HYDROGEN EMBRITTLEMENT IN GRAIN BOUNDARIES STUDIED BY FATIGUE CRACK PROPAGATION IN Al-Zn-Mg BICRYSTALS

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HYDROGEN EMBRITTLEMENT IN GRAIN BOUNDARIES STUDIED BY FATIGUE CRACK

PROPAGATION IN Al-Zn-Mg BICRYSTALS

A. Niegel, H.-J. Gudladt, V. Gerold

To cite this version:

A. Niegel, H.-J. Gudladt, V. Gerold. HYDROGEN EMBRITTLEMENT IN GRAIN BOUND-

ARIES STUDIED BY FATIGUE CRACK PROPAGATION IN Al-Zn-Mg BICRYSTALS. Journal

de Physique Colloques, 1988, 49 (C5), pp.C5-659-C5-664. �10.1051/jphyscol:1988585�. �jpa-00228082�

HYDROGEN EMBRITTLEMENT I N GRAIN BOUNDARIES STUDIED BY FATIGUE CRACK PROPAGATION IN Al-Zn-Mg BICRYSTALS

A, NIEGEL, H.-J. GUDLADT and V. GEROLD

Max-Planck-Institut fiir Metallforschung, Institut fur

Werkstoffwissenschaften, and Institut fur Metallkunde der Universitat Stuttgart, Seestrasse 92, 0-7000 Stuttgart 1, F.R.G.

La propagation des fissures de fatigue dans les joints de grains a Bt6 Btudiee dans des Bchantillons de Al-Zn-Mg

2

precipitation durcissante contenant un joint de grain unique perpendiculaire

2

l'axe de chargement. Dans le cas de bicristaux revenus au pic de duret6,sollicit6s en atmosphere d'azote contenant de la vapeur dleau,la pro- pagation apparait de type intergranulaire.Les mecanismes de "stress-corrosion crackingU(SCC) et de fatigue par corrosion intergranulaire (CF) contribuent tous deux

B

l'extension des fissures sous chargement cyclique.11 a BtB montre dans les deux cas que la propagation intergranulaire est influencee par la penetration de llhydrogBne dans le joint de grain au cours de chaque cycle de chargement, engendree dans le cas du mecanisme de "SCC" par llhydrog&ne dissous dans le joint de grain et les regions avoisinantes,alors que le mecanisme de "CF" intervient par le biais de llhydrog&ne piBg6.La discussion portera sur les aspects microstructuraux de ces deux processus.

ABSTRACT

High cycle fatigue crack propagation in grain boundaries was studied in precipita- tion hardened Al-Zn-Mg specimens containing a single grain boundary (gb) perpendicu- lar to the load axis. In peak-aged bicrystals tested in wet nitrogen atmospheres, cracks propagated in an intercrystalline manner. Under cyclic loading conditions two different mechanisms contribute to crack propagation which are termed stress cor- rosion cracking (SCC) and intergranular corrosion fatigue (CF). In both cases, it is well established that intercrystalline crack propagation is influenced by hydrogen penetxating into the gb during each load cycle. For SCC it is the hydrogen dissolved into the gb, resp. areas close to the boundary whereas for CF it is the trapped hydrogen which initiates the intergranular cracking. Both terms will be discussed under microstructural aspects.

1. INTRODUCTION

The role of microstructure in stress corrosion cracking (SCC) of Al-Zn-Mg alloys in gaseous surroundings has been discussed by various authors (1,Z). In most cases the experiments were performed under monotonic loading conditions in different atmo- spheres. More recently, attention has been paid to the transition from cracking under monotonic loading to the phenomenon of corrosion under cyclic loading condi- tions

( 3 , 4 ) .

It is well known that grain boundary (gb) embrittlement, influenced by hydrogen atoms penetrating into the gb, depends on its microstructure. In order to study this influence on the crack growth behavior, investigations of the gb region were undertaken using TEM, SEM and EDS.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988585

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2. EXPERIMENTAL DETAILS

The m a t e r i a l used f o r t h i s study was a h i g h p u r i t y A1-4.5wt%Zn

-

1.25wt%Mg a l l o y . B i c r y s t a l s w i t h a s i n g l e gb i n t h e middle o f t h e specimen were prepared by a modi- f i e d s t r a i n annealing technique. The c y l i n d r i c a l b i c r y s t a l s were machined b spark

!!

e r o s i o n i n t o f l a t specimens, having a r e c t a n g u l a r cross-section o f 1.5 x 6 mm

.

Usually, the specimen axes were o r i e n t e d f o r s i n g l e s l i p i n one o f t h e two g r a i n s w i t h t h e broad s u r f a c e p a r a l l e l t o the Burgers v e c t o r o f t h e primary s l i p system. I n most cases, t h e gb plane was normal t o t h e l o a d i n g a x i s . The m i s o r i e n t a t i o n o f t h e two g r a i n s t y p i c a l l y was below

l o 0

as revealed by Laue p a t t e r n and by e t c h i n g tech- niques. The gb d i d n o t show any special symmetry.

A l l samples were homogenized a t 480°C f o r 30 min, quenched i n t o water and e l e c t r o l y - t i c a l l y polished. To i n i t i a t e cracks i n t o t h e gb, notches having a depth o f 500 Hm were spark machined i n t o one o f t h e small faces o f t h e specimens. Thereafter, t h e f i n a l heat treatment a t 135OC was made e i t h e r f o r 100 h l e a d i n g t o t h e peak-aged s t a t e (PA) o r f o r 1000 h l e a d i n g t o an overaged c o n d i t i o n (OA). The crack propaga- t i o n t e s t s under c y c l i c l o a d i n g c o n d i t i o n s (100 Hz) were c a r r i e d o u t using an e l e c - trodynamic system. The experimental r e s u l t s o f i n t e r - and t r a n s g r a n u l a r crack growth as a f u n c t i o n o f t h e water vapor pressure p and t h e aging c o n d i t i o n have been described and discussed i n a previous paper ( 5 ) .

For TEM i n v e r j t i g a t i o n s , undeformed PA and OA b i c r y s t a l s w i t h a t h i c k n e s s o f about 100 vm were spark c u t from t h i c k specimens. Thin f o i l s were prepared from these f l a t samples using a j e t p o l i s h i n g method i n order t o i n v e s t i g a t e t h e m i c r o s t r u c t u r e o f t h e gb region, i .e., p r e c i p i t a t e f r e e zones (PFZ) and t h e morphology o f gb p r e c i p i - t a t e s . TEM i n v e s t i g a t i o n s were a l s o c a r r i e d o u t on f a t i g u e d specimens and i t was p o s s i b l e t o observe gb crack t i p s i n b i c r y s t a l s f a t i g u e d i n wet n i t r o g e n and c r y s t a l l o g r a p h i c stage I crack t i p s i n s i n g l e c r y s t a l s .

The concentration p r o f i l e o f Zn was measured across t h e gb w i t h an a n a l y t i c a l TEM (JEOL 200 CX) u s i n g energy d i s p e r s i v e X-ray spectroscopy (EDS). For t h e a n a l y t i c a l EDS as w e l l as f o r TEM examination t h e gb planes were always p a r a l l e l t o t h e e l e c - t r o n beam. The c a l c u l a t i o n o f t h e Zn c o n c e n t r a t i o n was based on t h e method o f C l i f f and Lorimer ( 6 ) , a p p l y i n g t h e i r c o r r e c t i o n f u n c t i o n f o r absorption.

3. EXPERIMENTAL RESULTS

F i g . 1 shows a TEM micrograph which i s t y p i c a l f o r PA b i c r y s t a l s . Besides t h e b u l k which contains coherent r l ' - p r e c i p i t a t e s a PFZ was observed. I t s t o t a l w i d t h depended on t h e aging c o n d i t i o n and was measured on t h e average t o be 70 nm f o r t h e PA speci- mens (Table 1 ) . The examination o f the gb r e v e a l s t h e existence o f o b l a t e d gb p r e c i - p i t a t e s w i t h v i s i b l e dimensions o f 110 and 25 nm p a r a l l e l and normal t o t h e gb.

t a t e f r e e zone f o r an A1-Zn-Mg random g r a i n boundary i n an b i c r y s t a l . The PFZ c o n t a i n s i n - overaged A1-Zn-Mg b i c r y s t a l . coherent MgZn2 p r e c i p i t a t e s f o r Large, i n c o h e r e n t p a r t i c l e s a r e t h e peak-aged c o n d i t i o n . formed a t t h e g r a i n boundary.

I n c o n t r a s t t o t h e gb m i c r o s t r u c t u r e o f t h e PA specimen t h e m i c r o s t r u c t u r e o f t h e OA sample was q u i t e d i f f e r e n t , as seen i n F i g . 2. T h i s m i c r o s t r u c t u r e i n d i c a t e s t h a t t h e p r e c i p i t a t e s i n t h e b u l k a r e much coarser and n o t as f i n e l y d i s t r i b u t e d as those o f t h e PA specimens. The average w i d t h o f t h e PFZ i n OA b i c r y s t a l s was measured t o be 175 nm. I n a d d i t i o n , F i g . 2 shows t h e s t r o n g coarsening o f t h e s t a b l e gb p r e c i p i - t a t e s . These l a r g e i n c o h e r e n t p a r t i c l e s have an average diameter o f 226 nm and an average t h i c k n e s s o f 85 nm, and show a marked g l o b u l a r c h a r a c t e r comparable t o t h e o b l a t e PA gb p r e c i p i t a t e s .

I n PA b i c r y s t a l s t e s t e d i n wet n i t r o g e n t h e crack always propagates i n t e r g r a n u l a r l y . F i g . 3 shows a micrograph o f a t y p i c a l gb crack s u r f a c e o b t a i n e d f r o a PA specimen f a t i g u e d i n wet n i t r o g e n a t crack propagation r a t e s g r e a t e r than m/cyc]e. The c r a c k s u r f a c e i s covered w i t h i n c o h e r e n t gb p r e c i p i t a t e s ( w h i t e d o t s ) , s i m i l a r t o those seen i n TEM observation. For small crack r a t e s c l o s e t o t h e t h r e s h o l d r e g i o n no such c o n t r a s t c o u l d be observed.

For OA b i c r y s t a l s i t was impossible t o i n i t i a t e an i n t e r g r a n u l a r c r a c k i n wet atmo- spheres. Therefore, F i g . 4 shows t h e i n t e r g r a n u l a r crack s u r f a c e from a coarse p o l y - c r y s t a l l i n e specimen, which was i n t h e same OA c o n d i t i o n as t h e b i c r y s t a l s (135OC f o r 1000 h ) . I n p o l y c r y s t a l l i n e specimens t h e i n t e r g r a n u l a r crack r e p r e s e n t s l e s s t h a n 30 p e r c e n t o f t h e t o t a l c r a c k surfaces. The s i z e , shape and d i s t r i b u t i o n o f t h e s t a b l e p r e c i p i t a t e s a r e v i s i b l e i n F i g . 4. Furthermore, t h e i r volume f r a c t i o n i s n e a r l y 2.5 times g r e a t e r t h a n t h a t o f t h e PA specimens. Hence, i t f o l l o w s t h a t t h e d i s t a n c e between t h e c e n t e r s o f gb p r e c i p i t a t e s i n t h e OA specimens i s t h r e e t i m e s g r e a t e r than t h e d i s t a n c e between t h e c e n t e r s i n t h e PA specimens.

F i g . 3 SEM examination o f a g r a i n Fig. 4 SEM examination o f a g r a i n boundary crack p l a n e i n peak- boundary crack plane i n over- aoed A1-Zn-Mg b i c r y s t a l . aged A1 -Zn-Mg p o l y c r y s t a l

.

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The Zn concentration p r o f i l e s was measured across t h e gb i n PA and OA b i c r y s t a l s u s i n g t h e m i c r o a n a l y t i c a l technique o f EDS. F i g . 5 shows t h e Zn c o n c e n t r a t i o n nor- malized t o t h e mean Zn c o n c e n t r a t i o n i n t h e b u l k f o r PA (open c i r c l e s ) and OA (open squares) c o n d i t i o n s p l o t t e d a g a i n s t the d i s t a n c e from t h e gb. The Zn c o n c e n t r a t i o n i s always d e f i n e d as t h e l o c a l average o f t h e Zn content o f both p r e c i p i t a t e s and depleted m a t r i x . For t h e PA specimens t h e f i r s t d e v i a t i o n from t h e mean value occurs a t a d i s t a n c e d o f 350 nm where w i t h decreasing d an increase o f t h e Zn content occurs which reaches i t s maximum value o f 1.3 a t d = 60 nm which i s close t o t h e border o f the PFZ (35 nm). For s h o r t e r distances t h e Zn content drops down t o 0.4 and shows a small peak immediately a t t h e gb ( t h e Zn content o f t h e gb p r e c i p i t a t e i s n o t included.

The f u r t h e r aging a t t h e same temperature t o t h e OA s t a t e leads t o a broadening o f t h e concentration p r o f i l e and a r e d u c t i o n o f t h e maximum value which i s now l o c a t e d a t d = 250 nm which i s f a r o u t s i d e t h e PFZ ( d = 90 nm). The minimum Zn content w i t h - i n t h e PFZ i s much s m a l l e r compared t o t h a t o f the PA c o n d i t i o n (Table I ) . These c o n c e n t r a t i o n p r o f i l e s r e s u l t from d i f f u s i o n o f t h e Zn atoms t o t h e gb p r e c i p i t a t e s and o u t o f t h e PFZ i n t o t h e b u l k .

-4

PFZ(OL\)

800 600 LOO

distance from grain boundary (nm)

Fig. 5 Zn c o n c e n t r a t i o n measurements across a g r a i n boundary i n PA and OA specimens 4. DISCUSSION

The crack propagation experiments d u r i n g c y c l i n g o f A1-Zn-Mg b i c r y s t a l s i n wet atmospheres have been described elsewhere ( 5 ) and f o r t h e PA specimens t h e f o l l o w i n g r e s u l t s were obtained:

( i ) Only f o r p a r t i a l water vapor pressures p above 0.8 kPa i n t e r c r y s t a l l i n e crack propaga1:ion was obtained.

( i i ) Close t o t h e t h r e s h o l d range (da/dN

<

m/cycle) t h e necessary s t r e s s i n t e n s i t y range AK depends on p r o u g h l y as 1/Jp.

dependency on p. Compared t o i n t e r c r y s t a l l i n e stage I 1 cracks t h e r a t e i s l a r g e r by one o r d e r o f magnitude.

( i v ) For s t i l l l a r g e r propagation r a t e s o n l y t r a n s c r y s t a l l i n e cracks a r e p o s s i b l e . From these f a c t s ( i - i v ) i t i s concluded t h a t under c y c l i c l o a d i n g c o n d i t i o n s two mechanisms a r e necessary i n o r d e r f o r m o i s t u r e t o a f f e c t i n t e r g r a n u l a r cracking.

For low crack growth r a t e s ( i i ) near t h e t h r e s h o l d a mechanism should dominate which depends on t h e p a r t i a l water vapor pressure and which leads t o a crack propagating c l o s e t o t h e g r a i n boundary b u t n o t w i t h i n it. The l a t t e r f o l l o w s from t h e f a c t t h a t no gb p r e c i p i t a t e s c o u l d be found i n SEM micrographs o f t h e corresponding crack s u r - faces. It i s suggested t h a t t h i s c r a c k i n g i s c o n t r o l l e d by d i s s o l v e d hydrogen which has d i f f u s e d i n t o t h e gb and spread i n t o areas c l o s e t o it. The e m b r i t t l i n g phenom- enon l e a d i n g t o i n t e r g r a n u l a r c r a c k i n g i s r e f e r r e d t o c y c l i c SCC. Under monotonic l o a d i n g c o n d i t i o n s SCC i s a w e l l e s t a b l i s h e d mechanism and i s discussed by d i f f e r e n t authors. A f t e r Gruhl ( 7 ) t h e s e n s i v i t y t o SCC increases w i t h i n c r e a s i n g Zn content.

Since t h e z i n c atoms a r e i m p o r t a n t f o r SCC i t i s b e l i e v e d t h a t t h i s c o n c e n t r a t i o n i s h i g h e s t i n areas o f h i g h z i n c c o n c e n t r a t i o n , i . e . , a t t h e r i m o f t h e PFZ ( F i g . 5 ) . T h i s should r e s u l t i n a c r a c k a t t h i s r i m between PFZ and m a t r i x as observed by SEM.

For h i g h crack propagation r a t e s ( i i i ) t h e second mechanism governs i n t e r g r a n u l a r cracking, b u t i n t h i s case t h e crack propagation r a t e i s independent from t h e par- t i a l water vapor pressure p. T h i s mechanism i s b e l i e v e d t o be due t o hydrogen which may have been trapped a t t h e gb p r e c i p i t a t e s . T h i s e f f e c t w i l l be c a l l e d i n t e r g r a n u - l a r CF i n c o n t r a s t t o SCC.

A f t e r Wei ( 3 ) , t h r e e mechanisms a r e necessary i n o r d e r f o r m o i s t u r e t o a f f e c t c r a c k i n g . These a r e t r a n s p o r t a t i o n o f water vapor t o t h e i n t e r g r a n u l a r crack t i p , a d s o r p t i o n o f water vapor on t h e newly c r e a t e d c r a c k s u r f a c e d u r i n g each h a l f load- c y c l e and i t s d i s s o c i a t i o n . The l a t t e r s t e p generate hydrogen atoms p e n e t r a i t i n g t h e gb v i a mobile d i s l o c a t i o n s d u r i n g c y c l i n g u n t i l t h e y were trapped by gb p r e c i p i - t a t e s . Two arguments can support t h i s idea. The f i r s t argument i s given by Tuck (8) who claimed a p o s s i b l e f o r m a t i o n o f h y d r i d e s a t t h e gb p r e c i p i t a t e s . An a l t e r n a t i v e p o s s i b i l i t y i s g i v e n by Christodoulou and Flower ( 9 ) who suppose an immediate t r a p - p i n g o f hydrogen by t h e p r e c i p i t a t e s which can be d e t e c t e d by TEM as bubbles.

P r e l i m i n a r y i n v e s t i g a t i o n s showed t h i s type o f c o n t r a s t i n t h e corresponding b i c r y s t a l s.

Secondly, i t has been found e x p e r i m e n t a l l y t h a t a f a s t crack propagating i n wet n i t r o g e n continues t o propagate i n an i n t e r c r y s t a l l i n e manner i f t h e atmosphere has been changed t o a d r y one (8). Even an annealing a t 135OC does n o t change t h e i n t e r - g r a n u l a r crack extension. U s u a l l y , o n l y t r a n s c r y s t a l l i n e cracks do occur under t h i s c o n d i t i o n i f t h e crack was i n i t i a t e d i n d r y atmospheres. Only a f u l l s o l u t i o n heat t r e a t m e n t f o l l o w e d by quenching and re-aging l e d t o t r a n s c r y s t a l l i n e crack propaga- t i o n . From these f a c t s i t must be concluded t h a t a c o r r o s i v e agent must have been trapped i n t h e gb d u r i n g c y c l i n g .

For OA b i c r y s t a l s n e i t h e r SCC nor CF has been observed d u r i n g c y c l i n g . Even i n wet atmospheres ( p

>

3 . 1 KPa) t h e crack propagates i n an t r a n s g r a n u l a r manner. I n t h i s case, both t h e c o n c e n t r a t i o n o f Zn i n t h e PFZ and t h e Zn enrichment a t t h e r i m o f t h e PFZ decreases w i t h i n c r e a s i n g annealing t i m e ( F i g . 5 ) and t h e d i s s o l v e d hydrogen p e n e t r a t e s f u r t h e r i n t o t h e b u l k . Therefore, t h e gb l o s e s i t s s e n s i t i v i t y t o SCC and t h e c r a c k i n i t i a t i o n becomes t r a n s g r a n u l a r . A f u r t h e r d i s c u s s i o n o f a l l e f f e c t s w i l l be g i v e n i n a subsequent paper.

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Table 1 Comparison o f C h a r a c t e r i s t i c Values f o r PA and OA Specimens

g b - p r e c i p i tal;e c h a r a c t e r i z a t i o n

w i d t h , nm 110 f 40

h i g h t , nm 2 5 2 4

mean gb p a r t i c l e spacing, nm 300 f 100

minimum Zn conc. c/cmatrix i n PFZ

crack propagation i n t e r g r a n u l a r

behavior SCC and CF

t o t a l w i d t h o f PFZ, nm peak Zn-conc. c/cmatri, d i s t a n c e from gb, nm

t r a n s g r a n u l a r s t r o n g r e s i s t a n c e t o SCC

ACKNOWLEDGEMENT 70 f 10

1.3 60

The authors g r a t e f u l 1 y acknowledge t h e f i n a n c i a l support o f t h e Deutsche Forschungs- gemeinschaft. The authors a r e o b l i g e d t o Dr. W. Mader and Mr. E. Feuerstein f o r t h e i r a s s i s t a r ~ c e i n performing t h e EDS measurements.

175 f 20 1.1 270

REFERENCES

(1) R i c h t e r , J . and Kaesche, H., Werkstoffe und Korrosion, 32 (1981) 174.

(2) Speidel, M.O., "The Theory o f Stress Corrosion Cracking i n A l l o y s " . Published by: NATO S c i e n t i f i c A f f a i r s D i v i s i o n Brussels 1971, 289.

(3) Wei, R.P. and Simmons, G.W., I n t . J. F r a c t . , 17 (1981) 235.

(4) _{. .} Jacko, R.J., Duguette, D.J., Hydrogen E f f e c t s i n Metals, Ed. Bernstein, I.M., . .

~hompson, A.I;I., (i98i) 477.

(5) Niegel, A., Gudladt, H.-J. and Gerold, V., submitted t o Proc. FATIGUE 87, C h a r l o t t c ! s v i l l e , Va., USA, June 28

-

J u l y 3, 1987.

(6) C l i f f , G. and Lorimer, G.W., J. Microsc., 103 (1975) 203.

(7) Gruhl, W.,

Z.

f. Metallkde., 75 (1984) 819.

(8) Tuck, C.D.S., M e t a l l . Trans. A, 10A (1985) 1503.

(9) Christodoulou, L., and Flower, H.M., Hydrogen E f f e c t s i n Metals, Ed. Bernstein, I.M. Thompson, A.W., (1981) 493.