DISLOCATION MOBILITY IN 5N ALUMINIUM DURING PUSH-PULL CYCLING STUDIED BY ULTRASONIC ATTENUATION MEASUREMENTS

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DISLOCATION MOBILITY IN 5N ALUMINIUM DURING PUSH-PULL CYCLING STUDIED BY ULTRASONIC ATTENUATION MEASUREMENTS

A. Vincent, A. Hamel, J. Chicois, R. Fougeres

To cite this version:

A. Vincent, A. Hamel, J. Chicois, R. Fougeres. DISLOCATION MOBILITY IN 5N ALU- MINIUM DURING PUSH-PULL CYCLING STUDIED BY ULTRASONIC ATTENUATION MEASUREMENTS. Journal de Physique Colloques, 1985, 46 (C10), pp.C10-321-C10-324.

�10.1051/jphyscol:19851071�. �jpa-00225456�

DISLOCATION MOBILITY IN 5N ALUMINIUM DURING PUSH-PULL CYCLING STUDIED BY ULTRASONIC ATTENUATION MEASUREMENTS

A. VINCENT* , A. HAMEL, J. CHICOIS AND R. FOUGERES

'Laboratoire de Traitement du Signal et d'Ultrasons, I N S A d e Lyon, Bdt. 502, 69621 Villeurbanne Cedex, France

Groupes d l E t u d e s d e MBtallurgie Physique et de Physique des MatBriaux, ( L A C N R S 341) I N S A de Lyon Bst. 502, 69621 V i l l e u r b a m e Cedex, France

Resum6 - Nous avons etudie, par mesures de variations d'attenuation ( A d

d'ondes ultrasonores, le comportement de l'aluminium pur sol licit6 en traction compression. Les cycles attenuation-deformation

f

sont prgsentes pour des amplitudes de deformation 10-5<

410-3. Nous avons observe l'influence de l'amplitude de deformation, du nombre de cycles et de l'etat initial de l'bchantillon (recuit ou prefatiguh) sur la forme des cycles f(€)). Les resultats sont interpret& en termes de desancrage des dislocations partir des defauts ponctuels, d' interaction entre dislocations, de creation ou de recombinaison de dislocations.

Abstract - The behaviour of pure aluminium during push pull cycling has been studied through ultrasonic attenuation change ( A d ) measurements. The attenuation-strain loops

in the range 1 0 - 5 ~

c10-3. The

f(&) loop shape is observed to be depending on the deformation amplitude, the cycle number and the initial state of the sample (annealed or prefatigued). These results are discussed in terms of dislocation depinning from point defects, dislocations interactions, creation or annihilation of dislocations.

INTRODUCTION

During the last 15 years the ultrasonic attenuation measurements superimposed on a slowly varying bias stress have been widely applied to the study of anelastic phenomena such as those due to dislocation depinning /1,2/ and kink mobility /3,4/

in pure metals. The aim of this paper is to present recent results obtained by this method applied in a wider amplitude deformation range extending up to the cyclic plasticity. This work is included within a more general study which goal is to contribute to the understanding of the dislocation mobility role on the fatigue behaviour of pure metals 151.

EXPERIMENTAL

This study was carried out on pure polycrystalline aluminium (99.998

annealed at 450°C for one hour after a cross section reduction of 7 0

(grain size u 1.5

the samples are cylindrical with

in diameter and they have a useful length of

The cyclic deformation was applied to the sample in a symmetrical push-pull mode on a specially designed apparatus working under completely controlled experimental conditions

particularly the test can be performed under-total strain

control or plastic strain ep control

in both cases the strain rate

or i is kept constant.

~ftrasonic attenuation measurements were carried out in real time during fatigue cycling at room temperature :longitudinal waves of 15 Mhz propagating in a direction parallel to the sample axis were used with the echo transmission technique

quartz transducers are glued to the sample extremities for emission and receiving of the ultrasonic waves

changes in the amplitude of the first transmitted echo are followed by a system developped in our laboratory 171 and stored in the computer memory

then the corresponding attenuation variations are computed from these data.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19851071

C10-322 JOURNAL DE PHYSIQUE

RESULTS

The evolutions of the ultrasonic attenuation cyclic changes versus deformation can be classified according to five typical figures presented for different deformation amplitudes and two initial states of the sample (annealed or prefatigued at 100 cycles with a plastic amplitude AEp=1.5 1 0 - 3 ) .

Low amplitude range : two cases can be distinguished in this range : the annealed state (figure 1 ) and the prefatigued one (figure 2) ; in the former one the attenua- tion changes are small, and the A d = f(E) cycles seem to remain rather stable (after 10 cycles) with a minimum value of &at € = 0 . In the latter one, if the variations are observed to be more important during the first cycles, on the contrary the at tenuation tends after 50 cycles towards a constant value during all the cycle.

-30 -20 -10 10 20 30 -30 -20 -10 10 20 30

F i g . 1

- Cyc&c evolLLtion

6

i n

- Cyc&c evollLtion

6

1 i n anneded aX&e

contrrol

phe6atigued d t a t e

conxhol w i t h

3 . 1 0 ~ ~

p u q ~ t u

cycle. Cycle . s = amplitude 0 6 o a c i t t a t i o n Emax

6

l i n ~atwtated heghe .

Cycle

d ( & I i n ~atwr&e.d heghe.

Medium amplitude ranse : In this range, as the number of cycles increases, the average attenuation increases rapidly in the case of the annealed state (figure 3),

400 300 200 100 100 200 300 400 F i g . 3

- C o n d i t i o ~

w a

- Condiaom ididentic&

06

excepz $oh

06

excep.t

a ptrevious

d u g o n e

the aample a p e v i o w tat undugone by the ample

a;t AEp = 2 5 . 1 0 - ~ . . at Em = 230.10'6.

d

decreases when the cycle number increases; a stationnary state is also obtained after about 100 cycles. In addition,in both states, the loops A&= f

are strongly irreversible, with an attenuation increasing graduately during the plastic stages (as labelled for example on figure 3,

and C e D respectively in traction and in compression for the stationnary cycle) and decreasing steeply during the quasi elas- tic unloading (B+C and D-+A figure 3).

High amplitude range

This range is characterized by

a quick stabilization of the loops Ad=

f(&) after few cycles

this is reported on figure 5 in the case of the annealed state.

a new shape of the loop

f

that it easily observed in the station- nary regime

during the plastic stage of the cycle the attenuation

mation goes on (BsC figure 5). In addition, it has to be noticed that the deformation amp1 itudes

limiting these three ranges are

slightly dependent on the state of

the sample

the limit between low and medium amplitude ranges is about 3.10-5 (annealed) or

(prefatigued)

that one between

medium and high amplitude ranges

-

acdC&

1 & 3, ^{6 0 1} d E p = I S O O . ~ O - ~

and

additional characteristics of

these ranges have been described

in an other paper /S/.Finally, it

has to be mentioned that the choice of a olastic deformation control or a total de- formation control does not affect signiPicantly the main characteristics of the Ad=f(€) loops above reported

so this point will not be discussed within the frame of this short paper.

DISCUSSION

The experimental results can be discussed qualitatively on the basis of the disloca- tion string model

rough12 the contribution of the dislocations to the attenuation is given by

K.A.1 , where

is a damping factor and

is the density of dislocations characterized by the free loop length 1

Thus an attenuation change can be interpreted in terms of a 1 variation, a A varia- tion or even of a

variation.

Dislocation-point defect interactions

In the low amplitude range, attenuation changes are ascribed to dislocation point

defect interactions, as suggested by previous studies in this range /2,9/. In the

case of the annealed sample, the cyclic depinning and repinning of dislocations from

their initial COTTRELL cloud of point defects lead to cyclic changes in the disloca-

tion loop length

the mean loop length is maximum at the maximum traction or

compression load and it is minimum at

o (figure 1).

similar mechanism is

expected from the prefatigued state, but being magnified due to the more numerous

preexisting dislocations (factorh) in that state

it is actually observed during

the first half cycle but the point defects are graduately swept out by the succes-

sive movements of the dislocations through the COTTRELL cloud, as indicated by the

quasi constant value taken by the attenuation after 50 cycles (figure 2). This dif-

ferent behaviour of the cloud has to be attributed mainly to its conditions of make

up

indeed, in this prefatigued state the cloud is developed during the subsequent

C10-324 JOURNAL DE PHYSIQUE

ageing at room temperature when it is made up from the treatment at 450°C in the case of the annealed state

so, in this latter case the cloud is expected to be more stable at room temperature.

In fact, the details of the attenuation changes, show that the cx, minimum ascribed to the repinning of dislocations by their initial COTTRELL cloud, disappears in a much more efficient way after the first half cycle in compression than after the previous one in traction (figure 2)

in addition ,this minimum is then significantly shifted towards positive deformation . This additional phenomenon observed only in the prefatigued sample can be explained in the following way

at the end of the prefatigue treatment , the sample was unloaded from a traction half cycle down to &=O for rest , thus inducing compressive internal stresses /5/. When the subsequent low amplitude test is carried on , the non relaxed part of these internal stresses helps the compressive external stress to induce some displacement of the so called hard pinning points (nodes or jogs)

the backward motion of these hard points would occur in an hysteretical way , thus contributing to the disappearance and the shifting of the& minimum .

creation and annihilation of dislocations

In the medium amplitude ranqe the attenuation chanqes are mainly due to chanqes in the dislocation density.

; in fact, an increase ;n A comes with-a de- crease in the loop length between nodes of the dislocations

although, this mecha- nism is likely to play a role in attenuation changes, it does not seems to affect the tendency of& variations, at least in this amplitude range.

Thus, on this basis, the cyclic evolutions of the attenuation are easily explained in the following way

during the plastic stage, the density of mobile dislocations increases graduately either by creation or by depinning of dislocations

on the contrary, during the elastic unloading, the density of mobile dislocation decreases either by annihilation or by dynamic repinning of dislocation by point defects /5/.

During the first evolutionary cycles the behaviour of the annealed sample and that of the prefatigued one are opposite

the average level of

increases due to the net increase of the dislocation density in the annealed sample

on the contrary this level decreases due to the net decrease of

in the prefatigued sample.

Actually, from the mechanical point of view, the former situation is accompanied by an hardening of the sample, when for the latter one a softening is observed.

Interactions dislocation-dislocation

The new characteristic of the attenuation behaviour in the high amplitude range is the saturation and even the decrease of

observed during the plastic stage of the cycles (figure 5). Two kinds of mechanism could contribute to this phenomenon

at first, an increase in the dislocation velocity, suggested by acoustic emission results /lo/, would reduce the tendency of increase in

Secondly, strong inter- actions arise between dislocations especially with those in the cell walls

these interactions give an high level of local internal stresses

leading to an increase of the restoring force acting on dislocations that could reduce the ultrasonic mobility and thus the damping factor K.

REFERENCES

/1/ Lenz, D., Edenhofer, B., Lucke, K., Scripta Met.

(1971) 387-395.

/2/ Vincent, A., Perez, J., Phil. Mag. 40 (1979) 377-397.

/3/

Deterre, P., Esnouf, C., Fantozzi, G., Peguin, P., Perez, J., Ritchie, I., Vanoni, F., Vincent, A., Acta Met. 3 (1979) 1779-1788.

/4/ Gremaud, G., Benoit, W., J. de Phys. C5, 10, 42 (1981) 163-168.

/5/ Chicois, J., Hamel, A., Guichon, G., Fougeres, R., Vincent, A., (to be pub1 ished) .

/6/ Chicois, J. et al. (to be published)

/7/ Vincent, A., Bouvier, J.L., Fleischmann, P., J. Phys. E, 15 (1982) 765-770.

/8/ Granato, A.V., Lucke, K., Stern, R.M., M6taux, 433 (1961) 299-319.

/9/

Gremaud, G., Benoit, W., J. de Phys. C5, l0, 42 (1981) 369-374.

/lo/ Slimani,

Chicois, J., Fougeres,

Fleischmann, P., to be published in the

proceedings of ICSMA 7, MONTREAL (1985).