HIGH-STRAIN-RATE BEHAVIOUR OF CP-271 ALUMINIUM-LITHIUM

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HIGH-STRAIN-RATE BEHAVIOUR OF CP-271 ALUMINIUM-LITHIUM

C. Chiem, Xin Zhou, W. Lee

To cite this version:

C. Chiem, Xin Zhou, W. Lee. HIGH-STRAIN-RATE BEHAVIOUR OF CP-271 ALUMINIUM-LITHIUM. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-577-C3-586.

�10.1051/jphyscol:1987367�. �jpa-00226598�

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JOURNAL DE PHYSIQUE

Colloque C3, supplement au n09, Tome 48, septembre 1987

HIGH-STRAIN-RATE B E H A V I O U R O F CP-271 ALUMINIUM-LITHIUM

C.Y. CHIEM, X.W. ZHOU and W.S. LEE

Ecole Nationale Superieure de MBcanique, Laboratoire des

Sciences des Materiaux de la MBcanique "ENSM-IMPACT", 1, Rue de l a No6, F-44072 Nantes Cedex, France

EXTENDED ABSTRACTS

Among advanced materials for transport, the low density aluminium-lithium alloy look particularly attractive for the aeronautic and aerospace industries.

The main topic of this study is aimed to focus the correlation of the fundamental mechanisms with respect to the high strain-rate behaviour of the CP-271 aluminium-

lithium alloy.

Specimens have been deformed in compression and torsion at strain-rate range from 1 0 - ~ s - l to ; the dynamic testing is done by the split-Hopkinson bars method.

Comparison of the results between quasi-static and dynamic behaviour is done. CP-271 aluminum lithium alloy is quite sensitive to strain-rate. The testing results and the microscopic observation prove that the heat-treatment conditions have a great influence on the dynamic rupture property of the aluminium-lithium alloys.

These basic mechanical properties and microstructure relationships will help to a better comprehension of the behaviour of these A1-Li alloys. I n addition, it helps to set up a guideline for the selection of a more promising microstructure for a better impact strength of the material.

1 . INTRODUCTION

Recently, adding lithium to high strengthaluminiumalloy has been widely know as one of the main materials for aircraft part-members due to its attractive combination of low density, high specific elastic modulus and high specific strength. So far, a num- ber of research works on the mechanical behaviour and physical characteristics of A1-Li alloys under quasi-static testing conditions have been carried out (I), however, the high strain rate behaviours, such as mechanical properties, microstructure cha- racteristics, dislocation and deformation mechanisms, are still considered as impor- tant works to be performed urgently.

-fany authors have made experiments to explain some F.C.C. metals behaviour at high strain rates. The split Hopkinson bar technique has been applied to investigate the dynamic behaviour in compression (2) or tension (3) of various materials and has shown great strain rates and strain-history effects on the behaviour of these metals.

Under these circumstances, there are athermal, thermally activated, diffusion- controlled and dislocation-drag controlled mechanisms (4) which control the deforma- tion rate of metals. Furthermore, when the strain rate is lower than 103s-I, the dominant rate controlling mechanism for dislocation motion shows the above mentioned mechanisms except drag. While the strain rate is higher than about 103s-I, the strain rate sensitivity of the flow stress increases rapidly.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987367

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C3-5 78 JOURNAL DE PHYSIQUE

During dynamic l o a d i n g , t h e change of m i c r o s t r u c t u r e occurs and t h e r e s u l t of t h i s change r e f l e c t s upon d i f f e r e n t s t r e s s s t a t e s , s t r a i n o r s t r a i n r a t e p a t h . A t high v e l o c i t y deformation, t h e deformation mechanism changes from s l i p t o twinning i n accordance with s t r e s s . Some of t h e r e s e a r c h e r s have explained t h e m i c r o s t r u c t u r e change by d i s l o c a t i o n theory (5) and d e s c r i b e d t h e d i s l o c a t i o n dynamics a t high s t r a i n r a t e . A c t u a l l y , t h e d i s l o c a t i o n d i s t r i b u t i o n c h a r a c t e r i s t i c s f o r shocked a l u - minium have been r e p o r t e d ( 6 ) , and c e l l s t r u c t u r e of d i s l o c a t i o n s was found i n dyna- mic deformation (7). For example, f o r copper (8), d i s l o c a t i o n c e l l s t r u c t u r e s a r e formed a t about 1 ~ 1 0 ~ s - ~ ; f o r aluminium s i n g l e c r y s t a l (9), t h e c e l l s i z e w i l l redu- c e i f t h e flow s t r e s s and t h e s t r a i n r a t e s e n s i t i v i t y a r e i n c r e a s e d . The o b j e c t i v e of t h i s i n v e s t i g a t i o n i s t o d e s c r i b e t h e experimental r e s u l t s which a r e obtained by t o r s i o n t e s t s i n t h e s t r a i n r a t e range of

^?.

10's-' and compression t e s t s w i t h i n

l o 2

^?.

10's-l. S e v e r a l o b s e r v a t i o n s of m i c r o s t r u c t u r e change a t d i f f e r e n t s t r a i n r a t e s a r e p r e s e n t e d .

2. EXPERIMENTAL PROCEDURES 2.1. P r e p a r a t i o n of specimens

The m a t e r i a l used i s provided by t h e Centre d e Recherche d e Voreppe, CEGEDUR PECHINEE I t s composition and h e a t t r e a t m e n t c o n d i t i o n s a r e l i s t e d i n t a b l e

I.

S t a t i c mecha- n i c a l p r o p e r t i e s a r e d e t a i l e d i n r e f e r e n c e s ( l o ) , (11).

Table I. Alloy composition ( i n weight p e r c e n t ) and h e a t t r e a t m e n t c o n d i t i o n s . The shape of t h e specimens t e s t e d f o r t h e t o r s i o n a l experiment i s shown i n Fig. 1 ; t h e specimens a r e machined i n a thin-walled tube with f l a n g e d ends which a r e cemented t o t h e Hopkinson b a r using an epoxy adhesive.

The specimens f o r t h e compressive t e s t s a r e c y l i n d r i c a l ; they a r e c l a s s i f i e d i n t o two groups, one i s p a r a l l e l t o t h e r o l l i n g d i r e c t i o n and t h e o t h e r p e r p e n d i c u l a r t o t h e r o l l i n g d i r e c t i o n . A f t e r deformation, t h e specimens were s l i c e d and polished i n o r d e r t o g e t t h i n f o i l s f o r t r a n s m i s s i o n e l e c t r o n microscope (TEII) o b s e r v a t i o n s . 2 . 2 . Mechanical t e s t i n g by t o r s i o n and compression Hopkinson b a r s

The specimens have been d e f o m e d i n compression and t o r s i o n a t s t r a i n - r a t e ranging from 102s-' t o 1 0 ~ s - I and 10-'s-I t o 10's-I r e s p e c t i v e l y . The split-Hopkinson b a r s a r e used f o r t h e t e s t s .

F i g . 1 and Fig. 2 show r e s p e c t i v e l y t h e g e n e r a l arrangement of t h e t o r s i o n a l and compression t e s t assemblies with Lagrangian wave propagation diagram on t h e t o r s i o n a l t e s t i n g d e v i c e .

The t e s t i n g systems a r e composed of t h r e e p a r t s : p u l s e g e n e r a t i o n system, p u l s e d e t e c t i n g and r e c o r d i n g system-and d a t a processing system. S i n c e t h e d e s c r i p t i o n of t h e a p p a r a t u s and experimental procedure have been d e t a i l e d i n r e f . (12) (13) and (14). Only a b r i e f account of them i s given h e r e .

Both t o r s i o n a l and compressive t e s t s a r e based on t h e theory of t h e propagation of

one-dimensional e l a s t i c - p l a s t i c s t r e s s wave i n b a r s (Fig. 1 ) . This l a t t e r has been

d i s c r i b e d c l e a r l y by H. Kolsky (15). The only d i f f e r e n c e between them i s t h e method

of t h e p u l s e g e n e r a t i o n . The t o r s i o n a l p u l s e i s s t i m u l a t e d by a sudden r e l e a s e of a

t o r q u e s t o r e d i n t h e i n c i d e n t b a r between t h e r o t a t i n g head and t h e clamp (Fig. I ) ,

and t h e compressive p u l s e i s generated by an impact between t h e p r o j e c t i l e and t h e

imput b a r (Fig. 2 ) . Then, t h e p u l s e propagates down t h e i n c i d e n t b a r toward t h e spe-

cimen. A t t h e i n c i d e n t bar/specimen and s p e c i m e n / t r a ~ s m i s s i o n b a r i n t e r f a c e s , r e f l e c -

t e d and t r a n s m i t t e d waves occur. The i n c i d e n t , r e f l e c t e d and t r a n s m i t t e d p u l s e s a r e

d e t e c t e d i n a conventional manner using e l e c t r i c - r e s i s t a n c e s t r a i n gages and a r e

recorded using a d i g i t a l o s c i l l o s c o p e . The average s t r e s s and s t r a i n i n t h e specimen

can b e c a l c u l a t e d from t h e r e f l e c t e d and t r a n s m i t t e d waves. I n t h e t o r s i o n a l t e s t (16)

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F i g . 1 . Hopkinson b a r s i n t o r s i o n and Lagrangian diagram

(:projectile 2:incident bar 3:transmission'bar 6:launching syptem 5:pressure c o m n d panel 6:apparatus signal recording system 7: data processing.

Fig. 2 . Compression t e s t s e t

where G i s t h e s h e a r modulus, re, t h e r a d i u s of t h e b a r s , e , r m and L being t h e w a l l t h i c k n e s s , t h e mean r a d i u s and t h e gauge l e n g t h of t h e specimen r e s p e c t i v e l y . CTis t h e v e l o c i t y of t h e t r a n s v e r s e wave i n t h e b a r , y T a n d y R being t h e t r a n s m i t t e d and r e f l e c t e d waves recorded by o s c i l l o s c o p e .

I n t h e compressive experiments (17)

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C3-580 JOURNAL DE PHYSIQUE

where E i s t h e Young's ~ o d u l u s , A , t h e c r o s s - s e c t i o n a l a r e a of t h e b a r s , L and A ,

t h e i n i t i a l l e n g t h and c r o s s - s e c t i o n a l a r e a of t h e specimen. s T and sR r e p s e s e n t ?he i n s t a n t a n e o u s amplitudes of t h e r e f l e c t e d and t r a n s m i t t e d p u l s e s r e s p e c t i v e l y . The c i u a s i - s t a t i c t e s t i n t o r s i o n i s performed a t t h e same a p p a r a t u s a s t h a t used i n dynamic t o r s i o n a l t e s t . The displacement i s d e t e c t e d by a DCDT ( D i r e c t Current d i s p l a - cement t r a n s d u c e r ) ( F i g . 3 ) . The t o r q u e i s measured by t h e e l e c t r i c a l - r e s i s t a n c e s t r a i n gage, t h e s t r e s s and s t r a i n can be obtained according t h e f o l l o w i n g formular

:

where, S i s t h e s e n s i t i v i t y of t h e t r a n s d u c e r , U being t h e v o l t a g e of t h e power supply. U i s t h e i n s t a n t a n e o u s a m p l i t i t u d e s of t g e displacement s i g n a l .

F i g . 3 . Displacement t r a n s d u c e r 2.3. M i c r o s t r u c t u r e o b s e r v a t i o n

The m i c r o s t r u c t u r e of t h e a l l o y s i s c h a r a c t e r i z e d by u s i n g scanning and t r a n s m i s s i o n e l e c t r o n microscope. Thin f o i l s f o r T.E.M. a r e prepared by e l e c t r o p o l i s h i n g 3 mm diameter by 0.3 mm t h i c k n e s s d i s c s which were c u t from compression specimens p a r a l l e l and p e r p e n d i c u l a r t o t h e r o l l i n g d i r e c t i o n . The d i s c s a r e e l e c t r o p o l i s h e d i n a double j e t p o l i s h i n g a p p a r a t u s operated a t 25V and room temperature with a c i r c u l a t i n g e l e c - t r o l y t e which c o n s i s t s of 7 8 m l p e r c h l o r i c a c i d , 700 m l e t h a n o l , 100 m l b u t y l c e l l o - s o l v e and 120 m l d i s t i l l e d water. Then they a r e observed i n a JEOL 120cx Temscan, o p e r a t i n g a t 100 kV. The f r a c t o g r a p h i c a n a l y s i s of t h e t o r s i o n specimens a r e examined i n a JEOL JSY-35C scanning e l e c t r o n microscope (S.E.N.) o p e r a t e d a t 15 kV.

3. RESULTS AND DISCUSSIONS 3.1. Shear s t r e s s - s t r a i n curves

Experimental r e s u l t s i n t o r s i o n a l t e s t s a r e presented i n F i g . 4 and Table 2.

Table 2 . Values taken from Fig. 4

F i g . 4 . Shear s t r e s s - s t r a i n curves

SHEAR STRAIN

(A)

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From F i g . 4, we can s e e t h a t 1416-14 (12h a t 190°C) and 1416-T (2h30 a t 160°C) have d i f f e r e n t behaviours. A t t h e same s t r a i n - r a t e , t h e maximum s t r e s s of 1416-M i s g r e a t e r than t h a t of 1416-T, and, 1416-P! shows a more b r i t t l e behaviour. I f , i n t h e t e s t s of 1416-M, we use t h e same specimen dimensions a s i n 1416-T t e s t s , we can never reach t h e same maximum s t r a i n - r a t e , even i f ve change t h e dimensions of specimens, we could not reach a s t r a i n - r a t e a s high a s t h a t i n 1416-T t e s t s . T h i s i s due t o t h e very small amount of p l a s t i c s t r a i n i n 1416-M.

lle can a l s o n o t i c e t h a t f o r both 1416->I and 1416-T, t h e dynamic curves l i e above t h e s t a t i c ones, and maximum s t r a i n i n s t a t i c t e s t s i s l e s s than t h a t i n t h e dynamic ex- periments. Among t h e dynamic s t r e s s - s t r a i n c u r v e s , t h e p l a s t i c s t r e s s i n h i s h e r s t r a i n r a t e i s a l i t t l e more g r e a t e r than t h a t i n lower s t r a i n - r a t e i n t h e c a s e of t h e same s t r a i n . For 1416-M, t h e maximum p l a s t i c s t r a i n i n h i g h e r s t r a i n - r a t e i s g r e a t e r than t h a t i n lower s t r a i n - r a t e (from 2%-8%), b u t f o r 1416-T, t h e maximum p l a s t i c s t r a i n i s almost t h e same i n each t e s t

(%

12%).

3 . 2 . S t r a i n - r a t e e f f e c t

The v a r i a t i o n of s h e a r s t r e n g t h and y i e l d s t r e s s w i t h s t r a i n r a t e from I O - ~ S - ' t o lo's-' a r e p r e s e n t e d i n Fig. 5 .

n

a

1 CP27I-92A-990

5

⁺

**

2 0 0

- --_--- ---h

m

+ - - -

,,2&

²⁰⁰

Ln -,--

P

W

-_---- -

w _----

t A/----

1 0 0

-

i 7

-

Y<T& 1 0 B

P:

a

A T - T C X )

I

0 ' " ' ' ' " 0

- 4 - 3 - 2 - 1 0 1 2 3 4

-

A

L O G C S T R R I N - R R T E c a - l ) )

' 4 - 3 - 2 - 1 0 1 2 3 4

B

L O G C S T R R T N - R R T E Cr-133

F i g . 5 . S t r a i n - r a t e dependence of s h e a r s t r e n g t h and y i e l d s t r e s s e s , A=1416-FI,F-l41fS For t h e 1416-T, we can s e e t h a t t h e curves a r e obviously d i v i d e d i n t o two p a r t s , a s most of t h e F.C.C. m e t a l s , t h e s h e a r s t r e n g t h and y i e l d s t r e s s i n c r e a s e r a p i d l y with

i n c r e a s i n g s t r a i n - r a t e from t o 10'

S - l .

However, f o r 1416-M, a t t h e whole range of s t r a i n - r a t e from 1 0 - ~ s - l t o t h e s t r e s s has almost a l i n e a r dependence t o t h e s t r a i n - r a t e .

Fig.-6 show t h e r e l a t i o n s h i p between s h e a r s t r e s s and s t r a i n - r a t e (from 1 0 ~ s - I to l o 4 s l ) a t v a r i o u s p l a s t i c s h e a r s t r a i n .

2 3 4 i? 3 4

A B

L O G ( S T R A I N - R R T E Cs-13) L O G ( S T R A I N - R R T E Cs-1))

Fie;. 6. Shear s t r e s s a s

a

f u n c t i o n of s t r a i n - r a t e , A : 1416-T, B

:

1416-If

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C3-582 JOURNAL DE PHYSIQUE

From t h e f i g u r e s , we could.notice t h a t a l i n e a r r e l a t i o n s h i p e x i s t s . I t f i t s an empi- r i c a l formula

^T=

A + Blogy where A and

B

a r e c o r r e l a t i o n c o e f f i c i e n t s , which were c a l c u l a t e d with t h e l e a s t square method. Table 3 g i v e s t h e v a l u e s of A and R .

Table 3

The s t r a i n - r a t e s e n s i t i v i t y i s known a s & . We can n o t e t h a t t h e c o e f f i c i e n t : B slog;

r e p r e s e n t s t h e s t r a i n - r a t e s e n s i t i v i t y . I n t h e s t r a i n - r a t e range from 1 0 ~ s - ' t o lo's-', t h e v a l u e s of B a r e almost independent of t h e s h e a r s t r a i n ; f o r 1416-M,

B

changes between 15 and 18PIPa. For 1416-T, from 25 t o 31 MPa, B can b e regarded a s a c o n s t a n t . I f we c o n s i d e r t h e s t r a i n - r a t e range from q u a s i - s t a t i c t o dynamic v a l u e s

( i . e . , t o 104s

I ) ,

CP271 (1416-M) and CP271 (1416-T) a r e both s t r a i n - r a t e s e n s i - t i v e ; however t h e second i s much more s e n s i t i v e than t h e f i r s t c a s e .

Lindholm has d e f i n e d t h e s t r a i n - r a t e s e n s i t i v i t y i n t h e range of t h e thermally a c t i - v a t e d mechanisms (18) under t h e following form :

where

ao

i s t h e flow s t r e s s f o r a known s t r a i n - r a t e . Using t h i s d e f i n i t i o n , Fig. 7 shows t h e comparison of s t r a i n - r a t e s e n s i t i v i t y of CP271 and t h a t of o t h e r a l l o y s ( l 9 ) .

This a l l o w s t o d i s t i n g u i s h t h e a l l o y i n g e f f e c t of t h e l i t h i u m on t h e aluminium m a t r i x and a l s o t h e h e a t - t r e a t m e n t e f f e c t . We can n o t i c e t h a t l i t h i u m shows a mediumalloying e f f e c t i n CP271. Concerning 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 s , t h e 1416-T y i e l d s a more important s t r a i n - r a t e s e n s i t i v i t y than t h e 1416-M c o n d i t i o n .

MAX STRESS A T ; = ls4: so ( K s i )

F i g . 7. Values of s t r a i n - r a t e sensitivity a s a f u n c t i o n of t h e maximum s t r e s s i n

v a r i o u s aluminium a l l o y s

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3 . 3 . F r a c t u r e a p p e a r a n c e of t h e specimens

Scanning e l e c t r o n micrographs of t h e s u r f a c e of t h e t o r s i o n specimens a r e shown i n F i g . 8 and F i g . 9 which show t h a t t h e f r a c t u r e s u r f a c e s i n a l l t h e t e s t c o n d i t i o n s a r e b r i t t l e and t h a t v a r i o u s c l e a v a g e s t r u c t u r e s e x i s t on t h e f r a c t u r e s u r f a c e , c o r - responding t o a b r i t t l e g r a c t u r e mode. F o r 1416-T c o n d i t i o n s (2h30 a t 160°C), when t h e s t r a i n r a t e i s low ( E

=

550 s - l , F i g . 8 a ) , numerous s m a l l c l e a v a g e s a r e s e e n s o e v i d e n t l y on t h e f r a c t u r e s u r f a c g t h a t t h e y a r e enoug& t o prove a low d u c t i l i t y . I f t h e s t r a i n r a t e i s i n c r e a s e d t o

E =

2000 s ( F i g . 8 b ) , t h e l a r g e c l e a v a g e f a c e t s a r e found. T h i s r e s u l t makes a c o u n t e r r e a c t i o n t o d u c t i l i t y . I n o t h e r words, t h e d u c t i - l i t y w i l l d e c r e a s e whe? s t r a i n r a t e i s i n c r e a s e d . F o r 1416-M c o n d i t i o n (12h a t 190°C), a t s m a l l s t r a i n r a t e ( E

=

350 s - l , , ~ i ~ . 9a) a lower s t r e n g t h and a poor d u c t i l i t y a p p e a r . But a t l a r g e s t r a i n r a t e ( E

=

1000 F i g . 9 b ) , we g o t r e v e r s e c o r r e l a t i o n between s t r e n g t h and d u c t i l i t y . Ariyway, t h e i r f r a c t u r e s u r f a c e s a r e s t i l l b r i t t l e f r a c t u r e s . On t h e o t h e r hand, a t

E =

1000 s-' t h e f r a c t u r e s u r f a c e shows a n i n t e r - s u b g r a n u l a r f r a c t u r e , comparing t h i s w i t h t h a t of F i g . 8b, t h e f r a c t u r e p r o c e s s a l - t e r s from t r a n s g r a n u l a r t o i n t e r s u b g r a n u l a r and t h e d u c t i l i t y d r o p s a c c o r d i n g l y . I n s h o r t , t h e f r a c t u r e s u r f a c e v a r i e s w i t h s t r a i n r a t e and h e a t t r e a t m e n t c o n d i t i o n , and t h e f r a c t u r e modes a r e r e l a t e d t o t h e s e two p a r a m e t e r s . Although t h e d u c t i l i t y v a r i a t i o n w i t h s t r a i n r a t e i s an a p p a r e n t tendency, t h e e f f e c t of h e a t t r e a t m e n t con- d i t i o n s h o u l d b e l a r g e r t h a n t h a t of s t r a i n r a t e , b e c a u s e , g e n e r a l l y , t h e h i g h e r h e a t t r e a t m e n t t e m p e r a t u r e and t h e l o n g e r h o l d i n g t i m e enhance a f a s t e r d e c r e a s e of t h e d u c t i l i t y .

3.4. M i c r o s t r u c t u r e s

The m i c r o s t r u c t u r e of compression specimens deformed a t room t e m p e r a t u r e u n d e r d i f f e - r e n t s t r a i n l e v e l s a r e shown i n F i g . 10 f o r t h e 1416-T (2h30 a t 160°C) and i n Fig.11 f o r t h e 1416-M (12h a t 190°C) and t h e i r d i f f e r e n t c h a r a c t e r i s t i c s have been evidenced by d i s l o c a t i o n a s p e c t s . A l s o , a l l t h e below mentioned samples a r e t a k e n p e r p e n d i c u l a r - l y t o t h e r o l l i n g d i r e c t i o n .

The undeformed specimens of t h e 1416-T (2h30 a t 160°C) show d i s l o c a t i o n s d i s t r i b u t e d evenly i n t h e m a t r i x a s i n F i g . 10a ; t h e m i c r o s t r u c t u r e c o n s i s t s of d i p o l e t r a i l s and p i n c h o f f d i p o l e s . F o r t h o s e specimens t e s t e d a t 3.8% s t r a i n , t h e r e a r e d i s l o c a - t i o n p i l e ups w i t h some arrangements which t e n d t o produce d i s l o c a t i o n c e l l s

( F i g . l o b ) . I f we i n c r e a s e t h e s t r a i n . t o 9.3%. a d i s l o c a t i o n c e l l s t r u c t u r e i s produced and a larger c e l l s i z e can be seen (Fig. 10c). When t e s t i s run to large strain value as 1'5.59. (Fig. lOd), the increase of dislocation density and p l a s t i c strain r e s u l t in a decrease of the mean dislocation c e l l s i z e , therefore, a larger s t r e s s i s needed for further deformation.

( a ) (b)

F i g . 8 . s c a n n i n g e l e c t r o n m i $ r o g r a ~ h s of t o r s i p n specimens of CP271 tempered f o r

2h30 a t 160°C, ( a )

E =

550 s-I , (b)

E =

2000 s l~x~OoO

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C3-584 JOURNAL DE PHYSIQUE

(a) ( b )

Fig. 9. Scanning electron picrographs of torsion specimens of CP271 tempered for 12h at 190°C, (a)

E =

350 s-l , ( b )

=

1000 ~ - ~ - x 1 0 0 0

Fig. 10. Transmission electron micrographs of specimens for 2 h 3 0 at 16OoC, deformed in room temperature to different strain levels.

(a) undeformed, ( b ) 3.8%, (c) 9.3%, (d) 15.5X.xS0000

For specimens of undeformed 1416-M (12h a t 190°C), t h e r e i s a

S'

phase p r e c i p i t a t i o n i n t h e matrix. T h i s phaseproduces.a s t r e s s f i e l d which c r e a t e s a l a r g e r s t r e n g t h a g a i n s t d i s l o c a t i o n motion, s o t h a t , t h e d i s l o c a t i o n s s u r r o n d i n g w i t h S' forms n e e d l e shape s t r u c t u r e (Fig. l l a ) . I f s t r a i n i s c o n t i n o u s l y i n c r e a s e d , t h e d i s l o c a t i o n s ga- t h e r a g a i n i n t o a m i c r o s t r u c t u r e of f o r e s t d i s l o c a t i o n s w i t h c l u s t e r e d d e f e c t s (Fig. I l b , Fig. I l c ) , f i n a l l y , d i s l o c a t i o n c e l l s a r e shaped a t 15% s t r a i n . By compa- r i n g t h e s t r a i n v a l u e w i t h Fig. 10 and F i g . 11, d i s l o c a t i o n c e l l s of F i g . 1 1 a r e formed much l a t e r than t h a t of F i g . 10. I n c a s e where samples a r e c u t p a r a l l e l t o t h e r o l l i n g d i r e c t i o n , t h e s i m i l a r r e s u l t can b e o b t a i n e b . T h i s s i t u a t i o n e x p l a i n s t h a t m i c r o s t r u c t u r a l changes v e r s u s s t r a i n v a l u e s have no concern with r o l l i n g d i r e c t i o n ,

they a r e much s e n s i t i v e t o h e a t treatment c o n d i t i o n s .

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( c ) (d)

F i g . 1 1 . T r a n s m i s s i o n e l e c t r o n micrographs of specimens f o r 12h a t 190°C, deformed i n room t e m p e r a t u r e t o d i f f e r e n t s t r a i n l e v e l s .

( a ) undeformed, ( b ) 4 . 4 X , ( c ) 8 . 4 X , (d) 1 5 % . ~ 5 0 0 0 n 4. CONCLUSIONS

T h r e e major r e s u l t s a r e o b t a i n e d from t h e mechanical b e h a v i o u r s t u d i e s : (1) A1-Li a l l o y s of t h e CP271 t y p e i s s t r a i n - r a t e s e n s i t i v e and a l s o v e r y s e n s i t i v e t o h e a t t r e a t m e n t c o n d i t i o n s . (2) a t two d i f f e r e n t h e a t t r e a t m e n t c o n d i t i o n s , 1416-T y i e l d s a more i m p o r t a n t s t r a i n r a t e s e n s i t i v i t y t h a n t h e 1416-M c o n d i t i o n . (3) by comparing t h e s t r a i n r a t e s e n s i t i v i t y of CP271 A1-Li a l l o y s t o t h a t of o t h e r l i g h t a l l o y s and p u r e o r commercial A l , i t h a s shown t h a t l i t h i u m ' s c o n t r i b u t i o n t o t h e s t r e n g t h e n i n g of t h e aluminium b a s e i s medium.

Higher h e a t t r e a t m e n t t e m p e r a t u r e and l o n g e r h o l d i n g t i m e s y i e l d a poor d u c t i l i t y f o r t h i s a l l o y s . The v a r i o u s d i s l o c a t i o n s t r u c t u r e s of undeformation samples a r e made by s o l u b i l i t y p r o d u c t s of l i t h i u m i n aluminium a s a f u n c t i o n of t e m p e r a t u r e . The d i s l o c a t i o n c e l l s formed under each c o n d i t i o n a t d i f f e r e n t s t r a i n v a l u e s , and t h e d e c r e a s e of c e l l s i z e i s r e l a t e d t o t h e i n c r e a s e of s t r e s s due t o s t r a i n r a t e e f f e c t . Cleavage f r a c t u r e s u r f a c e s of t h i s a l l o y s p r e s e n t low d u c t i l i t y c h a r a c t e r i s - t i c s and poor f r a c t u r e toughness f o r s t r a i n - r a t e higher 10~s-l. t h e r e f o r e , how t o improve t h e s e t w o weak p o i n t s t o obtain b e t t e r mechanical p r o p e r t i e s i s s t i l l a work needed t o be f u l f i l l e d f u r t h e r .

ACKNOWLEDGMENTS

The a u t h o r s wish t o thank M r . P h i l i p p e MEYER f o r h i s h e l p w i t h t h e e x p e r i m e n t a l works and t o Miss M a r t i n e DAMAS f o r t h e t y p i n g of t h e m a n u s c r i p t .

REFERENCES

(1) W.R.D. J o n e s and P.P. Das, J o u r n a l of t h e I n s t i t u t e of M e t a l s , v o l . 8 8 , 1959Q1960 pp. 435-443.

(2) A. Kumar and R.G. Kumble,

J.

Appl, P h y s i c s . 40-9 (1969), pp. 3475-3480.

(3) J.R. Klepaczko and J . Duffy, I n s t i t u t e of P h y s i c s Conf. S e r . n o 21 (1974).

(4) H. NeuhHuser, ICSMA.5, " S t r e n g t h of m e t a l s and a l l o y s 1 ' , v o l . 3 , pp. 1531-1550.

(5) R.W. H e r t z b e r g , "Deformation and f r a c t u r e mechanics of e n g i n e e r i n g m a t e r i a l s " ,

1976.

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M.F. Rose and T.L. Berger, Phil. Mag. 17, 1121 (1968).

J.W. Edington, Phil. Mag. 19 (1969), 1189-1206.

J. Shioiri, K. Satoh and K. Nishimura, IUTAM Symposium Tokyo/Japan, 1977, p.50-66.

C.Y. Chiem and J. Duffy, Brown University Report NSF ENG 75-1853219 (1979).

P. Meyer and B. Dubost, "Production of Aluminium-lithium alloy with high specific properties", 3rd International Aluminium-Lithium Conference, 8 July 1985,Oxford.

P. Sainfort, B. Dubost, P. Meyer, "Basic Hardening Mechanisms in Aluminium- Lithium Alloys",EuropeanMaterialsResearchSoc~etyFallMeeting 26-28 nov. 1985.

C.Y. Chiem and Z.G. Liu, 1985b, Annales des Composites, comportement au choc des materiaux et des structures composites, Villeurbanne, 12 decembre 1985.