DYNAMIC RESPONSE AND MICROSTRUCTURAL ASPECTS OF CP271 AND CP274 ALUMINIUM-LITHIUM ALLOYS

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DYNAMIC RESPONSE AND MICROSTRUCTURAL ASPECTS OF CP271 AND CP274

ALUMINIUM-LITHIUM ALLOYS

C. Chiem, W. Lee, P. Meyer

To cite this version:

C. Chiem, W. Lee, P. Meyer. DYNAMIC RESPONSE AND MICROSTRUCTURAL ASPECTS OF

CP271 AND CP274 ALUMINIUM-LITHIUM ALLOYS. Journal de Physique Colloques, 1988, 49

(C3), pp.C3-19-C3-28. �10.1051/jphyscol:1988303�. �jpa-00227719�

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JOURNAL DE PHYSIQUE

Colloque C3, Supplément au n°9, Tome 49, septembre 1988 C3-19

DYNAMIC RESPONSE AND MICROSTRUCTURAL ASPECTS OF CP271 AND CP274 ALUMINIUM-LITHIUM ALLOYS

C.Y. CHIEM, W . S . L E E and P. MEYER*

ENSM, Laboratoire des Sciences des Matériaux de la Mécanique

"ENSM-IMPACT", 1, rue de 2a Noë, F-44072 Nantes Cedex, France

Cégédur Péchiney, Centre de Recherche et Développement, BP 27, F-38340 Voreppe, France*

Résumé - Le comportement mécanique et la microstructure des alliages d'aluminium-lithium CP271 et CP274 en chargement dynamique par traction et par compression sont étudiés. L'effet de la vitesse de déformation sur le comportement mécanique est étudié dans la gamme des vitesses de déformation allant de 10 S à 10 S par des essais en traction à température ambiante.

Les essais dynamiques sont réalisés au moyen du dispositif de barres de Kolsky. Les faciès de rupture et les processus de rupture sont analysés en microscopie électronique à balayage. Les essais en compression sont faits dans la gamme des vitesses de déformation de 10 à 10 S à diférentes valeurs de déformation dans le but de trouver la relation entre la déformation et la variation des cellules de dislocations. L'aspect des dislocations de tous les échantillons est étudié en microscopie électronique à transmission. La relation entre les dislocations caractéristiques, la contrainte d'écoulement et la déformation plastique est déterminée et discutée en fonction de la microstructure de dislocations observées. Tous les résultats sont comparés avec ceux obterus dans les conditions quassi_stqtiques. Finallement, un modèle de loi de comportement est introduit afin de comparer les résultats expérimentaux aux résultats caculés pour décrire le co,portement dynamique de ces deux matériaux.

Abstract - The mechanical behaviour and microstructures of CP271 and CP274 Aluminium-Lithium alloys under dynamic loading by tension and compression were investigated. The strain rate effect on the mechanical behaviour has been studied by means of tension test in the strain rate range 10" S"1 to 10 S at room temperature. Dynamic tests were performed with a split- Hopkinson bar. The fracture surface and failure processes are analyzed by scanning electron microscoDy. Compression test were performed in the strain rate range from 10 to lU-^S"1 at different strain values with one of the purposes which emphasis is to find out a relationship between strain and dislocation cell variations. The dislocation configurations of all specimens which are produced by deformation at each test have been investigated with

the transmission electron microscopy technique. The relationship between dislocation characteristics, flow stress and plastic strain has been determined and is discussed in terms of the dislocation structure observed. All results are compared with those of quasi-static conditions. Finally, a constitutive model was introduced in order to compare the experimental and computed data and to describe the dynamic behaviour of those two materials.

1-INTR0DUCTI0N

During the last decade, considerable improvements have been made in the properties of Al-Li alloys. Because of their low density and high elastic modulus, Al-Li alloys have received great attention for aerospace applications. Especially, its lower density combined to a higher stiffness can lead to a weight reductions up to 15% in commercial aircraft. For the development of high-performance Al-Li, the main emphasis of this programme has been placed on the achievement of optimum properties by simple heat treatment process and high-strain-rate testing. Generally,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988303

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JOURNAL DE PHYSIQUE

dynamic plastic deformation cause fundamental mechanical properties and microstructure characteristics to change largely. Therefore, it is a must to examine cri- tically the effects of a series of factors, e.g, strain, strain rate, strain his- tory, temperature and grain size etc. which are customarily used to analyse and predict the deformation behaviour of materials. The technique of Hopkinson bars is used to perform high strain rate tests and the experimental results have been discussed from hoth macroscopic and microscopic aspects(l,2).

It is well known that the strain rate has effects on mechanical behaviour of materials, three zones of strain rate effect have been clearly determined(3). The strain rate effect on aluminium were first described by ~indholm(4). More recently Chiem and Duffy (5) have studied strain rate effects on monocrystalline aluminium.

Their conclusions are that the two above mentioned materials are sensitive to strain rate and show that flow stress increases with increasing strain rate. The results of other investigators (6-9) on the aluminium and some aluminium alloys have pointed out that strain rate dependence of flow stress can be rationalized with a simple theory for thermally activated motion of dislocation. Furthermore,

the principal effect of increased alloying is to increase athermal stress component.

During the loading process; metals and other crystalline solids can be deformed plastically by means o f different rnechanisms(l0). Some constitutive models have been proposed to calculate the rate of deformation (7,11-13). In order to describe the deformation process in terms of a constitutive model, both the form and the range of applicability of constitutive model must be defined. For both metals and alloys, the mechanical behaviour generally depends on its microstructure at the dislocation level. So, it is required to obtain a better understanding of the microstructural changes at the microscopic scale under deformation.

This paper presents the macro and micro experimental results for CP271 and CP274 A1-Li alloys and intends to ~ e v e a l the relationship between the microscopic obser- vetion of dislocations and the macroscopic response subjected to tension and compression. A constitutive model was introduced to describe the dynamic behaviour of these two materials.

2-EXPERIMENTAL PROCEDURES 2.1 Materials and specimens

Two kinds of materials listed in table 1 are used in this investigation. They are provided hy the Centre de Recherche de Voreppe. CEGEDUR PECHINEY, France. The detailed chemical composition and heat treatment conditions are also shown in the table.

Table 1: Chemical Analysis and Heat Treatment Conditions of Specimen Materials Specimens obtained from these two materials are parallel to the rolling direction except in the cases of 1416T and 1416M (V) which are perpendicular to rolling direction for compression test. Cylindrical compression specimens of llmm in diameter by 5.5mm high are prepared. Tensile specimens have been shaped directly with a gage length of ZOmm and a diameter of 3mm, the geometry and nominal dimen-

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s i o n s a r e shown i n F i g . 2C. A f t e r m e c h a n i c a l t e s t i n g , t h i n f o i l s w e r e p r e p a r e d f r o m t h e t e n s i o n a n d c o m p r e s s i o n s p e c i m e n s f o r T r a n s m i s s i o n E l e c t r o n M i c r o s c o p e E x a m i n a t i o n . R u p t u r e s u r f a c e s o f t h e t e n s i o n s p e c i m e n s a r e p r e p a r e d f o r S.E.M o b s e r v a t i o n s .

2 , 2 M e c h a n i c a l T e s t i n g

C o m p r e s s i o n t e s t s w e r e p e r f o r m e d on CP271 a n d CP274 a l l o y s s p e c i m e n s a n d d e f o r m e d t o d i f f e r e n t s t r a i n l e v e l s r a n g i n f r o 8% t 20%, s t r a i n r a t e was v a r i e d w i t h a w i d e r a n g e o f s t r a i n - r a t e f r o m LO'S - ' ' t o 10's-' a t room t e m p e r a t u r e . A I T S t e s t - i n g m a c h i n e w a s u s e d f o r t h e q u a s i s t a t i c t e s t s a n d a s p l i t H o p k i n s o n b a r a p p a r a t u s was f o r t h e d y n a m i c t e s t s . F i g . 1 shown a s c h e m a t i c d i a g r a m o f s p l i t H o p k i n s o n b a r f o r c o m p r e s s i o n t e s t s . H o p k i n s o n b a r p r i n c i p l e i s w e l l known ( 1 4 ) . As F i g . 1, i n c i d e n t b a r a n d t r a n s m i s s i o n b a r a r e made o f s t e e l w i t h a d i a m e t e r o f 20mm a n d a l e n g t h o f 1 M . I n o r d e r t o o b t a i n d i f f e r e n t s t r a i n v a r i a t i o n s u n d e r c o n s t a n t s t r a i n r a t e c o n d i t i o n , o n e s e r i e s o f p r o j e c t i l e s w i t h l e n g t h s f r o m 1 0 0 t o 4 0 0 mm a n d 20mm d i a m e t e r h a v e b e e n d e s i g n e d . E l e c t r i c r e s i s t a n c e s t r a i n g a g e s a r e s t i c k e d on i n - c i d e n t b a r a n d on t r a n s m i s s i o n b a r n e a r t h e s p e c i m e n . The i n c i d e n t , r e f l e c t e d a n d t r a n s m i t t e d p u l s e s a r e r e c o r d e d on a d i g i t a l o s c i l 1 o s c o p e and a r e a n a l y z e d by d a t a p r o c e s s i n g . From t h e r e c o r d s o f t h e t r a n s m i t t e d a n d r e f l e c t e d p u l s e s , t h e d y n a m i c s t r e s s - s t r a i n r e l a t i o n s c a n b e o b t a i n e d .

1: P r o j e c t i l e 2: Incident bar 3: Transmission bar 4 : Launching system

5: Pressure command panel 6 : Apparatus signal recording system 7 : Data processing F i g . 1 . C o m p r e s s i o n t e s t s e t

F o r t e n s i o n t e s t s , t h e t e n s i l e s p e c i m e n s o f t h e s e t w o m a t e r i a l s a r e e x a m i n e d a t s t r a i n r a t e r a n g e f r o m S-' t o

lo3s -'

a t room t e m p e r a t u r e . Q u a s i - s t a t i c t e s t s a r e c a r r i e d o u t i n "ADAMEL-LHOMARGY" m a c h i n e . Dynamic t e s t s a r e p e r f o r m e d i n t h e same s p l i t H o p k i n s o n b a r t h a t w a s u s e d f o r c o m p r e s s i o n t e s t s . H o w e v e r , t h e o n l y c h a n g e f o r d y n a m i c t e n s i o n t e s t s i s t h a t l e a d i n g t u h e a n d t r a n s m i t t e r b a r h a v e b e e n u s e d i n s t e a d o f i n c i d e n t b a r . F i g . 2 shows a g e n e r a l a r r a n g e m e n t o f t e n s i o n t e s t s a p p a r a t u s w i t h L a g r a n g e d i a g r a m . The s p e c i m e n i s s c r e w e d b e t w e e n t r a n s m i t t e r a n e l o a d i n g b a r s . A s t r a i n p u l s e i s g e n e r a t e d by a s u d d e n i m p a c t b e t w e e n t h e s t r i c k e r b a r a n d t h e l o a d i n g t u b e . As i t s t r i k e s t h e l o a d i n g b a r , i t i s p a r t i a l l y p r o p a - g a t e d t o w a r d t h e f r e e e n d o f t h e l o a d i n g b a r a n d p a r t i a l l y r e f l e c t e d b a c k i n t o t h e t r a n s m i t t e r b a r a s a t e n s i l e p u l s e . The p a s s a g e o f t h e P u l s e i s d e t e c t e d by e l e c - t r i c - r e s i s t a n c e s t r a i n g a g e s w h i c h a r e m o u n t e d on l o a d i n g a n d t r a n s m i t t e r b a r s a n d s i g n a l s a r e r e c o r d e d i n t h e d i g i t a l o s c i l l o s c o p e . B e i n g b a s e d on o n e d i m e n s i o n a l e l a s t i c wave p r o p a g a t i o n t h e o r y w i t h t h e r e c o r d s o f t h e l o a d i n g b a r a n d t r a n s m i t t e r b a r , t h e d y n a m i c s t r e s s - s t r a i n r e l a t i o n o f t h e s p e c i m e n i s d e t e r m i n e d .

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

T.E.M E x p e r i m e n t s w e r e p e r f o r m e d on t h i n f o i l s w e r e t a k e n f r o m t h e c o m p r e s s i o n s p e c i m e n s i n t h i n a r e a s w i t h i n 2mm o f t h e c o m p r e s s i o n f a c e and f r o m t h e t e n s i o n b a r n e a r t h e f r a c t u r e s u r f a c e o f e a c h t e s t . D i s c 3mm i n d i a m e t e r b y 0.3MM t h i c k n e s s w e r e c u t o u t by e l e c t r i c a l d i s c h a r g e m a c h i n e d (E.D.I.1) a n d e l e c t r o p o l i s h e d a t 25V w i t h a c i r c u l a t i n g e l e c t r o l y t e w h i c h c o n s i s t s o f 78 ml p e r c h l o r i c a c i d , 700 ml

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e t h a n o l , 100 ml b u t y l c e l l o s o l v e and 120 ml d i s t i l l e d w a t e r i n a T w i n - j e t e l e c t r o - p o l i s h e r a t room t e m p e r a t u r e . The t h i n f o i l s were o b s e r v e d i n a J e o l 120CX temscan o p e r a t e d a t 100KV. The f r a c t o g r a p h i c a n a l y s i s o f t h e t e n s i o n s p e c i m e n s i s examined i n a J o e l JSM-35C S c a n n i n g e l e c t r o n m i c r o s c o p e o p e r a t e d a t 15KV.

S t r i k e r Loading T r a n s m i t t e r Loading

b a r b a r b a r b a r

m! ^.--.--.- .

^{- -}^.

- - . - ¹

( b l D e t a i l e d "A"

( a ) L a g r a n g e d i a g r a m e ( c ) T e n s i o n s p e c i m e n s F i g . 2 S c h e m a t i c of t e n s i o n t e s t a p p a r a t u s

3-EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1 T e n s i o n t e s t s

3 . 1 . 1 T e n s i o n s t r e s s - s t r a i n c u r v e s

Average t e n s i o n s t r e s s s t r a i n c u r v e s f o r t h e CP271 and CP274 s p e c i m e n s t e s t e d u n d e r t h r e e s t r a i n r a t e s r e s p e c t i v e l y a r e p r e s e n t e d i n F i g . 3. A l l c u r v e s a r e somewhat s i m i l a r t o p a r a b o l i c c u r v e s and show b o t h t h e e f f e c t o f h e a t t r e a t m e n t c o n d i t i o n s and s t r a i n r a t e s . The a v e r a g e f l o w s t r e s s and f r a c t u r e s t r a i n show a s i g n i f i c a n t i n c r e a s e a s t h e s t r a i n r a t e f o r e a c h m a t e r i a l i s i n c r e a s e d . F o r CP271 m a t e r i a l , b e i n g t e s t e d a t t h e same s t r a i n r a t e ; 1416M ( 1 2 h a t 1 9 0 ° c ) shows a l a r g e r t e n s i l e s t r e s s t h a n t h a t o f 1416T ( 2 . 5 h a t 160°C). However; t h e d u c t i l i t y o f 1416M i s l e s s t h a n t h a t o f 1416T. F o r CP274, a v e r y s i m i l a r s t r e s s s t r a i n r e s p o n s e was found i n t h e c a s e of 1424Mi (12h a t 1 9 0 ' ~ ) and 1424N2 (12h a t 1 3 5 ' ~ ) . I f we compare t h e r e s u l t s o f CP271 and CP274, i t i s Found t h a t i n d e p e n d e n t l y of 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 t e n s i l e s t r e n g t h and t h e f r a c t u r e s t r a i n of CP274 a r e l a r g e r t h a n t h o s e o f CP271 ( s e e F i g . 3 a , 3c and 3 b , 3 d ) . On t h e o t h e r h a n d , u n d e r t h e c h a n g e s o f 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 d u c t i l i t v l o s s o f CP271 i s l e s s o b v i o u s t h a n t h a t o f

CP271

-

"

'OO-! ( a ) 1416T

t+ I

F i g . 3 S t r e s s - s t r a i n c u r v e s f o r CP271 and CPZ74 3.1.2 E f f e c t o f s t r a i n r a t e on t h e m e c h a n i c a l p r o p e r t i e s

The v a r i a t i o n between f l o w s t r e s s and s t r a i n r a t e f o r t h e t e s t e d s p e c i m e n s o f CP271 and CP274 i s shown i n F i g . 4 . I t i s o b v i o u s t h a t t h e s e two m a t e r i a l s h a v e two r e g i o n s which show d i f f e r e n t s t r a i n r a t e s e n s i t i v i t i e s o f t h e f l o w s t r e s s . A sudden c h a n g e o f t h e r a t e s e n s i t i v i t y a t a s t r a i n r a t e o f a b o u t 5 ~ 1 0 ~ ~ ' i s n o t i c e d . F o r any t e s t e d s p e c i m e n ; t h e f l o w s t r e s s i n c r e a s e s w i t h i n c r e a s i n g s t r a i n r a t e . However, i n t h e second r e g i o n , s t r a i n r a t e h a s a g r e a t i n f l u e n c e on f l o w s t r e s s and t h i s i n f l u e n c e c a u s e s a s h a r p i n c r e a s e of t h e s l o p . The g r e a t d i f f e r e n c e i n t h e s l o p e o f s t a g e 1 and 2 p r o b a l y r e f l e c t s a d i f f e r e n c e i n d e f o r m a t i o n mechanism. A t s t a g e 1, t h e r m a l l y a c t i v a t i o n i s t h e m a i n l y d e f o r m a t i o n c o n t r o l mechanism. When

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strain rate is over 10~s-l (stage 2), the deformation mechanism transitted from a lower velocity mechanism, as mentioned above, to a linear viscous drag mechanism which is associated with very high dislocation velocity.

LOG STRAIN RATE ( s " ) LOG STRAIN RATE (S-' j

Fig, 4 The variation of flow stress vs Fig. 5 Strain rate dependence of the the logarithm of the strain rate ductility of CP271 and CP274 for CP271 and CP274 alloys

The effect of strain rate on the fracture elongation for CP271 and CP274 is shown in Fig. 5. For these two materials, the highest elongation was obtained from 1424P11 (CP274-12h-135°C) and a lowest value was got from 1416M (CP271-12h-190°C). Under quasi-static condition, we can observe that the total elongation at fracture does not vary significantly with the increases of strain rate. In other words, the elongations of CP271 and CP274 almost remain constant even if the strain rate is increased to 10-'~-', this situation explains that strain rate does not influence elongations of CP271 and CP274 in the quasi static condition. In the dynamic range, the elongation at fracture is found to be lower than that of quasi static.

However, this elongation increases sharply with increases of strain rate. These different results also reflected that quantitative variations of ductility appear to be more sensitive to strain rate in the dynamic range than in the quasi static range.

3.1.3 Rupture behaviour

S.E.M investigation of the fracture surface of broken tensile specimens are shown in Fig. 6. At quasi static condition, the fracture surface shows a brittle aspect although some dimple structures are presented (Fig. 6 a , 6c). As in dynamic test at t. =103s-', both transgranular dimple and slip band fracture appeared in 1424M1 (Fig. 6d). For 1424M2 (Fig. be) specimens, a prodominant integranular fracture mode is revealed. Also, deep cracks in the subgrain boundary and many tearings

through matrix are observed. For 1416M (Fig. 6f), the material failed in a brittle manner and the larger cleavage facets corresponding to a brittle and having a poor ductility are seen on the fracture surface. For 1416T (Fig. 6b), intermediate behaviour was observed, both cleavage facets and dimpled region are apparent on the fracture surface.

If we compare the fracture surfaces and ductilities of these two materials on various tested conditions, the CP274 have a better combination at strength and ductility than CP271. Although it is apparent that ductility increases with increasing strain rate in dynamic range. The effect of compositions and heat treatment condition is larger than that of strain rate. This is generally due to best combination of mechanical properties and ductilities which correspond to a proper aging time ane aging temperature, furthermore, heat treatment conditions vary with different compositions. On the other hand, it had been reported that higher copper content will make higher strength in aluminium Lithium alloys, with regard to Li- thium, the higher Lithium contents mean a lower density result and a loss of ductility. That is the reason why CP274 has a more prodominant tendency to ductile fracture than CP271.

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( b ) 1416T ;=lo3s-' (f) 1416M <=lo's-'

Fig. b Fractographe for CP271 and CP274 Tensile specimens 3.1.4 Microstructure

The dislocation configurations of tensile specimens of CP271 and CP274 are shown in Fig. 7. Under quasi static condition, the dislocation are distrihuted uniformly in the matrix. For exemple, for 1416T (Fig. 7a), few dislocation tangles, dislocation dipoles as well as mixed dislocations were ohserved in the matrix. The dislocation pile ups in their slip planes against a grain boundary are observed. For 1424M1 (Fig. 7b), the material exhibits a number of helical and loop dislocations, and some dislocation segments which tangle together.

Under dynamic conditions (;=lo 3 ~ - 1 ) , for 1416T (Fig. 7c), many dislocation segments are evidenced at structure; some dislocations pile ups on slip planes with some arrangements and some of them shape as a network. For 141hM (Fig. 7f), needle shaped dislocations at quasi-static condition still exist with dislocation tangles and microstructure of forest dislocations is observed. For 1424M1 (Fig. 7d), the matrix consisted of many pairs of dislocations, also, long and nearly straight

screw dislocation pairs were seen. For 1424M2 (Fig. 7e), the tangle dislocation arrangement is well developed, long and short segments are formed by interactions

(a) 1416T

;=~o-'s-' &?!!!.

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Fig. 7 Transmission electron micrographs o f tensile specimens for CP271 and CP274 3.2 Compression tests

3.2.1 Stress strain curves

Stress strain curves of compression were obtained from three strain rates:

~ o - ~ s - ' ,

10~5-l and 2 x 1 1 1 ~ ~ ' and tested each at five strain levels: nominally R%, lo%, 13%, 16% and 20%. Here, we only prpsent results of CP271 and CP274 stress strain curves of 20% in Fig. 8. As indicated in the figu e we can see that the dynamic curves obtained at strain rates of lo3 S-' and 2X1.0

5

S" are always higher than static ones. Also, in the dynamic curves, at lower strain (below IS%), the flow stress at higher strain rate is greater than that of lower strain rate. Rut, at higher strain (above 15%), the flow stress of 2 x 1 0 ~ ~

-'

is only slightly stronger than that of 1 0 ~ s ' and they tend to reach a same value.

Fig. 8 Stress strain curves for CP271 and CP274.

tion. (V): perpendicular to rolling direc

900 -

----10-3.' - - -103-1 100 -

N O f ;

1 ,'

/

( d ) l . , ? 4 E i X l O

1; '

( e ) 1 4 2 4 ~ 2 ( ~ )

10%

I:

100 , o! , , '

ST21111 (7.)

(H): parallel to rolling direction.

3.2.2 Stress-strain relation

Fig. 9 a and Fig. 9 h shoz the relationship between stress and strain at different strain rates,

1n3s-'

and 2 x 1 0 ~ S"

.

From the figures, we can notice that a linear relationship exists. The flow stress levels under different strains in sequence from high to low are 1424M2 (H), 1416M (H), 1416M ( V ) , 1424M1 (H) and 1416T (V).

Under the same heat treatment condition, the flow stress level of 1424M2 is

stronger than that of 1416M. When the tested materiats are of the same composition, the flow stress of 1424M2 ( H ) is larger than that of 1424M1, and the 1416M is stronger than 1416T. These results indicate that the mechanical properties are related to the heat treatment conditions and compositions. Moreover, under the optimum aging conditions (i.e. time and temperature), a best conbination of stress and toughness can be obtained. From the results of 1416N (H) and 1416M (V), the flow stress in parallel to the rolling direction and is a little larger than that in the perpendicular direction, this difference probably is due to the isotropy and grain size effects.

900 900

-

700.-+

5

600 -

V) 0 1 4 2 4 ~ 2 (H)

V) "7

u 1424H2 ( H ) Y

P(

A 14161 (H) A 1416H (H)

% 500-

$

⁵⁰⁰

-

⁴¹⁴¹⁶¹ ^{( V )}

8 + 1416H ( V ) u x 142411 (H)

( a ] F-1000s-' x 142411 ( H ) 3 1 ( b ) i=2ooos-' 1416T (Y)

14167 ( V )

8 10 12 14 16 18 20 8 10 12 14 16 18 20

STR~IN 1%) s r m r a ( I )

Fig. 9: Effect of strain on flow stress for CP271 and CP274 (a) ;=1000 S-' (b) ;=

2000 s-1

3.2.3 Effect of strain rate

The effect of strain raft= o v the compressive stress for com?ression specimens is

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C3-26 JOURNAL

DE

PHYSIQUE

shown in Fig. 10a and Fig. 1% for CP271 and CP274 respectively. It is obviously apparent that these two materials exhibit two regions with different strain rate sensitivities and that the stress increases repidly with increasing strain rate from

~o's-'

to 104s

-'.

The reason of this has been described in the section of (3.1.2). According to Lindholm's definition of the strain rate sensitivity,B, it can he written as follows: @ = I/&-dU/dk. The slope du/di- was obtained under the strain range of 10-35' to l d S-'

,

^0,being the flow stress at

i

= IS-'

.

^{Thus, the}

values of 0.023, 0.021, 0.02, 0.018 can be obtained for 1416T (CP271), 1424M1 (CP274), 1416M (CP271), 1424M2 (CP274) respectively. From the above results, it is found that heat treatment conditions and alloying effect of lithium have a important influence on the strain rate sensitivity of CP271 and CP274. Considering those two factors, the relative rate sensitivity decreases with increasing mean strength of the alloy. The CP27l alloys reveals a more important strain rate sensitivity than that of CP274 alloys. If we compare the strain rate sensitivity of CP271 and CP274 alloys to those of other pure or commercial aluminiums, w e observed an important effect of strain rate sensitivity on these materials.

Fig. 10: The variation of flow stress vs the logarithm of the strain rate for CP271 and CP274 alloys.

3.2.4 Microstructure

From the results of T.E.M, by comparing the dislocations configurat-ions of CP271 and CP274, we can find that at the quasi static conditions there are no dislocation cell is observed even if these materials are deformed to 20% strain. For example, for 1416T (CP271), at 13% strain levels (Fig. lla), dislocations are distributed uniformly and screw dislocation pairs and pinched off dipoles are exhibited in the matrix. If the strain is increased to 20% (Fig. llh), typical planar dislocation arrays appear in their structure ane the dislocations are aligned along in their

1000

;

⁸⁰⁰

-

VI

g

600

C r

slip planes. However, at dynamic conditions; although the dislocation segments tangles or pile ups with some arrangements under lower strain levels, as Tlig. lle, all dislocation cells are formed at 20% for i=1000 S-' and 1 6 % for 2000 S

,

^except

for 1424M2 in which dislocation cells are formed at 16% for 1000 S - ' . Some dislocation cell structures in some of the above mentioned materials are shoivn in Fig.

Ild to llg. Although the generation of dislocation structures and formation of cells under compression loading depends on a number of parameters. Theoretically, as Fig. llh, the dislocation cell structures can be related to stacking fault free energy and other energetic considerations. These two elements cause cells to form as equilibrium arrays, however, they depend upon the dislocation character which are generated by appropriate applyed stresses and at the available time. I n our test, it is obvious that the dislocation configurations vary with strain levels and strain rate, anyways; strain rate has a more important influence on formation of dislocation cells than strains.

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

LOG STRAIN RATE (5-'1 LOG STRAIN RATE ( s - ' )

( a ) CP271 :A

- .Q

_i

-

@:

d

1000

- ;

⁸⁰⁰

-

rn 2 600 a

-

g

⁴⁰⁰

?A u

g

²⁰⁰

3.3 Constitutive equations

A constitutive equation of the following form is proposed to describe the dynamic flow stress of compression for CP271 and CP274.

r

(b) CP274

- .

_Q.^Y

- .

-

^o1424H2 ( H )

= 1424M1 ( H )

-

L ' 1 9 1 t * 1 *

2

⁴⁰⁰ ^11416M^{( H )}

u

z

^{2 0 0 -}

U

1416H ( V ) -1416T ( V )

I

(10)

k.here, A and K are constants, r is compression strain,n is strain hardening index, m is constant egual to eB, @ is strain rate sensitivity, toand

5

are strain rates at dynamic and quasi static respectively. The values of A and n are obtained from the stress strain curves of quasi-static tests by the least mean square method. The constant K was determined by trial and error until a close fit of the dynamic curve was obtained,@ is got from the section of 3.2.3. All these values are given in

table 2. Table 2

( g ) 1416M r=20%

a

(h) Model of cell structure(15)

?=lo3 S-I

Fig. 1 1 Transmission electron micrographs of compression specimens for CP271 and CP274.

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C3-28 JOURNAL DE PHYSIQUE

Comparison of the experimental and caculated curves are presented in Fig. 12, where a very good agreement to the experimental results can be found.

STRAIN ( % )

Fig. 12: The comparison between the experimental and caculated curves

4-

CONCLUSIONS

From the above results and discussions, some general conclusions under dynamic response can be established as follows: (1) The CP271 and CP274 A1-Li alloys are sensitive to strain rate, the stress and ductility always increase with strain rate in the dynamic range. (2) Under the same heat treatment conditions, by comparing mechanical properties, CP274 has higher strength, higher fracture strain and higher ductility than those of CP271. (3) All fracture surfaces depend on compositions and heat treatment conditions. Effects of strain rate on the overall fracture mode could be detected. (4) The dislocation arrangement and stress strain relationship indicate that there is a different rate controlling ,echanism at low and high strain rates. (5) The dislocation configuration and formation of dislocation cells vary with strain and strain rate, the latter have a important effect than the former in

this case.

REFERENCES

/I/ Campbell, J.D., Mat. Sci. Engg.

2

(1973) 3-21.

/2/ Lindholm, U.S., The Institute of Physics, Conference Series No. 21, London (1974) 3-21.

/3/ Campbell, J.D., Dynamic Plasticity of Metals, Udine (1970), Springer Verlay.

/ + / Lindholm, U.S., J. Mech. Phys. Sol.,

12

(1964) 317-335.

/5/ Chiem, C.Y. and Duffy, J., Matls, Sci. and Engr.,

57

(1983) 233-247.

/ h / Holt, D.L.; Rabcock, S.G.; Green, S.J. and Maiden, C.J., Trans-Am. Soc. Metals

6 0 (1967) 152-159.

/7/ c e g e r ; A., Phil. Mag.

5

(1955) 1194-1217.

/8/ Lindholm, U.S., and Yeakely, L.M., J. Mech. Phys. Solids

13

( & q - ( ) 41-53.

/9/ Shioiri, J;, Proc. 2nd Shock Technology Symposiup (1975) 115-122.

/lo/ Klahn, D ; , Mukherjee, A.K., and Dorn, J.E., 2nd Int. Conf. on Strength of Metals and Alloys, ASM (1970) 951-982.

1111 Clifton, R.J., J. of Appl. Mech.,

50

(1983) 941-952.

1121 Follansbee, P.S. and Zeertman, J., Mechanics of Materials, l(1982) 345-350.

1131 Evans, A.G. and Rawlings, R.D., Phys. Stat. Sol.

2

(1969) 9-31.

1141 Kolsky, H;, "Stress wave in solids", Dover; New York (1963)

1151 Shioiri, J., Satoh, K., and Kishimura, K.: "Experimental Studies on the Beha- viour of Dislocations in Copper at High Rates of Strain", High Velocity Deformation of Solids, Kawata, K., and Shioiri, J., (Eds.), Springerverlag, N.Y. (1978) 50.