INTERNAL FRICTION AT MEDIUM TEMPERATURES IN HIGH PURITY ALUMINIUM

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INTERNAL FRICTION AT MEDIUM

TEMPERATURES IN HIGH PURITY ALUMINIUM

M. Nó, C. Esnouf, G. Thollet, J. San Juan, Gilbert Fantozzi

To cite this version:

M. Nó, C. Esnouf, G. Thollet, J. San Juan, Gilbert Fantozzi. INTERNAL FRICTION AT MEDIUM

TEMPERATURES IN HIGH PURITY ALUMINIUM. Journal de Physique Colloques, 1987, 48 (C8),

pp.C8-161-C8-166. �10.1051/jphyscol:1987821�. �jpa-00227125�

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INTERNAL FRICTION AT MEDIUM TEMPERATURES IN HIGH PURITY ALUMINIUM

0

M.L. NO, C. ESNOUF' , G. THOLLET' , J. SAN JUAN and G. FANTOZZI' Dpto d e Fisica del Estado &lido, Facultad de Ciencias, Universidad del Pais Vasco, Aptdo 644, SP-48009 Bilbao, Spain 'GEMPPM-LA 341, INSA, Bdt. 502, F-69621 Villeurbanne Cedex, France

Resume'.- Afin de de'terminer 2e m&canisme qui e s t

12

2 'origine des relaxations PI e t P i nous avons compare' l e s r k u Z t a t s eqe'rimentaux de frottement i n t d r i e u r e t de mzcroscopie e'lectronique d'un

A2

99.9999%,obtenus autour de 0.5Tf,avec l e s previsions

thebriques des d i f f e ' r e n t s mode'les propose's duns l a litte'ratture.

Abstract.- I n order t o deternine which mechanism i s a t the o r i g i n of the PI and Pi r e l a x a t i o n s , the interma2 f r i c t i o n and electron microscopy experimental r e s u l t s of 99.9999% AZ around 0.5Tm have been compared t o the t h e o r e t i c a l forecasts of d i f - f e r e n t proposed models.

1.- INTRODUCTION

Under c e r t a i n experimental c o n d i t i o n s we can observe i n t h e h i g h e s t p u r i t y a l u - minium (99.9999%) t h e Pl and Pi r e l a x a t i o n s around 0.5Tm ( a t lHz) / 1 , 2 / . I n a lower p u r i t y aluminium (99.9999%+10ppm Cu o r Ag) we can observe a r e l a x a t i o n a t h i g h e r t e m p e r a t u r e s w h i c h c o u l d correspond t o PI r e l a x a t i o n of A 1 99.9999%(6N)/3/. Another k i n d o f r e s u l t s , which we w i l l d i s c u s s l a t e r on, have been o b t a i n e d by K I e t a l . on 6N A 1 /4/.

I n t h i s paper we w i l l try and a n a l y s e which model of a l l t h o s e proposed up t o now c o i n c i d e s w i t h t h e experimental behaviour of P1 and Pi r e l a x a t i o n s .

2.- EXPERIMENTAL CONDITIONS

I n t h i s work, we have used a 6N A 1 r e f i n e d by zone m e l t i n g . Some samples had been doped with lOppm o f Cu o r Ag. A l l of them have been produced by CECM of V i t r y / S e i n e , France. These samples have undergone v a r i o u s thermo-mechanical t r e a t m e n t s i n o r d e r t o c r e a t e d i f f e r e n t m i c r o s t r u c t u r e s :

-

Cold r o l l e d a t room temperature(RT1 and a t 77K.

-

T o r s i o n a l deformation a t RT and a t 77K.

-

Creep a t 200" and 300%.

-

Annealing a t 600K.

I n t e r n a l F r i c t i o n ( I F ) s p e c t r a have been t a k e n w i t h an i n v e r t e d t o r s i o n pendulum o s c i l l a t i n g around 1 Hz w i t h 2 x l 0 - ~ < E, < 5 x 1 0 - ~ and 6K < T < 730K. We can a l s o superimpose a s t a t i c t o r s i o n a l s t r e s s (us) upon t h e o s c i l l a t i n g s t r e s s (0,).

The m i c r o s t r u c t u r e s o f t h e samples h a s been analysed b e f o r e and a f t e r applying t h e s e thermo-mechanical t r e a t m e n t s , thanks t o a JEOL-2OOCX (AVWork=160Kv). The s t u d y f o c u s e s on t h o s e p a r t s of t h e samples where t h e t h i c k n e s s i s over 6000A s o t h a t they a r e r e p r e s e n t a t i v e of t h e bulk.

3 . - EXPERIMENTAL RESULTS

*

The samples deformed by t o r s i o n o r c o l d - r o l l i n g a t RT up t o 50% show two r e - l a x a t i o n s Pi and P i which a r e s t a b l e a f t e r a f i r s t a n n e a l i n g a t 600K (Fig.1 curve a ) . These r e l a x a t i o n s a r e n o t a f f e c t e d by a s t a t i c s t r e s s

(us)

superimposed upon t h e o s c i l l a t i n g s t r e s s (0,) and whose o r d e r of magnitude i s up t o 10 t i m e s ~ r ~ ( F i g . 2 ) / 1 , 2 , 5 / . T h e s t r e n g t h s of t h e s e r e l a x a t i o n s show a maximum v e r s u s t h e o s c i l l a t i n g amplitude ( f i g . 3 ) /1 , 2 , 5 / w h i l e a t t h e same time t h e peak s h i f t s towards low tem- p e r a t u r e s . The a c t i v a t i o n energy of t h e Pl r e l a x a t i o n was measured by two d i f f e r e n t ways : a ) I n a l o w frequency pendulum / 3 / . b ) By microcreep a t v a r i o u s t e m p e r a t u r e s , u s i n g Klam-s technique / 2 / .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987821

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We obtained the following results : Epl = l.leV, Hpi = 0.8eV.

By microcreep measurements and for different applied stress we have determi- ned the activation volume of the process : vpl E vpi z 1000b3.

The observation of the samples by Transmission Electron Microscopy indicates that the dislocations are organised in cells formed by tangled dislocations

(Fig-la)

.

The sampleswhichhave undergone creep through tension or torsion at 2000C or 3000C d o not show P1 and Pi relaxations ; however they show a strong background towards high temperatures (Fig.1 curve b) which corresponds to the low temperature side of a relaxation placed near the melting temperature/2,5/. In this case, the samples show a polygonized microstructure (Fig.lb). The size of the subgrains is about 30pm and the subboundaries are generally formed by families of dislocations that do not verify Frank-s law.

A deformation of at least 20% by torsion at RT, or a similar deformation by rolling at RT causes the reappearance of P1 and Pi relaxations. A later creep makes them disappear again /2,6/.

We could then conclude that P1 and Pi relaxations are associated to a deformation cells microstructure, while their disappearance is linked to a polygonized microstructure.

Experimental results on 99.9999%Al+lOppm of Cu or Ag have been reported in / 3 / and could be summarized as follows :

There is only one relaxation which occurs at higher temperature (T(Al+Ag)=526K, T(Al+Cu)=565K) than the P1 relaxation, whose activation energy is 1.27eV for Al+Ag and 1.54eV for Al+Cu. On the other hand a superimposed static stress does not affect internal friction spectra, and the internal friction maximum decreases versus oscillating amplitude. Furthermore, it is imaortant to remark that the relaxation dissap- pears by creep.

4.- DISCUSSION

In this section, we will try to bring together all the models proposed in order to account for the relaxations that appear around 0.5Tm in aluminium and other f.c.c. metals. We will also take into account all the models proposed to explain creep in stage 11, since the temperature range in which such stage occurs, as well as the activation energy and the activation volume either coincide or are close to those of P1 and Pi relaxation. So, we will analyse each model predictions, and will compare them to our experimental results.

a) Mechanisms connected with boundaries

KO et al. /4/ have worked on A1 6N and they have noticed that in the case of polycrystals there is a relaxation at 480K which they identify with KO's peak and they attribute to the intercrystalline sliding at boundaries. If the grain size

41

veri- fies $I > e (e being the sample's thickness), then the peak is replaced by another that they call "macrocrystalline peak" (T

"

50010, attributable to a mechanism in which polygonization and boundaries play important roles. A 0.4% deformation on the previous sample makes the macrocrystalline peak disappear, and makes the Pi peak appear at 450K.

We have previously proved /2,6/ that the background and P1 relaxation depend on the thermomechanical treatments and the oscillation amplitude. In this way we can obtain variations up to 60K in the peak temperature, depending on the experimental conditions. This makes us think that the three peaks mentioned by KO et a1./4/ that have never been noticed at the same time, are the same P1 relaxation. Furthermore, we have proved that both the appearance or disappearance of P1 and Pi, as well as their intensity are independent from the grain size. This makes us think that P1 and Pi can in no way be interpreted as a mechanism associated only with the boundaries as was formerly proposed by Woirgard /7/, who noticed Pi, P2 and P3 relaxa- tians in 5N A1 both in monocrystals and polycrystals.

b) Cross-slip on planes {111)

Among the models proposed to account for cross-slip Friedel-Escaig's stands

out / 8 / . Let us start from a constriction which has nucleated on a fault ribbon

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allowing some dislocationslength in between to be bowed out and to dissociate in the cross-slip plane as soon as the halves are brought apart (Fig.4). For some critical separation

,

AB, the configuration energy goes through a maximum, beyond which A and B are pushed apart under the work of stress, making the whole dislocation cross- slip. The activation energy falls between Uc and 2Uc (Uc being the constriction energy). If we consider that Uc is of order of a jog creation energy (Uc=0.42eV) we obtain that the activation energy of this process is very inferior to that of PI, Pi and P relaxations, so this process can not explain these relaxations.

The model proposed by Caillard /9/ and later generalised by Morris /10/(Fig.5) assumes that in the case of three dislocations which come to an intersection, their corresponding buraers vectors are coplanar This model explained before in /5/ was proposed to explain the extraction of dislocations out of the polygonization walls being these formed by three fainilies and we think

that

it could also be generalised to the deformation cells every time we have three coplanar vectors. Caillard shows that the stress that this mechanism needs to take place is very high (approximately 25% lower than Orowan's stress). Such stress can not be justified by means of the stress applied in IF measurements ; thus we consider that this process can not be responsible for P1 and Pi relaxations. On the other hand, the fact that the applied static stress superimposed upon the oscillation stress does not affect P1 and Pi relaxations

,

means that this mechanism does not explain these relaxations /2,5,6/.

Any applied static stress higher than the oscillation stress should produce the cross-slip and then, internal friction should disappear. Furthermore, the activation energy for this process is weak if we consider that the constriction nucleation energy is bore or less the same as the jog formation energy (Uc=0.42eV).

Later on, Morris et al. /lo/ observed that the extraction in the case of the three d i s l o c a t i o n s being on the same plane a s the burgers v e c t o r , can take p l a c e through a simple s1ipping.mechanism that needs climbing. We therefore think that in that case it is the climbing that controls the extraction mechanism.

C) Slip on non-compact planes

The experiments carried out by Carrard /11/ on monocrystals deformed by creep or tension along [112] directions, show that in the case of creep, the slipping on

{loo]

can be noticeable from 1800C onwards for 10-~s-l< E < 10-~s-l, whereas in the case of tension tests, it does not appear until 2790C for

t

= 1 2 ~ 1 0 - ~ s - ~ .

There are several models proposed to account for slipping on non-compact planes.

Among these, the appropriate one to explain the experimental results obtained up to this moment is glide on non-compact planes of dissociated dislocations on {Ill).

Such mechanism proposed by'friedel and later improved by Escaig /12/ assumes a dissociated dislocation on a plane (111) that joins together along a critical length AB, producing double kinks on a non-compact plane, and then it dissociates again on a plane (111) (Fig.6). This mechanism needs to create four constrictions and two dissociated segments on a non-compact plane. So,the activation energy will be :

2UK = 4Uc

+

2UR

,

UK being kink nucleation energy, Uc constriction nucleation energy and UR the energy needed to create a dissociated segment AA'or BB: If we consider the fact that Uc can be compared with the jog creation energy, and that UK is between 0.9 and 1.13eV /11/, the result is 1.3eV < 2UK < 1.7eV. The activation energy seems to be slightly high compared with the activation energy of P1 and Pi relaxations, noticed on 6N Al. On the other hand, the activation volume expected for this mechanism is between 350b3 < v < 700b3 /11/, a value' which is slightly low compared with the activation volume measured in our micro-creep tests 900b3 < v < 1600b3 /I/. We must also point out that such mechanismsh~~ld give anIFpeak similar to Bordoni's. Therefore we could expect non linear effects like those described by Esnouf /13/ for Bordoni relaxation. Thus, in the case of weak internal stress, both the internal friction peak strength

$

and its correspondent temperature must have a maximum versus 0,. So,

41

and TM increase as Pare's condition is not fulfilled, in such a way that if we apply a static stress superimposed upon the oscillating stress, the strength of the relaxation will increase. Our experimental results show a maximum

$

versus 0, but on the contrary T, decreases with a non linear dependen- cy versusUmrhile Us does not affect P1 and Pi relaxations, as opposed to the theo- retical predictions.

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A 1 samples doped with lOppm of Cu o r Ag have v a l u e s f o r t h e a c t i v a t i o n energy o b t a i n e d f o r P r e l a x a t i o n and IF r e s u l t s /3/ t h a t can be explained through F r i e d e l - Escaig's mechanism. However t h e s t u d y w i l l be incomplete u n t i l t h e m i c r o s t r u c t u r e o f t h e samples i s throughly analysed and i n t e r n a l f r i c t i o n is s t u d i e d i n d e t a i l i n o r - d e r t o p r e c i s e t h e i n f l u e n c e of t h e a p p l i e d t e n s i o n . A s we w i l l s e e l a t e r on, t h e r e s u l t s o b t a i n e d up t o now a l s o c o i n c i d e with a d i s l o c a t i o n g l i d e mechanism c o n t r o l - l e d by jog climb.

I f , i n s t e a d of c o n s i d e r i n g f r e e d i s l o c a t i o n s we considered a subboundary w i t h t h r e e f a m i l i e s of d i s l o c a t i o n s v e r i f y i n g Frank's law, then one of t h e t h r e e f a m i l i e s i s screw. Under s t r e s s , mixed d i s l o c a t i o n s can g l i d e on two p l a n e s { I l l ) while screw d i s l o c a t i o n s can do i t on (100) o r on {110) a f t e r a Friedel-Escaig mechanism i n which o n l y one kink w i l l be n u c l e a t e d s i n c e node 0 is a b l e t o g l i d e f r e e l y ( p i g . 7 ) / 1 5 / . The a c t i v a t i o n energy expected w i l l be : 0.65eV < UK < 0.85eV, which i s t o o low t o account f o r PI r e l a x a t i o n . ' M o r e o v e r , we have n o t i c e d through o u r experiments t h a t Pi r e l a x a t i o n d i s a p p e a r s from polygonized samples,which seems t o i n d i c a t e t h a t t h i s mechanism i s n o t t h e c a u s e o f P1 r e l a x a t i o n . On t h e o t h e r hand, it seems u n l i k e l y

t h a t i n t h e c a s e of non polygonized samples with a t a n g l e d d i s l o c a t i o n s t r u c t u r e t h e r e i s a high number of screw d i s l o c a t i o n s t h a t cause a r e l a x a t i o n a s i n t e n s e a s P1 o r P i . N e v e r t h e l e s s a more thorough a n a l y s i s of t h e m i c r o s t r u c t u r e would be advi- s a b l e t o v e r i f y t h i s p o i n t .

d ) c o n t r o l l e d by p i p e - d i f f u s i o n

u s suppose t h e r e i s a screw o r mixed d i s l o c a t i o n with a s e s s i l e jog. The d i s l o c a t i o n cannot g l i d e u n l e s s t h e jog climbs thanks t o a non c o n s e r v a t i v e movement which p r o v i d e s v a c a n c i e s c r e a t e d e i t h e r i n t h e bulk o r n e a r t h e d i s l o c a t i o n (Fig.8!.

In t h i s p r o c e s s , t h e a c t i v a t i o n energy i s around t h e a u t o - d i f f u s i o n energy (H,,=l.SeV) o r t h e p i p e - d i f f u s i o n (Hd=l.leV), and it depends s l i g h t l y on t h e t y p e of d i s l o c a - t i o n /14/. I n such c o n d i t i o n s H,, i s t o o high t o be r e s p o n s i b l e f o r t h e r e l a x a t i o n s we a r e d e a l i n g with. However, r e l a x a t i o n s P1 and Pi can be explained i n terms of p i p e - d i f f u s i o n . I n t h i s mechanism, a s t a t i c s t r e s s superimposed upon a m cannot a f f e c t t h e r e l a x a t i o n because i t w i l l o n l y change t h e o s c i l l a t i o n c e n t r e of t h e d i s l o c a t i o n l i n e . T h i s p r e d i c t i o n a g r e e s with t h e experimental r e s u l t s . On t h e o t h e r hand, from t h e v a l u e 0, we could e x p e c t a d e c r e a s e i n t h e i n t e r n a l f r i c t i o n maximum v e r s u s t h e o s c i l l a t i n g s t r e s s . For high s t r e s s , t h e number of v a c a n c i e s absorbed o r e m i t t e d d u r i n g h a l f c y c l e of t h e o s c i l l a t i n g s t r e s s is l i m i t e d by t h e number o f v a c a n c i e s , Nv, t h a t may be c r e a t e d n e a r a j o g a t a given temperature. T h e r e f o r e , t h e number of jumps N t h a t t h e d i s l o c a t i o n may have, amount t h a t must be i n c r e a s e d with o s c i l l a - t i n g s t r e s s , w i l l be l i m i t e d by t h i s p r o c e s s . Thus, t h e energy d i s s i p a t e d i n a c y c l e

( a t a given temperature) s t a y s t h e same whereas t h e e l a s t i c energy i n c r e a s e s w i t h s t r e s s and a s a consequence t h e maximum of i n t e r n a l f r i c t i o n d e c r e a s e s w i t h s t r e s s . T h i s behaviour a g r e e s w i t h t h e experimental r e s u l t s ( s e e F i g . 3 ) / 2 , 6 / .

On t h e o t h e r hand, we could e x p e c t t h a t i n t h e c a s e of polygonized samples, t h e d i s l o c a t i o n s a r e more s t r a i g h t ( t h e p o l y g o n i z a t i o n w a l l s a r e f l a t t o minimize t h e energy) and t h e r e f o r e t h e r e w i l l h a r d l y be any deformation jogs and s o , t h e ~ would have t o be c r e a t e d . I n t h i s c a s e t h e a c t i v a t i o n energy would correspond t o t h a t of a dou- b l e jog n u c l e a t i o n , which i s very high / 5 / .

I n t h i s way, t h i s mechanism seems capable of accounting f o r a l l r e s u l t s concer- ning r e l a x a t i o n s p1 and i n 6N AI. For 6N AI doped with ~ g o r C U , t h e e x p e r i m e n t a l r e s u l t s o b t a i n e d i n i n t e r n a l f r i c t i o n can a l s o be e x p l a i n e d i n terms of t h i s

mechanism. Indeed,%n t h i s c a s e , t h e i m p u r i t i e s w i l l t e n d t o p l a c e on t h e jogs modi- f y i n g t h e climb energy of t h e jog. Therefore t h e a c t i v a t i o n energy of t h e process must be s l i g h t l y h i g h e r than t h a t of p i p e - d i f f u s i o n , i n agreement with t h e experimental r e s u l t s .

5.- CoNcLusIONS

Considering our experimental r e s u l t s and t h e models proposed s o f a r , we can conclude t h a t t h e only mechanism c a p a b l e of accounting f o r P1 and P i r e l a x a t i o n s i n 6N A 1 i s d i s l o c a t i o n g l i d e (screw f o r Pi, mixed f o r P') c o n t r o l l e d by jog c l i m b and vacancy p i p e - d i f f u s i o n . However, f o r 6 N A 1 doped wlth i m p u r i t i e s , we need some 1 supplementary i n t e r n a l f r i c t i o n t e s t s and a c h a r a c t e r i z a t i o n of t h e samples micros- t r u c t u r e i n o r d e r t o s t a t e i f t h e observed P r e l a x a t i o n i s t h e same a s PI r e l a x a t i o n which has s h i f t e d towards high t e m p e r a t u r e s due t o t h e presence of i m p u r i t i e s , o r

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REFERENCES

1

.-

M.L.N6, J.San Juan, C.Esnouf, G.Fantozzi, A-Bernalte; J. de Phys.44(1983)C9-751 2

.-

M.L.N6; Thesis University of Lyon (France) (1985)

3

.-

C.Esnouf, A.Kulik, J.F.Theumann,M.L.N6; This conference

4

.-

T.S.K.5, P.Cui, S.C.Yan, Q.Huang; Phys.Stat.Sol.(a)86 (1984) 593

5

.-

M.L.N6, C.Esnouf, J.San Jcsn, G.Fantozzi; J. de Phys. 46 (1985) C10-347 6

.-

M.L.N6, C-Esnouf, J.San Juan, G.Fantozzi; in press.

7

.-

J.Woirgard; Phil.Mag. 33 (1976) 623 8

.-

B.Escaig; J. de Phys. 35 (1974) C7-151

9

.-

D.Caillard, J.L.Martin; Acta Met. 31 (1983) 813 10.- M.A.Morris, J.L.Martin; Acta Met 32 (1984) 549 11.- M .Carrard; Thesis University of Lausanne (1985) 12.- B.Escaig; Phys. Stat. Sol. 28 (1968) 463

13.- C.Esnouf; Thesis University of Lyon (1978)

14.- M.L.N6, C.Esnouf, J.San Juan, G.Fantozzi; Scripta Met. 21 (1987) 213 15.- D.Caillard; Phil. Mag. A51 (1985) 157

Fig.1.-

Internal Friction spectra for different microstructural states and their corresponding transmission electron micrographics ( E _m=8x1

o - ~ ) .

a) samples deformed by rolling or torsion at RT or at 77K.

b) Samples deformed by creep at 2000C or at 30032.

200 300 400 500 600 700 TEMP.

K

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I

200 300 400 500 600 TEMP.

K

Fiq.2.-

us

influence on Internal Friction Fig.3.- Evolution of the Pl and Pi relaxations strength versus Om for spectrum. um=2x10-6p a)uS=O# b)uS=10-5v different themechanical treatments c ) ~ ~ = 2 x 1 0 - ~ p , d)0~=4x10-~p 121.

Fiq.4.- Cross-slip in the Escaig's model.

Fig.5.- Cross-slip mechanism at the nodes of subboundaries : we can see the cross-slip of dislocation 3 /9/.

Fig.6.- Friedel-Escaig-s mechanism /11/.

Fig.7.- Glide motion in PI, P2, and ~ig.8.- Glide controlled by pipe-diffusion:

P3 planes due to nucleation and 1) Initial state. 2) Loop of dislocation under migration of a kink on the {lo01 stress. 3 ) Final state after creating and plane (PI) /IS/. emitting a vacancy out of a jog.