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The effect of composition on the stored energy of cold work and the deformation behavior of gold-silver alloys
THE
EFFECT AND
OF
THE
COMPOSITION
ON
DEFORMATION P.
THE
BEHAVIOR
GREENFIELDtS
and
STORED OF M.
ENERGY
OF
GOLD-SILVER B.
COLD
WORK
ALLOYS*
BEVERt
The energy stored in chips formed by drilling at room temperature and at - 195°C has been determined for five gold-silver alloys in the composition range from 35 to 98 atomic per cent Au. The stored energy varies with composition by a factor of nearly 5 at room temperature and by nearly 3 at - 195°C. The stored energy also varies with the temperature of deformation and, after working at - 195’C, is approximately two to five times as large as after working at room temperature. The energy expended in the deformation is larger at the lower temperature, and increases with increasing concentration of solute. Only about 1 y0 of this energy is stored.
L’EFFET DE L.4 COMPOSITION SUR L’ENERGIE EMMAGASINEE L’ECROUISSAGE ET LE COMPORTEMENT A LA DEFORMATION ALLIAGES OR-ARGENT
LORS DE DES
L’energie emmagasinee dans des copeaux recueillis per forage It la temperature ordinaire et b - 195% a Qte determinee pour cinq alliages or-argent de la gamme 3%98% at. de Au. Cette Qnergie varie avec la composition: du simple au quintuple a la temperature ordinaire, au triple a - 195°C. Elle varie egalement avec la temperature de deformation et est approximativement de deux it cinq fois plus grande it - 195°C qu’it la temperature ordinaire. L’energie d&pens&e lors de la deformation est plus importante a la temperature la plus basse et elle s’accroit avec la concentration en solute: un pourcent seulement de cette bnergie est retenue.
DER
EINFLUSS DER ZUSAMMENSETZUNG AUF DIE BE1 KALTBEARBEITUNG GESPEICHERTE ENERGIE UND DAS VERFORMUNGSVERHALTEN VON GOLD-SILBER-LEGIERUNGEN
An fiinf Gold-Silber-Legierungen im Konzentrationsgebiet zwischen 35 und 98 At. - y0 Au wurde die in bei Raumtemperatur bsw. bei - 195°C hergestellten Bohrspanen gespeicherte Energie bestimmt. Die gespeicherte Energie variiert mit der Zusammensetzung und zwar bei Raumtemperatur urn annahernd einen Faktor 5, bei -195°C urn etwa einen Faktor 3. Sie iindert sich such mit der Verformungstemperatur und ist nach Kaltbearbeitung bei -195°C niiherungsweise 2 bis 5 ma1 so gross als nach Bearbeitung bei Raumtemperatur. Die gesamte Verformungsarbeit ist bei der tieferen Temperatur grosser und nimmt mit wachsender Konzentration der Zulegierung zu. Nur etwa 1 y0 dieser Arbeit wird gespeichert.
INTRODUCTION
stored energy during the annealing of brass of various zinc contents and of other alloys of copper, but few of
Measurements of the energy stored during cold working have been made on various metals and alloys.
his results can be compared
with each other because
different
had been applied
In general, the results are not comparable, however, because of differences in the methods of cold working
specimens.
and also in the precision of the methods of measuring
energy stored by these alloys and reported the values
the stored energy.
Knowledge
the stored
on the nature
energy
of the dependence of the
metal
work
has
been
done
to
investigate
the rate of evolution
1
METALLURGICA,
VOL.
-5, MARCH
1957
to the of the
a,gainst the work of torsion.
an increase in zinc content was associated with a,n increase in the ratio of the stored to the total energy.
With one minor exception,
Quinney and Taylor(2) reported a corresponding difference between copper and 70-30 brass. Clarebrough et aZ.t3) found that an increase in
the
analyzed
impurities
in copper from 0.012 to 0.033%
affected the rate of evolution of the stored energy during annealing, but had little effect on the total These investigators also compared their amount.
of the
*This investigation was sponsored by the US Atomic Energy Commission under Contract AT(30-l)-1002. Received Feburary 15, 1956; in revised form May 21, 1956. t Department of Metallurgy, Massachusetts Institute of Technology. $ Present address: The English Electric Co., Ltd., Whetstone, Leicestershire. ACTA
the amounts
is
effect of composition on the energy stored by alloys representing a wide composition range within a single-phase region. Satoo) investigated
He also determined
of
limited, therefore, and little is known about the effect of change in composition. In particular, no extensive
degrees of torsion
results with the larger amounts of stored energy measured by Taylor and Quinneyc4) in copper believed to contain about 0.6% of impurities.(3) Clarebrough et al. attributed this difference in the stored energy 125
126
ACTA
to the difference gation,
in composition.
Clarebrough
the energy
stored
METALLURGICA,
In a later investi-
et aZ.(5) observed in copper
containing
again
that
0.45%
im-
purities was greater than the energy stored in copper of higher purity. They also found that the rate of evolution of the stored energy was different for the two grades of copper. In
the
present
investigation,
gold-silver
alloys
VOL.
5,
1957
addition
to the bath.
suitably
chosen,
the
If the relative heat
effect
amounts
of the
addition could be reduced to a minimum.
were
combined
Gold, which
is exothermic upon dissolution in tin, could be used to produce thermal compensation with those silverrich alloys which dissolved in tin with insufficient heat evolution. Silver, which is endothermic upon dissolution in tin, could be used with alloys evolving
an
containing from 35.4 to 98.2 atomic per cent Au were drilled under controlled conditions at -195°C
excess of heat. When equal quantities of the compensating element were added with the annealed and
and at room temperature,
the cold-worked
chips was determined.
and the energy stored in the
The energies expended
deformation were also measured. made at both temperatures. EXPERIMENTAL
Cold
zLorking.
during
Tensile tests were
The
cold-worked
and at room temperature or water by a procedure
specific
The compensating
samples
were
under carbon tetrachloride in detail previ-
fered
with
the prompt
attainment
measured by a strain-ring assembly.(7)
all runs in which compensating
The stored-energy
as the
produced
difference
by the addition
values were deter-
between
the
of a worked
heat
effect
sample to a
bath of tin and the effect produced by an annealed standard. The accuracy of the method required that these heat effects were small. This condition resulted if the heat evolved on dissolution of a sample balanced the heat absorbed the tin. As in previous
in raising it to the temperature work,(63 ** s) gold-silver
used, but in this investigation
of
alloys were
the compositions
were
varied over a wide range. Thermal compensation for a given composition can be attained only over definite temperature
intervals.
The temperature
from which
the samples were added was fixed by the temperature of the working samples;
process
at -195°C
in order to standardize
for one set of the conditions
of
measurement, the samples worked at room temperature were also added from -195°C. The temperature of the bath ranged from 232”C, the melting-point
of tin,
to about 35O”C, considered the upper limit for satisfactory operation. Changing the bath temperature, therefore,
could not give full thermal
compensation
except in the composition range from about 62 to about 83 atomic per cent Au. To extend the composition limits for determining the stored energy, the experimental procedure was modified. A pure metal which, upon dissolution in tin, had a heat effect opposite in sign to the effect of the
alloy
specimen,
was included
for a set
In all runs in which an element was added with the alloy specimen, differences in the solution rate interthermal balance.
Calorim,elry.
additions
than 1 mg.
o~sly.(~) The total energy expended in the drilling operation was found from the torque, which was
mined
in the values
of worked and annealed samples were weighed against each other; their weights differed by less
under liquid nitrogen
described
uncertainties
of gold and silver and their
heats had little effect on the stored-energy
measurements.
PROCEDURES
prepared by drilling at -195°C
additions,
of the heats of solution
as part
of
the
of the
desired
These differences could be minimized
by operating at the highest possible temperature, were carried
out at 350°C.
additions
and
were made
The proportion
of the
compensating addition to the alloy specimen was 1 part of silver to 625 parts of alloy for the 98.2 atomic per cent Au specimen, and 1 part of gold to 0.35 parts of alloy for the 35.4 per cent Au specimens. The la,tter proportion was considered the limit compatible with the desired accuracy,
and alloys of lower gold content
were not investigated. The amount of gold required for compensation increased more rapidly than that of silver,
because
the
contribution
of
the
gold
was
reduced by the heat required to raise it to the temperature of the bath, while the silver was effective both through the sensible heat requirement and the heat of solution. The three alloys in the center of the composition range, containing 62.1, 72.2, and 83.0 atomic per cent Au, did not require additions. The technique
of transferring
compensating
the samples
to the
calorimeter, the calorimetric procedure, and methods of calculation have been described.c6) Tensile tests.
Tensile tests were made at -195°C
and at room temperature
with small strip specimens of
each of the compositions examined calorimetrically a procedure previously described.c6) EXPERIMENTAL
Stored energy
the
measurements.
by
RESULTS
The
values
of
the
energy stored, Esl, at room temperature and at -195°C are listed in Tables 1 and 2. They are plotted against composition in Fig 1. The data for the alloys
GREE~F~ELD
AND BEVER:
STORED
ENERGY
127
TABLE I. Energy cold working gold-silver alloys of various compositions ---___ ___--- relations in ____-_ . ..-..-_- -.-_____ -..-.___ Calorimetric run No.
Composition, at. 0/0 Au
Stored energy E,,, cetl/gram-atom
j room temperature _____
Energy of working E,, cal/gram-atom
----_
-
164 188
98.2 98.2
142 167
83.0 83.0
55 45
112 113 114 I56 116
72.2 72.2 72.2 72.2 72.2
93 76 93 94 86
62.1 62.1
40 41a
35.4 35.4
190 191
I
-~---.-. TABLE
..- -
_
2. Energy
____--
5530 6640
0.31 0.39
8790
1.1
8740
0.94
9780
1.03
--_~ ._.--
.-
_-
~____I -__.
relations in cold working gold-silver alloys of various oomposil;io-.._____I ns at - 195% - -.____ __ -.-_ __-.---. ___ 1
Calorimetric ruu No. ___
Composition, at. 76 Au
1 Stored energy E,,, Cal/gram-atom
..-.. .__--
12,280
0.92
205 206 207
19,580
17,400 17,400
1.05 1.18 1.19
62. I 62.1 62.1
240 247 240
17,810” 17,810* 17,810*
I.35 1.39 1.35
35.4 35.4 35.4
228 189 226
19,730 20,900
1.16 0.90
98.2 98.2
113 88
143 144 165
s3.0 83.0 83.0
148 166 154
104 101 106
72.2 72.2 72.2
:; 94 184 189 191
-=
______~_ * Average for run No. 87, 92, 94.
E 280 3 0 ; 240
Weight 80
60
50
and 72.2 atomic per cent Au have been reported previously in different contexts.‘“> lo) Energy expendd during drilling. Some values of the energy expended during the drilling operation E, me included in Tables I and 2. In one instance, the figure reported is the average of several tests. An uncertainty of at least 10% is Iikely for all values of the expended energy. The tables also give the ratios of the stored to the expended energy, ~~~~~~,expressed as percentages. Tensile tests. Figs. 2 and 3 show the true stresstrue strain curves at room temperature and --I95’C for all compositions investigated calorimetrically. The curves for two of the alloys have already been
40
% 80 40 I 100
90
80
100
70
60
Composition,
ZZ
containing 62.1
% Au 70
ii 2200 0 160 S g 120 0) 0 x
V.&,jK#)f
-
177 181
90
Energy of working E,, cal/gram-atom
50 atomic
40 % Au
30
FIQ. 1. The energy stored in gold-silver alloys cold-worked at two temperatures as a function of composition
ACTA
1%
METALLURGICA,
2”15
20.000
I-
2
10 10.000 5
0
0
0% 0.24 ,True strain
008
o-32
FIG. 2. Trua stress-true stmin curvcx for various gold.silver
.11__.__I ..__.._ L^_..____i..-^
,
,
* AP.c)
r’l
0 FIG.
/
At % Au
940
I 0.08
11
i .
I(
0.16 0.24 True stroin
,
Rm00
2 2
IJ 0.32
’
3. True stress-true strain curves for various gold-silver
x-blloysat - 198°C.
reported.t6$ lo) The curves were not carried to maximum stress, since necking prevented measurement of true stress beyond this point. A possible size effect resulting from the small thickness of the specimens should be kept in mind in considering the absolute values of the stress. DISCUSSION (hzeral. The energy stored by gold-silver alloys depends appreciably on composition and at both temperatures of working increases with decreasing gold content in the range from 98 to about 50 atomic per cent Au. The data for lower gold contents are limited owing to experimental difficulties already mentioned, but it is very probabIe that the stored energy goes through a maximum near the equi-atomic composition. At - 195”C, the energy stored increases from approximately 100 ~al~gram-untornat the goldrich end to a probable maximum of approximately
VOL.
6, 1957
250 Cal/gram-atom. The corresponding figures for working at room temperature are 20 and 100 oal/gramatom. The effect of composition on the stored energy is, therefore, relatively larger at, room temperature. The energy expended in forming chips by drilling increases with decreasing gold content of the alloys at both temperatures. Because the data are limited in extent and accuracy, it has not been established that the energy expended reaches a maxirnllln at the equi-atomic composition. The energy expended at the temperature of liquid nitrogen is approximately twice as large as at room temperature. The ratio of the stored energy to the energy expended in the working process is of the order of 1% both at room temperature and at -195°C. The only exceptions are the lower ratios found with the specimens containing 98.2% Au worked at room temperature. In earlier work with the 62.1 atomic per cent Au alloy, the ratio of the stored to the expended energy was determined as a function of the strain. When this ratio had fallen to 1.3% in rolling(*) and to 0.7% in orthogonal cutting,(i’~ the stored energy approached saturation. In the present investigation, the measured values of the ratio E7,,/E, never exceeded 1.4%, and in many instances mere much lower. It is concluded that the values of t’he stored energy for all compositions represent essentially satiation. I&rpretation of results. The main findings of this investigation are the marked composition-dependence of the stored energy and the effect of temperature on this dependence. For an ~~~retation, possible correlations of these findings with other observations must be tested. In particular, the composition dependence of the strength and of the energy expended should be compared with the behavior of the stored energy. The manner in which the compositiondependence of these quantities varies with temperature may also be significant. In terms of the mechanism of the storing of energy, the destruction of short-range order, the role of vacancies at, low temperature, elastic energy, the generation of subgrain boundaries, and the effect of imperfections generally should be considered. Earlier investigators have found that the strength properties of gold-silver alloys at room temperature go through a maximum near the equi-atomic composition.(i2-r4) The true stress-true strain curves, shown in Figs. 2 and 3 for the alloys investigated in the present work, indicate an increase in strength with decreasing gold eontent in the gold-rich alloys at room temperature and at -195°C. The curves are
GREENPIELD
90
Weight % Au 80 70 60
AND
50
BEVER:
40
$ 240 , E’ 200 & \ jj 760
$? 60 c” LlJ 40 0 100 QO
80 70 60 50 40 30 Composition. atomic % Au
20
Ra. 4. The energy stored in cold-worked samples of goldsilver alloys and the energy due t*o +!iOyOdestruction of short-range order. also compatible with the existence of maxima of the strength properties near the equi-atomic composition. The strength properties may be analysed by comparing the true stress at a given stmin for the various alloys. Such an analysis shows that the difference of the strength properties at room temperature and -195’C is relatively smaller and varies less with composition than t,he difference in the energy stored at the two ~m~eratures, which is markedly co~n~osition-dependent, as can be seen from Pig. 4. Any simple connection between the mechanical strength and the stored energy at, either t,emperature is thus precluded. Expsessed as functions of composition, the stored energy and the energy expended increase parallel to each other. The effect of temperature on the two sets of energy values differs, however: the energy expended changes by a factor of approximately 2 between room temperature and -195°C for all compositions, while the factor for the stored energy is considerably larger and varies with composition. No simple relation between the stored and expended energy can thus be assumed. Norman and WarrerP) have shown that shortrange order exists in gold-silver alloys. The shortrange order parameter varies with composition and has been measured in alloys containing 25, 50, and 62.1 atomic per cent Au as -0.05, -0.08, and -0.06, respectively.(gl 15) Severe deformation by filing a 62.1 per cent Au alloy reduced the short-range order parameter by about 50%. ts) Drilling may be assumed to reduce the parameter by approximately the same amount as filing. It is significant in this connection that the energies stored in filing and drilling at room temperat~ureare almost equal.
STORED
ENERGY
129
The energy associated with changes in shortrange order may be calculated by the quasi-chemical theory.(l”j Fig. 4 shows the effect due to a destruction of short-range order of 50% as a function of composition. (Changes in short-range order of different amounts would have propo~ional energy effects.) These c&lculations show that the dependence of the stored energy on composition can be attributed only in small part to the destruction of short-range order. A considerable portion of the change of the stored energy with composition must therefore be associated with other mechanisms. It should further be noted in Fig. 4 that the portion of the stored energy which is not attributable to short-range order destruction is itself composition-dependent and goes through a maximum near 50 atomic per cent. Fig. 4 also shows that the difference in the energies stored at -195% and at room temperature varies with composition and has a maximum near the equiatomic composition. A recent investigation of the evolution of the energy stored by the 72.2 atomic per cent Au alloy cold-worked at -195°C and at room temperature suggests that part of the difference in the energy stored at these temperat.ures is attributable to vacancies formed at the low temperature.(lO) It is not known whether the number of vacancies resulting from low-temperature deformation and the energy associated with their formation depend on composition. The contributions of the various mecha~sms of energy storage may be summarized as follows. The destruction of short-range order can account at most for only a small part of the difference in the energy stored by gold-silver alloys of different compositions. The role of vacancies is probably restricted to low temperatures. Local elastic distortion was shown previously to account only for a negligible fraction of the energy stored in a 62.1 per cent Au alloy.@) Since these mechanisms thus cannot explain the total amount of energy stored, the unaccoun~ balance must be a function of the number and distribution of imperfections other than vacancies. A previous investigation has shown that most of the energy stored during working a 62.1 per cent Au alloy at room temperature is associated with the presence of sub-boundaries.cO) The composition dependence of the stored energy found in the present work suggests an effect of solute atoms on the characteristics of the subgrain structure. An effect of solute atoms on the inherent interfacial energy of sub-boundaries is possible, but it is more probable that the size of subgrains formed during deformation depends on the composition of the alloys.
230
ACTA
M~T~I~LURG~~A,
ACKNOWLEDGMENTS
The authors express their appreciation of the support of this investigation by the US Atomic Ener,oy Commission under Contract No. AT(30-l)-1002. They are also grateful to Mr. Benjamin Howell for his assistance in the ex~e~mental work. REFERENCES 8. Sara Sci. Rep. Tolmku. Imp. Univ. 20, 140 (1931), H. QUINNEY and G. I. TAYLOR Proc. Roy, Sm. (London) l@A, 157 (1937). L. M. CLAREBROUGH,M. E. HAROREAVES, D. MI~EELL. and G. W. WEST Prac. Roy. Sot. (London) 215A, 607 (1962). G. 1. TAYLOR and H. QUINNEY Proc. Roy. I%C. (London) l48A, 307 (1934). L. M. CLAREBROUOH,M. E. HARGREAVES, rend G. W. WEYT Proc. Roy. Sac. (London) 232A, 252 (1955).
VOL,
5,
18.57
6. J. S. L. LEACH, E. G. LOE~EN, and M. B. BEVER J. Appl. Phys. 28, 728 (1955). 7. E. G. LOEWEN ebnd M. C. S~aw Imtmments 28, 560 (1950). 8. M. B. BEVER and L. B. TICKN~R Acta Metallurgica 1, 116 (1953). 9. B. L. AVERBACR, M. B. BEVER, iM. F. COWERFORD,and J. S. L. &ZACH Acta &fetnllurgim 4, 477 (1950). 10. P. GREENFIELD and M, B. I~EYER Aeta ~~~~l~ur~~c~ 4, 433 (1956). 11. RI. B. BEVER. E. R. MARSHX&. and L. B. TICKNOR J. Appl. Phyi24, 1176 (1953). ’ 12. N. S. KURNAKOW and S. I?. ZEMCZUZNY 2. nnorg. Chem. so, 1 (1908). 13. L. STERNER-RAINER 2.1MetaEk.18, 143 (1926). 14. G. SACKS and J. WEERTS 2. Physik 62, 473 (1930). 16. N. NORMAN and B. E. WARREN J. Appl. Phys. 22, 483 (1951). 16. P. S. RUPEAAN and 8. L. AVERBACK B&n ~~fe~~~~~rg~e~ 4, 383 (1956).
EFFECT AND
OF
THE
COMPOSITION
ON
DEFORMATION P.
THE
BEHAVIOR
GREENFIELDtS
and
STORED OF M.
ENERGY
OF
GOLD-SILVER B.
COLD
WORK
ALLOYS*
BEVERt
The energy stored in chips formed by drilling at room temperature and at - 195°C has been determined for five gold-silver alloys in the composition range from 35 to 98 atomic per cent Au. The stored energy varies with composition by a factor of nearly 5 at room temperature and by nearly 3 at - 195°C. The stored energy also varies with the temperature of deformation and, after working at - 195’C, is approximately two to five times as large as after working at room temperature. The energy expended in the deformation is larger at the lower temperature, and increases with increasing concentration of solute. Only about 1 y0 of this energy is stored.
L’EFFET DE L.4 COMPOSITION SUR L’ENERGIE EMMAGASINEE L’ECROUISSAGE ET LE COMPORTEMENT A LA DEFORMATION ALLIAGES OR-ARGENT
LORS DE DES
L’energie emmagasinee dans des copeaux recueillis per forage It la temperature ordinaire et b - 195% a Qte determinee pour cinq alliages or-argent de la gamme 3%98% at. de Au. Cette Qnergie varie avec la composition: du simple au quintuple a la temperature ordinaire, au triple a - 195°C. Elle varie egalement avec la temperature de deformation et est approximativement de deux it cinq fois plus grande it - 195°C qu’it la temperature ordinaire. L’energie d&pens&e lors de la deformation est plus importante a la temperature la plus basse et elle s’accroit avec la concentration en solute: un pourcent seulement de cette bnergie est retenue.
DER
EINFLUSS DER ZUSAMMENSETZUNG AUF DIE BE1 KALTBEARBEITUNG GESPEICHERTE ENERGIE UND DAS VERFORMUNGSVERHALTEN VON GOLD-SILBER-LEGIERUNGEN
An fiinf Gold-Silber-Legierungen im Konzentrationsgebiet zwischen 35 und 98 At. - y0 Au wurde die in bei Raumtemperatur bsw. bei - 195°C hergestellten Bohrspanen gespeicherte Energie bestimmt. Die gespeicherte Energie variiert mit der Zusammensetzung und zwar bei Raumtemperatur urn annahernd einen Faktor 5, bei -195°C urn etwa einen Faktor 3. Sie iindert sich such mit der Verformungstemperatur und ist nach Kaltbearbeitung bei -195°C niiherungsweise 2 bis 5 ma1 so gross als nach Bearbeitung bei Raumtemperatur. Die gesamte Verformungsarbeit ist bei der tieferen Temperatur grosser und nimmt mit wachsender Konzentration der Zulegierung zu. Nur etwa 1 y0 dieser Arbeit wird gespeichert.
INTRODUCTION
stored energy during the annealing of brass of various zinc contents and of other alloys of copper, but few of
Measurements of the energy stored during cold working have been made on various metals and alloys.
his results can be compared
with each other because
different
had been applied
In general, the results are not comparable, however, because of differences in the methods of cold working
specimens.
and also in the precision of the methods of measuring
energy stored by these alloys and reported the values
the stored energy.
Knowledge
the stored
on the nature
energy
of the dependence of the
metal
work
has
been
done
to
investigate
the rate of evolution
1
METALLURGICA,
VOL.
-5, MARCH
1957
to the of the
a,gainst the work of torsion.
an increase in zinc content was associated with a,n increase in the ratio of the stored to the total energy.
With one minor exception,
Quinney and Taylor(2) reported a corresponding difference between copper and 70-30 brass. Clarebrough et aZ.t3) found that an increase in
the
analyzed
impurities
in copper from 0.012 to 0.033%
affected the rate of evolution of the stored energy during annealing, but had little effect on the total These investigators also compared their amount.
of the
*This investigation was sponsored by the US Atomic Energy Commission under Contract AT(30-l)-1002. Received Feburary 15, 1956; in revised form May 21, 1956. t Department of Metallurgy, Massachusetts Institute of Technology. $ Present address: The English Electric Co., Ltd., Whetstone, Leicestershire. ACTA
the amounts
is
effect of composition on the energy stored by alloys representing a wide composition range within a single-phase region. Satoo) investigated
He also determined
of
limited, therefore, and little is known about the effect of change in composition. In particular, no extensive
degrees of torsion
results with the larger amounts of stored energy measured by Taylor and Quinneyc4) in copper believed to contain about 0.6% of impurities.(3) Clarebrough et al. attributed this difference in the stored energy 125
126
ACTA
to the difference gation,
in composition.
Clarebrough
the energy
stored
METALLURGICA,
In a later investi-
et aZ.(5) observed in copper
containing
again
that
0.45%
im-
purities was greater than the energy stored in copper of higher purity. They also found that the rate of evolution of the stored energy was different for the two grades of copper. In
the
present
investigation,
gold-silver
alloys
VOL.
5,
1957
addition
to the bath.
suitably
chosen,
the
If the relative heat
effect
amounts
of the
addition could be reduced to a minimum.
were
combined
Gold, which
is exothermic upon dissolution in tin, could be used to produce thermal compensation with those silverrich alloys which dissolved in tin with insufficient heat evolution. Silver, which is endothermic upon dissolution in tin, could be used with alloys evolving
an
containing from 35.4 to 98.2 atomic per cent Au were drilled under controlled conditions at -195°C
excess of heat. When equal quantities of the compensating element were added with the annealed and
and at room temperature,
the cold-worked
chips was determined.
and the energy stored in the
The energies expended
deformation were also measured. made at both temperatures. EXPERIMENTAL
Cold
zLorking.
during
Tensile tests were
The
cold-worked
and at room temperature or water by a procedure
specific
The compensating
samples
were
under carbon tetrachloride in detail previ-
fered
with
the prompt
attainment
measured by a strain-ring assembly.(7)
all runs in which compensating
The stored-energy
as the
produced
difference
by the addition
values were deter-
between
the
of a worked
heat
effect
sample to a
bath of tin and the effect produced by an annealed standard. The accuracy of the method required that these heat effects were small. This condition resulted if the heat evolved on dissolution of a sample balanced the heat absorbed the tin. As in previous
in raising it to the temperature work,(63 ** s) gold-silver
used, but in this investigation
of
alloys were
the compositions
were
varied over a wide range. Thermal compensation for a given composition can be attained only over definite temperature
intervals.
The temperature
from which
the samples were added was fixed by the temperature of the working samples;
process
at -195°C
in order to standardize
for one set of the conditions
of
measurement, the samples worked at room temperature were also added from -195°C. The temperature of the bath ranged from 232”C, the melting-point
of tin,
to about 35O”C, considered the upper limit for satisfactory operation. Changing the bath temperature, therefore,
could not give full thermal
compensation
except in the composition range from about 62 to about 83 atomic per cent Au. To extend the composition limits for determining the stored energy, the experimental procedure was modified. A pure metal which, upon dissolution in tin, had a heat effect opposite in sign to the effect of the
alloy
specimen,
was included
for a set
In all runs in which an element was added with the alloy specimen, differences in the solution rate interthermal balance.
Calorim,elry.
additions
than 1 mg.
o~sly.(~) The total energy expended in the drilling operation was found from the torque, which was
mined
in the values
of worked and annealed samples were weighed against each other; their weights differed by less
under liquid nitrogen
described
uncertainties
of gold and silver and their
heats had little effect on the stored-energy
measurements.
PROCEDURES
prepared by drilling at -195°C
additions,
of the heats of solution
as part
of
the
of the
desired
These differences could be minimized
by operating at the highest possible temperature, were carried
out at 350°C.
additions
and
were made
The proportion
of the
compensating addition to the alloy specimen was 1 part of silver to 625 parts of alloy for the 98.2 atomic per cent Au specimen, and 1 part of gold to 0.35 parts of alloy for the 35.4 per cent Au specimens. The la,tter proportion was considered the limit compatible with the desired accuracy,
and alloys of lower gold content
were not investigated. The amount of gold required for compensation increased more rapidly than that of silver,
because
the
contribution
of
the
gold
was
reduced by the heat required to raise it to the temperature of the bath, while the silver was effective both through the sensible heat requirement and the heat of solution. The three alloys in the center of the composition range, containing 62.1, 72.2, and 83.0 atomic per cent Au, did not require additions. The technique
of transferring
compensating
the samples
to the
calorimeter, the calorimetric procedure, and methods of calculation have been described.c6) Tensile tests.
Tensile tests were made at -195°C
and at room temperature
with small strip specimens of
each of the compositions examined calorimetrically a procedure previously described.c6) EXPERIMENTAL
Stored energy
the
measurements.
by
RESULTS
The
values
of
the
energy stored, Esl, at room temperature and at -195°C are listed in Tables 1 and 2. They are plotted against composition in Fig 1. The data for the alloys
GREE~F~ELD
AND BEVER:
STORED
ENERGY
127
TABLE I. Energy cold working gold-silver alloys of various compositions ---___ ___--- relations in ____-_ . ..-..-_- -.-_____ -..-.___ Calorimetric run No.
Composition, at. 0/0 Au
Stored energy E,,, cetl/gram-atom
j room temperature _____
Energy of working E,, cal/gram-atom
----_
-
164 188
98.2 98.2
142 167
83.0 83.0
55 45
112 113 114 I56 116
72.2 72.2 72.2 72.2 72.2
93 76 93 94 86
62.1 62.1
40 41a
35.4 35.4
190 191
I
-~---.-. TABLE
..- -
_
2. Energy
____--
5530 6640
0.31 0.39
8790
1.1
8740
0.94
9780
1.03
--_~ ._.--
.-
_-
~____I -__.
relations in cold working gold-silver alloys of various oomposil;io-.._____I ns at - 195% - -.____ __ -.-_ __-.---. ___ 1
Calorimetric ruu No. ___
Composition, at. 76 Au
1 Stored energy E,,, Cal/gram-atom
..-.. .__--
12,280
0.92
205 206 207
19,580
17,400 17,400
1.05 1.18 1.19
62. I 62.1 62.1
240 247 240
17,810” 17,810* 17,810*
I.35 1.39 1.35
35.4 35.4 35.4
228 189 226
19,730 20,900
1.16 0.90
98.2 98.2
113 88
143 144 165
s3.0 83.0 83.0
148 166 154
104 101 106
72.2 72.2 72.2
:; 94 184 189 191
-=
______~_ * Average for run No. 87, 92, 94.
E 280 3 0 ; 240
Weight 80
60
50
and 72.2 atomic per cent Au have been reported previously in different contexts.‘“> lo) Energy expendd during drilling. Some values of the energy expended during the drilling operation E, me included in Tables I and 2. In one instance, the figure reported is the average of several tests. An uncertainty of at least 10% is Iikely for all values of the expended energy. The tables also give the ratios of the stored to the expended energy, ~~~~~~,expressed as percentages. Tensile tests. Figs. 2 and 3 show the true stresstrue strain curves at room temperature and --I95’C for all compositions investigated calorimetrically. The curves for two of the alloys have already been
40
% 80 40 I 100
90
80
100
70
60
Composition,
ZZ
containing 62.1
% Au 70
ii 2200 0 160 S g 120 0) 0 x
V.&,jK#)f
-
177 181
90
Energy of working E,, cal/gram-atom
50 atomic
40 % Au
30
FIQ. 1. The energy stored in gold-silver alloys cold-worked at two temperatures as a function of composition
ACTA
1%
METALLURGICA,
2”15
20.000
I-
2
10 10.000 5
0
0
0% 0.24 ,True strain
008
o-32
FIG. 2. Trua stress-true stmin curvcx for various gold.silver
.11__.__I ..__.._ L^_..____i..-^
,
,
* AP.c)
r’l
0 FIG.
/
At % Au
940
I 0.08
11
i .
I(
0.16 0.24 True stroin
,
Rm00
2 2
IJ 0.32
’
3. True stress-true strain curves for various gold-silver
x-blloysat - 198°C.
reported.t6$ lo) The curves were not carried to maximum stress, since necking prevented measurement of true stress beyond this point. A possible size effect resulting from the small thickness of the specimens should be kept in mind in considering the absolute values of the stress. DISCUSSION (hzeral. The energy stored by gold-silver alloys depends appreciably on composition and at both temperatures of working increases with decreasing gold content in the range from 98 to about 50 atomic per cent Au. The data for lower gold contents are limited owing to experimental difficulties already mentioned, but it is very probabIe that the stored energy goes through a maximum near the equi-atomic composition. At - 195”C, the energy stored increases from approximately 100 ~al~gram-untornat the goldrich end to a probable maximum of approximately
VOL.
6, 1957
250 Cal/gram-atom. The corresponding figures for working at room temperature are 20 and 100 oal/gramatom. The effect of composition on the stored energy is, therefore, relatively larger at, room temperature. The energy expended in forming chips by drilling increases with decreasing gold content of the alloys at both temperatures. Because the data are limited in extent and accuracy, it has not been established that the energy expended reaches a maxirnllln at the equi-atomic composition. The energy expended at the temperature of liquid nitrogen is approximately twice as large as at room temperature. The ratio of the stored energy to the energy expended in the working process is of the order of 1% both at room temperature and at -195°C. The only exceptions are the lower ratios found with the specimens containing 98.2% Au worked at room temperature. In earlier work with the 62.1 atomic per cent Au alloy, the ratio of the stored to the expended energy was determined as a function of the strain. When this ratio had fallen to 1.3% in rolling(*) and to 0.7% in orthogonal cutting,(i’~ the stored energy approached saturation. In the present investigation, the measured values of the ratio E7,,/E, never exceeded 1.4%, and in many instances mere much lower. It is concluded that the values of t’he stored energy for all compositions represent essentially satiation. I&rpretation of results. The main findings of this investigation are the marked composition-dependence of the stored energy and the effect of temperature on this dependence. For an ~~~retation, possible correlations of these findings with other observations must be tested. In particular, the composition dependence of the strength and of the energy expended should be compared with the behavior of the stored energy. The manner in which the compositiondependence of these quantities varies with temperature may also be significant. In terms of the mechanism of the storing of energy, the destruction of short-range order, the role of vacancies at, low temperature, elastic energy, the generation of subgrain boundaries, and the effect of imperfections generally should be considered. Earlier investigators have found that the strength properties of gold-silver alloys at room temperature go through a maximum near the equi-atomic composition.(i2-r4) The true stress-true strain curves, shown in Figs. 2 and 3 for the alloys investigated in the present work, indicate an increase in strength with decreasing gold eontent in the gold-rich alloys at room temperature and at -195°C. The curves are
GREENPIELD
90
Weight % Au 80 70 60
AND
50
BEVER:
40
$ 240 , E’ 200 & \ jj 760
$? 60 c” LlJ 40 0 100 QO
80 70 60 50 40 30 Composition. atomic % Au
20
Ra. 4. The energy stored in cold-worked samples of goldsilver alloys and the energy due t*o +!iOyOdestruction of short-range order. also compatible with the existence of maxima of the strength properties near the equi-atomic composition. The strength properties may be analysed by comparing the true stress at a given stmin for the various alloys. Such an analysis shows that the difference of the strength properties at room temperature and -195’C is relatively smaller and varies less with composition than t,he difference in the energy stored at the two ~m~eratures, which is markedly co~n~osition-dependent, as can be seen from Pig. 4. Any simple connection between the mechanical strength and the stored energy at, either t,emperature is thus precluded. Expsessed as functions of composition, the stored energy and the energy expended increase parallel to each other. The effect of temperature on the two sets of energy values differs, however: the energy expended changes by a factor of approximately 2 between room temperature and -195°C for all compositions, while the factor for the stored energy is considerably larger and varies with composition. No simple relation between the stored and expended energy can thus be assumed. Norman and WarrerP) have shown that shortrange order exists in gold-silver alloys. The shortrange order parameter varies with composition and has been measured in alloys containing 25, 50, and 62.1 atomic per cent Au as -0.05, -0.08, and -0.06, respectively.(gl 15) Severe deformation by filing a 62.1 per cent Au alloy reduced the short-range order parameter by about 50%. ts) Drilling may be assumed to reduce the parameter by approximately the same amount as filing. It is significant in this connection that the energies stored in filing and drilling at room temperat~ureare almost equal.
STORED
ENERGY
129
The energy associated with changes in shortrange order may be calculated by the quasi-chemical theory.(l”j Fig. 4 shows the effect due to a destruction of short-range order of 50% as a function of composition. (Changes in short-range order of different amounts would have propo~ional energy effects.) These c&lculations show that the dependence of the stored energy on composition can be attributed only in small part to the destruction of short-range order. A considerable portion of the change of the stored energy with composition must therefore be associated with other mechanisms. It should further be noted in Fig. 4 that the portion of the stored energy which is not attributable to short-range order destruction is itself composition-dependent and goes through a maximum near 50 atomic per cent. Fig. 4 also shows that the difference in the energies stored at -195% and at room temperature varies with composition and has a maximum near the equiatomic composition. A recent investigation of the evolution of the energy stored by the 72.2 atomic per cent Au alloy cold-worked at -195°C and at room temperature suggests that part of the difference in the energy stored at these temperat.ures is attributable to vacancies formed at the low temperature.(lO) It is not known whether the number of vacancies resulting from low-temperature deformation and the energy associated with their formation depend on composition. The contributions of the various mecha~sms of energy storage may be summarized as follows. The destruction of short-range order can account at most for only a small part of the difference in the energy stored by gold-silver alloys of different compositions. The role of vacancies is probably restricted to low temperatures. Local elastic distortion was shown previously to account only for a negligible fraction of the energy stored in a 62.1 per cent Au alloy.@) Since these mechanisms thus cannot explain the total amount of energy stored, the unaccoun~ balance must be a function of the number and distribution of imperfections other than vacancies. A previous investigation has shown that most of the energy stored during working a 62.1 per cent Au alloy at room temperature is associated with the presence of sub-boundaries.cO) The composition dependence of the stored energy found in the present work suggests an effect of solute atoms on the characteristics of the subgrain structure. An effect of solute atoms on the inherent interfacial energy of sub-boundaries is possible, but it is more probable that the size of subgrains formed during deformation depends on the composition of the alloys.
230
ACTA
M~T~I~LURG~~A,
ACKNOWLEDGMENTS
The authors express their appreciation of the support of this investigation by the US Atomic Ener,oy Commission under Contract No. AT(30-l)-1002. They are also grateful to Mr. Benjamin Howell for his assistance in the ex~e~mental work. REFERENCES 8. Sara Sci. Rep. Tolmku. Imp. Univ. 20, 140 (1931), H. QUINNEY and G. I. TAYLOR Proc. Roy, Sm. (London) l@A, 157 (1937). L. M. CLAREBROUGH,M. E. HAROREAVES, D. MI~EELL. and G. W. WEST Prac. Roy. Sot. (London) 215A, 607 (1962). G. 1. TAYLOR and H. QUINNEY Proc. Roy. I%C. (London) l48A, 307 (1934). L. M. CLAREBROUOH,M. E. HARGREAVES, rend G. W. WEYT Proc. Roy. Sac. (London) 232A, 252 (1955).
VOL,
5,
18.57
6. J. S. L. LEACH, E. G. LOE~EN, and M. B. BEVER J. Appl. Phys. 28, 728 (1955). 7. E. G. LOEWEN ebnd M. C. S~aw Imtmments 28, 560 (1950). 8. M. B. BEVER and L. B. TICKN~R Acta Metallurgica 1, 116 (1953). 9. B. L. AVERBACR, M. B. BEVER, iM. F. COWERFORD,and J. S. L. &ZACH Acta &fetnllurgim 4, 477 (1950). 10. P. GREENFIELD and M, B. I~EYER Aeta ~~~~l~ur~~c~ 4, 433 (1956). 11. RI. B. BEVER. E. R. MARSHX&. and L. B. TICKNOR J. Appl. Phyi24, 1176 (1953). ’ 12. N. S. KURNAKOW and S. I?. ZEMCZUZNY 2. nnorg. Chem. so, 1 (1908). 13. L. STERNER-RAINER 2.1MetaEk.18, 143 (1926). 14. G. SACKS and J. WEERTS 2. Physik 62, 473 (1930). 16. N. NORMAN and B. E. WARREN J. Appl. Phys. 22, 483 (1951). 16. P. S. RUPEAAN and 8. L. AVERBACK B&n ~~fe~~~~~rg~e~ 4, 383 (1956).