EFFECTS OF PLASTIC DEFORMATION ON THE INTERNAL FRICTION SPECTRUM OF Al-Mn ALLOYS

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EFFECTS OF PLASTIC DEFORMATION ON THE INTERNAL FRICTION SPECTRUM OF Al-Mn

ALLOYS

C. Diallo, R. Schaller, W. Benoit

To cite this version:

C. Diallo, R. Schaller, W. Benoit. EFFECTS OF PLASTIC DEFORMATION ON THE INTERNAL

FRICTION SPECTRUM OF Al-Mn ALLOYS. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-

765-C9-769. �10.1051/jphyscol:19839116�. �jpa-00223351�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 44, dkembre 1983 page C9-765

EFFECTS OF P L A S T I C DEFORMATION ON THE INTERNAL F R I C T I O N SPECTRUM OF A1-Mn ALLOYS

C. Diallo, R. Schaller and W. Benoit

I n s t i t u t de Ggnie Atornique, Swiss FederaZ I n s t i t u t e of TechnoZogy Luusanne, PHB-EcubZens, CH-1015 Lausanne, SwitzerZad

RQsumd

-

Dans les thdories classiques, il est admis que la recristallisation ddpend de forces de freinage dues aux interactions des dislocations,des joints ou sous-joints de grains avec les autres ddfauts tels que atomes deso- lut6, prdcipitds. Les rdsultats obtenus dans A1-Mn montrent que la force de freinage qui contrzle effectivement les premiers stades de la recristallisa- tion est liCe 2 l'dpinglage des dislocations.

Des modsles ont 6td dlabor6s qui rendent compte des interactions des dislocations avec les atomes de solutd ("fond" de frottement intdrieur) ou avec les prdcipitds (pic de frottement intdrieur). Le "fond" et le pic de frottement intdrieur dans A1-Mn sont donc lids 2 la mobilitg des dislocations et suscep- tible~ de fournir des informations sur les forces actives pour la recristallisation.

Abstract

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In the classical theories it is assumed that the recrystallization behaviour depends on the retarding forces which result from the interactions between the dislocations, the grains or subgrains boundaries and other defects as solute atoms or precipitates. The results obtained in A1-Mn show that the retarding force which effectively controls the first stages of recrystallization is connected with the dislocations pinning.

Models for the internal friction evolution due to the interaction of the dislocations with the solute atoms (internal friction "background") or with the precipitates (internal friction peak) have been developed

.

The background and the peak are then connected with the mechanisms which control the dislocations mobility and can provide informations on the recrystallization forces.

INTRODUCTION

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It is generally assumed that the recrystallization kinetics in alloys are controlled by retarding forces due to the solute atoms or the precipitates which act on the subgrains and grains boundaries [I].

On the other hand, the main driving force for recrystallization is connected withthe energy stored in the dislocations which have been created during the deformation.The decrease of the free energy of the crystal is achieved by a decrease in the dislocations density during recrystallization. The rearrangement and the annealing out of the dislocations give rise to deep changes in the microstructure.

Internal friction measurements have been then performed in deformed A1-Mn alloys in the aim to study the mobility of the dislocations and specially the influence of the solute atoms and the precipitates on their rearrangement during the recovery stages.

STAGES IN THE EVOLUTION OF THE INTERNAL FRICTION

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During isochronal annealings,two stages appear in the evolution of the internal friction spectrum of supersaturated and deformed A1-Mn alloys (fig. 1).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19839116

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JOURNAL DE PHYSIQUE

Isochronal treatments o after 10' at 473 K 7

..

^., .. s n ~ - '10

x

..

^650K

- 1.05

- 0.95

, T(K) 0.90

350 450 550 650

Fig.l : Effects of isochronal annealings on the internal friction in an A1-Mn industrial alloy.

Isochronal (IOrnn) annealtngs

Fig. 2 : Evolution of the residual electrical resistivity and the thermoelectric power during isochronal annealings at T

R '

Fig. 3 : Internal friction spectrum of A1-l%Mn.

curve a : after deformation curve b : after annealing lOmn at

5733.

The first one, observed after an annealing at 573K, is characterized by an increase of the internal friction. This stage can be associated with precipitation. Effecti- vely, the recovery of the residual electrical resistivity and the thermoelectric power [21 is maximum in the same domain of

ageing temperatures (fig. 2).

It is possible to give an account for the increase of the internal friction by the increase during precipitationofthe length of the dislocations loops, which were pinned by the Mn solute atoms.

The second stage is characterized by a decrease of the internal friction. Optical microscopy observations show that this stage is due to recrystallization [31

.

^The

decrease of the internal friction is then due to the diminution of the dislocations density.

After the decrease of the internal friction, it is possible to detect the presen- ce of a relaxation peak which appears at

420K. In fact, this peak appears alrea- dy at the first stage of the evolution of the internal friction (fig. 3).

But it is only detectable when the internal friction is relatively low. On fig.3, the maximum of the internal friction which appears at higher temperature (QJ600K) associated with a minimum in the frequency, gives an account for the recrystallization starting.

CHARACTERISTICS OF THE RELAXATION PEAK

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The relaxation peak observed at QJ420K appears after annealings in a temperature range where the G1 precipitates (Al12Mn- bcc) are formed [4].

It disappears when these precipitates transform in the A16Mn stable ones.

In addition, the activation energy of the peak (l.leV/at) and the limit frequency

% 1 0 1 2 ~ z give an account for a relaxation mechanism controlled by atomic movements

(diffusion of Mn in A1 [ 5 ] ) .

From this point of view, the peak seems to be similar to the "precipitates peaks" observed in other aluminium alloys [6,71.

But,if we consider the relaxation strength, the peak is different from the classical

"precipitates peaks" because its height is not proportional to the concentration of the alloy (fig.4). ~ffectively the relaxation strength depends strongly on the dislocations density (fig. 5) and, the peak appears only in deformed samples.

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I

¹^Aged2 L h r s al 573K

I

2 60% deformed

/

³Aged 30mn at 573K.6Wk deformed " 24hrs " ' ', "

1

F i g . 4 : E f f e c t of t h e Mn c o n c e n t r a t i o n F i g . 5 : E f f e c t s o f p l a s t i c d e f o m a t i o n a n d on t h e peak h e i g h t i n A1-Mn : no simple p r e - a g e i n g t r e a t m e n t s o n t h e a p p e a r a n c e o f c o r r e l a t i o n i s observed t h e r e l a x a t i o n p e a k inAl-lwt%Mn.

The c o n d i t i o n s f o r t h e appearance of t h e peak a r e t h e n a p l a s t i c deformation f o l l o - wed by an ageing which a c t i v a t e s t h e formation of t h e Grtype p r e c i p i t a t e s ( f i g . 5 curve 2 ) .

I n a d d i t i o n i t i s p o s s i b l e t o s e e ( f i g . 5 curve 4) t h a t t h e peak i s g r e a t e r when t h e deformation i s preceeded by a f i r s t a g e i n g . The f i r s t ageing forms p r e c i p i t a t e s w h i c h a r e a b l e t o hold t h e d i s l o c a t i o n s d u r i n g t h e deformation. The second ageing l i b e r a - t e s t h e s e d i s l o c a t i o n s from t h e i r p i n n e r s o l u t e Mn atoms. The d i s l o c a t i o n s can t h e n i n t e r a c t w i t h t h e p r e c i p i t a t e s , and t h i s i n t e r a c t i o n can g i v e an account f o r t h e re- l a x a t i o n peak.

MODEL - I n t h e s o l i d s o l u t i o n , t h e d i s l o c a t i o n s a r e pinned by t h e Mn s o l u t e atoms.

During p r e c i p i t a t i o n , t h e d i s l o c a t i o n s loops l e n g t h i n c r e a s e s due t o t h e c l u s t e r i n g of t h e s e p i n n e r s s o l u t e atoms ( f i g . 6 ) . T h i s g i v e s r i s e t o a n i n c r e a s e of t h e i n t e r - n a l f r i c t i o n background.

I t i s p o s s i b l e t o a t t r i b u t e t h e o r i g i n of t h e

" i n t e r n a l f r i c t i o n background" t o t h e movement of t h e d i s l o c a t i o n s i n t h e s o l i d s o l u t i o n . The i n t e r a c t i o n s of t h e s e d i s l o c a t i o n s w i t h t h e so- l u t e atoms l e a d t o t h e following e x p r e s s i o n f o r t h e i n t e r n a l f r i c t i o n Q-' :

where E i s t h e m i g r a t i o n energy of t h e Mn a- toms,

A

t h e d i s l o c a t i o n s d e n s i t y and R t h e

- -

^N ^C

..

Mn p i n n e r atoms. This e x p r e s s i o n g i v e s a good l e n g t h of t h e d i s l o c a t i o n s loops between two Fig. 6 : I n c r e a s e of t h e l e n g t h of account f o r t h e i n c r e a s e of t h e i n t e r n a l f r i c - t h e d i s l o c a t i o n s loops (R2>R1) du- t i o n d u r i n g t h e c l u s t e r i n g of t h e Mn atoms : r i n g t h e c l u s t e r i n g of t h e s o l u t e R i n c r e a s e s ( f i g . 6 ) . On t h e o t h e r hand, t h e a t oms d e c r e a s e of t h e i n t e r n a l f r i c t i o n d u r i n g re-

c r y s t a l l i z a t i o n can b e explained by t h e de- c r e a s e of A

.

Now, i f we c o n s i d e r t h e i n t e r a c t i o n s of t h e d i s l o c a t i o n s w i t h t h e Gltype-precipita- t e s , i t i s p o s s i b l e t o show t h a t t h e d i s l o c a t i o n s induce a p e r t u r b a t i o n i n t h e ordered s t r u c t u r e of t h e p r e c i p i t a t e s ( f i g . 7 ) .

On f i g . 7b), i t i s p o s s i b l e t o s e e t h a t ~ e d i s l o c a t i o n c a n o c c u p y t w o e q u i v a l e n t egui- l i b r i u m p o s i t i o n s : A o r B . I f t h e d i s l o c a t i o n i s i n A, t h e Mn atom marked by an a r - row i s i n B , and v i c e v e r s a . Also when t h e d i s l o c a t i o n moves under t h e a p p l i e d s t r e s s from A t o B , t h e Mn atom must jump from B t o A i n o r d e r t o m a i n t a i n t h e ordered s t r u c t u r e . This g i v e s r i s e t o a r e l a x a t i o n mechanism c o n t r o l l e d by t h e m i g r a t i o n of t h e Mn atom which t a k e s p l a c e a t t h e boundary of t h e p r e c i p i t a t e .

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C9-768 JOURNAL DE PHYSIQUE

Fig.7 : Illustration of the structure of the G I precipitates (AllnMn-bcc). The dark circles represent the Mn atoms. The dislocation in b) induces a perturbation in the ordered structure.

This relaxation can be calculated with a formalism similar to the Schiller's model [81. Then, the relaxation peak can be expressed by :

where T is the relaxation time associatedwi~hthejumpof the Mn atoms in the precipitates.

If we consider the relaxation strength, it is possible to understand the similar sen- sitivity to the thermomechanical treatments of the height of the peak and of the

"background" (fig

.

⁵⁾

RECRYSTALLIZATION

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The internal friction is then sensitive to the dislocations microstructure by the term

AR

which is present as well in the expression of the peak as of the "background". Now,this term hR gives an account for the driving force for the annealing out of the dislocations. Effectively A is proportional to the driving force for recrystallization and

R

is inversely proportional to the pinning force which acts on the dislocations.

Our results show that recrystallization is easier when the product AR is great (fig. 8).

Effectively the appearance of a high relaxa-

A I 1% Mn f ( H r ) A g o tion peak associated with a high increase of

the internal friction during precipitation gives rise to a low temperature of recrys- . tallization. On the contrary when the peak is too small, recrystallization is not possible. As during precipitation,A can be con-

- 7 0 sidered as a constant, it is possible to at-

tribute the increase of the internal friction to the increase of R, i.e. to the decrease of the dragging force which acts on the dislocations. From this point of view,

ao T(K) recrystallization is only possible after

300 500 700 the liberation of the dislocations which Fig. 8 _:A high value of the internal have to be re-arranged.

friction leads to a low temperature of recrystallization.

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CONCLUSIONS

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The effective force which acts on recrystallization, is a function of the driving force due to the energy stored in the dislocations and the dragging force due to the pinning of these dislocations. In the A1-Mn alloys, recrystallization is favoured by precipitation which gives rise to the decrease of the dragging force which was due to the pinning of the dislocations by the solute atoms. The evolution of this dragging force can be followed very well by the evolution of the internal friction spectrum.

REFERENCES

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[l] "Recrystallization and Grain Growth of Multi-Phase and Particle containing Materials"

ed. HANSEN N., JONES A.R., LEFFERS T. Risb (1980) [ 2 ] BORRELLY R., Mem. Sci. Rev. Met.

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(1979), 37 [3] DIALLO C., SCHALLER R., BENOIT W., MONDINO M.

Mem. Sci. Rev. Met.

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(1982), 33 [4] DIALLO C., MONDINO M., BENOIT W.,

J. Phys.

2

(1981), C5-957 [5] LUNDY T.S. and MURDOCK J.F.,

Journal of Appl. Phys.

2

(1962), 1671 [61 SCHALLER R. and BENOIT W.,

Proc. of third europ., conf. on Internal Friction and Ultrasonic Attenuation, Ed., C.C. Smith, Pergamon Press, Oxford (1980), 319

[7] SCHOECK G., BISOGNI E.,

Phys. stat. sol.

2

(1969), 31 [8l SCHILLER P.,

Phys. stat. sol.

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(1964), 391