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CORRELATION BETWEEN AGEING HEAT TREATMENTS, MICROSTRUCTURE AND STRESS CORROSION PROPERTIES OF Al-Li-Cu-Mg ALLOYS

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Submitted on 1 Jan 1987

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CORRELATION BETWEEN AGEING HEAT

TREATMENTS, MICROSTRUCTURE AND STRESS CORROSION PROPERTIES OF Al-Li-Cu-Mg ALLOYS

Mumtaz Ahmad

To cite this version:

Mumtaz Ahmad. CORRELATION BETWEEN AGEING HEAT TREATMENTS, MICROSTRUC-

TURE AND STRESS CORROSION PROPERTIES OF Al-Li-Cu-Mg ALLOYS. Journal de Physique

Colloques, 1987, 48 (C3), pp.C3-871-C3-879. �10.1051/jphyscol:19873102�. �jpa-00226536�

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JOURNAL DE PHYSIQUE

C o l l o q u e C3, s u p p l 6 m e n t a u n 0 9 , Tome 48, s e p t e m b r e 1 9 8 7

CORRELATION BETWEEN AGEING HEAT TREATMENTS, MICROSTRUCTURE AND STRESS CORROSION PROPERTIES OF Al-Li-Cu-Mg ALLOYS

M. AHMAD

D e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g , L i n k o p i n g I n s t i t u t e

o f

T e c h n o l o g y , S - 5 8 1 8 3 , L i n k o p i n g , Sweden

Abstract

St.ress c o r r o s i o n cracking (SCC) behaviour of 8090 a l l o y (Al-Li-Cu-Mg) has been s t u d i e d u s i n g precracked double c a n t i l e v e r beam (DCB) specimens and tuning f o r k type smooth t e s t specimens. The a l l o y was SCC t e s t e d i n s e v e r a l ageing

c o n d i t i o n s . Although s t r e s s c o r r o s i o n cracks r e a d i l y i n i t i a t e when t h e a l l o y i s exposed t o

3.5

% NaCl environment, crack growth i s d i f f i c u l t . The SCC

performance of t h e 8090 a l l o y has been compared with t h a t of 7075-T651 a l l o y . F r a c t u r e toughness of t h e a l l o y i n a v a r i e t y of ageing h e a t treatment

c o n d i t i o n s was determined by u s i n g chevron-notched s h o r t bar specimens. Ageing response of t h e a l l o y has been determined by hardness measurements and

e l e c t r i c a l c o n d u c t i v i t y measurements.

I n t r o d u c t i o n

S t r e s s c o r r o s i o n crack growth i n aluminium a l l o y s i s s t r o n g l y a f f e c t e d by composition and h e a t treatment (1). I n 7000 s e r i e s a l l o y s , underaged and peak- aged tempers a r e h i g h l y s u s c e p t i b l e t o SCC, while overageing produces s t r e s s c o r r o s i o n r e s i s t a n t a l l o y s (1).

Recently, SCC of lithium-containing aluminium a l l o y s have been t h e s u b j e c t of number of papers (2-9). Work by Rinker e t a 1 ( 5 ) on a l l o y 2020 have shown t h a t ageing t h e a l l o y from t h e underaged t o t h e peak-aged c o n d i t i o n r e s u l t s i n s i g n i f i c a n t r e d u c t i o n s i n t h e p l a t e a u v e l o c i t y . T h i s change i n SCC behaviour was a t t r i b u t e d t o t h e p o t e n t i a l d i f f e r e n c e between g r a i n boundary T

p r e c i p i t a t e s and t h e g r a i n i n t e r i o r . Christodoulou and co-workers

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s t u d i e d SCC of A 1 - L i b i n a r y a l l o y s . It was suggested t h a t hydrogen embrittlement may play a r o l e i n t h e SCC mechanism. Holroyd e t a 1 ( 6 ) i n v e s t i g a t e d t h e SCC behaviour of binary A1-Li, t e r n a r y A l - L i - Z r and q u a r t e r n a r y Al-Li-Cu-Mg a l l o y s . Various mechanisms p o s s i b l y involved i n t h e s t r e s s c o r r o s i o n cracking of A 1 - L i a l l o y s were discussed.

The o b j e c t i v e of t h e p r e s e n t work was t o study t h e e f f e c t s of complex ageing h e a t treatments on t h e f r a c t u r e toughness and t h e s t r e s s c o r r o s i o n r e s i s t a n c e o f t h e 8090 a l l o y .

M a t e r i a l s and experimental procedures

A 44 mm t h i c k 8090 p l a t e produced by B r i t i s h Alcan Aluminium was obtained. The as-received p l a t e was i n t h e peak aged c o n d i t i o n i . e . aged f o r 16 hours a t 1 9 0 ~ ~ . The chemical composition of t h e a l l o y p l a t e i s given i n t a b l e 1.

Table 1

Chemical composition of 8090 a l l o y p l a t e (by weight % )

L i Cu Mg Z r Fe S i A 1

2.36 1.08 0.67 0 . 1 1 0.05 0.03 balance

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19873102

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C3-872 JOURNAL DE PHYSIQUE

The plate was sawn along the rolling direction into bars with cross-section dimension of 44 mm x 28 mm. These bars were solution heat-treated in an air furnace for 1 hour at 530°c, quenched in room temperature water, stretched 2.5

%, and aged.

Fracture toughness tests were conducted on chevron-notched short bar specimens machined from the mid-thickness of plate. The geometry of short bar fracture toughness specimens has been described by Barker (10). The short bar specimens used in the present study were with a thickness, B of 25.4 mm. They were loaded in a tensile testing machine with the help of suitable grips. A clip gage was mounted on the grips. The load and the load point opening displacement were '

recorded on an X-Y plotter. This test procedure has been shown to provide toughness values for aluminium alloys that are closely related with the plane- strain fracture toughness K measured by the ASTM E 399 test method for toughness below

35

~ ~ a ] m (117. Fracture toughness data are based on the average of two tests.

The short transverse stress corrosion resistance of the alloy was studied using smooth tuning fork type specimens (Fig. la) and precracked double cantilever beam (DCB) specimens (Fig.lb). Tuning fork specimens were alternately immersed in a neutral 3.5 % sodium chloride solution. Alternate immersion cycle was

1 2

/ I I

Dimensions in mm

Fig. 1 Geometry of tuning fork (a) and double cantilever beam (b) specimens for SCC testing.

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accomplished s u c h t h a t t h e specimens were soaked i n t h e s o l u t i o n f o r 10 minutes followed by a 50 minutes d r y p e r i o d i n a i r . The specimens were i n s p e c t e d e v e r y second o r e v e r y t h i r d day w i t h a b i n o c u l a r a t about 20x m a g n i f i c a t i o n . A specimen was c o n s i d e r e d f a i l e d i f a c r a c k was e v i d e n t a t 20 times

m a g n i f i c a t i o n . A l l t h e t u n i n g f o r k specimens were i n i t i a l l y loaded t o a maximum s t r e s s o f 120 MPa with t h e h e l p of s t e e l b o l t s and a s t r a i n gage.

S t r e s s c o r r o s i o n c r a c k p r o p a g a t i o n r e s i s t a n c e o f t h e a l l o y i n t h e v a r i o u s a g e i n g tempers was e v a l u a t e d u s i n g DCB specimens. The s i z e o f t h e DCB specimens was chosen i n accordance w i t h recommendations made i n r e f . ( 1 2 ) . The p r e s e n t d e s i g n does n o t s i g n i f i c a n t l y d i f f e r from t h a t o f H y a t t ( 1 3 ) . Specimens were f a t i g u e p r e c r a c k e d a t 10 Hz and t h e maximum s t r e s s i n t e n s i t y was l e s s t h a n 80 p e r c e n t o f K F a t i g u e c y c l i n g continued u n t i l a c r a c k had grown 2 t o 2.5 mm beyond t h e 96C;legree chevron notch. A c l i p gage was a t t a c h e d and t h e specimens were b o l t loaded t o pre-determined s t a r t i n g s t r e s s i n t e n s i t y l e v e l s . Loaded specimens were placed v e r t i c a l with t h e notched end up i n a c l i m a t i s e d room, and a few d r o p s of 3.5 % NaCl s o l u t i o n were added t o t h e n o t c h once a hour with t h e h e l p o f a squeeze pump and p l a s t i c t u b e s . Crack l e n g t h s were measured o p t i c a l l y (500X m a g n i f i c a t i o n ) with t h e h e l p of a t r a v e l l i n g microscope on both s i d e s of t h e specimen and t h e measurements averaged.

The f o l l o w i n g e q u a t i o n was used t o c a l c u l a t e t h e stress i n t e n s i t y KI ( 1 2 ) :

where E = modulus o f e l a s t i c i t y , 79 GPa f o r 8090 a l l o y and 7 1 . 1 GPa f o r 7075-T651 a l l o y .

2V = d e f l e c t i o n of specimen arms a t t h e l o a d l i n e . H = 1 / Z specimen h e i g h t .

a = c r a c k l e n g t h measured from t h e l o a d l i n e . R e s u l t s and Discussion

Fracture toughness

The v a r i o u s a g e i n g t r e a t m e n t s included i n t h e i n v e s t i g a t i o n and corresponding h a r d n e s s and e l e c t r i c a l c o n d u c t i v i t y v a l u e s a r e summarized i n t a b l e 2. F r a c t u r e toughness d a t a o f t h e a l l o y i n d i f f e r e n t a g e i n g c o n d i t i o n s a r e a l s o shown i n t a b l e 2. The r e s u l t s show t h a t f r a c t u r e toughness d e c r e a s e s w i t h ageing. T h i s

Table 2

R e s u l t s o f f r a c t u r e toughness t e s t s Specimen Ageing o f

Number 8090 Alloy

Vickers E l e c t r i c a l KSB ( a ) Hardness C o n d u c t i v i t y ( M P ~ J m)

(10 KP) ( % IACS)

4h/190°c 130.0

16h/190°c 142.9

80h/190°c 139.3

190h/130°c 133.1

5 0 h / 1 3 0 ~ ~ + 1 6 h / l 9 0 ~ ~

137.1

4 m i n / 2 5 0 ~ ~ + 1 6 h / l 9 0 ~ ~ 141.7 a s r e c e i v e d (16h/190°c) 142.5 a s r e c e i v e d ( 1 6 h / 1 9 0 ~ ~ )

-

7075-T651

-

( a ) F r a c t u r e toughness t e s t o r i e n t a t i o n was S-L, e x c e p t t h e No 8 a (T-L).

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C3-874 JOIIRNAL DE PHYSIQUE

i s t y p i c a l f o r many o t h e r aluminium a l l o y s w i t h o r w i t h o u t l i t h i u m (14-17). I n a d d i t i o n i t i s s e e n t h a t a s l i g h t improvement i n f r a c t u r e toughness of t h e a l l o y can be achieved by a g e i n g a t lower temperatures. This i s c o n s i s t e n t with t h e r e s u l t s of C u r t i s e t a 1 ( 1 8 ) . Ashton e t a 1 (19) and P e e l e t a 1 ( 2 0 ) . Two d i f f e r e n t double a g e i n g t r e a t m e n t s (No. 6 and No. 7 ) i n c l u d e d i n t h e

i n v e s t i g a t i o n d i d n o t show any s i g n i f i c a n t improvement of t h e f r a c t u r e toughness o f t h e a l l o y a t t h e peak s t r e n g t h l e v e l . The a l l o y specimen r e - s o l u t i o n t r e a t e d and aged t o t h e peak s t r e n g t h (No. 2 ) showed somewhat h i g h e r f r a c t u r e toughness compared t o t h e a s - r e c e i v e d c o n d i t i o n (No. 8 ) .

S t r e s s c o r r o s i o n c r a c k i n i t i a t i o n

The s u s c e p t i b i l i t y t o s t r e s s corrosi.on c r a c k i n i t i a t i o n was e v a l u a t e d u s i n g smooth specimens o f t u n i n g f o r k t y p i i , 'l'lle t e s t r e s u l t s a r e p r e s e n t e d i n t a b l e

3.

A s can b e s e e n , most of t h e Al-1.i specimens were f a i l e d w i t h i n s i x days of t e s t i n g . A l l o t h e specimens o f t h e 7075 a l l o y and o f t h e 8090 a l l o y aged f o r 190 h a t 130 C s u r v i v e d 29 days a l t e r n a t e immersion i n 3 . 5 NaCl s o l u t i o n . The s u r v i v i n g 7075 a l l o y was s e v e r e l y p i t t e d . However, t h e s u r f a c e of aluminium- l i t h i u m a l l o y was c l e a n and showed very l i t t l e g e n e r a l c o r r o s i o n . The A 1 - L i a l l o y aged 4h/190°C had t h e l o w e s t r e s i s t a n c e t o s t r e s s c o r r o s i o n c r a c k

i n i a t i o n . The overaged temper (80h/190°c) showed somewhat g r e a t e r r e s i s t a n c e t o SCC i n i t i a t i o n t h a n t h e underaged (4h/190°c) and t h e peak-aged ( 1 6 h / 1 9 0 ~ ~ ) temper. However, t h e r e was a g r e a t d e a l o f s p r e a d i n t h e time t o f a i l u r e d a t a of t h e overaged temper. P r e v i o u s l y Reynolds and co-workers (21) r e p o r t e d t h a t t h e p e r i o d t o c r a c k i n i a t i o n o f t h e 8090 a l l o y can be s i g n i f i c a n t l y i n c r e a s e d by overageing.

Table 3

R e s i s t a n c e t o s t r e s s - c o r r o s i o n c r a c k i n g o f 8090 and 7075-T651 a l l o y : Tuning f o r k smooth t e s t specimens exposed t o 3 . 5 % NaCl a l t e n a t e immersion,

s h o r t t r a n s v e r s e s t r e s s 120 MPa.

Specimen number

Ageing t r e a t m e n t s

Number t e s t e d

Fail-ure t i m e s , days

4h/190°C 2

16h/190°C 3

8 0 h / l g 0 ~ C 3

l90h/ 1 3 0 ~ ~ 3

5 0 h / l 3 0 ~ ~ + 1 6 h / 1 9 0 ~ ~

3

4 m i n / 2 5 0 ~ ~ + 1 6 h / l g 0 ~ ~ 3 a s r e c e i v e d ( 1 6 h / 1 9 0 ~ ~ )

3

7075-T651

3

( a ) A f t e r 1 0 days of t e s t i n g , exposure f o r one of t h e specimens was d i s c o n t i n u e d f o r m e t a l l o g r a p h i c examinations.

The t e n s i l e s u r f a c e s of t u n i n g f o r k specimens a f t e r 1 0 days a l t e r n a t e immersion i n aqueous 3 . 5 % NaCl s o l u t i o n a r e shown i n f i g 2. P r i o r t o photographing, t h e s u r f a c e o f t h e 7075 a l l o y specimen was chemically c l e a n e d by u s i n g 65 % HNO The overaged specimen shows more p i t t i n g c o r r o s i o n a t t a c k and m u l t i c r a c k 3' i n i t i a t i o n , w h i l e t h e underaged shows l e s s p i t t i n g c o r r o s i o n a t t a c k and a f e w . b u t l o n g e r s t r e s s c o r r o s i o n c r a c k s . The specimen pre-aged a t t h e h i g h

t e m p e r a t u r e

( 4

min/250°c + 1 6 h / 1 9 0 ~ ~ ) shows a s i n g l e l o n g c r a c k t h a t h a s grown a c r o s s t h e width ( 8 mm) of t h e t u n i n g f o r k specimen. A f t e r f r a c t u r i n g , i t was observed t h a t t h e s t r e s s c o r r o s i o n a t t a c k i n t h e above specimen was deeper compared t o t h e o t h e r specimens aged d i f f e r e n t l y . It h a s been r e p o r t e d t h a t a s i m i l a r two-step a g e i n g t r e a t m e n t c a n improve SCC r e s i s t a n c e o f 2214 a l l o y

( 2 2 ) . A f t e r 10 days of a l t e r n a t e immersion, some o f t h e t u n i n g f o r k specimens were s e c t i o n e d i n o r d e r t o examine t h e t y p e of c o r r o s i o n a t t a c k . O p t i c a l photomicrographs of t h e c r o s s s e c t i o n of t h e a s - r e c e i v e d 8090 a l l o y and t h e

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7075 a l l o y a r e shown i n f i g . 3. A t y p i c a l i n t e r g r a n u l a r type s t r e s s corrosion crack i s seen i n t h e 8090 a l l o y . The 7075 a l l o y shows s e v e r e c o r r o s i o n p i t s but crack i n i t i a t i o n has n o t occured.

S t r e s s c o r r o s i o n crack propzgation

The s t r e s s c o r r o s i o n crack growth versus exposure t i m e curves a r e shown i n f i g .

4 .

The DCB specimens were made from t h e 8090 a l l o y given d i f f e r e n t ageing treatments and t h e 7075-T651 a l l o y . Values of t h e s t a r t i n g s t r e s s i n t e n s i t y f a c t o r a p p l i e d t o t h e DCB specimens a r e l i s t e d i n t a b l e

4.

It is e v i d e n t from f i g . 4 t h a t t o t a l growth i n t h e 7075 a l l o y is much h i g h e r compared t o t h e 8090 a l l o y , even though t h e s t a r t i n g stress i n t e n s i t y f a c t o r f o r 7075-DCB specimen was s l i g h t l y lower. An i n t e r e s t i n g f e a t u r e o f t h e crack extension curves f o r t h e aluminium-lithium a l l o y i s the occurrence of numerous s t e p s . This i n d i c a t e s t h a t crack growth i n t h e aluminium-lithium a l l o y might be discontinuous.

Christodoulou e t a 1 ( 3 ) s t u d i e d t h e SCC behaviour of t h e A1-2.8% Li a l l o y . They n o t i c e d banded t e x t u r e on t h e SCC-fracture s u r f a c e s and a t t r i b u t e d i t t o t h e discontinuous crack propagation. I t was suggested t h a t hydrogen embrittlement may play a r o l e i n t h e cracking mechanism. Average growth r a t e s based on a t o t a l exposure period of 22 days were c a l c u l a t e d and t h e r e s u l t s a r e presented i n t a b l e

4.

The overaged specimen (No. 3) and t h e low temperature underaged (No.

5 )

d i d n o t show any measurable crack growth f o r s i x weeks of exposure.

These two ageing treatments performed b e t t e r even i n t h e smooth specimen s t r e s s c o r r o s i o n t e s t s . However, i t should be noted t h a t t h e t h r e s h o l d K f o r t h e overaged temper i s r a t h e r low due t o i t s lower f r a c t u r e toughness1gg!9 ~ ~ a j m ) . The specimens aged t o t h e peak s t r e n g t h v i a d i f f e r e n t ageing treatments (No. 2 , 6 , 7 ) had n e a r l y t h e same average growth r a t e s . The DCB specimens have s o f a r been t e s t e d f o r 6 weeks. During t h i s period t o t a l growth i n t h e underaged A 1 - L i specimen (No. 1) i s g r e a t e r than t h e o t h e r A1-Li specimens. However. during t h e f i r s t one month of t e s t i n g growth i n t h e underaged specimen was slower than t h e o t h e r A 1 - L i pealc aged specimens (No. 2 . 6 , 7 ) . This e a r l y slow growth i n t h e underaged specimen is probably due t o i t s high f r a c t u r e toughness.

Fig. 2 The s u r f a c e appearance of tuning f o r k smooth specimens a f t e r 10 days a l t e r n a t e immersion i n aqueous 3.5 % NaCl s o l u t i o n .

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C3-876 JOURNAL

DE

PHYSIQUE

F i g .

3

C r o s s - s e c t i o n o f t h e t u n i n g f o r k specimens a f t e r 10 days a l t e r n a t e immersion i n

3.5

% NaCl s o l u t i o n , ( a ) a s - r e c e i v e d 8090 a l l o y (No.

8

)

-

s t r e s s c o r r o s i o n c r a c k , ( b ) 7075-T651 a l l o y

-

p i t t i n g c o r r o s i o n .

Table

4

S t r e s s c o r r o s i o n c r a c k p r o p a g a t i o n d a t a of f a t i g u e - p r e c r a c k e d DCB specimens, p e r i o d i c moistening w i t h aqueous 3.5 % NaCl s o l u t i o n .

Specimen Number

Ageing t r e a t m e n t s

I n i t i a l s t r e s s i n t e n s i t y ( a ) ~ p d m

F i n a l s t r e s s i n t e n s i t y

(b) ~ p d m

T o t a l c r a c k growth Aa (mm)

Crack growth r a t e ( c ) m / s x 1 0 - l ~

( a ) C r a c k - t i p s t r e s s i n t e n s i t y f a c t o r a p p l i e d b e f o r e environmental exposure.

( b ) C r a c k - t i p s t r e s s i n t e n s i t y f a c t o r a f t e r 22 days exposure.

( c ) Average growth r a t e based on an exposure p e r i o d o f 22 days.

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0

10 20 30 40

50

Exposure time (days)

Fig.

4

S t r e s s c o r r o s i o n crack growth versus exposure time curves f o r t h e 8090 a l l o y i n d i f f e r e n t ageing c o n d i t i o n s and t h e 7075-T651 a l l o y . P e r i o d i c moistening with 3.5 % NaCl s o l u t i o n .

The law-temperature underaged specimen ( 1 9 0 h / 1 3 0 ~ ~ ) showed b e s t combination of f r a c t u r e toughness and SCC. A p o s s i b l e explanation t o t h i s might be t h a t a t t h i s low temperature (130'~) p a r t i c l e s of type S t (A12CuMg), T1 (A12CuLi) and 8 (AlLi) have n o t y e t p r e c i p i t a t e d a t g r a i n boundaries and subgrain boundaries.

It i s known t h a t S' and T1 p a r t i c l e s p r e f e r e n t i a l l y p r e c i p i t a t e s o a t t h e g r a i n and t h e subgrain boundarigs i n t h e e a r l y s t a g e s of ageing a t 190 C ( 2 3 ) . The underaged temper ( 4 h/190 C ) i s then expected t o be more s u s c e p t i b l e t o SCC than t h e peak-aged and t h e overaged temper due t o a g r e a t e r p o t e n t i a l d i f f e r e n c e between g r a i n boundaries and t h e g r a i n i n t e r i o r . There a r e some i n d i c a t i o n s f o r t h a t (Table 3 and Fig. 4 ) , but t h e d a t a gathered s o f a r a r e too l i m i t e d t o make d e f i n i t e conclusion. Urushino e t a 1 (24) measured t h e p i t t i n g p o t e n t i a l of 2024 a l l o y (Al-Cu-Mg) i n a de-aerated I M NaCl s o l u t i o n . I t was suggested t h a t t h e d i s s o l u t i o n of S (A12CuMg) phase should occur while SCC proceeds, b u t t h e important r 6 l e i n SCC of t h e a l l o y i s played by t h e p i t t i n g d i s s o l u t i o n of solute-denuded zones along t h e g r a i n boundaries.

T e s t i n g on t h e DCB specimens i s continuing. The scanning e l e c t r o n microscopic study on t h e f r a c t u r e d s h o r t b a r specimens, and t h e TEM study of t h e

m i c r o s t r u c t u r e s a r e planned. More extended r e s u l t s w i l l be published elsewhere.

Conclusions

1) The S-L f r a c t u r e toughness of t h e 8090 a l l o y decreases with ageing a t 1 9 0 ~ ~ from

33

~ ~ a ] m (underaged) t o

8.9

M P ~ ] m (overaged)

.

2 ) S t r e s s c o r r o s i o n cracks r e a d i l y i n i t i a t e i n t h e underaged and peak- aged temper of t h e 8090 a l l o y when exposed t o 3.5 % N a C l a l t e r n a t e immersion. On t h e o t h e r hand e q u i v a l e n t exposure of t h e 7075-T651 a l l o y r e s u l t s i n severe p i t t i n g a t t a c k b u t no SCC.

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C3-878 JOURNAL

DE

PHYSIQUE

3 ) From t h e s t r e s s corrosion crack growth s t u d i e s on t h e DCB specimens i t is concluded t h a t t h e 8090 a l l o y i s s u p e r i o r t o the 7075-T651 a l l o y .

4) A low-temperature underageing treatment ( 1 9 0 h / 1 3 0 ~ ~ ) of 8090 a l l o y showed b e s t combination of f r a c t u r e toughness and SCC r e s i s t a n c e . Acknowledgements

The author would l i k e t o thank Alcan I n t e r n a t i o n a l Ltd f o r providing t h e 8090 a l l o y p l a t e . The author would a l s o l i k e t o thank D r . B. Jaensson and M r . B.

Svensson of Saab-Scania AB. A i r c r a f t Division f o r conducting stress corrosion tests on tuning f o f k type specimens a t t h e i r t e s t i n g f a c i l i t y a t Saab.

Encouragement and support given by Prof. T. Ericsson and D r . S. Johansson i s g r a t e f u l l y acknowledged. The t e c h n i c a l a s s i s t a n c e of M r . N. Larsson i s g r e a t l y appreciated. This research was funded by Saab-Scania AB. A i r c r a f t Division and the National Swedish Board f o r Technical Development.

References

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