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THE EFFECTS OF AN INTERFACIAL SHEAR STRESS ACTING ON Al/Al2Cu BOUNDARIES
M. Ignat, S. Kaddour, M. Dupeux
To cite this version:
M. Ignat, S. Kaddour, M. Dupeux. THE EFFECTS OF AN INTERFACIAL SHEAR STRESS ACT- ING ON Al/Al2Cu BOUNDARIES. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-651-C2-656.
�10.1051/jphyscol:1985471�. �jpa-00224726�
THE EFFECTS O F AN I N T E R F A C I A L SHEAR S T R E S S A C T I N G ON A l / A l
2C u B O U N D A R I E S
M. Ignat, S. Kaddour and M. Dupeux
Inst-itut National Polytechnique de Grenoble, L.T.P.C.M., C.N.R.S. , E.N.S.E.E.G., B.P. 75, 38402 Saint-Martin-d'Heres, France
Résumé - Les études fondamentales sur les propriétés mécaniques des joints de phases sont peu nombreuses. Cependant, l'importance des joints lors de la déformation à haute température d'alliages biphasés est reconnue.
Afin de mieux mettre en évidence le rôle des joints de phases, nous avons choisi de les solliciter en cisaillement parallèlement à leur plan, et ceci sur deux types d'éprouvettes : des monograins lamellaires de 1'eutectique Al/Al
2Cu et des bicristaux Al/Al
2Cu.
Les repères tracés sur les faces de ces échantillons mettent en évidence, en fluage ou lors d'essais dynamiques, la contribution des joints, ainsi que le cisaillement de la phase aluminium.
Les résultats obtenus pour les deux types d'échantillons montrent que, à haute température, les bicristaux semblent plus résistants au cisaillement que les échantillons d'eutectique, tandis qu'aux températures les plus basses de nos essais, les bicristaux présentent un comportement fragile.
Abstract - The important role of interphase boundaries during high tempera- ture Reformation of two-phase alloys is now well established. This article presents the first results of shear experiments on two-phase samples. For the purpose of analyzing the interphase deformation mechanisms, shear stress was imposed parallel to the phase boundary planes.
Creep and constant strain rate tests were performed on two different types of specimens : Al/Al-Cu lamellar eutectic monograins and Al/Al^Cu two-phase bicrystals.
Thin grooves marked on the specimens faces revealed the respective contribut- ion of the phase boundary sliding and the shear of the aluminium phase.
For the eutectic samples, microstructural observations show that interlamel- lar sliding is the predominant deformation mechanism, acting principally at singular phase boundaries. Creep results are discussed in terms of the mea- sured activation energy. Comparing the behaviour of the bicrystalline to the eutectic samples, we note that, at the high temperatures the bicrystals are more resistant to shearing while, at lower temps, they exhibit brittle beha- viour.
I -INTRODUCTION
Most engineering materials used in advanced technologies are two phase alloys. Metal/
metal composite materials or nickel based superalloys are two typical examples. The high temperature deformation of metals is often interpreted using semi-empirical parameters deduced from mechanical tests, e.g. : the activation energy and volume, the stress exponent, etc... These are related to a possible mechanism which controls the deformation ; generally described in terms of dislocation motion. Similarly, for two phase alloys, the control of deformation is frequently attributed to a single mechanism active in only one of the phases. Despite numerous investigations of two phase materials and discussion of the importance of interfacial processes, this in- terpretation is still prevalent in current litterature. For instance, in the case of composite materials, analysis of their deformation is based on the reinforcing phase
+
L . A . 29
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985471
C4-652 JOURNAL DE PHYSIQUE
concept [l, 21. Contrary t o t h i s k i n d o f a n a l y s i s , i n a r e c e n t work on a l a m e l l a r composite, we showed t h a t t h e " r u l e o f m i x t u r e " f a i l s a t h i g h temperatures 131.
Moreover, t h e coherent i n t e r p r e t a t i o n o f h i g h temperature behaviour o f o u r l a m e l l a r m a t e r i a l , obtained from two d i f f e r e n t deformation t e s t s , was o n l y p o s s i b l e by consi- d e r i n g t h e deformation mechanisms o p e r a t i n g a t t h e i n t e r f a c e [4]. I t i s worth n o t i n g t h a t f o r two phase a l l o y s , the o n l y case i n which t h e interphase mechanisms a r e con- s i d e r e d as p l a y i n g a predominant r o l e i s i n s u p e r p l a s t i c systems, i.e., a p a r t i c u l a r s i t u a t i o n : f i n e microduplex s t r u c t u r e and s p e c i a l l y imposed experimental c o n d i t i o n s [5, 6, 71.
Before presenting o u r f i r t s r e s u l t s i t i s u s e f u l t o review b r i e f l y t h e e x i s t i n g e v i - dence f o r t h e i n f l u e n c e o f phase boundaries.
I n t h e case o f two-phase a l l o y s - For p o l y g r a i n e d a - Bbrass s l i p accomodation was observed near boundaries p e r p e n d i c u l a r l y o r i e n t e d t o t h e d i r e c t i o n o f t h e a p p l i e d s t r e s s [8]. Research i n t o s l i p p a r a l l e l t o t h e i n t e r f a c e i n l a m e l l a r e u t e c t i c s has been performed w i t h Pb/Sn p o l y c r y s t a l s 19, 101 , and w i t h A1 /A12Cu monocrystal
S11 l] .
I n these works c o n s t i t u t i v e deformation laws and observation o f i n t e r l a m e l l a r s l i p were analysed.
I n t h e case o f two-phase b i c r y s t a l s - S l i p systems a c t i v a t e d i n the phases, deform- a t i o n e n o m e n a were s t u d i e d u s i n g a p p l i e d s t r e s s e i t h e r
p a r a l l e l o r perpendicular t o t h e phase boundary, 112 t o 161 . L i t t l e research, however, has been r e p o r t e d concerning b i c r y s t a l l i n e interphase s l i p , i n v o l v i n g a - Bbrass b i c r y s t a l s obtained by d i f f u s i o n bonding [17,18].
I 1 - EXPERIMENTAL PROCEDURE i ) M a t e r i a l s
Two types o f samples ; b o t h using t h e same two phases and subjected t o shear p a r a l l e l t o phase boundaries :
Lamellar monograins o f A1/A12Cu e u t e c t i c composite
A f t e r u n i d i r e c t i o n a l s o l i d i f ic a t i o n (Bridgman me.thod) o f as-cast e u t e c t i c ingots, rods o f l a m e l l a r monograins were obtained so t h a t each l a m e l l a o f about 2 pm wide was a s i n g l e c r y s t a l . Furthermor$, the l a m e l l a e had a w e l l d e f i n e d c r i t i c a l o r i e n t - a t i o n : t h e t e t r a g o n a l A1,2Cu (2 11) planes p a r a l l e l t o t h e face centered cubic A1
(111) planes, t h e l a t t e r being c l o s e t o t h e mean interphase plane. But t h e lamel- l a r s t r u c t u r e i s o n l y p e r f e c t w i t h i n a 20 Mm range, t h a t i s as blocks d e f i n e d by a l t e r n a t i n g low angle boundaries across the l a m e t l a e w h i l e l o n g i t u d i n a l l y l i m i t e d by s i n g u l a r phase boundaries. This d e f i n e s subgrains w i t h i n the s i n g l e c r y s t a l .
A1/A12Cu b i c r y s t a l s
These were obtained by d i f f u s i o n bo nding o f s i n g l e c r y s t a l s o f each phase. T h e i r r e l a t i v e c r y s t a l l o g r a p h i c o r i e n t a t i o n can be chosen using. Laue back r e f l e x i o n on each s i n g l e c r y s t a l . For t h e p r e l i m i n a r y s e t o f experiments, however, the r e l a t i v e o r i e n t _ a t i o n o f the two phases i s n o t c_o_nsistent w i t h t h a t found i n t h e e u t e c t i c , i n s - tead (120)A12Cu planes a r e p a r a l l e l t o (135)Al planes b o t h p a r a l l e l t o the boundary plane w i t h i n which [0011 A1 2Cu i s p a r a l l e l t o p321 A1 (+3O).The bonding o f t h e two s i n g l e c r y s - t a l s was achieved by a h e a t treatment o f 100 hours a t 535OC - a technique p r e v i o u s l y described and analysed 1191.
i i ) The samples
Shearing specimens o f l a m e l l a r e u t e c t i c composite were d i r e c t l y c u t from u n i d i r e c t - i o n a l l y s o l i d i f i e d rods, s i m i l a r l y b i c r y s t a l l i n e samples were c u t from l a r g e d i f f u s - i o n bonded b i c r y s t a l s . The dimensions o f t h e r e s u l t i n g p a r a l l e p i p e d s were : 8
X8
X8 mm3 f o r the e u t e c t i c and 8 X 8
X4 mm3 f o r the b i c r y s t a l s . Two c u t s o f roughly
0.5 mm t h i c k n e s s were then introduced perpendicular t o t h e i n t e r f a c e and separated
by 4 mm (see Fig. 1). Thus producing an area o f shear s t r e s s i n t h e interphase
r e g i o n when t h e specimen i s subjected t o compression, as shown by p h o t o e l a s t i c
models [22].
Fig. 1 - Schematic view o f the shear samples and o r i e n t a t i o n o f the interphase boundary planes a) case o f l a m e l l a r A1 /A1 2Cu e u t e c t i c b ) case o f two-phase b i c r y s t a l s . Dark arrows i n d i c a t e t h e d i r e c t i o n o f t h e compressive f o r c e , r e s u l t i n g i n a shear s t r e s s i n t h e c e n t r e o f the specimen ( d o t t e d arrows)
Creep t e s t s performed w i t h the c l a s s i c a l i n t e r p e n e t r a t i n g c o m p r e s s i o n j a w s w i t h t h e specimen p o s i t i o n e d between p a r a l l e l alumina p l a t e s . Any displacement was recorded using an a m p l i f i e r w h i l e a furnace p e r m i t t e d incremental temperature changes w i t h t r a n s i s t o r y periods l e s s than t e n minutes.
Constant s t r a i n t r a t e t e s t s were executed i n t h e same manner w i t h an INSTRON machine.
A thermocouple s i t u a t e d a t 1 mm from t h e specimen allowed c a r e f u l c o n t r o l o f t h e temperature although, u n t i l now, o n l y two d i f f e r e n t temperatures have been e x p l o r e d : 0.52 TE and 0.82 TE.
(TE = A1/A12Cu e u t e c t i c temperature = 821 K) I 1 1 - RESULTS AND DISCUSSION
I n the case o f t h e e u t e c t i c samples, f o r phase g l i d i n g was e v i d e n t w i t h scanning e l microscouv. A t h i a h maanif i c a t i o n observat
100th creep and compression t e s t s , i n t e r - e c t r o n microscopy (S.E.M.) o r even o p t i c a l :ion o f u r e v i o u s l v marked arooves showed
Fig. 2 - A d e t a i l o f t h e observed sheared zones(~EM) ,the l icjk,ter l a m e l l a e correspond toA12Cu pnase,thedarkerareAl lamellae.
Fig. 3 - D e t a i l e d view o f i n t e r l a m e l l a r d i s -
placement a t o r d i n a r y phase boundaries (aland
intral amellar shearing i n aluminium l amellae(b)
C4-654 JOURNAL DE PHYSIQUE
i) Important boundary shifting between segments of l ines are only present between bundles of lamellae corresponding to a displacement of one lamellar block with res- pect to another. Within these blocks slight interlamellae displacement exists which seems to be more intense at 673 K (0.82 TE).
ii) On the lamellae, the lines grooved on-the A1 Cu phase remained straight, whereas those marked on the A1 phase were distorted (at feast close to the boundaries) indic- ating an active intralamellar shear.
iii) Many of the lamellar faulted zones showed void formation.
Consequently, the shear deformation of the sample may be separated into three dis- tinct contributions : a) the plastic shear of A1 lamellae, b) the slip of ordinary phase boundaries, c) the slip of singular phase boundaries which describe the la- lamellar blocks. This last contribution being the most important of the three as previously shown [l l] .
For the creep tests, we define a creep strain rate as half of the ratio between the instantaneous compression rate, - Al/At (obtained directly from registered curves where - A1 is the compressive displacement which occurs in time at) with the width, c, of the sheared zone.
In the case of the eutectic samples
After transitory periods of two to three hours, creep strain rates were obtained as following (orders of magnitude) ;
IO-~S-' at 0.5 TE (673 K), 17 mPa
IO-~S-' at 0.82 TE (423 K), 2 mPa
Through a classical - ~ o r n method apparent activation energy was approximatively 140 kJ mole-' and 80 kJ mole-' respectively. The first of these values is close to the aluminium self-diffusion energy, indicating that the deformation should occur in that phase. But, as we observed, interphase gliding was predominant, and aluminium lamellae were only deformed in certain zones close to the phase boundaries. the second activation energy value measured at 0.82 TE is close to the value of copper phase boundary diffusion, as estimated previously in the same eutectic 1217. These considerations would indicate that at the lower temperature interlamellar sliding is always accompanied with important deformation of adjacent zones of aluminium lamellae, whereas at the higher, temperature interphase sliding, activated by intense interphase diffusion, controls the shear of the sample. Most importantly, this low temperature result is contradictory to the metallographic observations so far reported.
The registered curves from constant strain rate compression tests show, depending on the temperature, very different values for yield stresses corresponding .to 0.2 %
strain ( - Al/C) :
at 0.52 TE, a mean value of 27 mPa, at 0.82 TE, a mean value of 0.4 mPa.
Furthermore, increasing the strain past 0.2 % produced very different results ; at 0.52 TE cracks appeared parallel to the lamellae and brittle failure occured, whereas at 0.82 TE the strain was extended to 30 % while preserving a constant stress of 1.5 mPa.
In the case of the bicrystals
A creep test performed at 0.52 TE and 17 mPa (as with the eutectic samples) lead to a rapid failure of the sample along the phase boundary, through, this experiment is yet to be repeated.
For creep conditions corresponding to 0.82 TE and 2 mPa the bicrystalline specimen
did not produce any detectable deformation.~p~lied stress was increased progressi-
75 Mm. As i l l u s t r a t e d in Fig. 4, t h i s displacement was d i s t r i b u t e d in i n t e r f a c i a l s l i d i n g but also in shearing of the aluminium phase. S l i p t r a c e s observed in t h i s phase a r e compatible with (1 11)Al planes.
Constant s t r a i n r a t e t e s t s confirmed t h i s behaviour ; a t 0.52 TE, the sample f a i l e d , during e l a s t i c loading, by b r i t t l e part- ing a t the interface ; meanwhile, a t 0.82 TE, the resulting load increased regularly up t o a constant level of 5.5 mPa - t h a t i s g r e a t e r than the eutec- t i c smaples deformed under similar condi- tions. In summary, i t seems t h a t , a t 0.82 T E , the b i c r y s t a l s exhibit greater resistance to shear than the e u t e c t i c whereas, a t 0.52 TE, b r i t t l e f a i l u r e occurs.
Simply, we can account f o r these r e s u l t s by considering the action of the phase boundaries. The b i c r y s t a l s contain only one j o i n t of an imperfect nature ; having been diffusion bonded, so t h a t the shear s t r e s s i s concentrated a t c e r t a i n points.
Fig. 4 - An Al/Al,Cu two-phase bi- Thus, the shearing of the specimen' i s
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