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VUL 20 1965
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THE EFFECT OF RADIATION ON ATOMIC
REARRANGEMENTS IN Fe-N, Cu-Ni, AND Cu-Al ALLOYS
J. T. Stanley, J. M. Williams, and M. S, Wechsler
PATENT CLEARANCE OBTAINED. RELEASE TO
JHE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IN THE RECEIVING SECTION.
.
SOLID STATE DIVISI ON
OAK RIDGE NATIONAL LABORATORY
Operated by
UNION CARBIDE CORPORATION
for the
U. S. Atomic Energy Commission
Oak Ridge, Tennessee
June, 1965
TEE EFFECT OF RADIATION ON ATOMIC
REARRANGEMENTS IN Fe-N, Cu-Ni, AND Cu-Al ALLOY S
J. T. Stanley, J. M. Williams, and M. S. Wechsler
Solid State Division, Oak Ridge National Laboratory
Oak Ridge, Tennessee
ABSTRACT
The effect of neutron and electron irradiations at below room tempera-
ture on the precipitation, segregation, and short-range ordering processes
in Fe-N, Cu-Ni, and Cu-Al alloys, respectively, has been studied. Neutron
· irradiation, but not electron irradiation, is found to accelerate the precip-
itation of nitrides, which is attributed to the introduction of nucleation
sites. However, at high electron doses, evidence is found for the trapping
of nitrogen by radiation-produced point defects. For the segregation process
in Cu-Ni and short-range ordering in Cu-Al, the reaction rate is increased by
irradiation with neutrons and electrons. These are believed to be cases of
radiation-enhanced diffusion, which occurs when radiation-pzo duced defects
become mobile at tempe ra tures above -50°C. However, the identity of the de-
fect responsible for the diffusion has not been definitely established.
*Research sponsored by the U. 8. Atomic Energy Commission under contract
witb Union Carbide Corporation.
THE EFFECT OF RADIATION ON ATOMIC
REARRANGEMENTS IN Fe-N, Cu-Ni, AND Cu-Al ALLOYS
J. T. Stanley, J. M. Williams, and M. S. Wechsler
I. Introduction
The relative arrangement of the different types of atoms in alloys
often has a profound effect on properties. Therefore, it is important
to understand better how and why radiation causes atomic arrangements in
alloys to be altered. The three alloys considered here, Fe-N, Cu-Ni, and
Cu-Al, exhibit three different types of metallurgical reactions : precipi-
tation, segregation, and short-range ordering, respectively. In each case,'
the effect of neutron and electron radiations on these processes has been .
studieå.
The principal, mechanisms whereby radiation-produced defects affect,
the kinetics of metallurgical reactions are by the enhancement of diffusion.
and by providing additional nucleation sites, A comparison of the relative
effects of neutron and electron bombardments is helpful in determining the
relevant mechanism in particular cases because large clusters of defects
should be necessary for nucleation and these are produced upon neutron irra-
diation but not upon electron irradiation. A third mechanism that may play
-
-
--
.
..
- ...
a role is the trapping of solute atoms by radiation-produced defects. We
will see that trapping appears to be a factor in the precipitation of nitro-
gen in electron-irradiated iron, but only after relatively high exposure.
-
-
.
-
-
-
II.
Precipitation in Fe-N Alloys:
-
For the work on Fe-N alloys, the amount of nitrogen in solid solution
.
---
was determined from the magnitude of the Snoek internal friction peak. The
foil samples of Ferrovac E iron, 1 1/2-in. x 3/16-in. x 0.005-in, in dimen-
sions, were de carburized and nitrided to give a concentration of about
0.015 wt.:% nitrogen. The internal friction was measured in flexure at a
. ------
skúseno matatanda na wanita
s imme
met
2.
frequency of about 35 cps. At this frequency the nitrogen internal friction
peak occurs at 65°C. The details of sample treatment and measurement have
been given elsewhere."
No change in the temperature of the internal friction peak is observed
upon irradiation, which demonstrates that the nitrogen jump rate is not
affected. However, as is shown in Fig. 1, the loss of nitrogen from super-
saturated solid solution in alpha iron at 65°C 18 accelerated by a factor
of five followi.ng neutron irradiation below -120°C for 23 days. Furthermore,
it was found that the same degree of acceleration takes place upon irradia-
tion under similar circumstances for only one day, corresponding to a neutron
dose of about 6 x 10+) neutrons/cm? (E > 0.6 Mev). Now, the accelerated loss
i of nitrogen from solid solution upon neutron irradiation 13 most likely re
to enhanced nucleation of the precipitating nitride particles. The nuclea-
tion site is probably the damage cluster which results from a primary neutron-
ator collision. Therefore, a dose of 6 x 10+) neutrons/cm corresponds to a
density of damage clusters of about 2 x 10°°, since the cross section for a
i
!
primary collision is about 3 x 10°*cm in iron. It is interesting that the
concentration of precipitate particles in a normal aging experiment is about
(1-5) x 10”. An increase by ten-fold in the number of particles will pro-
duce a decrease in the inter-particle spacing by a factor of (10)" and a
decrease in the time for a given loss of nitrogen at the particles by a factor
of (10)4/9 = 5, which is consistent with the five-fold decrease in the reaction
time observed upon neutron irradiation. However, the reason for the saturation
in the radiation-enhanced reaction rate is not entirely clear, although it may
be a consequence of the rapid reduction in supersaturation in the neighborhood
of the damage clusters when a large number of them is present.
As we bave said, electron irradiation should not give rise to enhanced
mucleation of the precipitation process, inasmuch as the electron damage
-3.
consists largely of isolated interstitial-vacancy pairs rather than damage
clusters as for neutron irradiation. That this 18 80 for electron-irradiated
Fe-N 18 shown in Fig. 2, where the aging curves at 65°C are seen to coincide
for the unirradiated sample and for the sample irradieted with 2-Mev electrons
at 0°C to a dose of 1.8 x 1016 electrons /cm2. The fractional number of dis-
placements produced by this electron Irradiation, about 10, corresponds
roughly to that for the neutron irradiation discussed above. Although some
..
clustering of the defects may have occurred upon electron irradiation at 0°C,
.
.....
apparently no clusters large enough to nucleate the precipitation reaction
were formed.
We have also mentioned that solute atoms may be trapped by radiation- '
produced defects, due to a binding energy between the defect and the solute
atom. In fact, this mechanism provides an explanation of certain observations
for irradiated Fe-cs. However, trapping is not likely to be an important
factor unless the concentration of defects approaches the concentration of
Les
male Monte Sandakinenin armonii Werone minimam.
solute atoms, which is about 6 x 104 atomic fraction nitrogen for our Fe-N
samples. Therefore, it is not surprising that the irradiation of 1.8 x 100
electrons/cm (Fig. 2), producing about 10*° defects, did not give rise to
any trapping, as is seen in the fact that in the irradiated state the sample
exhibited the same internal friction at the outset of the aging reaction as
it did in the unirradiated condition. However, an irradiation at 77°K to a
dose of 7.8 x 1010 electrons/cm caused about a 16% decrease in the internal
friction measured in the temperature range 25-65°c (Fig. 3, closed circles).
This indicates that between 77°K and 25°c the radiation-produced defects mi-
grated to the nitrogen atoms and trapped about 16% of them. This is perhaps
reasonable since the dose of 7.8 x 1040 electrons/cm? 18 expected to produce
about 6 x 10-4 defects, which 18 of the same order as the nitrogen concentra-
tion. A st111 higher dose of 1.5 x 2019 electrons/com? gave a decrease in
e
h
S
internal friction of about 38% (Fig. 3, open squares).
A further poi.nt should be made with reference to Fig. 3. As the legend
indicates, after the sample was irradiated to 7.8 x 10° electrons/cm, it
was aged for 31.6 hrs at 65°C; then it was annealed for 15 min at 370°C and
water quenched. The closed triangle points show that the internal friction
returned to the value it had after the irradiation to 7.8 x 101° electrons/cm.
Thus, the heat treatment at 370°C was sufficient to redissolve into solid
solution those nitrogen atoms that had not been trapped upon irradiation or
upon raising the temperature to the test temperature following the irradia-
tion, but the nitrogen atoms that became trapped upon irradiation remained.
trapped when the 370°C heat treatment was applied. On the other hand, the i
closed squares in Fig. 3 show that a heat treatment at 450°C was sufficient
to free most of the nitrogen atoms from the radiation-produced traps and put
them back into solid solution. Finally, the open triangle point shows that,
once the nitrogen atoms are freed from their traps, a heat treatment of 370°C
will put them back into solid solution. The major point underlying the above
remarks is that the defect-solute atom complex probably dissociates at a tem-
perature between 370°C and 450°C.
There remains the question of the kinetics of the aging at 65°C following
the irradiation to 7.8 x 2010 electrons/cm (Step 5, Fig. 3) as compared to
that for the sample in the unirradiated condition (Step 2, Fig. 3). The aging
curves, normalized to the initial nitrogen in solid solution, are shown in
Fig. 4. As for the lower electron dose (Fig. 2), no acceleration of the reac-
tion was observed. In fact, a slight retardation is noted in Fig. 4, which
· may be due simply to the lower concentration of nitrogen in solid solution
after irradiation because of the loss of nitrogen from solution by trapping.
-5.
III. Segregation in Cu-N1 Alloys:
In the past, it has been thought that the Cu-N1 system displays a con-
timuous series of FCC substitutional solid solutions. However, as a result
of magnetic susceptibility and other measurements, & tendency toward
segregation of the copper and nickel at lower temperatures has been noted.
Under usual conditions, the atomic mobilities are too low to allow the seg-
regation to occur to any appreciable extent. But when the alloy 18 irrad-
iated, the segregation process 18 accelerated and the segregation takes place
at lower temperatures than 18 normal].y possible. The decrease in electrical
resistivity of Cu-37 at. $ Ni upon neutron irradiation at 70-100°C has been
ascribed to the radiation-enhanced segregation.'
In experiments at the Oak Ridge National Laboratory, Cu-N1 foil samples
of compositions 37, 48, 62, end 77 at. % nickel were irradiated at -180°C to
a dose of about 10% neutrons/cm? (E > 0.6 Mev). Upon subsequent 1sochronal
annealing, the three samples of higher nickel content underwent an anomalous
decrease in resistivity. The largest decrease was observed for the Cu-62
at. % Ni alloy. Figure 5 shows that the decrease in resistivity for this
sample started at about -50°C and reached its largest value (-1.0 wobm-cm)
at about 300°C. A similar control sample given the same isochronal annealing
treatment showed a maximum decrease in resistivity of only 0.3 mohm-cmº. The
decrease in resistivity for the unirradiated sample indicated that there is
a tendency toward segregation even in the absence of radiation. However, the
process occurs at a lower temperature and to a greater extent in a sample pre-
viously irradiated at -180°C.
· When a Cu-62 at. % Ni sample is irradiated with 2-Mev electrons at -196°C
to a dose of 9 x 10+ electrons/cm, the isochronal annealing bebavior 18
very similar to that observed for the neutron-irradiated sample (Fig. 5).
-6.
The two irradiations are expected to have produced comparable numbers of
defects. The fact that the same result was observed after electron irradi-
ation suggests that the radiation-enhanced segregation 18 not caused by
mucleation sites introduced upon irradiation. Instead it would appear that
the enhancement is due to the diffusion attendant upon the motion of puint
defecte above -50°C.
The annealing curves in Fig. 5 show a gradual decrease
in resistivity over the large temperature interval of -50 to 300°c. Since
the annealing spectrum is so diffuse, it is difficult to identify the de-
fect or defects responsible. Isochronal annealing experiments on quenched
and on quenched-and-irradiated saraples have also been conducted in the attempt
, to better understand the mechanism of the radiation-enhanced segregation in
Cu-Ni alloys.
IV. Ordering in Cu-Al Alloys:
Although no superlattice has been observed for alpha solid solutions of
alumimum in copper, several x-ray diffuse scattering studies to have demon-
strated the presence of short-range order for alloys with aluminum contents
near 15 at. Yoo furthermore, it has been shown that upon neutron irradiation
at 100°C the degree of short-range order is increased. Neutron't,ta and
electron'irradiation at 100ºC is known also to give rise to a decrease in
electrical resistivity, which is therefore attributed to an increase in short-
range order.
In view of the previous discussio1 of the low-temperature electron and
neutron irradiations in precipitating and segregating systems, it is interest-
ing to examine the results of similar irradiations in the short-range ordering
system Cu-Al. In Fig. 6, we see that the 18ochronal annealing curves are quite
similar following neutron irradiation at -120°C to about 10+' neutrons/cm?
(E > 0.6 Mev) and 1-Mev electron irradiation at -196°C to about 2 x 1077
electrons/cm2. The decrease in resistivity sets in at about -50°C, as was true
-7-
or Cu-Ni (Fig. 5), but the magnitude of the decrease in resistivity 18 muco
smaller.
Since low-temperature electron irradiation is effective in stimu-
lating the short-range ordering, we conclude that in Cu-Al as for Cu-Ni the
introduction of nucleation sites is not an important aspect of the effect of
the irradiation. Again, the explanation of the radiation-enhancement is sought
in terms of the atomic rearrangement brought about upon the motion of the radi-
ation-produced point defects. +
V. Concluding Remarks:
A comparison of the results of annealing (or aging) experiments in Fe-N,
Cu-Ni, and Cu-Al alloys following low-temperature neutron and electron irrad-
iation leads to the conclusion that the precipitation reaction in Fe-N is .
accelerated by virtue of the introduction of nucleation sites, whereas the
segregation process in Cu-Ni and short-range ordering in Cu-Al are enhanced
by the diffusive motion of radiation-produced point defects.
This 18 perhaps
not surprising, since the segregation and ordering processes constitute atomic
rearrangements on the matrix crystal lattice, and therefore the reactions are
not limited by the need to overcome an interfacial energy between two crystal
structures. On the other hand, the precipitation of aitrides in Fe-N does in-
volve the formation of a second phase.' Therefore, nucleation is expected to
be an important factor. Furthermore, the diffusion of nitrogen in iron is
interstitial in character and, thus, radiation-produced defects do not provide
a simple mechanism for enhanced diffusion. However, isolated point defects
may serve as trapping centers for the nitrogen atoms. Our experiments indicate
that trapping may occur upon electron irradiation, provided that the concentra-
tion of radiation-produced defects approaches the nitrogen concentration.
commerce
-8.
References
1. J. T. Stanley, Diffusion in Body-Centered Cubic Materials, p. 349,-
American Society for Metals, Metals Park, Ohio, 1965.
2. A. 8. Keh and 1. A. Wriedt, Trans. Met. Soc. AIME, 1962, vol. 224,
560-572.
3. H. Wagenblast and A. C. Damask, J. Phys. Chem. Solids, 1962, vol. 23,
221-227.
4. F. E. Fujita and A. C. Damask, Acta Met., 1964, vol. 12, 331-339.
5. F. M. Ryan, E. W. Pugh, and R. Smoluchowski, Phys. Rev., 1959, vol. 116,
2106-1112.
6. A. Ascoli, M. G. Beiloni, and G. T. Queirolo, Phys. Lett., 1964, vol. 9,
305-306.
Energy Agency, Vienna, 1962.
8. W. Scłale, B. C. Kelley, M. S. Wechsler, and J. M. Williams, ORNL-3676,
Oak Ridge National Laboratory, Oak Ridge, Tennessee, October, 1964.
9. C. B. Houska and B. L. Averbach, J. Appl. Phys., 1959, vol. 30, 1525-
1531.
B. Borie and C. J. Sparks, Jr., Acta Cryst., 1964, vol. 17, 827-835.
11. M. S. Wechsler and R. H. Kernohan, J. Phys. Chem. Solids, 1958, vol. 7,
307-326.
12. R. I. Kernoban and M. S. Wechsler, J. Phys. Chem. Solids, 1961, vol. 18,
.275-180.
13. M. S. Wechsler and R. H. Kernohan, Radiation Damage in Solids, pp. 81-103,
International Atomic Energy Agency, Vienna, 1962.
14. M. Šo Wechsler, J. H. Barrett, Jr., and J. M. Williams, to be published.
*
Figure Captions
แอน
2.
The fractional amount of nitrogen remaining in solid solution versus
aging time at 65°C.
Internal friction measurements at about 35 cps
-
on unirradiated and neutron-irradiated Fe-0.015 wt. % N.
44.
imi
disin
2. Internal friction versus aging time at 65°C for unirradiated and
electron-irradiated Fe-0.015 wt. % N. Irradiation temperature, oºc..
3. Internal friction at about 35 cps versus test temperature for Fe-0.015
wt. % N after various treatments. Electron energy, 2 Mev.
4. The effect of 2-Mev electron irradiation on the precipitation of nitro-
gen in iron. Fractional amount of nitrogen remaining in solid solution
versus aging time at 65°C. Internal friction measurements at about
35 cps on Fe-0.015 wt. % N.
5. Change in resistivity of Cu-62 at. % N1 during isochronal annealing
following neutron and electron irradiation. All measurements made at
liquid nitrogen temperature.
Change in resistivity of Cu-15 at. % Al upon isochronal annealing
(t = 30 min) following neutron and 1 Mev electron irradiation. All
measurements made at liquid nitrogen temperature.
aindikisins
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from the statements
misition
.
.
.
. .
UNCLASSIFIED
ORNL-DWG 63-6902R
TION OF NITROGEN REMAINING
IN SOLUTION
F16. /
.
• UNIRRADIATED
LO IRRADIATED 23 days
AT TS-120°C
1000

TIME
10
100 .
TIME AT TEST TEMPERATURE ( min )
Aging of the Nitrogen Peak in Iron-Nitrogen
Alloy at 65°C.
UNCLASSIFIED
ORNL-DWG 64-5945R

0.024
0.020
0.016
0.012
-
FIG. 2.
0.008
O ELECTRON IRRADIATED 1.8 x 1016
2 Mev ELECTRONS/cm2
-• UNIRRADIATED
0.004
:
0
1000
100
7, AGING TIME AT 65°C (min)
ORNL-DWG 65-7021

SPECIMEN HISTORY
(1) O UNIRRADIATED
(2) AGED 18.9 hr AT 65°C .
(3) SOLUTION ANNEALED AT 590°C, 1 hr ; W.Q.
(4) O IRRADIATED TO 7.8 x1018 electrons/cm2 AT 770K
(5) AGED. 31.6 hr AT 65°C
(6) A SOLUTION ANNEALED AT 370°C, 15 min; W.Q.
(7) O SOLUTION ANNEALED AT 450°C, 15 min; W.Q.
(8) A AGED 24 hr AT 65°C; SOLUTION ANNEALED
AT 370°C, 15 min; W.Q.
(9) O IRRADIATED TO 1.5 x 10'electrons/cm2 AT 170°K
0.014
0.012
..
·
Qº', INTERNAL FRICTION
0.002
20
30
40 50 60
TEST TEMPERATURE (°C)
70
80
3 The Effect of 2-Mev Electron Irradiation on the
Nitrogen Internal Friction Peak in Iron.
ORNL-DWG 65-7022
-0-0-
eos
FRACTION OF NITROGEN REMAINING IN SOLUTION
FIG. 4
O UNIRRADIATED
O IRRADIATED TO 7.8x1018 electrons/cm2
AT 770K
.
1000
10
100
,AGING TIME AT 65°C (min)
The Effect of 2-Mev Electron Irradiation on the Precipitation
of Nitrogen in Iron.

menanti
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.
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.
..
.
.
ORNL-DWG 64-9467
F16.5
Ap, CHANGE IN RESISTIVITY (Mohm-cm)
ELECTRON IRRADIATED
. NEUTRON IRRADIATED
-200
-100
.
0 100 200 300 400
T, ISOCHRONAL ANNEALING TEMPERATURE (°C)
500
600

•
:.


UNCLASSIFIED
ORNL-LR-DWG 46293
- CONTROL
F
ELECTRON IRRADIATED
AT - 196°C
7
.
i
.
NEUTRON IRRADIATED
AT -120°C-
Ap , CHANGE IN RESISTIVITY (pohm-cm)
991
-0.12 L
-200 -150 - 400 -50 0 50 100 150 200
T, ISOCHRONAL ANNEALING TEMPERATURE (°C).
Change in Resistivity of Cu-Al ( 15 at. % AI) Upon Isochronal
Annealing (p = 30 min ) Following Low Temperature Reactor Neutron
Irradiation and 1 Mev Electron Irradiation.

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