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Quench sensitivity of the anelastic effects due to the precipitation and dissolution in AlAg
Scripta METALLURGICA
Vol. 20, pp. 1689-1694, 1986 Printed in the U.S.A.
Pergamon Journals Ltd All rights reserved
QUENCH SENSITIVITY OF THE ANELASTIC EFFECTS DUE TO THE PRECIPITATION AND DISSOLUTION IN AIAg S. Kiss +
R
Schaller
and W. Benoit
Institut de Genie Atomique, Swiss Federal Institute PHB-Ecublens, CH-IOI5 Lausanne, Switzerland +Department of Solid State Physics, H-4010 Debrecen, Hungary
Kossuth
of Technology,
Lajos University,
P.O.Box
2,
(Received July 21, 1986) (Revised September 25, 1986)
Introduction When investigating a diffusion controlled phenomenon in a quenched material, the influence of the quenched in vacancies or other defects should always be taken into consideration. Regarding the widely studied and very important solute diffusion in different A1 alloys, an accelerating effect due to the excess vacancies can be observed in the case of long range diffusion (clustering for instance) (I) and short range reorientation (2) as well. Not counting these direct effects connected with the vacancy mechanism of the diffusion, other indirect effects due to quench can also be expected, mainly in the case of the long range diffusion process. Concerning the precipitation process such an indirect effect can be caused by secondary defects formed from the quenched in vacancies ()). These vacancies often condensate forming a monolayer disc, which having collapsed, can produce a Frank sessile dislocation loop. This ~ other loops formed in a similar way during the absorption of the vacancies can be transformed into more stable dislocation configurations depending on the stacking fault energy for instance. Since this last parameter is influenced by the solute concentration, and the resulting defects can act as nucleation sites for the precipitation, a dependence of the precipitation process on the quench circumstances can be expected. Because of the high sensitivity of the anelastic measurements to the crystal defects this method seems to be suitable for the investigation of the influence of quenching on the precipitation and dissolution processes. Since the quench can affect the distribution, the size, the surface morphology, or even the type of the precipitates, the anelastic effects connected with these parameters may change as well. This may cause change in the peak parameters caused by the precipitates present, but also a significant effect can be expected during the investigation of the anelaetic behaviour during the precipitation solution process. Since rather large and characteristic anelastic changes were found due to this transition in several AiAg alloys (4), the goal of this work is to investigate the influence of the quench on this phenomenon. Experimental Internal friction (i.f.) and reIative dynamic modulus measurements were made on AiAgSw% alioy using an inverted torsionai penduium. The 140 mm long
1689 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals
Ltd.
1690
ANELASTIC
EFFECTS
IN AIAg
Vol.
20, No. 12
specimens having a form of wire with a diameter of i mm were recrystallized and homogenized in a vertical furnace with argon atmosphere at 800 K for 4 hours. The water quench was made from the homogenisation or a lower temperature. The amplitude of the relative deformation during the continuous ~ibration and at the beginning of the measuring period in a free decay was i0- . The measuring frequency was about 0.8 Hz at room temperature. The heating and cooling rate used was i-1.5 K/min in most cases. Heat treatments between 700 and 800 K for 20-30 min in the pendulum at the end of the measuring period were applied as well. Results Results obtained on samples exposed to the same previous treatment but siowiy cooied in a few hours from the homogenization temperature wiii be used as a comparison. In the siow cooied case a sharp increase of the i.f. was observed at about 600 K during heating up. This increase was foiiowed by a sudden decrease between 640 and 660 K. The height of the sharp peak appearing in this way was about 20xlO "5 subtracting the estimated background. This peak is accompanied by a large decrease of the measuring frequency. The frequency change is at about 0.08 Hz which is more than i0% of the frequency measured before the peak. No significant effect in the above temperature range was found during cooling from about 640 K or higher temperatures. But i.f. peak and moduius increase simiiar to that during heating appeared at about 605 K in the spectrum measured during cooiing. These two effects were found to be strongiy coupled. Either of them appears oniy in case we have passed through the temperature range of the other one during the previous heat treatment. The above very characteristic and considerable effect right near the soivus temperature was much reduced for the sampies quenched after homogenisation. But the temperature and the character of them remained the same as it was for the slowIy cooied sampies (Fig.i). The difference can be attributed mainly to the change of the anelastic behaviour above the
-~ 1 0 5 4OF quenched
/ /
- - - s t o w t y cooled
/ /
30
~
~O
/ ./ > • ~-~
/~ ~
_
~
/
~
/
~7
we can find that this sudden frequency change after quench is about i/3 of that obtained after a slow cooling. (The f(515K) and f(685K) values are the frequencies measured at that temperature. The O.04Hz correction substracted is the extrapolated de-
crease of the f r e q u e n c y i n the 515 K - 685K t e m p e r a t u r e range, d i s r e g a r d i n g the sudden
effect under consideration.) Since this considerable change in ~f compared to the / / slowly cooled case is originated mainly J from the much less decrease of the f(685K) value, we can say that the alloy "remembers" even by hundred degrees above the solubility @OO 600 T[K) 700 0.6 limit, that it was quenched or slowly cooled from 800 K. Fig.l. Much reduced effect at the solvus This last statement is confirmed by a temperature after quench from great number of measurements on samples 800 K Quenched from 800 K and heat treated beIC
/ /
I
Lf peak t e m p e r a t u r e . The f r e q u e n c y and the IHz) damping curves i n t h i s range are u s u a l l y c o n s i d e r a b l y h i g h e r i n the quenched c o n d i t i o n s . This can be c o n f i r m e d by the q u a n t i t a t i v e a n a l y s i s of the f r e q u e n c y or modulus ~@ change c o r r e s p o n d i n g j u s t to the narrow transition t e m p e r a t u r e range. I n t r o d u c i n g the r e p r o d u c i b l e p a r a m e t e r , 6 f , which does not depend on the h e a t i n g r a t e
"""
/
"~'----~;
Vol.
20, No. 12
ANELASTIC
EFFECTS
IN AIAg
1691
tween room temperature and 750 K. The main character of the spectra and the value of df was the same after different subsequent heat treatments below 750 K. The quench suppresses the development of the significant peak and the significant change in the modulus observed on slow cooled samples. This effect of the quench can be partially or totally eliminated if annihilation heat treatment (AHT) between 750 and 800 K for about 20-30 minutes has been applied after the quench from 800 K. (The maximum cooling rate after these AHT was 8 K/min.) This elimination of the effect of the quench, i.e. the increase of the ~f value for instance is more effective when the AHT temperature is higher as it can be seen on the curves in Fig.2 where the annihilation temperatures are indicated. The i.f. and frequency curves measured after the AHT in this temperature range represent a transition from that b~x10 ~ , ! measured after quenching to that observed f (H~ after slow cooling. The higher the AHT tem750 0.$ perature the nearer it is to the slowly cooled case. These results are characteris3( /~ tic of the AHT temperature (Fig.3). As it
~
/
20
_
/
~
can be expected, these curves do not change after a new AHT of the same sample at a temperature lower than i t was before. But using subsequently higher and higher AHT temperatures and the same heating or cooling
'
~
rate in every case, a typical series of curves mentioned above can be obtained. ~.7
/#/ / S I-~ 10
~'~50 " ~ ~~,~ $
~5.~-7~n
~5
It should be noted that for the same sample the high temperature background above the peak usually decreased after the subsequent AHT. But the increasing peak height due to the increasing AHT temperature mostly can be attributed to the higher i.f. values below the peak.
A portion of the measurements was made on samples quenched from temperatures lower 600 TIK} 700 0.6 than 800 K. In order to keep the same starting conditions a l l s a m p l e s for studying the quench effect were recrystallized and homoFig.2. The effect of the annealing tem- genized at 800 K, and they all were quenched perature on the curves during from this temperature into water. After putheating ting back one of these samples to 800 K for about 20 minutes it was slowly cooled with the furnace to T temperature and it was quenched into water from this temperature. Measuring q these samples up to T ~ T ~ temperature Q-I and frequency curves characteristic of ~ can be obtained, but these results are a bit influenced by the sample mounting as well. More unambigous results charscteristic of the quench temperature can only be obtained during cooling these samples from about 700 K (where they were heat treated for 30 minutes) following the first measuring period of heating (Fig.4). The higher the quench temperature the higher the blocking effect on the frequency change. Comparing these results (obtained on different samples with the same geometrical parameters) a curve similar to the one obtained from that got after different annihilation treatments can be found but in the opposite sense. As it can be seen in Fig.5, the effect at the solvus temperature is higher if T is lower. The most intensive change of this curve is nearly in the same tem- q perature range as it was for the annealing experiments.
1692
ANELASTIC EFFECTS IN A1Ag
,
-,..
[Hz) 0~ -x-----..,..___~4~_
Vol.
,o
f (515K) p _ . . ^ _ ~c.-
$0
/I//t
_,.o
f (685 K) - x ~ ~_
~f
^~.~.-
' 0@
• 04
---~ 730
-~ 10
4
.02
Fig.3.
°'
/
0.65
o,60 76o
20, No. 12
640~
7~o "ron,~otEK)S'oo
S6o
The higher the annealing temperature the bigger the frequency change above the peak
Fig.4.
~------~90 30
/,80
660
TtK)
7&
ResuIts during cooling on samples quenched before from the temperatures marked
~f .10 Hz]
[ -
.05
k ~ ~'-0--
5'00
6()0
760
Fig.5. The e f f e c t of the quench temperature on the frequency change due to the t r a n s [ tion p r o c e s s
Tquench (~}~ Discussion
The considerabie increase of the modulus in the case of the siowiy cooied aampies has been attributed to the presence of precipitates and to their interaction with the iattice dislocations (4). The model suggested considers two possibiiities of the moduius increase by the precipitates. One part can be attributed to the higher moduius of the precipitates themselves in the two phase precipitated sample (5). This part is proportionai to the voiume fraction of the precipitates present. Another part was supposed to be connected to the disiocation pinning by the precipitates in. this high temperature region. It was estimated that the whoie possibie additionaI strain due to the disiocation motion in a Frank network is reduced by the pinning (6). This part of the modu-
Vol.
20, No. 12
ANELASTIC
lus disappears right in a narrow tates dissolve.
EFFECTS
temperature
IN AIAg
1693
range where the pinning
precipi-
The significant decrease of the damping near the solvus has been attributed to the disappearence of the precipitates which are thought to act as effective sources of the high temperature i.f. background. Not counting the possible relaxation inside or at the surface of the precipitates (which processes can produce an i.f. peak on the spectrum), the shape change of the precipitates due to the torsional stress during the measurement has been considered. This long range diffusion controlled process can result in an exponentially increasing background. In this case the mean diffusional length of the material transport necessary for the process has an important role. This length is the mean precipitate diameter. In this case a vacancy (and a resulting atomic) flux is supposed to occur from one part of the precipitate to the other one. The reason for this can be the different vacancy over or under saturation at the precipitate interfaces as vacancy sources under the action of the stress. This is similar to the process of the Nabarro-Herring or Coble creep that has been supposed to be a possible source of the high temperature i.f. background (6, 7). The resulting damping is inversely proportional to the second or third power of the characteristic diffusion length, respectively. The damping increases steeper than exponentially because of the decrease of the diameter during the dissolution but it goes to zero when the precipitate interfaces considered as vacancy sources disappear. An additional heating or cooling rate dependent i.f. part has also been observed in this transition temperature range. In order to explain our results on the quenched samples, the above considerations for the slowly cooled case can be used but they should be a bit modified because the defect structure can be different in the quenched state. Dislocation loops due to the quench enclosing stacking fault and attracting Ag atoms have been observed for a long time in AiAg alloy (8). The diffusion of the Ag-atoms to the loops Iowers the stacking fault energy and stabilises the loop by hindering its dissociation. The helical and the Frank sessile dislocations are known to be nucIeation sites for the ~' precipitates but the equilibrium ~ precipitates usually nucleate on the grain boundaries. Investigating the dissoiution of the 7' , a successive removal of the layers and a corresponding dislocation emission were observed (9). This dislocation can act as a nucieation place in a new precipitation process during cooling. Considering the above behaviour of the quenched AiAg alloy at not too high temperatures, we can suppose at least two types of defects containing agglomerated Ag atoms: the usual precipitates and another very stable precipitates probably nucleated heterogeneously on the different quenched in defects. The distinction was made because of their supposedly different thermal stability. The existence of these two types of precipitates has to be prooved by other more direct methods (e.g. X-ray diffraction or electron microscopy). We can explain our results supposing that a part of these defects disappears at the normal solubility limit during heating, but the other part of the defects due to the quench remains stable up to about 750 K, and they dissolve continuously mainly in the 750-800 K temperature range. Taking into consideration this behaviour, the model for the slowly cooled case can be applied. For the pinning model of the modulus change we obtain three characteristic mean values for the displacement of the dislocations under I solv < T ~ Tqs; and Us > uq f o r stress: Up+~ for T < Tsolv ; u ~ >u p+~,, f o r T ~Tqa where T i s t h e supposed s o l v u s t e m p e r a t u r e r e g i o n " f o r the defects qs c o n n e c t e d w i t h the quench. ( u and u~ are t h e d i s p l a c e m e n t of t h e d i s l o c a t i o n i n s o l i d s o l u t i o n s t a t e ~nd i n t h e s t a t e c o n t a i n i n g o n l y t h e quenched i n defects respectively. The u i s t h e d i s p l a c e m e n t of t h e d i s l o c a t i o n i n the P+~ m a t r i x c o n t a i n i n g both t y p e s of p r e c i p i t a t e s s u p p o s e d . ) The a d d i t i o n a l strain due to t h e d i s l o c a t i o n m o t i o n can be s e p a r a t e d i n a s i m i l a r way. Using t h e s e
obd be
t
s
values,
three stages o, the modulus e v a l u a t i o n
. It means that only a smaller decrease
of the modulus
can be
can
1694
ANELASTIC
EFFECTS
IN AIAg
Vol.
20, No.
12
observed during the normal dissolution of the precipitates. The missing part of the modulus defect can be detected during the dissolution of the quenched in defects between about 750 and 800 K. The effect of the different quench temperatures on the modulus can be explained in a similar way. We suppose that the number of the quenched in complexes and/or their effectivity increases with the temperature of quench. The interpretation of this i.f. can be made also in a similar way as for the siow cooled samples but here we can suppose that the defects due to quench can also act as a source of the damping. (These defects can be counted as vacancy sources for instance.) So their size, activity and volume fraction must be taken into consideration as well. Supposing that this last part of the i.f. background can be high enough, we are in agreement with the experimental results when a high and a steep background was found after the quench. If these defects compared to the ordinary precipitates are the dominant part of the damping sources at the transition temperature, a considerabiy lower damping peak can aiso be understood. The results of the AHT at different temperatures can be explained in a similar way. Supposing that the quenched in defects can be favourable centers of the precipitation nucieation, we can expect that the nucleation is more homogeneous and the precipitates are smaIler if a higher part of the quenched in complexes has been annihilated. For these precipitates the contribution to the high temperature damping in the precipitated state under consideration should be higher as it was really observed. The same considerations can be used for the interpretation of the effect of the quench temperature. The lower damping for the more drastically quenched samples in the precipitated range can be connected with a not homogeneous nucleation and with higher characteristic diffusional distances inside the precipitates. It should be noted that the results obtained in different samples can not be compared in all cases because of the influence of the sample mounting conditions. But usually a higher damping was also found above the peak for higher Tq values. This can be attributed to the higher concentration and activity of the vacancy sources in this region. Acknowledgements The authors are grateful to professor F.J. Kedves for his helpfui discussion and criticaI reading of the manuscript. This work was performed under the Swiss-Hungarian culturai scientific cooperation.
References (1) D. T u r n b u l l , H.S. Rosenbaum and H.N. T r e a f t i s , Acta Met. J , 277 (1960) (2) R. de B a t i s t , I n t e r n a l F r i c t i o n of S t r u c t u r a l D e f e c t s in C r y s t a l l i n e S o l i d s , N o r t h - H o l l a n d , Amsterdam (1972) (3) R.M.O. C o t t e r i l l et a l . , L a t t i c e D e f e c t s in Quenched M e t a l s , Academic Press, New York (1965) (4) S. K i s s , R. S c h a l l e r , W. B e n o i t , A n e l a s t i c e f f e c t s due to p r e c i p i t a t i o n (5) (6) (7) (8) (9)
and dissolution in AIAg alloys (Acta Met. in press) F. Fouquet, These INSA-Lyon (i977) J. Friedel, Oisiocations, Pergamon Press, Oxford (1964) A. Monzen, K. Suzuki, A. Sato, T. Mori, Acta Met. )l, 519 (l~B3) R.B. Nichoison and 3. Nutting, Acta Met. ~, 332 (1961) J.A. Hren and G. Thomas, Trans. Met. Soc. AIME 227, 308 (196))
Vol. 20, pp. 1689-1694, 1986 Printed in the U.S.A.
Pergamon Journals Ltd All rights reserved
QUENCH SENSITIVITY OF THE ANELASTIC EFFECTS DUE TO THE PRECIPITATION AND DISSOLUTION IN AIAg S. Kiss +
R
Schaller
and W. Benoit
Institut de Genie Atomique, Swiss Federal Institute PHB-Ecublens, CH-IOI5 Lausanne, Switzerland +Department of Solid State Physics, H-4010 Debrecen, Hungary
Kossuth
of Technology,
Lajos University,
P.O.Box
2,
(Received July 21, 1986) (Revised September 25, 1986)
Introduction When investigating a diffusion controlled phenomenon in a quenched material, the influence of the quenched in vacancies or other defects should always be taken into consideration. Regarding the widely studied and very important solute diffusion in different A1 alloys, an accelerating effect due to the excess vacancies can be observed in the case of long range diffusion (clustering for instance) (I) and short range reorientation (2) as well. Not counting these direct effects connected with the vacancy mechanism of the diffusion, other indirect effects due to quench can also be expected, mainly in the case of the long range diffusion process. Concerning the precipitation process such an indirect effect can be caused by secondary defects formed from the quenched in vacancies ()). These vacancies often condensate forming a monolayer disc, which having collapsed, can produce a Frank sessile dislocation loop. This ~ other loops formed in a similar way during the absorption of the vacancies can be transformed into more stable dislocation configurations depending on the stacking fault energy for instance. Since this last parameter is influenced by the solute concentration, and the resulting defects can act as nucleation sites for the precipitation, a dependence of the precipitation process on the quench circumstances can be expected. Because of the high sensitivity of the anelastic measurements to the crystal defects this method seems to be suitable for the investigation of the influence of quenching on the precipitation and dissolution processes. Since the quench can affect the distribution, the size, the surface morphology, or even the type of the precipitates, the anelastic effects connected with these parameters may change as well. This may cause change in the peak parameters caused by the precipitates present, but also a significant effect can be expected during the investigation of the anelaetic behaviour during the precipitation solution process. Since rather large and characteristic anelastic changes were found due to this transition in several AiAg alloys (4), the goal of this work is to investigate the influence of the quench on this phenomenon. Experimental Internal friction (i.f.) and reIative dynamic modulus measurements were made on AiAgSw% alioy using an inverted torsionai penduium. The 140 mm long
1689 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals
Ltd.
1690
ANELASTIC
EFFECTS
IN AIAg
Vol.
20, No. 12
specimens having a form of wire with a diameter of i mm were recrystallized and homogenized in a vertical furnace with argon atmosphere at 800 K for 4 hours. The water quench was made from the homogenisation or a lower temperature. The amplitude of the relative deformation during the continuous ~ibration and at the beginning of the measuring period in a free decay was i0- . The measuring frequency was about 0.8 Hz at room temperature. The heating and cooling rate used was i-1.5 K/min in most cases. Heat treatments between 700 and 800 K for 20-30 min in the pendulum at the end of the measuring period were applied as well. Results Results obtained on samples exposed to the same previous treatment but siowiy cooied in a few hours from the homogenization temperature wiii be used as a comparison. In the siow cooied case a sharp increase of the i.f. was observed at about 600 K during heating up. This increase was foiiowed by a sudden decrease between 640 and 660 K. The height of the sharp peak appearing in this way was about 20xlO "5 subtracting the estimated background. This peak is accompanied by a large decrease of the measuring frequency. The frequency change is at about 0.08 Hz which is more than i0% of the frequency measured before the peak. No significant effect in the above temperature range was found during cooling from about 640 K or higher temperatures. But i.f. peak and moduius increase simiiar to that during heating appeared at about 605 K in the spectrum measured during cooiing. These two effects were found to be strongiy coupled. Either of them appears oniy in case we have passed through the temperature range of the other one during the previous heat treatment. The above very characteristic and considerable effect right near the soivus temperature was much reduced for the sampies quenched after homogenisation. But the temperature and the character of them remained the same as it was for the slowIy cooied sampies (Fig.i). The difference can be attributed mainly to the change of the anelastic behaviour above the
-~ 1 0 5 4OF quenched
/ /
- - - s t o w t y cooled
/ /
30
~
~O
/ ./ > • ~-~
/~ ~
_
~
/
~
/
~7
we can find that this sudden frequency change after quench is about i/3 of that obtained after a slow cooling. (The f(515K) and f(685K) values are the frequencies measured at that temperature. The O.04Hz correction substracted is the extrapolated de-
crease of the f r e q u e n c y i n the 515 K - 685K t e m p e r a t u r e range, d i s r e g a r d i n g the sudden
effect under consideration.) Since this considerable change in ~f compared to the / / slowly cooled case is originated mainly J from the much less decrease of the f(685K) value, we can say that the alloy "remembers" even by hundred degrees above the solubility @OO 600 T[K) 700 0.6 limit, that it was quenched or slowly cooled from 800 K. Fig.l. Much reduced effect at the solvus This last statement is confirmed by a temperature after quench from great number of measurements on samples 800 K Quenched from 800 K and heat treated beIC
/ /
I
Lf peak t e m p e r a t u r e . The f r e q u e n c y and the IHz) damping curves i n t h i s range are u s u a l l y c o n s i d e r a b l y h i g h e r i n the quenched c o n d i t i o n s . This can be c o n f i r m e d by the q u a n t i t a t i v e a n a l y s i s of the f r e q u e n c y or modulus ~@ change c o r r e s p o n d i n g j u s t to the narrow transition t e m p e r a t u r e range. I n t r o d u c i n g the r e p r o d u c i b l e p a r a m e t e r , 6 f , which does not depend on the h e a t i n g r a t e
"""
/
"~'----~;
Vol.
20, No. 12
ANELASTIC
EFFECTS
IN AIAg
1691
tween room temperature and 750 K. The main character of the spectra and the value of df was the same after different subsequent heat treatments below 750 K. The quench suppresses the development of the significant peak and the significant change in the modulus observed on slow cooled samples. This effect of the quench can be partially or totally eliminated if annihilation heat treatment (AHT) between 750 and 800 K for about 20-30 minutes has been applied after the quench from 800 K. (The maximum cooling rate after these AHT was 8 K/min.) This elimination of the effect of the quench, i.e. the increase of the ~f value for instance is more effective when the AHT temperature is higher as it can be seen on the curves in Fig.2 where the annihilation temperatures are indicated. The i.f. and frequency curves measured after the AHT in this temperature range represent a transition from that b~x10 ~ , ! measured after quenching to that observed f (H~ after slow cooling. The higher the AHT tem750 0.$ perature the nearer it is to the slowly cooled case. These results are characteris3( /~ tic of the AHT temperature (Fig.3). As it
~
/
20
_
/
~
can be expected, these curves do not change after a new AHT of the same sample at a temperature lower than i t was before. But using subsequently higher and higher AHT temperatures and the same heating or cooling
'
~
rate in every case, a typical series of curves mentioned above can be obtained. ~.7
/#/ / S I-~ 10
~'~50 " ~ ~~,~ $
~5.~-7~n
~5
It should be noted that for the same sample the high temperature background above the peak usually decreased after the subsequent AHT. But the increasing peak height due to the increasing AHT temperature mostly can be attributed to the higher i.f. values below the peak.
A portion of the measurements was made on samples quenched from temperatures lower 600 TIK} 700 0.6 than 800 K. In order to keep the same starting conditions a l l s a m p l e s for studying the quench effect were recrystallized and homoFig.2. The effect of the annealing tem- genized at 800 K, and they all were quenched perature on the curves during from this temperature into water. After putheating ting back one of these samples to 800 K for about 20 minutes it was slowly cooled with the furnace to T temperature and it was quenched into water from this temperature. Measuring q these samples up to T ~ T ~ temperature Q-I and frequency curves characteristic of ~ can be obtained, but these results are a bit influenced by the sample mounting as well. More unambigous results charscteristic of the quench temperature can only be obtained during cooling these samples from about 700 K (where they were heat treated for 30 minutes) following the first measuring period of heating (Fig.4). The higher the quench temperature the higher the blocking effect on the frequency change. Comparing these results (obtained on different samples with the same geometrical parameters) a curve similar to the one obtained from that got after different annihilation treatments can be found but in the opposite sense. As it can be seen in Fig.5, the effect at the solvus temperature is higher if T is lower. The most intensive change of this curve is nearly in the same tem- q perature range as it was for the annealing experiments.
1692
ANELASTIC EFFECTS IN A1Ag
,
-,..
[Hz) 0~ -x-----..,..___~4~_
Vol.
,o
f (515K) p _ . . ^ _ ~c.-
$0
/I//t
_,.o
f (685 K) - x ~ ~_
~f
^~.~.-
' 0@
• 04
---~ 730
-~ 10
4
.02
Fig.3.
°'
/
0.65
o,60 76o
20, No. 12
640~
7~o "ron,~otEK)S'oo
S6o
The higher the annealing temperature the bigger the frequency change above the peak
Fig.4.
~------~90 30
/,80
660
TtK)
7&
ResuIts during cooling on samples quenched before from the temperatures marked
~f .10 Hz]
[ -
.05
k ~ ~'-0--
5'00
6()0
760
Fig.5. The e f f e c t of the quench temperature on the frequency change due to the t r a n s [ tion p r o c e s s
Tquench (~}~ Discussion
The considerabie increase of the modulus in the case of the siowiy cooied aampies has been attributed to the presence of precipitates and to their interaction with the iattice dislocations (4). The model suggested considers two possibiiities of the moduius increase by the precipitates. One part can be attributed to the higher moduius of the precipitates themselves in the two phase precipitated sample (5). This part is proportionai to the voiume fraction of the precipitates present. Another part was supposed to be connected to the disiocation pinning by the precipitates in. this high temperature region. It was estimated that the whoie possibie additionaI strain due to the disiocation motion in a Frank network is reduced by the pinning (6). This part of the modu-
Vol.
20, No. 12
ANELASTIC
lus disappears right in a narrow tates dissolve.
EFFECTS
temperature
IN AIAg
1693
range where the pinning
precipi-
The significant decrease of the damping near the solvus has been attributed to the disappearence of the precipitates which are thought to act as effective sources of the high temperature i.f. background. Not counting the possible relaxation inside or at the surface of the precipitates (which processes can produce an i.f. peak on the spectrum), the shape change of the precipitates due to the torsional stress during the measurement has been considered. This long range diffusion controlled process can result in an exponentially increasing background. In this case the mean diffusional length of the material transport necessary for the process has an important role. This length is the mean precipitate diameter. In this case a vacancy (and a resulting atomic) flux is supposed to occur from one part of the precipitate to the other one. The reason for this can be the different vacancy over or under saturation at the precipitate interfaces as vacancy sources under the action of the stress. This is similar to the process of the Nabarro-Herring or Coble creep that has been supposed to be a possible source of the high temperature i.f. background (6, 7). The resulting damping is inversely proportional to the second or third power of the characteristic diffusion length, respectively. The damping increases steeper than exponentially because of the decrease of the diameter during the dissolution but it goes to zero when the precipitate interfaces considered as vacancy sources disappear. An additional heating or cooling rate dependent i.f. part has also been observed in this transition temperature range. In order to explain our results on the quenched samples, the above considerations for the slowly cooled case can be used but they should be a bit modified because the defect structure can be different in the quenched state. Dislocation loops due to the quench enclosing stacking fault and attracting Ag atoms have been observed for a long time in AiAg alloy (8). The diffusion of the Ag-atoms to the loops Iowers the stacking fault energy and stabilises the loop by hindering its dissociation. The helical and the Frank sessile dislocations are known to be nucIeation sites for the ~' precipitates but the equilibrium ~ precipitates usually nucleate on the grain boundaries. Investigating the dissoiution of the 7' , a successive removal of the layers and a corresponding dislocation emission were observed (9). This dislocation can act as a nucieation place in a new precipitation process during cooling. Considering the above behaviour of the quenched AiAg alloy at not too high temperatures, we can suppose at least two types of defects containing agglomerated Ag atoms: the usual precipitates and another very stable precipitates probably nucleated heterogeneously on the different quenched in defects. The distinction was made because of their supposedly different thermal stability. The existence of these two types of precipitates has to be prooved by other more direct methods (e.g. X-ray diffraction or electron microscopy). We can explain our results supposing that a part of these defects disappears at the normal solubility limit during heating, but the other part of the defects due to the quench remains stable up to about 750 K, and they dissolve continuously mainly in the 750-800 K temperature range. Taking into consideration this behaviour, the model for the slowly cooled case can be applied. For the pinning model of the modulus change we obtain three characteristic mean values for the displacement of the dislocations under I solv < T ~ Tqs; and Us > uq f o r stress: Up+~ for T < Tsolv ; u ~ >u p+~,, f o r T ~Tqa where T i s t h e supposed s o l v u s t e m p e r a t u r e r e g i o n " f o r the defects qs c o n n e c t e d w i t h the quench. ( u and u~ are t h e d i s p l a c e m e n t of t h e d i s l o c a t i o n i n s o l i d s o l u t i o n s t a t e ~nd i n t h e s t a t e c o n t a i n i n g o n l y t h e quenched i n defects respectively. The u i s t h e d i s p l a c e m e n t of t h e d i s l o c a t i o n i n the P+~ m a t r i x c o n t a i n i n g both t y p e s of p r e c i p i t a t e s s u p p o s e d . ) The a d d i t i o n a l strain due to t h e d i s l o c a t i o n m o t i o n can be s e p a r a t e d i n a s i m i l a r way. Using t h e s e
obd be
t
s
values,
three stages o, the modulus e v a l u a t i o n
. It means that only a smaller decrease
of the modulus
can be
can
1694
ANELASTIC
EFFECTS
IN AIAg
Vol.
20, No.
12
observed during the normal dissolution of the precipitates. The missing part of the modulus defect can be detected during the dissolution of the quenched in defects between about 750 and 800 K. The effect of the different quench temperatures on the modulus can be explained in a similar way. We suppose that the number of the quenched in complexes and/or their effectivity increases with the temperature of quench. The interpretation of this i.f. can be made also in a similar way as for the siow cooled samples but here we can suppose that the defects due to quench can also act as a source of the damping. (These defects can be counted as vacancy sources for instance.) So their size, activity and volume fraction must be taken into consideration as well. Supposing that this last part of the i.f. background can be high enough, we are in agreement with the experimental results when a high and a steep background was found after the quench. If these defects compared to the ordinary precipitates are the dominant part of the damping sources at the transition temperature, a considerabiy lower damping peak can aiso be understood. The results of the AHT at different temperatures can be explained in a similar way. Supposing that the quenched in defects can be favourable centers of the precipitation nucieation, we can expect that the nucleation is more homogeneous and the precipitates are smaIler if a higher part of the quenched in complexes has been annihilated. For these precipitates the contribution to the high temperature damping in the precipitated state under consideration should be higher as it was really observed. The same considerations can be used for the interpretation of the effect of the quench temperature. The lower damping for the more drastically quenched samples in the precipitated range can be connected with a not homogeneous nucleation and with higher characteristic diffusional distances inside the precipitates. It should be noted that the results obtained in different samples can not be compared in all cases because of the influence of the sample mounting conditions. But usually a higher damping was also found above the peak for higher Tq values. This can be attributed to the higher concentration and activity of the vacancy sources in this region. Acknowledgements The authors are grateful to professor F.J. Kedves for his helpfui discussion and criticaI reading of the manuscript. This work was performed under the Swiss-Hungarian culturai scientific cooperation.
References (1) D. T u r n b u l l , H.S. Rosenbaum and H.N. T r e a f t i s , Acta Met. J , 277 (1960) (2) R. de B a t i s t , I n t e r n a l F r i c t i o n of S t r u c t u r a l D e f e c t s in C r y s t a l l i n e S o l i d s , N o r t h - H o l l a n d , Amsterdam (1972) (3) R.M.O. C o t t e r i l l et a l . , L a t t i c e D e f e c t s in Quenched M e t a l s , Academic Press, New York (1965) (4) S. K i s s , R. S c h a l l e r , W. B e n o i t , A n e l a s t i c e f f e c t s due to p r e c i p i t a t i o n (5) (6) (7) (8) (9)
and dissolution in AIAg alloys (Acta Met. in press) F. Fouquet, These INSA-Lyon (i977) J. Friedel, Oisiocations, Pergamon Press, Oxford (1964) A. Monzen, K. Suzuki, A. Sato, T. Mori, Acta Met. )l, 519 (l~B3) R.B. Nichoison and 3. Nutting, Acta Met. ~, 332 (1961) J.A. Hren and G. Thomas, Trans. Met. Soc. AIME 227, 308 (196))