THE INTERNAL FRICTION BEHAVIOUR OF A COLD-WORKED Al-Mg-Si ALLOY AT 1 Hz

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THE INTERNAL FRICTION BEHAVIOUR OF A COLD-WORKED Al-Mg-Si ALLOY AT 1 Hz

E. Bertling-Berrens, P. Mccormick, G. Sokolowski, K. Lücke

To cite this version:

E. Bertling-Berrens, P. Mccormick, G. Sokolowski, K. Lücke. THE INTERNAL FRICTION BE-

HAVIOUR OF A COLD-WORKED Al-Mg-Si ALLOY AT 1 Hz. Journal de Physique Colloques,

1987, 48 (C8), pp.C8-143-C8-148. �10.1051/jphyscol:1987818�. �jpa-00227122�

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J O U R N A L DE P H Y S I Q U E

C o l l o q u e C8, s u p p l b m e n t au n 0 1 2 , T o m e 4 8 , d b c e m b r e 1987

THE I N T E R N A L F R I C T I O N B E H A V I O U R OF A C O L D - W O R K E D Al-Mg-Si A L L O Y A T 1 Hz

E. BERTLING-BERRENS, P.G. MCCORMICK*, G. S O K O L O W S K I and K. LiJCKE

Institut fiir Allgemeine Metallkunde und Metallphysik der R W T H , 0-5100 Aachen, F.R.G.

'Department of Mechanical Engineering, University of Western Australia, Perth 6009 Nedlands, Australia

Abstract

-

A commercial Al-Hg-Si alloy has been used for the present investigation.

After tensile deformation at about 100 K modulus and decrement have been measured simultaneously between 100 and 400 K with a torsion pendulum. The internal friction effects were found to be not very reproducible from sample to sample. However, continuous dislocation pinning between 90 and 300 K and two pinning stages between 300 and 350 K were observed. The deformation-induced damping peaks appeared between 115 and 265 K. Their annealing behaviour has been investigated. All peaks dis- appeared after annealing at PO0 K.

I . .I INTRODUCTION

Damping measurements in Al-Mg alloys have been used to study the segregation of solutes to dislocations at temperatures below bulk diffusion of solutes / I / . The aim of the present experiments is to investigate similar effects in a commercial Al-Mg- Si alloy deformed at low temperatures. The present alloy was chosen because it is the same material in which previously at 300 K the Portevin-Le Chatelier effect has been studied / 2 / , and because in literature no internal friction measurements at low temperatures exist for such an alloy. Additionally modulus and decrement will be measured between 100 and 400 K, to get from the modulus values a direct information about dislocation pinning. In Al-Mg this information was derived from decrement measurements only.

After 8% tensile deformation at about 100 K modulus and decrement were measured during linear heating as a function of temperature, using an inverted torsion pendulum /3/ at about 2

85.

A computer evaluated the data on-line. The amplitude of measurement was 2x10 , to demonstrate the amplit~dg~dependence of the internal fric- tion effects sometimes the lower amplitude of 8x10 was used. The samples.had a diameter of 0.4 mm and a length of 10 m. The material was a commercial A1 alloy with 0.57% Mg, 0.55% Si, 0.24% Fe, and 0.0006% Mn, supplied by-gne of the authors

(P.C. McCormick). All samples were annealed in vacuum (2.6~10 Pa) for 1 h at 773 K , and cooled down to room temperature within about 5 h. After this treatment the grain size was homogeneous and much smaller than the diameter of the sample.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987818

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Precipitates of Mg Si and Al-Fe-Si phases could be found. The state of the samples will be noted as

~3

(undeformed), D (deformed), and A (annealed at a certain temperature). An additional arrow up or down indicates, if necessary, whether the measurement was done during heating or cooling.

(1) Modulusl Curve 1 in Fig.1 (l.D, squares) gives the temperature dependence of shear modulus for sample 1 after 8.8% deformation at about 100 K, curve 2 (1.A340 K, circles) depicts the same after an anneal at 340 K. At low temperatures the deformation-induced modulus defect can be recognized. A comparison of the slopes reveals that during the first heating run after deformation (curve 1) the modulus defect is decreased by a continuously smeared out dislocation pinning and both curves approach at high temperatures. In this continuous pinning regions of more pronounced pinning, pinning, stages can be seen between 160 and 200 K, at 300 K, and above 330 K. Modu- lus decreases, corresponding to relaxation effects are concealed by the continuous pinning.

A similar behaviour can be seen in Fig.2 for sample 2 after 8.5% deformation. Curve 1 (2.D, squares) gives the first heating run, curve 2 (2.A310 K+, circles) has been measured during cooling down. Again both curves approach at higher temperatures, but here appears betqgen 96 and 120 K a very pronounced pinning stage. Curve 3

(2.A310 K*, 8x10 , triangles) has been measured with the amplitude of 8x10-~. At this lower amplitude the state of pinning is somewhat higher, curve 3 lies above curve 2. Pinning stages can be seen here at 320 - 330 K and 340

-

^{350 K.}

While all these pinning effects have been irreversible, the modulus curve 3 in Fig.3 O.D, circles, right scale) for sample 3 after 8.2% deformation shows a strong pinning effect between 278 and 291 K with (AG/G) . = +1.8%, that recovers comple-

pin.

Temoeroture [ K I Temperature l K 1

Elg..l Modulus as function of Flq.2 Modulus as function of temperature

temperature for sample 1.D after sample 2.D after 8.5gdeformation (curve I ) , 8.8% deformation (curve 1) and for 2.A310 R L during cooling down after an for 1.A340 K after an anneal anneal at 310 K-gcurve 2), and for

at 340 K (curve 2). 2.A310 Kc, 8x10 measured with the smaller amplitude during heating (curve 3 ) .

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100 150 2M) 250 300 350 Temperature I K I F k 3 - Temperature dependence of decre- ment for sample 3.UD (curve I), 3.0 after 8.2% deformation (curve 2), and of modulus (3.0, curve 3, right scale)

O' 100 150 200 250 300 350

Temperature [ K 1

Fiq.A Decrement as function of temperature for sample 4.D (curve 1, 8.5% deformation), 5.D (curve 2, 8.2% deformation), and

.

6.D (curve 3, 8.5% deformation).

tely with (AG/G) =

-

^1.8%between 291 and 302 K. The corresponding changes in decrement will bge8f b u s s e d later.

The comparison of the modulus values at 100 K between state 3.UD (not shown in Fig.3) and state 3.D gives a deformation-induced modulus defect of (6G/G) = -19%.

Ui-Decrement

after defor-mat-1.. . The decrement curve 1 of Fig.3 gives the result for the undeformed sample 3 (3.UD. solid line) and curve 2 that after 8.2% deforma- tion (3.D, broken line). I m c u r v e 2 one recognized an increase of background damping

(66 = 100% at 100 K) with a superimposed deformation-induced peak structure, that is not present in curve 1. Corresponding to the dislocation pinnigg (see curve 3) between 278 and 291 K the decrement decreaggs by 6 6 = -6.8~10 and it increases again between 291 and 302 K by A6 = +11x10

.

Fig.4 demonstrates the variety of the deformation-induced peak structures: curve 1 (4.D. 8.5% deformation) , curve 2 (5.D.

8.2% deformation), and curve 3 (6.D, 8.5% deformation). Even for identical pre- treatment of the samples and nearly identical degree of deformation damping back- ground and the peak structure are not reproducible. The main damping peaks appear at 115, 240, and 263 K (curve 1). at 130 and about 245 K (curve 2). and at 210 and 250 K (curve 3). Values of the peak heights are difficult t o obtain because of the uncertainties in the determination of background damping.

For sample 7, measured at 8x10 -6 , curve 1 in Fig.5 (7.D, solid line) shows after 8.2% deformation a large peak below 100 K and smaller ones at 220 and 250 K. The latter has a slowly decreasing high-temperature flank with the indication of an additional peak at 270 K. Smaller peaks appear at 178 and 200 K.

Decrement after an-nea-1-1,ng-, After an anneal at 300 K curve 2 in Fig.5 (iii)

(7.A300 K, broken line) gives the second heating run. The peaks at 178, 220, and 250 K are reduced in height and shifted to lower temperatures, while that at 200 K seems to be unchanged. The third heating run (curve 3, 7.A400 K, dotted curve) after an anneal at 400 K gives a strong reduction of background damping and a recovery of all peak structures.

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1

O% I & 260 250

3k

³⁵⁰

Temperature I K I

F u Decrement as function of tem- perature for sample 7.D after 8.2%

deformation (curve I), 7.A300 K after an anneal at 300 K (curve 21, and 7.A400 K after an anneal at 400 K (curve 3). The-gmplitude of measurement was 8x10

.

I .

O 100 150 200 250 300 350 I

Temperature I K I -6 Decrement as function of tempera- ture for sample 4.D deformed by 8.5%

(curve 1) and after annealings at 310 K and 370 K. Curves 3 and 5 were measured with the smaller amplitude.

A detailed investigation of the annealing is shown in Fig.6 for sample 4. Curve 1 (4.D) is the first heating run after deformation as given in Fig.4 (curve 1). Curve 2 was measured during cooling down after 5 min at 310 K (4.A310 K+). The main fea- tures are that the peak at 263 K in curve 1 is recovered and that a new peak appears at 210 K. A third heating rug with the same state of pinning as ig6curve 2 is shown as curve 3 (4.A310 K+, 8x10 ) with a measuring amplitude of 8x10

.

The background damping is reduced and the peaks of curve 2 at 210 and 230 K are strongly reduced at this lower amplitude. An additional peak appears at 158 K, but the increase to this peak can already be seen in curve 2. After annealing up to 313 K curve 4 (4.A370 K) gives a fourth heating run with the normal amplitude of 2x10

.

The dislocation pinning above 300 K (see for instance the step in the increase of high temperature damping in curve 3 at about 340 K) recovered all the small peak structures of ctrve 3, only the peak at 264 K appears again very strongly. Curve 5 (4.A370

K6

^8x10 )

gives the same state of pinning as in curve 4, but was measured at 8x10

.

^{At this}

smaller.amplitude a reduction of background damping and a much smaller peak at 264 K can be seen, indicating that the peak at 264 K is amplitude dependent.

JIV) ;sradiat$on effects. Irradiating a cold-worked sample with 3 MeV-electrons (5x10 e /cm ) gave no irradiation-induced changes of modulus and decrement, both values reproduced the pre-irradiation values.

For sample 3 after 8.2% tensile deformation at about 100 K a deformation-induced modulus defect (AG/G) = -19% has been found at 100 K. This is the same order of magnitude as those observed after 5% tensile deformation at 78 K: (PG/G) = -4.8% for Cu and (AGIG) = -11% for Cu+O.O3at% Au /4/. During heating after low temperature deformation this modulus defect recovers continuously by dislocation pinning between 100 and 300 K (see Figs.1 and 2). While in deformed pure metals (see for instance Cu

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/5/) the main part of dislocation pinning occurs in few definite stages, here the effects are smeared out due to the presence of a high concentration of different solutes which act as traps for the deformation-induced point defects or point de- fect-solute complexes. Superimposed to this continuous pinning process in sample 1

(Fig.1) pinning stages appear at 160 to 200 K, at 300 K, and above 330 K. For sample 2 (Fig.2) a very pronounced pinning stage is found between 96 and 120 K, and smaller ones at 320

-

330 K and at 340

-

³⁵⁰^K. The different pinning behaviour at low temperatures for these samples cannot be explained at the present time. The present results cannot be compared with other pinning experiments at the same alloy. Only a comparison with decrement measurements of Schwarz /1/ in Al+O.lat% Ug deformed at 120 K is possible. Here the observed decrement changes were interpreted by a small pinning stage between 140 and 183 K and a large one between 250 and 280 K. The latter stage does not coincide with the present results. Additionally no continuous pinning process as in the present measurements was found, but a depinning process between 220 and 250 K. These discrepancies may be due to the different alloys used for both experiments or due to differences in the amplitude of measurement (this amplitude is not given in /I/). Differences in the spectrum of solute atoms would give different binding energies for point defects that are trapped at these solutes.

Different amplitudes would detect pinning points of different strength to pin the dislocations, because the weaker ones can become inactive as pinning points by break-away processes.

Positron annihilation studies indicate, that vacancy recovery occurs between 220 and 250 K in pure Al, between 200 and 285 K in A1+0.5at% Hg, and between 270 and 370 K in Al+O.lat% Si /6/. The comparison with the present pinning stages shows that a pinning due to the freely migrating vacancy is not observed. This is obvious, be- cause of the high concentration of solute atoms that trap vacancies. The present pinning at 300 K or above 300 K agrees with he vacancy recovery observed in Al+O.4at% Si. By Hbssbauer spectroscopy of 5'Co-doped A1 after low temperature de- formation it was observed that vacancy migration already starts between 70 and 100 K by pipe diffusion along dislocations /7/. It cannot be decided definitely, whether the pronounced pinning between 96 and 120 K in sample 2 is caused by the same pro- cess.

All pinning effects discussed till now were of irreversible nature. However, in sample 3 (see Fig.3) a reversible pinning effect was observed. The pre-treatment of this sample differs from that of all others, because during the measurement of the undeformed state (3.UD) the sample was heated up to 370 K before deformation. Simul- taneous changes of modulus and decrement show pinning between 278 and 291 K followed by a depinning between 291 and 302 K. While the modulus increase is identical to the following modulus decrease, the corresponding changes in decrement differ by 62%.

This difference is not yet understood. The ratios (A6)/(AG/G) = -0.038 and (A&) / (AG/G)

.

⁼-0.061 have a negative sign because of thgihposite signs of the changes in $ 8 8 h h s and decrement. The values of the ratios are in good agreement with those observed during recovery of electron-irradiated Cu /8/. They can be un- derstood by dislocation loops dragging mobile pinning points which are pinned by immobile pinning points (modulus increase, decrement decrease). Afterwards this immobile pinning points are lost by migration into other pinning points or into dislocation nodes (modulus decrease, decrement increase) /8/. However, the effect of reversible pinning in sample 3, certainly related to its special treatment, cannot be explained in detail.

Now the decrement results will be discussed. The comparison of curve 1 for the undeformed sample with curve 2 for the deformed one (Fig.3) shows that the deforma- tion increases the background damping and causes a peak structure, both effects cannot be reproduced from sample to sample. Even for a nearly identical degree of deformation Fig.4 gives strong differences of the decrement curves. As can be seen from Figs.1 and 2 there exist also differences in dislocation pinning at low tempe- ratures. But no correlation between the differences in dislocation pinning and the different peak structures could be found. It has to be concluded, that the identical annealing treatment before deformation did not give an identical internal state of

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the samples.

According to Figs.5 and 6 the main damping peaks after deformation appear between 100 to 150 K and between 220 and 270 K. The peaks between 150 and 220 K are smaller and not very reproducable. Therefore they will not be discussed here. The peak teaperature of the first main peak varies from below 100 K (7.D, Fig.5) to 130 K

(5.D, Fig.4). This certainly reflects the irreproducibility of the effects and not an amplitude-dependence, for 7.D has been measured with the smaller amplitude. This peak at low temperatures is only found in the first heating run after deformation.

After an anneal at about 300 K (curve 2 in Fig.5 and curve 3 in Fig.6) it is no longer present.

In the temperature range from 220 to 270 K larger peaks appear at 263 K (curve 1, Fig.61, at 245 K (curve 2, Fig.41, and at 250 K (curve 1, Fig.51. These peaks are considered to be identical, but the shift of their peak temperature from sample to sample cannot be explained. The peaks are missing after an anneal at about 300 K

(curves 2, Fig.5 and Fig.6). This should be caused by the pinning of dislocations below 300 K. but an anneal up to 370 K (curve 4, Fig.6) restores the peak to a height even greater than in the first heating run. This is the effect of dislocation pinning above 300 K, which can be seen as a step in the increase of high temperature damping at 340 K. Therefore the defects or defect-solute complexes responsible for this damping peak are present at the dislocations during the first heating run, become inactive by the pinning up to 310 K, and are again present at the dislocation after the pinning above 310 K. They may be released from traps at temperatures above 310 K. But the type of the defects or defect-solute complexes cannot be specified from the present measurements. However, the process underlying-fhis peak is clearly amplitgde dependent, as can be seen by comparing curve 4 (2x10 1 with curve 5 (8x10 ) . Here the state of pinning of the dislocations should be identical and is determined by the same annealing temperature of 370 K. At the lower amplitude (cugve

5 ) the peak is much smaller. A rest of the pgak is also present in curve 3 (2x10 )

at the same temperature as in curve 1 (2x10 ) . Therefore only the peak height and not the peak temperature is amplitude dependent.

After an anneal at 370 K (curve 5 in Fig.6) or 400 K (curve 3 in Fig.5) dislocation pinning has strongly reduced the background damping and has recovered all damping peaks.

The irradiation of a deformed sample with 5x1017 e-/cm gave no irradiation-induced 2 changes of modulus and decrement. Such a dose would give clear changes of these quantities in the case of deformed pure Cu 181. One has to assume that in the present case the high concentration of solute atoms traps the larger fraction of the irradiation-induced point defects and/or that the point defects that arrive at the dislocations do not form irradiation-induced pinning points. At the dislocations the point defects may be lost to other pinning points or cannot act as pinning points for the present amplitude of measurement, which is higher than that used in the case of irradiated Cu.

REFERENCES

/1/ R.B. Schwarz, Journal de Physique 46, (1985). C10-207 121 P.G. HcCormick, Acta metall. 30, (1982), 2079

/3/ A. Schnell, G. Sokolowski, 8. Brumme, J.of Phys. E9, (19761, 833 /4/ A.J. Brouwer, C. Groenenboom-Eygelaar, Acta metall. 15, (1967), 1597

151 E. Trager, K. Lticke, G. Schrdder, G. Sokolowski, Internal Friction and Ultraso- nic Attenuation in Solids, Ed. C.C.Smith, Pergamon Press, (1980). 79

/6/ R.N. Vest, A. Alan, Point Defect Interactions in Metals, Yamada Science Founda- tion, Universtiy of Tokyo Press, (1982). 469

171 K. Sassa, V. Petry, G. Vogl, Phil. Mag. A48, (19831, 41

181 A. Schnell, G. Sokolowski, K. Liicke, Crystal Lattice Defects 8, (1980). 201