d l e p o i n t energy has t o be provided only by thermal a c t i v a t i o n ( a t 2 0 = 0 ) t h i s wou d g i v e

_-4

W ( o = 0 ) N 500 eV with R = 200

1,

b = 2.8A, $a = 140mJ/m

. ^r

⁼

1 . 3 4 ~ 1 0 J/m. Ampure thermally a c t i v a t e d s h e a r i n g during t h e s t o r a g e of specimens b e f o r e TEM o b s e r v a t i o n s is t h e n q u i t e u n l i k e l y . On t h e c o n t r a r y , t h e c r i t i c a l r e s o l v e d s h e a r s t r e s s 0 which reduces t h e energy b a r r i e r t o z e r o is given by :

-

0

.

^(,

- .r- 1,

d 5 0 HPa c b

a

R

which corresponds t o an a p p l i e d s h e a r s t r e s s of about 500 MPa ( t a k i n g i n t o account an average Schmid's f a c t o r ) . This s t r e s s l e v e l is n o t reached during f a t i g u e (maximum s t r e s s ?u_ 150 MPa), b u t i f two loops p i l e up around a p r e c i p i t a t e on t h e same s l i p p l a n e , t h e APB is reduced t o t h e s u r f a c e between t h e loops while t h e s t r e s s on t h e l e a d i n g d i s l o c a t i o n is s i g n i f i c a n t l y i n c r e a s e d . Shearing is t h e n p o s s i b l e f o r a l o c a l s t r a i n E = n . b / 4 R, ( w i t h n = 2 ) N 0.7%, which can be e a s i l y reached a t

X

^As

=o.&

by a s l i g h t s t r a i n l o c a l i z a t i o n .

Orowan looping and subsequent p r e c i p i t a t e s h e a r i n g by Orowan loops can o p e r a t e even without t h e h e l p of any thermal a c t i v a t i o n i n t h e p r e s e n t c o n d i t i o n s . The domains o f s t r e s s i n which t h e s e two p r o c e s s e s occur can e v e n t u a l l y o v e r l a p , s i n c e t h e Orowan l o o p i n g is determined by t h e s t r e s s and t h e spacing between p r e c i p i t a t e s , and t h e s h e a r i n g by Orowan loops by t h e s t r e s s , t h e l i n e t e n s i o n

,

t h e a n t i p h a s e boundary energy and t h e s t r a i n l o c a l i z a t i o n . T h i s s t r a i n l o c a l i z a t i o n is both t h e cause and t h e consequence of p r e c i p i t a t e s h e a r i n g , and t h e r e f o r e i n c r e a s e s during f a t i g u e , while l a r g e volumes of t h e specimen c o n t r i b u t e t o t h e f a t i g u e s t r a i n by small and q u a s i r e v e r s i b l e movements of d i s l o c a t i o n s . Another reason f o r s t r a i n l o c a l i z a t i o n is t h a t PFZ's have grown a l o n g g r a i n boundaries during annealing, and most of t h e s t r a i n is observed t o l o c a l i z e d i n t h e s e PFZ, where d i s l o c a t i o n s move much more f r e e l y t h a n i n t h e g r a i n s .

CONCLUSION

The f a t i g u e behaviour of t h e s o l i d s o l u t i o n A l L i i s c h a r a c t e r i z e d by an homogeneous and i r r e v e r s i b l e motion of d i s l o c a t i o n s i n a w e l l defined c e l l s t r u c t u r e . On t h e o p p o s i t e , a l l o y s hardened by small 6

'

p r e c i p i t a t e s e x h i b i t a s t r o n g s t r a i n l o c a l i z a t i o n i n narrow s l i p bands i n which p r e c i p i t a t e s become r e a d i l y u n s t a b l e by accumulation o f APB's and d i s s o l v e .

I n t h e c a s e of l a r g e p r e c i p i t a t e s , Orowan looping p r e v a i l s i n t h e g r a i n s , but Orowan loops can e a s i l y s h e a r t h e p r e c i p i t a t e s i f s t r a i n is somewhat l o c a l i z e d . S t r a i n l o c a l i z a t i o n a l s o occurs i n PFZ's, where d i s l o c a t i o n s move much more f r e e l y t h a n i n t h e i n t e r i o r of g r a i n s .

C3-712 JOURNAL

DE

PHYSIQUE

' i g u r e 1 O p e r a t i o n of a c e l l s t r u c t u r e

F i g u r e 2 i n s i t u zvalanches (B)

F i g u r e 3 d a r k f i e l d s u p e r l a t t i c e (-15)

C3-714 JOURNAL DE PHYSIQUE

F i ~ u r e 4 EELS f i l t e r e d images ( B )

Figure

5

Orowan bypassing of precipitates

(in-situ straining

)

-Alloy C

Figure

6

Orowan's loops (weak

beam

image

)

and sheared precipitates (dark f i e l d

on

a

surstructure spot

110

)-Alloy

C

C3-716 JOURNAL DE PHYSIQUE

ACKNOWLEDGMENTS The a u t h o r s t h a n k s Aluminium Pechiney Company f o r having provided t h e m a t e r i a l s and f o r f i n a n c i a l s u p p o r t o f o f them (Y.B.).

(1) S. HAN, B. WACK T h i s c o n f e r e n c e ("paper I " )

( 2 ) Y. BRECHET, E'. LIVET, T h i s c o n f e r e n c e ("paper 111") ( 3 ) J DHERS, T h e s i s S t E t i e n n e 1986

( 4 ) Y. BRECHET, F. LOUCHET, J.L. VERGER-GAUGRY, P h i l . Mag. under p r e s s . (5) P. SAINFORT-Thesis Grenoble U n i v e r s i t y , 1985.