INTERACTION MECHANISMS BETWEEN DISLOCATIONS, IMMOBILE POINT DEFECTS AND MOBILE POINT DEFECTS IN PURE ALUMINIUM NEAR THE ROOM TEMPERATURE RANGE

(1)

HAL Id: jpa-00223436

https://hal.archives-ouvertes.fr/jpa-00223436

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTERACTION MECHANISMS BETWEEN

DISLOCATIONS, IMMOBILE POINT DEFECTS AND MOBILE POINT DEFECTS IN PURE ALUMINIUM

NEAR THE ROOM TEMPERATURE RANGE

G. Gremaud, Li Ho, W. Benoit

To cite this version:

G. Gremaud, Li Ho, W. Benoit. INTERACTION MECHANISMS BETWEEN DISLOCATIONS,

IMMOBILE POINT DEFECTS AND MOBILE POINT DEFECTS IN PURE ALUMINIUM NEAR

THE ROOM TEMPERATURE RANGE. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-581-

C9-586. �10.1051/jphyscol:1983986�. �jpa-00223436�

(2)

INTERACTION MECHANISMS BETWEEN DISLOCATIONS, IMMOBILE POINT DEFECTS AND MOBILE POINT DEFECTS IN PURE ALUMINIUM NEAR THE ROOM TEMPERATURE RANGE

G. Gremaud, Li Ping Ho and W. Benoit

Institut de Gdnie A tomique, Swiss Federal Inskitute of TeehnoZogy, PHB-EcubZens, CH- 101 5 Lausanne, Switzer Zand

RQsumb

-

BasQe sur une Qtude dCtaillCe du pic PC, du fond de fond de frotte- ment intsrieur et de mesures ultrasonores sous dbformation cyclique, une nou- velle interprbtation des mscanismes d'interaction entre dislocations, dbfauts ponctuels imobiles et d6fauts ponctuels mobiles dans l'aluminium pur est prb- sentbe.

Abstract

-

Based on a detailed study of the PC peak, of the internal friction background and of harmonic bias stress experiments, a new

interpretation of the interaction mechanisms between dislocations, immobile point defects and mobile point defects in pure aluminium is presented.

I. INTRODUCTION - After plastic deformation of pure aluminium, an internal friction peak appears in the room temperature range. This peak, called the PC peak by Perez

[I], is superposed to an important internal friction (IF) background. Vincent and Perez [I] have attributed the PC peak to a mechanism of breakaway from the substitutional impurities and the increase of the IF background as a function of the temperature to a mechanism of dragging of the same impurities.

In this paper, a detailed analysis of the behaviour of the IF spectrum as a function of the strain measurement amplitude, based on the results obtained in theoretical paper [31, leads to an opposite interpretation : the PC peak is due to a mechanism of dragging of mobile point defects and the IF background is due to a mechanismofbreak- away from other point defects, which are not mobile. This interpretation is strongly supported by the results obtained during harmonic bias stress experiments performed in the same temperature range.

11. INTERNAL FRICTION RESULTS - Internal friction measurements have been performed in two kinds of aluminium samples : 5N A1 containing essentially 15 pprn of Si and less than 1 pprn of other substitutional impurities, and 6N A1 containing 0.3 pprn Ca, 0.1 pprn Fe, 0.1 pprn Mg and 0.2 pprn Si.

11.1. Effects of the plastic deformation and of the annealings

-

The PC peak appears only after plastic deformation. Its height, which does not depend strongly on the kind of plastic deformation (tension, torsion), increases strongly when the plastic deformation increases between 0 and 3%, and less strongly for higher plastic defor- mations.

With the 6N A1 samples, after low temperature plastic deformation, the PC peakcanbe observed only after an annealing near 360K, and it disappears after higher temperature annealings. With the 5N A1 samples, the Pc peak is immediately observed after plastic deformation at low temperature, andaflrst very short annealing at 350K increases its height. But during the following annealings at 350K, its height decreases progressively (fig. 1,a). On the other hand, the peak temperature is almost the same in 5N and 6N A 1 (fig.l,b), and also in the case of measurements done by otherau- thors [1,4,5]

.

Concerning the IF background in 5N and 6N Al, it increases with the amplitude of the plastic deformation (fig. l,c), and it decreases exactly in the Sa- me way than the PC peak does during the successive annealings.

Concerning the Bordoni relaxation, a Bordoni peak is always observed during thefirst

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983986

(3)

C9-582 JOURNAL DE PHYSIQUE

i n c r e a s e of t h e temperature j u s t a f t e r t h e low temperature p l a s t i c deformation.When t h e Bordoni peak d i s a p p e a r s a f t e r t h e f i r s t a n n e a l i n g a t 350K i n 5N A l , i t i s a l w a y s observed i n 6N A 1 a f t e r s e v e r a l a n n e a l i n g s a t 350K, b u t i t s h e i g h t i s l i g h t l y smal- l e r .

11.2 E f f e c t s of t h e s t r a i n measurement amplitude - The dependance on t h e s t r a i n am- p l i t u d e Eo i s very i n t e r e s t i n g : i n 6N A l , t h e peak h e i g h t d e c r e a s e s s t r o n g l y and simultaneously t h e I F background i n c r e a s e s s t r o n g l y when t h e s t r a i n amplitude in- c r e a s e s from t o ( f i g . 1 , d ) . I n 5N A l , e x a c t l y t h e same behaviour i s observed, b u t s h i f t e d f o r s t r a i n amplitudes between 1 0 - ~ a n d On t h e o t h e r hand, the- s e simultaneous s t r a i n amplitude dependances of t h e PC peak and of t h e I F b a c k g r o u n d a r e p e r f e c t l y r e v e r s i b l e ( f i g . 1 , d ) .

111. INTERACTION MECHANISM RESPONSIBLE FOR THE PC PEAK - A mechanism of breakaway from immobile PD cannot e x p l a i n t h e f a c t t h a t t h e peak h e i g h t i s maximum a t t h e lo- wer s t r a i n amplitudes. A t t h e c o n t r a r y , a mechanism of dragging of mobile PD can e x p l a i n t h e s t r a i n amplitude dependance of t h e P peak, and t h i s i n t e r p r e t a t i o n i s s t r o n g l y supported by harmonic b i a s s t r e s s experlments perfonned i n t h e temperature F.

range of t h e PC peak.

111.1. S t r a i n amplitude dependance of t h e P, peak - I f d i s l o c a t i o n s e g m e n t s o f l e n g t h Q ' , pinned by e q u a l l y spaced (Q) p o i n t d e f e c t s ( f i g . 2 , a ) a r e c o n s i d e r e d , i t has been shown i n paper [31 t h a t oO-T diagrams ( a p p l i e d s t r e s s amplitude v e r s u s tempera- t u r e ) can be p l o t t e d and d i v i d e d i n f o u r d i f f e r e n t domains i n which t h e i n t e r a c t i o n mechanisms a r e completely d i f f e r e n t ( f i g . 3 : ( I ) r i g i d pinning, (11) breakaway,

(111) p u r e dragging, (IV) simultaneous dragging and breakaway). I n t h e temperature range of t h e dragging mechanism, between T I and T p , a pure dragging r e l a x a t i o n peak t a k e s p l a c e i n domain 111, a t low a p p l i e d s t r e s s amplitudes. By i n c r e a s i n g t h e s t r e s s amplitude, t h e domain I V w i l l be reached. I n domain IVa, a f r a c t i o n of t h e d i s l o c a - t i o n segments of l e n g t h Q ' moves by a breakaway mechanism when t h e o t h e r moves by a pure dragging mechanism, and i n domain IVb, a l l t h e d i s l o c a t i o n segments move by a breakaway mechanism. This l e a d s t o a d e c r e a s e , i n domain IVa, of t h e I F l o s s e s due t o t h e dragging mechanism, and t o a v a n i s h i n g of t h e s e l o s s e s i n domain IVb. From paper [31 i t i s p o s s i b l e t o c a l c u l a t e t h e s e e f f e c t s . The r e s u l t s a r e p l o t t e d i n f i g . 3 i n t h e c a s e of each Go-T diagram.

The e f f e c t of t h e s t r a i n amplitude on t h e PC peak h e i g h t i s very s i m i l a r t o t h i s t h e o r e t i c a l e f f e c t of t h e a p p l i e d s t r e s s amplitude on t h e h e i g h t of t h e dragging peak. Even t h e shape of t h e PC peak i n 6N A 1 ( f i g . 1 , b : t h e PC peak seems t o be s p l i t t e d i n two peaks) i s s i m i l a r t o t h e one of t h e t h e o r e t i c a l peaks of f i g . 3 ( e ) . 111.2 Harmonic b i a s s t r e s s experiments

-

Bias s t r e s s experiments a r e v e r y s e n s i t i v e t o t h e depinning-repinning p r o c e s s e s , because t h e u l t r a s o n i c a t t e n u a t i o n Act and t h e u l t r a s o n i c v e l o c i t y d e f e c t AV/V depend syrongly-on t h e changes of t h e average l e n g t h

X between t h e pinning PD (dependance i n Q 4 and Q 2 r e s p e c t i v e l y ) . When performed w i t h an harmonic a p p l i e d s t r e s s (coupling t e c h n i q u e [ 6 ] ) , t h e b i a s s t r e s s experiments a l - low t o o b t a i n p l o t s of Aa(0) and Av/v(u). Such curves have been c a l l e d t h e "signa- t u r e c u r v e s " of t h e i n t e r a c t i o n mechanisms because t h e i r shape a r e q u i t e c h a r a c t e - r i s t i c of t h e i n t e r a c t i o n mechanism which c o n t r o l s t h e d i s l o c a t i o n m o b i l i t y [71.

I n paper [31, i t i s shown t h a t t h e i n t e r a c t i o n mechanisms i n domain IVb of t h e Do-T diagrams l e a d t o a r e d i s t r i b u t i o n of t h e PD i n t h e g l i d e p l a n e s of t h e d i s l o c a - t i o n segments. Near t h e temperature T

,

t h i s mechanism p r e s e n t s a long time t r a n s i - t o r y s t a g e , which l e a d s f i n a l l y t o a t y p i c a l s t a t i o n n a r y s t a t e f o r which t h e PD a r e d i s t r i b u t e d i n two rows s i t u a t e d n e a r t h e e x t r e m i t i e s of t h e g l i d e a r e a s w e p t b y t h e d i s l o c a t i o n segments. Such a mechanism must p r e s e n t t y p i c a l s i g n a t u r e c u r v e , c a l l e d t h e llmoustache-shaped curves" [ 3 , 7 ] . The behaviour of t h e I F spectrum of 6N A 1 c l e a r l y shows t h a t t h e domain I V b of t h e oo-T diagram must be reached f o r a tempe- r a t u r e range s i t u a t e d near 290K and f o r t h e s t r a i n amplitude range of And i t i s p r e c i s e l y i n t h i s temperature and a p p l i e d s t r a i n amplitude range t h a t "moustache -shaped curves", w i t h t h e i r t r a n s i t o r y s t a g e , have been e x p e r i m e n t a l l y observed i n 0,4% p l a s t i c a l l y deformed 6N A 1 ( f i g . 4 , a ) .

On t h e o t h e r hand, i n t h e t r a n s i t i o n zone between domains IVb and IVa, a n o t h e r com-

(4)

shaped curve" [3,71.

The IF spectrum of the 6N A1 shows that the transition zone between domains IVb and IVa could be reached by decreasing strongly the measurement frequency at 290K and for E, = It is precisely in such conditions that "eight-shaped curves" have been experimentally observed in 0,4% plastically deformed 6N A1 (fig. 4,b).

Finally, it is also interesting to note that very simiZar "moustache-shaped curves"

have been obtained on the two Hasiguti peaks PA and PB in 6N aluminium

[a].

These signature curves, which suddenly disappear exactly in the same temperature range than the annealing temperatures of the two Hasiguti peaks, allow to attribute the Hasiguti peaks to a similar interaction mechanism than the one responsible for the PC peak (dragging in domain 111 and IV of the oo-T diagram), due to an interaction with other PD (intrinsic PD).

IV. INTERACTION MECHANISMS RESPONSIBLE FOR THE TF BACKGROUND

-

The PC peak must be due to a mechanism of dragging of PD which become mobile in the temperature range of T1 (fig. 3,a). Below this temperature, and depending on the applied stress amplitude, a mechanism of breakaway from the same PD can take place, and this breakaway mechanism is responsible for the IF background situated below the PC peak (see fig.

1,d).

But the IF background situated above the PC peak is not explained by this model.

Another kind of PD, which are not mobile in the temperature range of the PC peak, is necessary to explain this IF background.

IV.l Existence of another kind of PD

-

The IF background above the PC peak, which increases almost exponentially with the temperature, increases also strongly with increasing strain amplitudes. Such a behaviour cannot be interpreted by a dragging mechanism, but at the contrary, it is quite typical of a thermally activated mecha-

nism of breakaway from immobile point defects.

These PD cannot be the same PD than those responsible for the PC peak, because above the PC peak, the PD responsible for the PC peak are so mobile that they can ea- sily follow the dislocation segments. One has then to introduce two kinds of PD : the point defects PD1 which become mobile near T I and which are responsible for the PC peak, and the point defects PD2 which are not mobile in the temperature range of the PC peak.

The internal friction measurements clearly show that the breakaway mechanism from the immobile PD2 can appear only when the PDI are sufficiently mobile (fig.l,d : same value of the IF in the low temperature range of the PC peak for different values

of the strain amplitude). This observation leads to the model represented in fig.2 (e) : the dislocation segments Q' drag the mobile PD1, and this mechanism is responsible for the PC peak. On the otherhand, the dislocation segment L can breakaway from the PD2 (flg.Z,f), and this mechanism is responsible for the IF background a- bove the PC peak.

Two experimental observations corroborate this model of interaction with two kinds of point defects, as that is shown in the next two paragraphs.

IV.2 Correlated behaviour of the P, peak and the IF background - Experimentally,one observes that the PC peak height decreases exactly when the IF background above the peak increases. This can be completely explained by the proposed model.

The mechanism of breakaway from immobile PD leads to an increase, depending on the strain amplitude, of the average length Q' of the dislocation segments which contri- bute to the mechanism of dragging of mobile PDI (fig.2,e and f). This increase of the average length k ' , which should increase the relaxation strength of the dragging peak, contributes in fact to a decrease of the critical stress which limits the domains 111 and IV in the 0,-T diagram of the interaction mechanisms with the mobile PD,(fig. 3 : effect of an increase of the average length Q'). This effect explains perfectly why the PC peak height decreases exactly when the IF background above the peak increases, and also why these effects are reversible.

(5)

C9-584 JOURNAL DE PHYSIQUE

IV.3 Observation of the mechanism of breakaway from PD2 - In 0,4% plastically deformed 6 N A1 samples, which have been aged six months at room temperature, the mechanism of breakaway from immobile PD2 has been experimentally observed at room temperature by harmonic bias stress experiments.

During harmonic bias stress experiments performed in six months aged 6 N A1 samples as a function of the temperature [81, "bow-shaped eum)es" appear at about 150K, and their amplitude increases with the temperature (fig. 4,c). Such signature curve has been associated to a breakaway mechanism under the effect of the compression applied bias stress [3,7,81. The temperature of 150K corresponds precisely to the temperature at which the IF background below the PC peak begins to increase. Thesetwoeffects (bow-shaped curves and IF background below the PC peak) must be associated with the breakaway mechanism from the PD1, which takes place in domain I1 of the oo-T diagrams of fig. 3.

From about 240K, these "bow-shaped curves" are progressively modified and are repla- ced at room temperature by the stationnary state of the signature curves represented in fig. 4(d). This temperature of 240K is precisely the temperature at which the PC peak begins to increase and corresponds then to the temperature at which the PD become mobile. On the other hand, the stationnary state of the signature curves in six months aged samples is not symetrical (fig. 4,d) : it is composed in fact by a super- position of a "moustache-shaped curve" (which is due to the redistribution of the mobile PDI in the glide planes) and of a "bow shaped curve", which i s due t o the mecha- nism of breakaway from the i m o b i l e PD2.

V. ROLE OF THE PLASTIC DEFORMATION AND OF THE ANNEALINGS

-

Harmonic bias stress experiments performed in freshly plastically deformed (0,4%) 6 N A1 present perfectly symetrical signature curves (fig. 4,a and b). Takingthemechanism of breakaway from PD2 into account (fig. 2,e and f) this can be explained only if the average length R is much smaller than the average length R' in fig. 2(e).

This means that, i n freshly deformed samples, the concentration (on the dislocation segments) of mobile PD, ismuch higher than the concentration of immobile PD,.

On the other hand, during the six months aging at room temperature, the PD migrate to the dislocations and the "moustache-shaped curves" become asyrnetrical (fig. 4,d).

From these observations, the role of the plastic deformation and of the annealings for the increase and the decrease of the IF background and of the Pcpeakseemsclear:

(i) the plastic deformation allows to decrease strongly the concentrations of PDI and PD2 on the dislocation segments

(ii) the first annealing at 350K, which is necessary to observe the PC peak, allows the migration of the mobile PD1 towards the dislocations.

(iii) during the annealings at higher temperatures, the PD2 can migrate towards the dislocation segments, which leads to the decrease of the PC peak.

The behaviour of the Bordoni relaxation just after the plastic deformation and after the first annealing corroborates this interpretation.

V I . CONSIDERATIONS ON THE NATURE OF THE POINT DEFECTS

-

The exact nature ofthe point defects is not known, but some important considerations concerning their nature can be done :

(i) the strain amplitude dependance of the IF background and the annealing temperature of the PC peak depend both strongly on the purity of the samples. This means that the concentration of PD, responsible for the I F background above

the PC peak depends on the impurity concentration of the samples.

(ii) Moreover in specially treated zone refined aluminium furnished by Dr. Revel of Vitry, recent results obtained by Bujard [9] show that the "moustache-shaped

curves" are completely missing in the room temperature range. This means that, in this very pure aluminium the concentration of PDl is very small. As a con- sequence the concentration of PD, responsible for the PC peak depends a l s o on the impurity concentration of the samples.

(iii) the PC peak, due to the dragging of PD1, appears at about the same temperature in all the measured A1 samples. This means that the migration energy of the PD1 should be almost the same in the different aluminium samples, and, as

(6)

6 -

(1) before defarmatlon (2) after 10% deformatton (3) after etghl successwe L -

2 -

0

1 0 0 2 0 0 300

0-I . l o 3 6 N Al

.

deformed 7% (d)

Figure 1 : IF measurements : (a) effect of successive annealings at 350K in 5 N Alaf-- ter plastic deformation, (b) comparison of the PC peaks obtained in 5 N and 6 N Al, (c) effects of the plastic deformation and of the annealings in 5N Al, (d) effect of the strain measurement amplitude on the PC peak and the IF background in 6 N Al.

Figure 2 : Interaction mechanisms between dislocation segments of length 9,' (a) and equally spaced point defects : (b) breakaway in domain 11, (c) dragging in domain I11 and (d) complex mechanism in domain IV of the Do-T diagram-Interaction mechanisms of dislocation segments of length L (e) with mobile PD1 and immobile PD2 : breakaway from PD2 and dragging of PDI (f).

(7)

C9-586 JOURNAL DE PHYSIQUE

2U,

dlb

12yd

-

leZb

F i g u r e 3 : E f f e c t of a n i n c r e a s e o f t h e d i s l o c a t i o n segment l e n g t h R' on t h e Go-T diagrams of t h e i n t e r a c t i o n mechanisms w i t h t h e m o b i l e PDI. Corresponding t h e o r e t i - c a l b e h a v i o u r of t h e I F due t o t h e mechanism of d r a g g i n g of PD1, f o r d i f f e r e n t v a l u e s of t h e a p p l i e d s t r e s s a m p l i t u d e .

'I,

F i g u r e 4 : s i g n a t u r e c u r v e s o b t a i n e d by harmonic b i a s s t r e s s e x p e r i m e n t s a t ~ ~ = 1 0 - ~ : ( a ) moustache-shaped c u r v e s o b t a i n e d a f t e r 360 c y c l e s i n p l a s t i c a l l y deformed 6N A 1 ( 0 . 1 Hz, 290K), ( b ) e i g h t - s h a p e d c u r v e s o b t a i n e d a f t e r 2 c y c l e s i n p l a s t i c a l l y deformed 6N A 1 (0.001 Hz, 290K), ( c ) bow-shaped c u r v e s o b s e r v e d i n a r o o m t e m p . a g e d 6NA1 between 150 and 240K ( 0 . 1 H z ) , (d) s i g n a t u r e c u r v e s o b s e r v e d i n a r o o m t e m p . a g e d 6N A1 above 240K ( 0 . 1 Hz)

.

REFERENCES -

[ I ] PEREZ J.M., t h e s i s , INSA-Lyon (1970)

[2] VINCENT A . , PEREZ J . , P h i l . Mag. A , (1979) 377 [31 GREMAUD G., t o b e p u b l i s h e d i n t h e s e p r o c e e d i n g s [41 TIRBONOD B . , t h e s i s , EPF-Lausanne (1977)

[51 BAUDRAZ F . , t h e s i s , EPF-Lausanne (1980) [61 GREMAUD G . , J. d e Phys.,

62,

(1981) C5-1141

[7] GREMAUD G . , BENOIT W . , J . d e Phys.

42

(1981) C5-369 [8] GREMAUD G . , t h e s i s , EPF-Lausanne (1981)

[9] BUJARD M., u n p u b l i s h e d r e c e n t r e s u l t s