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ULTRASONIC ATTENUATION MEASUREMENTS OF DISLOCATION DAMPING IN COPPER SINGLE
CRYSTALS BETWEEN RT AND 15 K UNDER (COMPRESSION/TENSION) BIAS STRESS
C. Becker, D. Lenz, K. Lücke
To cite this version:
C. Becker, D. Lenz, K. Lücke. ULTRASONIC ATTENUATION MEASUREMENTS OF DISLOCA-
TION DAMPING IN COPPER SINGLE CRYSTALS BETWEEN RT AND 15 K UNDER (COM-
PRESSION/TENSION) BIAS STRESS. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-293-
C8-298. �10.1051/jphyscol:1987842�. �jpa-00227146�
ULTRASONIC ATTENUATION MEASUREMENTS OF DISLOCATION DAMPING IN COPPER SINGLE CRYSTALS BETWEEN RT AND 15 K UNDER (COMPRESSION/TENSION) BIAS STRESS
C. BECKER, D. LENZ and K. LUCKE
Institut fiir Allgemeine Metallkunde u n d Metallphysik d e r RWTH Aachen, 0-5100 Aachen, F.R.G.
Abstract.- Measurements of ultrasonic attenuation under slowly varying cyclic com- pression/tension loading of the samples ("cyclic bias stress"=CBS) provide informa- tion on the interaction between dislocations and pinning points. The present paper describes an apparatus for CBS measurements at cryogenic temperatures, some manda- tory experimental techniques and presents several results. Attention is focused on the following requirements and techniques: (i) continuous loading of the samples under variable rate at stresses below yield and steady transition from pure tension to pure compression through F O by a computer controlled preloaded disk spring ar- rangement and a double membrane centering of the loading bar at its cryogenic end;
(ii) stress free mounting and temperature changes of very soft samples provided by Ga soldering of the precision lapped samples to precision lapped joining pieces;
(iii) elimination of mismatch stresses due to different thermal expansion of sample and attached ultrasonic transducer by application of novel PVDF (=polyvinylidene- fluoride) foil transducers.
I. INTRODUCTION
Measurements of dislocation damping by ultrasonic techniques under cyclic compres- sion/tension bias stress (CBS) offer important information on the dislocation-pin- ning point interaction which cannot be obtained by other techniques /I/. In order to get reliable CBS results, especially at low temperatures, considerable experimental difficulties must be overcome: (i) Transition through zero-stress free from play and avoidance of bending moments on the sample especially under compression.
(ii) Stress-free, rigid mounting of soft single crystals and avoidance of parasitic thermal stresses. (iii) Avoidance of sample deformation caused by different thermal expansion of ultrasonic transducer and sample. The present techniques were developed for experiments on copper single crystals. To our belief they can be readily modi- fied for studies of other materials.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987842
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11. A X R A T U S FOR CBS MEASUREMENTS AT LOW TEMPERATURES Fig.1 shows a schematic cross section of the com- pressionltension apparatus (CTA) for CBS measure- ments at cryogenic temperatures. The sample #1
(c.f. Fig.1) is mounted inside the sample chamber
#2 at the lower end of a steel tube #3 (55 cm length) fitting into a Leybold He -evaporisation cryostat #4. The single crystalline sample (for details c.f.121) is rigidly attached to the two joining pieces # 5 and 116, the lower of-which is connected to the bottom mounting plate of the sam- ple chamber. The upper one is fixed to the inner movable steel bar # 7 (compression/tension bar) inside the outer load-carrying tube #3. To reduce thermal expansion mismatch the joining pieces and all parts of the sample chamber are made of cop- per. Outside the cryostat the compression tension bar is fixed to the load cell # 8 (Hottinger Bald- win) which in turn is screwed to the steel plate 19. This plate is mounted between double stacks of preloaded disk springs #lo. The length of these stacks is varied by the up/down movement of steel plate #I1 which is accomplished by a screw spindle
#12 driven by the stepping motor #14 through the worm gear 113. The preload of the spring stacks ensures single sided tooth engagement irrespective of upldown movement of plate #9. Thus its up- movement results in tension, its down movement in compression on the sample with maximum load li- mited by the disk spring preload. To avoid bending moments on the sample, the compression/tension bar is vertically guided immediately above the sample by a pair of precipitation hardened Cu-Be mem- branes #15 (0.2 mm thickness) separated by a pre- cision-lapped distance ring #16. The action of the stepping motor is controlled by the difference between the voltage of the load measuring ampli-
fier and the reference voltage provided by a mi- Fia.1: Schematic cross section crocomputer (Rockwell AIM 6 5 ) . Thus o(t) functions of the compression/tension whatever can easily be programmed. The present apparatus
standard is a "sawtooth" a(t). The fastest load rate is 5 Nls. The load range is +/-500 N with the standard disk spring stacks and preload.
111. MOUNTING O_F...&A~PeLEEEETO. JOINING PIECES
To fix the soft and stress sensitive copper single crystals rigidly to the joining pieces ( # 5 and #6 in Fig.1) came out to be one of the most intriguing problems. Any type of threads or clamping devices were ruled out because the accompanying stresses readily disturb the dislocations under investigation. The only promising way was the look for suitable adhesives. However, experiments with different organic glues showed strong irreversible and glue dependent attenuation increases during cooling
(Fig.2) instead of the expected continuous attenuation decrease due to the T-depen- dence of dislocation damping /3/ (c.f. also Pig.3). Since during these measurements tlie upper joining piece was got attached to the loading bar any apparatus effect is ruled out. The form of the a(T)-dependence in Fig.2 resembles that under external load i.e. dislocation break-away. As Fig.3 shows, a sample covered only with a layer of glue on the two joining faces but without joining pieces attached exhibits the undisturbed reversible q(T) behaviour. Thus the adhesively fixed joining pieces cause the disturbing break-away stress in the sample. Since the adjacent surfaces
Temperature T I K 1
Fia.2: Attenuation effects due to dis- location break away caused by organic adhesives between sample and joining pieces; curve a: G.E. varnish (7031) curve b: Bees wax/rosin mixture
are neither microscopically nor macrosco- pically ideally flat (c.f. Fig.4) the roughness as well as the wedge shaped misfit volume is filled with the adhe- sive. On cooling the adhesive (having a factor of 10 larger thermal contraction then copper) pulls the adjacent surfaces strongly together. Thus the surface aspe- rities of the joining piece stress the adjacent sample. More long-range stresses occur since the adjacent surfaces cannot be lapped to perfect flatness but show different curvature with wedge opening of 0.5-2fi (c.f. Fig.4). Thus any organic glue is ruled out as adhesive; a joining material with a better matched thermal expansion is to be used i.e. a metallic solder. It has been shown /4/, that cop- per can be successfully soldered at RT by applying a mixture of liquid Gallium
(melting at 29.S°C) and copper powder.
We found, that the use of copper powder as filler was not necessary with the pre- sent precision lapped surfaces. Instead a small amount of molten Gallium was spread between the well cleaned surfaces to be connected. By gently rubbing the surfaces across each other both became completely wetted by a thin Ga film and any excess Gallium was squeezed out at the border.
The parts then were aligned and left at rest for about 6 hours after which time the solder layer became solid and strong enough to resist tensional stresses in the needed range ( 5 150 N/c&). Fig. 5
(curve b) shows a(T) measured during cooling of a sample which was Gz-soldered to the two joining pieces. Comparison with a(T) before soldering (Fig.5, curve a) shows that no stress induced a-effects
Temperature T I K 1
Fia.3: Temperature dependence of at- tenuation of Cu (same sample as in Fig.2) with joining faces covered with G.E. varnish but not attached to the joining pieces
joining piece
\
Fia.4: Schematic cross section of sample joining piece interface
I
220 240 260 280 300 Temperature T [ K I Fia.5: Comparison of a(T) before
(curve a) and after (curve b) Ga- soldering the sample to the joining pieces
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are observed (compare also with Fig. 4 for such effects). However after soldering (curve b) the total attenuation is slightly increased probably due to the rubbing manipulations. This has to be accepted and taken into account if very stress sensi- tive samples as in the present case are to be Ga-soldered. Once a Ga-bond is formed at RT it cannot be detached up to at least 650 C. In some case this is a handicap, e.g. if the sample is to be plastically deformed in the course of a CBS measurement, the Ga technique is hardly practicable. On the other hand annealing studies have been performed on samples which
after
deformation were Ga soldered and (together with the joining pieces) annealed up to 650 C /5/.IV. INSTALLATION OF THE SAMPLE/JOINING PIECE UNIT IN THE CTA A useful installation technique must (i) allow for
small differences in length of different sample/
joining piece units and (ii) allow for small devia- tions of the unit from the exact vertical and middle position after fastening the upper joining piece and
(iii) avoid any torque or bending stress during fastening of the bottom joining piece. Our present method which fulfills these requirements is described with the help of Fig. 6 depicting the sample chamber with the sample/joining pieces in place. To install the sample in the empty chamber first the bottom plate P is removed and the upper joining piece A1 is screwed to the compression/tension bar B below the centering membranes M. The lower joining piece A2 ends in a cylindrical rod T with circumferential saw- tooth notches (a thread would do as well). Rod T hangs freely down into the socket S which has circum- ferential sawtooth notches on its
inner
bore. During fastening of the bottom plate one has to make sure that T never gets into contact with S and becomes precisely centered inside S. The lower end of S then is closed by a cellulose stopper C and the gap bet- ween T and S is filled from above with molten Salol supercooled to RT. On contact with the stopper theSalol readily starts to solidify. Solidification then C P continues slowly upwards through the gap between T
and S. It is important to have the Salol in this way
simultaneously crystallizing around T; otherwise its Fiq.6: Cross section of volume shrinkage results in bending forces on the sample chamber
sample.
'4. THEREAL ESMA_T_CH STRESSES BETWEEN TRANSDUCE-R A_N_D_SA@L_E
Disturbing mismatch stresses between quartz transducer (thermal expansion coeffi- cient P(Quartz) ~5.5 -~o-'/K) and sample (B(Cu) =l. 7 . ~ O - ~ / K ) arise as soon as the acoustic bond between transducer and sample becomes solid during cooling (e.9. below 220K with Nonaq bonds c.f. Fig.7). These mismatch stresses act on the dislocations below the transducer. The resulting attenuation effects ("transducer-sample-defor- mation (TSD)-effects") are due to break-away and dislocation multiplication. They can be avoided by use of piezoelectric polyvinylidene fluoride (PVDF) transducer foils /6/. PVDF is a thermoplast which after cold stretching (to increase crystalli- nity) and poling in an electric field exhibits a strong piezoelectric effect; the high electric strength of PVDF allows the use of thin foils (610~) for pulse echo measurements with typically up to 103 Volts transmitter peak voltage. That TSD-free a(T) measurements of highly stress sensitive samples can be obtained with PVDF- transducers is mainly due to the very small thickness of the foil and its easy deformability. For details see /6/. We have to mention that the use of PVDF-foil
their T-dependence, undisturbed a(T) measurements of dislocation damping in the temperature range between RT and 4.2 K have been obtained. In Fig.7 a(T) measure- ments on two Cu samples at 30 MHz with a 10 MHz quartz transducer and different bonds (curves a, b, c and f) are compared to measurements using a 9~ PVDF-foil transducer on the same samples (curve d and curve e). It can be seen that in con- trast to quartz measurements (curve b depicts the typical TSD problem) the measure- ments with the PVDF-foil are completely reversible and thus represent the true tem- perature dependence 9f the dislocation damping.
m:
Temperature dependence of attenuation due to dislocation resonance measured with PVDF- transducers (curves d,e) compared with differently bonded quartz transducers (curves a-c and f).A = solidification temperature of bond
- .- .-
non dislocation background curve a : Salol bond\.
&-- - - - -. curves b, f: Nonaq bond
curve c : Silicon oil bond
50 100 150 200 250 300 curves e, d: PVDF-foil bonded by
Temperature T IK1 cured epoxy-resin
Using the technique described above a pure copper single crystal was Ga-soldered to the joining pieces and a 9p-PVDF transducer was applied. Fig.8, comparing a(T) mea- sured on the sample soldered to the joining pieces before (dashed curve) and after mounting in the CTA (solid curve), shows (i) that stress effects due to the instal- lation procedure in the CTA and to TSD have been avoided by the described techniques and (ii) that unavoidable stresses due to slightly different cooling rates of the the outer load carrying tube and the inner compression/tension bar are quite effec- tively balanced out by the load control. We remind that the strong a(T) increase below 50K is due to phonon-electron interaction / 7 , 8 / and does not indicate para- sitic stresses on the sample. Fig.8 also shows superimposed on a(T) some "butter- flyw-shaped a(@) curves between RT and 15 K. These curves exhibit no influence of TSD below 200 K; furthermore they stay essentially symmetric about FO in the whole temperature range which proves the absence of parasitic bending or torque stresses.
The observed T-effects on amplitude and hysteresis of the a(@) loops will be discus- sed in a later publication.
I 0 50 100 150 200 250 300
I I
Temperature T lK1
Fia.8: Attenuation as function of bias stress at different tempera- tures below RT superimposed on the a(T) dependence measured with PVDF transducer (solid lines)
.
Dashed curve: a(T) for same sample before installation in the CTA.
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Acknowledgement: Work supported by the Deutsche Forschungsgemeinschaft (Sonder- forschungsbereich 125, Aachen-Julich-K6ln).
REFERENCES
[I1 G. Gremaud, M. Bujard; "Proc. 8th ICIFUAS" (A.V. Granato, G. Mozurkewich, C.A. Wert, Eds.) J. de Physique, Tome 46, Dec.1985, pp. C.10-315
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[8] A.B. Pippard; "Ultrasonic Attenuation in Metals", Phil. Mag. Vol. 46 (1955) p. 1104
