HYDROGEN EMBRITTLEMENT IN Al-Li-Cu-Mg

ALLOYS (8090-T651)

F. Binsfeld, M. Habashi, J. Galland, J. Fidelle, D. Miannay, P. Rofidal

To cite this version:

F. Binsfeld, M. Habashi, J. Galland, J. Fidelle, D. Miannay, et al.. HYDROGEN EMBRIT-TLEMENT IN Al-Li-Cu-Mg ALLOYS (8090-T651). Journal de Physique Colloques, 1987, 48(C3), pp.C3-587-C3-596. <10.1051/jphyscol:1987368>. <jpa-00226599>

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JOURNAL DE PHYSIQUE Colloque C3, suppl6ment au n09, Tome 48, septembre 1987

HYDROGEN EMBRITTLEMENT IN Al-Li-Cu-Mg ALLOYS (8090-T651)

F. BINSFELD, M. HABASHI, J. GALLAND, J.P. FIDELLE*, D. MIANNAY* and P. ROFIDAL*

Ecole Centrale des Arts et Manufactures, F-92295 Ch.Eitenay-Malabry Cedex, France *CEA, B.P. 511, F-75752 Paris Cedex 15, France

ABSTRACT

This paper describes the hydrogen embrittlement (HE) o f an A1-Li a l loy

aged a t 190°C and with d i f f e ren t durations o f ageing (10, 15, 20 and 30 hr).

Two techniques were employed t o measure HE : a) cathodic polar izat ion i n a molten sa l ts bath with -3 VIAg on tens i le specimens ; b ) gaseous hydrogenation

on disks. Hydrogen charging was achieved a t 190°C. The resul ts show that HE i s

important when the a1 1 oy i s i n the over-aged condition.

INTRODUCTION

A1-Li a l loys have gathered strong in teres t especial ly i n the a i r c r a f t

industry. Compared t o conventional high-strengh a1 umini um a1 loys o f the 2000

or 7000 series, i t i s known tha t A1-Li a l loys o f fe r a 10% increase i n Young's

modulus along wi th a 10% decrease i n speci f ic weight, thus making them rather

competitive t o new non-metal 1 i c materi a1 s 1 i ke carbon f i b r e reinforced '

composi tes. Moreover, t h e i r mechanical properties are equivalent t o those o f conventional high-strength A1 . a1 loys.

I n the binary Al-Li a l loy system, the 6 ' (A13Li) pa r t i c l es which

prec ip i ta te throughout the matrix are responsible fo r strengthening. I n the

A1-Li-Cu-Mg a l loys apart from the 6 I , semi coherent, S t (AlzCuMg) and TI

(A12CuLi) phases prec ip i ta te during a r t i f i c i a l ageing treatment. 6 ' i s a

metastable phase' which forms as spherical par t ic les which remain coherent wi th the matrix ( 1 ). During p las t i c deformation, these par t ic les may be cut by

moving dislocations such tha t fur ther deformation along the same s l i p plane i s

favoured ; s l i p becomes coplanar (2, 3) and leads t o poor toughness and

ducti 1 i ty. The s l i p cop1 anari t y gives ri se t o stress concentrations a t grain

boundaries i nduci ng intergranul ar f a i 1 ures (4). The growth ra te o f 6 depends

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987368

JOURNAL DE PHYSIQUE

on vacancy concen t ra t ion which i n t u r n i s a f f e c t e d by high s o l u t i o n t r ea tmen t

t empera tu re f o l 1 owed by c o l d water quenched (5 1. The s t r e n g t h enhancement

du r ing room temperature ageing appears t o be due t o t h e p r e c i p i t a t i o n o f 6' phase. Subsequent a r t i f i c i a l ageing g i v e s r i s e t o a g r a i n boundary REZ,

( P r e c i p i t a t i o n F ree Zone) i n d i c a t i n g t h a t $ p r e c i p i t a t i o n i s dependent upon

t h e presence of excess vacancies (5). The P.F.Z. grows according t o t 1 / 3 1 aw,

where t i s t h e ageing t ime a t a given ageing temperature ( 6 ) . Attempts t o

improve toughness o f A1-Li a1 loys have been -based on t h e change i n g r a i n

s t r u c t u r e t o i n f l u e n c e t h e f r a c t u r e process. Addi t ion of manganese i s known a s

a g r a i n - r e f i n e r b u t a s an improvement t o g r o s s i n t e r g r a n u l a r f a i l u r e (7).

Zirconium add i t ion of l e s s than 0.2% forms p ' (A13Zr) p r e c i p i t a t i o n which

g i ves r i s e to g r a i n--ref i nement, i n h i b i t i o n of r e c r y s t a l 1 i s a t i o n and decreas ing

shea red 6 ' p a r t i c l e s by t h e moving d i s l o c a t i o n s (8). However, zirconium g i v e s

h igh ly t e x t u r e d s t r u c t u r e s ( 5 ) i n A1-Li a l loys .

The Cu and Mg p r e c i p i t a t e heterogeneous1 y dur ing a r t i f i c i a l ageing as S '

(A1 2CuMg) and TI (A1 2CuLi ) . The nuc lea t ion sites of t h e s e p r e c i p i t a t e s a r e

d i s l o c a t i o n s formed dur ing quenching, d i s l o c a t i o n s in t roduced dur ing

s t r e t c h i n g and low-angle subgrain boundaries. Grain boundary p r e c i p i t a t i o n of

6 (AlLi ) and complex A1-CU-Mg phases occur dur ing a r t i f i c i a l ageing ( 9 ) . a

s t r e t c h be fo re a r t i f i c i a l ageing i n c r e a s e s t h e r a t e of ageing and then

i n c r e a s e s t h e y i e l d s t r e n g t h and t h e u l t ima te t e n s i l e s t r e n g t h b u t reduces

t h e e longa t ion t o f r a c t u r e . For each chemical composition of A1-Li a l l o y ,

t h e r e i s a s p e c i f i c ageing hea t t r ea tmen t cond i t ion (peak aged c o n d i t i o n ) a t

which t h e mechanical p r o p e r t i e s a r e optimum. In t h e under-aged cond i t ion , t h e

deformation i s l o c a l i z e d w i t h i n s l i p bands whi le a t t h e peak c o n d i t i o n t h e

deformation i s homogeneous and due to t h e increased volume f r a c t i o n of S 1

throughout t h e matrix. In t h e overaged cond i t ion , t h e p r e c i p i t a t i o n of 'S' is a t once wi th in t h e matr ix and i n t h e sub-grain boundaries (9) .

On t h e o t h e r hand, conventional h igh-s t rength aluminium a l l o y s such as A1-Zn-Mg, Al-Zn-Mg-Cu and A1-Mg s u f f e r a r e v e r s i b l e hydrogen embri t t lement

whereas A1-Cu and al-Cu-Mg appear to be r e s i s t a n t to hydrogen in t roduced

e i t h e r by ca thod ic charging o r by exposure to water con ta in ing environments

(10).

To t h e b e s t of our knowledge, A1 -Li-Mg-Cu-Zr (8090 a l l o y ) has not been

s t u d i e d so f a r . In t h i s paper, hydrogen embri t t lement of 8090 - T651 was then

inves t iga t ed .

EXPERIMENTAL

The chemical composition and t h e th ickness t of t h e a l l o y s s t u d i e d i n

t h i s i n v e s t i g a t i o n a r e given i n t a b l e I .

Table I : Chemical composition and th ickness of t h e a l l o y s s tud ied (wt%) . ................................................................

I Li I Mg I Cu I Zr I S i I Ti I Fe INa,ppmlt ,mml

1 2.7 1 1.1 1 1 .3 1 0.09 1 0.02 1 0.02 1 0.02 1 3 1 0 .8 1 1 2.9 1 1.1 1 1 .3 1 0.09 1 0.02 1 0.02 1 0.02 1 3 1 1 .6 1

The experimental ma te r i a l was suppl i ed by CEGEDUR PECHINEY i n t h e form of

0.8 and 1.6 nun r o l l e d p l a t e i n t h e s o l u t i o n t r e a t e d (535OC dur ing I h r and then

co ld water quenched) and 3% s t r e t c h e d , condi t ion T-351. Hydrogen charging was

achieved by two methods :

1 ) Cathodical ly on t e n s i l e specimens having 1.6 mmthickness i n a molten

s a l t s bath (1 1 ) a t 190°C dur ing 10, 15, 20 and 30 h r and a t d i f f e r e n t cathodic

p o t e n t i a l s : -1 .5, -2.0, -2.5 and -3.0 V/Ag. Af te r hydrogenation, outgasing

was achieved, and QH was measured a t 520°C and 1000°C. Uncharged and hydrogen

c a t h o d i c a l l y charged specimens were t e n s i l e t e s t e d a t room temperature , us ing

a s t r a i n r a t e o f 2 . 7 . 1 0 - ~ s - ~ t o s tudy t h e e f f e c t of hydrogen charging on

t e n s i l e p roper t i e s . Reference specimens were those aged a t 1 90°C i n fu rnace

dur ing 10, 15, 20 and 30 hr .

2 ) Gaseous hydrogen on d i sks having 0 .8 mm th ickness . The d e t a i l s of t h i s

method were descr ibed elsewhere (12). The re fe rence gaz was helium. The

f o l l owing procedures were achieved :

a ) inc reas ing t h e r a t e of hydrogen pressure A P/ A t from 0.007 t o 188 -4

M%hi&&ternal hydrogen). The a l l o y was i n T651 cond i t ion ;

b) thermal hydrogen o r helium charging on both s i d e s a t 1 90°C, dur ing

20 h r . Gas p ressu re was 400 Pa ( i n t e r n a l hydrogen) ;

c ) c a t h o d i c a l l y hydrogen charging a t 190°C dur ing 20 h r and with

-3V/Ag ( i n t e rna l hydrogen).

Tn t h e two l a s t cond i t ions e i t h e r he1 ium ( b ) or/and hydrogen gas ( c ) were usedtochieve f a i l u r e .

All t h e f a i l u r e s were c a r r i e d o u t a t 20°C. F rac tu re su r faces were examined using a scanning e l e c t r o n microscope.

JOURNAL DE PHYSIQUE

EXPERIMENTAL RESULTS

1 ) Cathod ica l l y hydrogen charging

F igure 1 shows t h a t t h e outgased QH i s an inc reas ing f u n c t i o n as t h e

a p p l i e d c a t h o d i c p o t e n t i a l l e v e l and t h e t ime of hydrogen charging a r e

inc reased whatever t h e outgas ing temperature i s 520°C o r i s 1 000°C. However,

QH measured a t 1000°C i s h igher than t h a t obta ined a t 520°C showing t h a t

molecular hydrogen t r app ing occurs and i n c r e a s e s with t h e s e v e r i t y o f hydrogen

cha rg ing cond i t ion . QH p r i o r t o hydrogen charging i s measured a t 520°C t o be

about 5 ppm f o r t = 1.6 m and about 1 0 ppm f o r t = 0.8 nun, and i s due t o quenching t h e a l l o y from 535°C i n cold water.

F igu re 2 shows t h e v a r i a t i o n of t h e y i e l d s t r e n g t h Re a s a func t ion of

hydrogen charging t ime i n t h e molten s a l t s bath a t f r e e p o t e n t i a l and a t

-3V/Ag, and a s a func t ion of ageing time when t h e ageing t r ea tmen t i s achieved

i n t h e furnace with o r wi thout vacuum. The y i e l d s t r eng th is independent on

t h e ageing t i m e and on t h e ca thod ic p o t e n t i a l level and dxceeds t h o s e due t o ageing h e a t t r ea tmen t i n fu rnace dur ing va r ious t imes ; t h e i r Re ach ieves a

maximum f o r 15 h r ageing.

Maximum t e n s i l e s t r e n g t h Rm v a r i a t i o n a s a func t ion of ageing t ime i s

very s e n s i t i v e t o t h e ageing h e a t t r ea tmen t cond i t ion , f i g u r e 3 . However, Rm va lues measured i n t h e molten s a l t s bath a r e higher than t h a t ob ta ined when

us ing t h e furnace t o achieve t h e ageing hea t t rea tment . Independently o f t h e

h e a t t r e a t m e n t c o n d i t i o n , t h e maximum stress Rm i s a l s o maximum f o r 1 5 h r

ageing.

D u c t i l i t y l o s s F % due t o hydrogen charging is about 20% f o r ageing

t ime varying f r o m l O t o 2 0 h r , f i g u r e 4 . % d r a s t i c a l l y i n c r e a s e s u p t o

75% f o r a 30 h r ageing time.

2 ) Gaseous hydrogen charging

For a1 1 test cond i t ions , t h e d i sks c e n t r a l p a r t (under p res su re ) breaks

i n t o numerous 1 i t t l e p i eces , f i g u r e 5 , i n d i c a t i n g t h e mater ia l b r i t t l e

cond i t ion . we have t o remember t h a t t h e r e t a i n e d QH p r i o r gaseous hydrogen

charging was measured t o be about 10 ppm. P a r t l y f o r t h i s reason and c e r t a i n l y

due t o t h e e f f e c t i v e n e s s of s u r f a c e oxides , no s i g n i f i c a n t e m b r i t t l i n g e f f e c t

o f hydrogen could be evidenced, f i g u r e 6 .

DISCUSSION

The results show that we have introduced high concentrations of hydrogen

in A1 -Li alloy by using the molten sal ts b a t h technique a t the optimum ageing

temperature. To the best of our knowledge, no attempt has been done to measure

the hydrogen diffusion coefficient DH in A1-Li alloys a t 190°C. However,

NAKASHIMA and coll. (13) have measured DH in binary A1-Li system as a function

of Li percentage and of temperature. Their results show great scattering and

DH i s estimated as about 1 o - ~ cm2. s-1 in the temperature range 1 80 t o 480°C,

i ndependentl y of Li%. I n our conditions, hydrogen may diffuse a t a distance d

from the surface of the metal equal to about 85 t o 150 pm for charging time

vary'ing from 10 t o 30 h r respectively and according t o the relation :

d 5 . That i s t o say hydrogen may diffuse up t o or behind lithium

depletion zone (14) (S 100 pm), which i s far beyond the oxide thickness (032 pm) (14).

As mentionned above, S1 phase i s initiated on dislocations sites. Knowing

that internal hydrogen promotes dislocations density (1 5, 16), the probabil i ty

of S' formation i s then more important in the presence of hydrogen than

without hydrogen. Tensile stresses are increased when the A1 -Li a1 loy i s

cathodical ly hydrogen charged. This increase does depend on hydrogen

concentration, namely the polarisqtion time a t 190°C. Table I1 gives the

percentage increase in tensile properties due t o hydrogen charging for two

durations 15 and 30 h r . The reference alloy i s taken as t h a t heat treated in

furnace under vacuum.

Table I I : Relative variation in the mechanical properties due to time of

hydrogen charging (ageing time) a t -3.0 V/Ag. ....................................... I t, hr I A Re% I A R ~ % I F%* I 1----_--1__---_--_1---------I--------- I 1 1 5 1 6 1 5 1 2 3 1 \ 30 1 115 1 19 1 75 1

The effect of internal hydrogen on the mechanical properties i s very

important when the alloy i s in an overaged condition (190°C, 30 hr).klithout

internal hydrogen and ageing heat treatment, the fai 1 ure surfaces show ductile

rupture with f l a t surfaces containing s l ip lines, figure 7a. Figure 7b shows that when the alloy i s aged heat treatment a t 190°C during 30 hr in the

C3-592 JOURNAL DE PHYSIQUE

fu rnace under vacuum, a mixed r u p t u r e t y p e occurs . Deep c racks i n t h e

sub-gra in boundar ies and many t e a r i n g s throughout t h e m a t r i x a r e observed.

Th i s r e s u l t p o i n t s o u t t h a t coa r sed 6' p a r t i c l e s ( 8 ) and S ' phase wi th in t h e

ma t r ix ( 9 ) and a l s o , S ' ( ~ 1 2CuMg) ( 9 ) , T2 (A12Cu~i ) ( 1 7) and A16(Fe, Cu) (18 )

p a r t i c l e s may be p r e s e n t i n t h e sub-grain boundaries. With i n t e r n a l hydrogen

and a t 500 pm from t h e s u r f a c e , t h e r u p t u r e t ype changes from i n t e r g r a n u l a r

(10 h r ) t o mixed ( i n t e r + t r a n s c r i s t a l l i n e ) (1 5 and 20 h r ) and f i n a l l y becomes

i n t e r g r a n u l a r (30 h r ) , f i g u r e 8. However, a t 100 pm from t h e s u r f a c e , t h e

f a i l u r e h a s an i n t e r g r a n u l a r f e a t u r e , independent ly o f t ime of ageing. The

l a s t r e s u l t s prove t h a t hydrogen has d i f f u s e d and e m b r i t t l e d t h e metal a t a

d i s t a n c e equal t o o r g r e a t e r than 100 pm from t h e m e t a l l i c su r f ace . The

s u r f a c e ox ides i s t hen overcome du r ing c a t h o d i c charging i n molten s a l t s ba th .

T h i s i s not t h e c a s e whi le gaseous hydrogen charging. However t h e r e s t r i c t e d

number o f d i s k s l e f t a v a i l a b l e d i d no t al low t h e necessary i n v e s t i g a t i o n o f

t h e p r e s s u r e i n c r e a s e r a t e i n f luence . The re fo re , t h e s c a r c e tests on d i s k s

hydrogen charged by t h i s t echn ique a r e i nconc lus ive but ought t o b e resumed

more c o n s i s t e n t l y , e s p e c i a l 1 y w i th t h i c k e r d i sks .

CONCLUSION

I n t h i s work we have i n v e s t i g a t e d t h e hydrogen ernbr i t t lement o f an A1-Li

a1 loy (8090). The r e s u l t s ob ta ined lead t o t h e fo l lowing conc lus ions :

1 ) Using t h e molten s a l t s ba th t echn ique and c a t h o d i c a l l y hydrogen

charged t h e 8090 - T351 a1 loy a t t h e peak tempera ture (1 90°C). The outgased

q u a n t i t y of hydrogen i s an i n c r e a s i n g f u n c t i o n as t h e t ime of cha rg ing ( age ing

t ime) and c a t h o d i c p o t e n t i a l l eve l i nc rease . Molecular hydrogen t r app ing i s

po in t ed out.

2) Hydrogen may promote d i s l o c a t i o n s d e n s i t y which a r e t h e i n i t i a t i o n

s i t e s o f S ' phase. T e n s i l e stresses and d u c t i l i t y l o s s a r e t hen increased. The

e f f e c t of hydrogen i s impor tant when t h e a l l o y i s i n t h e overaged cond i t i on .

3 ) Hydrogen promotes a l s o i n t e r g r a n u l a r f a i l u r e , e s p e c i a l 1 y i n t h e

overaged cond i t i on . P r e c i p i t a t i o n o f S ' , T2 and A16(Fe, Cu) p a r t i c i e s i n t h e

sub-gra in boundar ies may i n t e r a c t wi th hydrogen atoms and t h e embri t t l e m e n t i s favoured.

REFERENCES

( 1 ) 5. NOBLE, G.E. THOMSON - Metal S c i . J. 5, 114, (1 971).

( 2 ) T.H. SANDERS, E.A. STARKE - Acta Meta l l . 30, 927, (1982).

(3 ) 8. NgBLE, S.J. HARRIS, K . DINSDALE - Metal S c i . J . 16, 425, (1982).

(4) T.H. SANDERS - Proc. 1s t I n t . A1-Li Conf., p. 63, Meta l l . Soc. AIME,

(1981).

(5) H.M. FLOWER, P.J. GREGSON, C.N.J. TITE, A.K. MUKHOPADHYAY - P ~ o c .

Al-A1 loys, t h e i r physical and mechanical proper t ies - Ed i t o r s E.A. STARKE

J r and T.H. SANDERS Jr , Un ivers i t y o f V i r g i n i a , Char lo t tesv i l l e , USA,

June (1986), p . 743.

(6) I.M. LIFSHITZ, V.V. SLYOZOV - J. Phys. Chem. Sol ids, 19, 35, (1961).

(7) K. DINSDALE, S.J. HARRIS, B. NOBLE - I b i d . 4, p. 101.

(8) P.J.E. BISCHLER, J.W. MARTIN - Ib id . 5, p. 963.

(9) S.J. HARRIS, B. NOBLE, K. DINSDALE, M. PRIDHAM - Ib i d . 5, p. 755.

(10) G.M. SCAMANS - Proc. Hydrogen E f f ec t s i n Metals, ed i ted by I.M. BERNSTEIN

and A.W. THOMPSON, The Meta l l . Soc. o f AIME, Carnegie-Mellon Univers i ty ,

PIH, PENNSYLVANIA (1980), p. 467.

(1 1 ) A. ELKHOLY, J. GALLAND, P. AZOU, P. BASTIEN - C.R. Acad. Sci., 284, s6 r i e

C, (1977), p. 363.

(12) J.P. FIDELLE - "Hydrogen embrittlement t es t i ng " ASTM STP 543, 31 (1974),

p. 243.

(13) M. NAKASHIMA, M. SAEKI, Y. ARATONO, E. TACHIKAWA - Journal o f Nuclear

Mater ia ls , 116, (1983), p. 141.

(14) F. DEEREVE, Ph. MEYER, N. THORNE - Pertes en 1 i th ium des a l l i ages

A1-Li-X, Rapport in te rne de PECHINEY, mars (1 986).

(15) J. EASTMAN, T. MATSUMOTO, N. NARITA, F. HEUBAUM, H.K. BIRNBAUM - Conf.

Proc. on "Hydrogen i n Metals" Edi ted by I. M. BERNSTEIN, A.W. THOMPSON

Carnegie - Me1 l o n Un ivers i t y , P i t tsburgh, Pennsylvania, U.S.A. (1980),

p. 397.

(16) K.S. SHIN, C.G. PARK, M. MESH11 - I b i d 15, p.209.

( I T ) F.S. LIN, S.B. CHAKRABORTTY, E.A. STARKE J r - Met. Trans. 13A (1982),

p. 401.

(18) N.J. OWEN, D.J. FIELD, E.P. BUTLER - Ib id . 4, p. 1217.

JOURNAL DE PHYSIQUE

Figure 1 : Variat ion of Q desorded a t B 5 2 0 ' ~ and 1000 C as a funct ion of cathodic p o t e n t i a l l eve l and of hydrogen charging (ageing time) ; hydrogen charging temperature = 19O0C.

0 1151 (0lDITlOl

DLIW sun ur* s I, 11w~cl + mna sun urn AT -1 114 (1W.O . N U T S "1- .IW 1I)O.O 0 ,mu([ "IT"ILY(IWC1

I , h r Figure 3 : Variat ion of maximum

t r u e s t r e s s a s a funct ion of e i t h e r t h e hydrogen charging time (with and without cathodic po la r iza t ion) o r ageing time : peak temperature = 190°C.

t . h r Figure 2 : Variat ion of y i e l d

s t reng th as a funct ion of e i t h e r the hydrogen charging time (with and without cathodic po la r iza t ion) o r ageing time ; peak temperature = 190°C.

Figure 4 : Variat ion of maximum s t r a i n a s a funct ion of e i t h e r the hydrogen charging time (with and without cathodic po la r iza t ion) o r ageing time ; peak temperature = 190°C.

Figure 5 : View of a d i sk f a i l u r e by inc reas ing hydrogen gas- 1 pressure a t 0.1 MPa.min .

1

Figure 6 : Disk f a i l u r e pressure of hydrogen o r helium gas versus the f a t e of inc reas ing of the gas and of p r i o r i n t e r n a l hydrogen charging : ist s e r i e s corresponds t o n a t u r a l ageing u n t i l 1985, 2nd s e r i e s corresponds t o n a t u r a l ageing u n t i l 1986.

Figure 7a : Surface f a i l u r e by t e n s i l e Figure 7b : Surface failure by tensile t e s t a t room temperature of t e s t a t room temperature of 8090-~351 a l l o y . 8090 a l l o y aged a t 190°C

during 30 hr .

JOURNAL DE PHYSIQUE

H CHARGING T I M E ( - 3 V / A g , 1 9 0 ° C )

Figure 8 : Var ia t ion of the type of rup tu re i n 8090 a l l o y c a t h o d i c a l l y hydrogenated (-3V/Ag) a s a func t ion of

lo hr

time of hydrogenat ion and the depth from the specimen su r face ; peak temperature = 190°c.