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On the interruption yield point of irradiated aluminum single crystals
ACTA
140
METALLURGICA,
is even worse when the electron microscopy results are compared to the equilibrium data of Simmons and ?3aluffi.@) This is not unexpected since estimates of vacancy concentrations in quenched metals using electron microscopy are always likely to be on the low side, Therefore, it does not seem reasonable to compare our quenching results with those from e~u~~ib~~u?n rneasurenzents.‘“’ Whether or not there really is a discontinuity in vacancy concentration at the melting point cannot be decided until it becomes possible to measure equilibrium vacancy ~oneentrations in liquid metals. G. THOMAS Department of Mineral Technology University of California Berkeley, CaliJomia R. H. WILLENS W. M. Kech Engineering Laboratory Calqornia Institute of Technology Pasadena, California Now at: Bell Telephone Resewch Labs. New Jersey References 1. R. W. BALUFFI and R. 0. SIMMONS, Acta Met. 12,957
VOL.
13,
1965
cations. In the irradiated specimens, a fraction of the irradiation induced interstitials are trapped by impurity atoms before reaching the dislocations. During the subsequent plastic deformation, these interstitials are released by gliding dislocations from their impurity trapping sites and, therefore, become free to migrate to the dislocations. The above interpretation implies that interstitials are not readily absorbed by dislocations upon their Instead, they first form an atmosphere arrival. around the dislocations and later precipitate in the form of superjogs. Experimentally, this should cause a transition from an interruption yield point to a flow stress increase as the time of the interruption is lengthened. It is the purpose of this letter to report that this actually is the situation. A study of strain aging kinetics is also reported and the result again conforms to the scheme outlined above. Aluminum single crystals of 99.999 % initial purity (& = 43”, 0, = 54”) were irradiated at about 86°K with electrons having an incident energy of 1 MeV to a total dose of 5 x I@’ ejcm2. Six specimens were used, one being tested without irradiation. To determine the flow stress prior to the irradiation, the
(1964). 2. G. THOMAS and R. H. WILLENS, ibid. 12, 191 (1964). 3. R. 0. SIMMONS and R. W. BALUFFI, Phys. Rev. 117, 5% 4. ~‘~‘~~A=s~A~
and S. PEARSON, Phil. &fag. 2,570
(1957).
* Received August 24, 1964.
550 On
the interruption yield point of irradiated aluminum single crystals*
In a recent paper by Ono et al.,(l) the flow stress increase of aluminum single crystals following electron irradiation at 80*K was attributed to the irradiation induced interstitials which migrated to dislo~tions. The interstitials were presumed to precipitate in the form of superjogs on the dislocations when a critical supersaturation of the interstitials was reached and these superjogs were dragged along during subsequent motion of the dislocations thereby raising the flow stress. In addition to the hardening observed immediately after the irradiation, an interruption of the tensile test after a small amount of deformation resulted in a sign&ant yieId point in the irradiated specimens. The yield point is usually called an “interruption yield point” and has been previously observed in heavily deformed aluminum and other f.o.c. metals,(s4J This interruption yield point was interpreted to be due to Gottrell atmospheres of interstitials around dislo-
450 500 WE > 400 z &oC
65C
55C
45C
, 0.2%
,
Extension Fra. 1 Effect of interruption time on shape of the interrupt.ion yield point.
LF,TTERS
specimens
w-ere slightly
st,raRRstrain
defornled test
THE
EDITOR
141
in tNension and the
relat,ion wias recnrded. tensile
TO
Following
was resumed
the
and
was
irradiation
the
interrupted
after a shear strain c~f 0.7 I::;_ The int.erof 77°K
ruption was carried out at a test temperature
under stress for times ranging from 5 EW to 5 min. The unloading during the interruptSion was about 20 y/,. A similar test bias carried out with the unirradiated cryst,al using an interruption time of 1 min. A typical yield point resulting from MI interruption of the tensile tost of an irradiated less is shown
in Fig.
l(a).
interruytionY
the
flow
amount aa,.
Deformation
rate of work hardening
crystal for 1 Tin or
On reksting
stress
after
was increased
tShf?nproceeded
by
the an
with a low
FIG. 2. Amount of the inbrruption yield point, with respect to to&l shear
until the flaw curve rejoined
the extrapolation of t,he stress-strain curve prior to IVhen an interruption time of mare the interruption. than 1 min wa,s employed for the irradiated crystals,
first, then saturated
the stress.-strain
of the yield point, Aq,, decreased
olated curve.
curve did not return to the extrap-
Instead,
a permanent
flow stress level, Ao~, was observed.
increase in the As can be seen in
Fig, l(b) and 1 (c), this change occurred gradually. transient state was observed after an interruption
duration and the change was almost complete
2 min
after 5 min.
to
A of
As the amount of the deformation prior was increased following the
interruptirm
m
irradiation, aging
this transition
from ACJ~tt> PM, at long
times was no lunger observed.
normal
interruption
Instead,
F-icld point (withut
t.he
permanent
against time, t, in Fig. 3.
maximum
the
impurity steeping
of
irradiation permanent disappear. the amount the impurity
induced
yield point, which results in the increase at Ionqer aging Cmes, should According to the previous calculation,(l’ of the deformation trapped
times,
the
obeys a relation-
parameter diffusion
by Harp@ for strain exp (-KKs2’3] with E =
f = 1 -
In t#he present
of interskitial-dislocation coefficient
case,
of an interstitial,
k the Boltzmann’s
constant
I> can be rewritten
A
interaction,
is a D
p dislocation
and T tcmpera-
as
a% exp (AS/k) - exp ( -EE,‘jkT), where a is a lattice
parameter,
frequency
and AX a,nd E,I
and
(lP/sec),
activation
energy
of
v atomic
vibration
are entropy
interstitial
motion,
required to release all
discussed
with
below.
In Fig. 2, the change in flow stress, Au (-ha, ACJ,
short- aging
= AaJAa,
interstitIials from t,hc trapping
sites is about 2_5’A which is in good agreement the observation
For
aging, 3p(~;rr/2)‘I”(ADlkT)213.
the
trapped interstitials is decreased by the action of the gliding dislocations and the
level.
change in Au,f namely
ture.
number
the
line in
ship similar t’o that obtained
density,
the
at 1 min, as shown by the dotted
flow stress
fractional
increase in the flow stress level) was found even after
proceeds,
The magnitude
after reaching
Fig. 3. The decrease in ACT at longer interruption periods is, of course, due to the permanent increase in
a G-min interruption. This is expected from the already me:ntSioned model. As the deformatinn
ha increased with time at
at about, 1 min.
due to an interruption,
is plotted
against
-t_ the
strain at, which the interruptjon was made. Results in the figure were obtained from the irradiated and nnirradiated crptals for an interruption time of T min. ho for the irradiated crystals was about twice that of the unirradiated crystal in the early stages of deformation, but the differences in AO between the twa
crystals
decreased
with
increasing
&rain,
as
anticipated from the preceding discussion. The effect of interruption t’imc on Aa is plotted
J
,
,
50
100
, --y----;____J, 150
200
250
f (secl Frc. 3. Amount of the interruption yield point, AU, in irradiated crystals m. interruption time, t. &oken line indicstes change in Aa,.
142
ACTA
METALLURCICA,
VOL.
13, 1965
tzh (see+)
ence of strain aging kinetics on strain, the activation energy for the strain aging process and the maximum yield point obtainable from strain aging have been de~rmin~. The activation energy for strain aging and the maximum yield effect are compared with those for the Ag base Al(l) and Ag base Zn(2) cases. Ag base Sn alloys were prepared by induction melting 99.99 % Ag with 99.9 % Sn in graphite molds under a high purity argon atmosphere. Single crystals of the alloy were then grown from the melt in ,an argon atmosphere by passing the alloy in its graphite mold through a hot zone in a resistance wound furnace. Gauge lengths 2 cm long and 0.3 cm in diameter FIG. 4. Logarithm of fractional change in Au, f, vs. @/a. Sensitivity of the stress measurement is +2 g/mm* and were introduced into the specimen using an electricindicated in the figure. discharge lathe; the machined crystals were then electropolished in a 5 o/oKCN solution to a diameter respectively. From the data of Corbett et aZ.,(+Q of 2.8 mm. The orien~tions of the crystals were exp (Amok) was est~ated to be about 5.1. Together determined using the back-reflection method. The with Em1 = 0.12 eV, (‘I D = 1.8 X 1O-1ocm2/sec was crystals were then annealed in high purity argon obtained. By taking A = 3.43 x lo-8 eV - cm(i) and atmospheres at 800°C for 24 hr and allowed to furT = 77°K p = 2.4 x 10s/om2 was obtained from the slope of the straight line in Fig. 4. Therefore, it nace cool. All tensile tests were performed on a tloor appears reasonable to attribute the aging process to model Instron using a strain rate of 2.2 x lo-* se&. Test tem~ratures of 273O and 294’, 314*K were interstitial diffusion. obtained through the use of ice water and constant The authors are grateful to Prof. J. W. Kauffman for arranging usage of the Northwestern University, Materials Research Center, Van de Graeff accelerator % and to Mr. R. Kloske for reading the manuscript. 0
A
$”
KANJX ON0 M. MESHII &knee The Technologiud Institute Northwestern University Earn Ill~~~~ References I. K. ONO. M. MESE~Iand J. W. KAUFFMAN.Acta Met. 19. ~e~rt~~
2. 3. 4. 5. 6.
7.
of ~~e~~
361 (1964). A. R. C. WESTWOOD and T. BROOM,Acta Met. 5, 77 (1957). H. K. BIRNBAUM,J.Appl. Phya. 84, 2175 (1963). J. TAKAMURA, K. FURUKAWA, S. MIURAand P. H. SHINUU, J. Phys. Sot. Japan 18, Suppl. III, 7 (1963). S. HARPER,Phys. Rev. 85, 709 (1951). J. W. CORBEL, R. B. SNITE and R. M. WALKER,Phya. Rev. 114, 1460 (1959). K. HERSCEBACH, P&N. Rev. 150,554 (1963).
3 E UJ
2-
%
Y In 8
3 H
1
IO
20
30
% SHEAR
STRAIN
40
50
60 t
* Received August 10, 1964. This research was supported by the Advanced Research Projects Agency of the Department of Defence and the United States Atomic Energy Commission.
Strain aging in Ag base Sn solid solutions* This communication describes the yield point behavior obtained from strain aging ex~riments on Ag-4 at. % Sn solid solutions crystals. The depend-
Fro. 1. AT vs. strain for a constant glide strain increment (2 Ok).a constant aging time (4 min) and a constant aging temperature (314K). The shear stress-strain curve is also shown (top half).
140
METALLURGICA,
is even worse when the electron microscopy results are compared to the equilibrium data of Simmons and ?3aluffi.@) This is not unexpected since estimates of vacancy concentrations in quenched metals using electron microscopy are always likely to be on the low side, Therefore, it does not seem reasonable to compare our quenching results with those from e~u~~ib~~u?n rneasurenzents.‘“’ Whether or not there really is a discontinuity in vacancy concentration at the melting point cannot be decided until it becomes possible to measure equilibrium vacancy ~oneentrations in liquid metals. G. THOMAS Department of Mineral Technology University of California Berkeley, CaliJomia R. H. WILLENS W. M. Kech Engineering Laboratory Calqornia Institute of Technology Pasadena, California Now at: Bell Telephone Resewch Labs. New Jersey References 1. R. W. BALUFFI and R. 0. SIMMONS, Acta Met. 12,957
VOL.
13,
1965
cations. In the irradiated specimens, a fraction of the irradiation induced interstitials are trapped by impurity atoms before reaching the dislocations. During the subsequent plastic deformation, these interstitials are released by gliding dislocations from their impurity trapping sites and, therefore, become free to migrate to the dislocations. The above interpretation implies that interstitials are not readily absorbed by dislocations upon their Instead, they first form an atmosphere arrival. around the dislocations and later precipitate in the form of superjogs. Experimentally, this should cause a transition from an interruption yield point to a flow stress increase as the time of the interruption is lengthened. It is the purpose of this letter to report that this actually is the situation. A study of strain aging kinetics is also reported and the result again conforms to the scheme outlined above. Aluminum single crystals of 99.999 % initial purity (& = 43”, 0, = 54”) were irradiated at about 86°K with electrons having an incident energy of 1 MeV to a total dose of 5 x I@’ ejcm2. Six specimens were used, one being tested without irradiation. To determine the flow stress prior to the irradiation, the
(1964). 2. G. THOMAS and R. H. WILLENS, ibid. 12, 191 (1964). 3. R. 0. SIMMONS and R. W. BALUFFI, Phys. Rev. 117, 5% 4. ~‘~‘~~A=s~A~
and S. PEARSON, Phil. &fag. 2,570
(1957).
* Received August 24, 1964.
550 On
the interruption yield point of irradiated aluminum single crystals*
In a recent paper by Ono et al.,(l) the flow stress increase of aluminum single crystals following electron irradiation at 80*K was attributed to the irradiation induced interstitials which migrated to dislo~tions. The interstitials were presumed to precipitate in the form of superjogs on the dislocations when a critical supersaturation of the interstitials was reached and these superjogs were dragged along during subsequent motion of the dislocations thereby raising the flow stress. In addition to the hardening observed immediately after the irradiation, an interruption of the tensile test after a small amount of deformation resulted in a sign&ant yieId point in the irradiated specimens. The yield point is usually called an “interruption yield point” and has been previously observed in heavily deformed aluminum and other f.o.c. metals,(s4J This interruption yield point was interpreted to be due to Gottrell atmospheres of interstitials around dislo-
450 500 WE > 400 z &oC
65C
55C
45C
, 0.2%
,
Extension Fra. 1 Effect of interruption time on shape of the interrupt.ion yield point.
LF,TTERS
specimens
w-ere slightly
st,raRRstrain
defornled test
THE
EDITOR
141
in tNension and the
relat,ion wias recnrded. tensile
TO
Following
was resumed
the
and
was
irradiation
the
interrupted
after a shear strain c~f 0.7 I::;_ The int.erof 77°K
ruption was carried out at a test temperature
under stress for times ranging from 5 EW to 5 min. The unloading during the interruptSion was about 20 y/,. A similar test bias carried out with the unirradiated cryst,al using an interruption time of 1 min. A typical yield point resulting from MI interruption of the tensile tost of an irradiated less is shown
in Fig.
l(a).
interruytionY
the
flow
amount aa,.
Deformation
rate of work hardening
crystal for 1 Tin or
On reksting
stress
after
was increased
tShf?nproceeded
by
the an
with a low
FIG. 2. Amount of the inbrruption yield point, with respect to to&l shear
until the flaw curve rejoined
the extrapolation of t,he stress-strain curve prior to IVhen an interruption time of mare the interruption. than 1 min wa,s employed for the irradiated crystals,
first, then saturated
the stress.-strain
of the yield point, Aq,, decreased
olated curve.
curve did not return to the extrap-
Instead,
a permanent
flow stress level, Ao~, was observed.
increase in the As can be seen in
Fig, l(b) and 1 (c), this change occurred gradually. transient state was observed after an interruption
duration and the change was almost complete
2 min
after 5 min.
to
A of
As the amount of the deformation prior was increased following the
interruptirm
m
irradiation, aging
this transition
from ACJ~tt> PM, at long
times was no lunger observed.
normal
interruption
Instead,
F-icld point (withut
t.he
permanent
against time, t, in Fig. 3.
maximum
the
impurity steeping
of
irradiation permanent disappear. the amount the impurity
induced
yield point, which results in the increase at Ionqer aging Cmes, should According to the previous calculation,(l’ of the deformation trapped
times,
the
obeys a relation-
parameter diffusion
by Harp@ for strain exp (-KKs2’3] with E =
f = 1 -
In t#he present
of interskitial-dislocation coefficient
case,
of an interstitial,
k the Boltzmann’s
constant
I> can be rewritten
A
interaction,
is a D
p dislocation
and T tcmpera-
as
a% exp (AS/k) - exp ( -EE,‘jkT), where a is a lattice
parameter,
frequency
and AX a,nd E,I
and
(lP/sec),
activation
energy
of
v atomic
vibration
are entropy
interstitial
motion,
required to release all
discussed
with
below.
In Fig. 2, the change in flow stress, Au (-ha, ACJ,
short- aging
= AaJAa,
interstitIials from t,hc trapping
sites is about 2_5’A which is in good agreement the observation
For
aging, 3p(~;rr/2)‘I”(ADlkT)213.
the
trapped interstitials is decreased by the action of the gliding dislocations and the
level.
change in Au,f namely
ture.
number
the
line in
ship similar t’o that obtained
density,
the
at 1 min, as shown by the dotted
flow stress
fractional
increase in the flow stress level) was found even after
proceeds,
The magnitude
after reaching
Fig. 3. The decrease in ACT at longer interruption periods is, of course, due to the permanent increase in
a G-min interruption. This is expected from the already me:ntSioned model. As the deformatinn
ha increased with time at
at about, 1 min.
due to an interruption,
is plotted
against
-t_ the
strain at, which the interruptjon was made. Results in the figure were obtained from the irradiated and nnirradiated crptals for an interruption time of T min. ho for the irradiated crystals was about twice that of the unirradiated crystal in the early stages of deformation, but the differences in AO between the twa
crystals
decreased
with
increasing
&rain,
as
anticipated from the preceding discussion. The effect of interruption t’imc on Aa is plotted
J
,
,
50
100
, --y----;____J, 150
200
250
f (secl Frc. 3. Amount of the interruption yield point, AU, in irradiated crystals m. interruption time, t. &oken line indicstes change in Aa,.
142
ACTA
METALLURCICA,
VOL.
13, 1965
tzh (see+)
ence of strain aging kinetics on strain, the activation energy for the strain aging process and the maximum yield point obtainable from strain aging have been de~rmin~. The activation energy for strain aging and the maximum yield effect are compared with those for the Ag base Al(l) and Ag base Zn(2) cases. Ag base Sn alloys were prepared by induction melting 99.99 % Ag with 99.9 % Sn in graphite molds under a high purity argon atmosphere. Single crystals of the alloy were then grown from the melt in ,an argon atmosphere by passing the alloy in its graphite mold through a hot zone in a resistance wound furnace. Gauge lengths 2 cm long and 0.3 cm in diameter FIG. 4. Logarithm of fractional change in Au, f, vs. @/a. Sensitivity of the stress measurement is +2 g/mm* and were introduced into the specimen using an electricindicated in the figure. discharge lathe; the machined crystals were then electropolished in a 5 o/oKCN solution to a diameter respectively. From the data of Corbett et aZ.,(+Q of 2.8 mm. The orien~tions of the crystals were exp (Amok) was est~ated to be about 5.1. Together determined using the back-reflection method. The with Em1 = 0.12 eV, (‘I D = 1.8 X 1O-1ocm2/sec was crystals were then annealed in high purity argon obtained. By taking A = 3.43 x lo-8 eV - cm(i) and atmospheres at 800°C for 24 hr and allowed to furT = 77°K p = 2.4 x 10s/om2 was obtained from the slope of the straight line in Fig. 4. Therefore, it nace cool. All tensile tests were performed on a tloor appears reasonable to attribute the aging process to model Instron using a strain rate of 2.2 x lo-* se&. Test tem~ratures of 273O and 294’, 314*K were interstitial diffusion. obtained through the use of ice water and constant The authors are grateful to Prof. J. W. Kauffman for arranging usage of the Northwestern University, Materials Research Center, Van de Graeff accelerator % and to Mr. R. Kloske for reading the manuscript. 0
A
$”
KANJX ON0 M. MESHII &knee The Technologiud Institute Northwestern University Earn Ill~~~~ References I. K. ONO. M. MESE~Iand J. W. KAUFFMAN.Acta Met. 19. ~e~rt~~
2. 3. 4. 5. 6.
7.
of ~~e~~
361 (1964). A. R. C. WESTWOOD and T. BROOM,Acta Met. 5, 77 (1957). H. K. BIRNBAUM,J.Appl. Phya. 84, 2175 (1963). J. TAKAMURA, K. FURUKAWA, S. MIURAand P. H. SHINUU, J. Phys. Sot. Japan 18, Suppl. III, 7 (1963). S. HARPER,Phys. Rev. 85, 709 (1951). J. W. CORBEL, R. B. SNITE and R. M. WALKER,Phya. Rev. 114, 1460 (1959). K. HERSCEBACH, P&N. Rev. 150,554 (1963).
3 E UJ
2-
%
Y In 8
3 H
1
IO
20
30
% SHEAR
STRAIN
40
50
60 t
* Received August 10, 1964. This research was supported by the Advanced Research Projects Agency of the Department of Defence and the United States Atomic Energy Commission.
Strain aging in Ag base Sn solid solutions* This communication describes the yield point behavior obtained from strain aging ex~riments on Ag-4 at. % Sn solid solutions crystals. The depend-
Fro. 1. AT vs. strain for a constant glide strain increment (2 Ok).a constant aging time (4 min) and a constant aging temperature (314K). The shear stress-strain curve is also shown (top half).