Several alloys (Hastelloy X, AISI 347, A-286 bolts, Inconel 718, Al 7039-T63 and Ti-5Al-2.5Sn ELI) were irradiated in liquid nitrogen (140 R) to neutron fluences between 10 to the 17th power and 10 to the 19th power nvt (E greater than 1.0 Mev). After irradiation, tensile properties were obtained in liquid nitrogen without permitting any warmup except for some specimens which were annealed at 540 R. The usual trend of radiation damage typical for materials irradiated at and above room temperature was observed, such as an increase in strength and decrease in ductility. However, the damage at 140 R was greater because this temperature prevented the annealing of radiation-induced defects which occurs above 140 R

Davidson, M. J.

Dixon, C. E.

Funk, C. W.

English

NASA Technical Reports Server

EFFECT OF CRYOGENIC IRRADIATION
ON NERVA STRUCTURAL ALLOYS
C. E. Dixon (1) , M. J. Davidson (2) , and C. W. Funk (3)
ABSTRACT*
Several alloys (Hastelloy X, AISI 347, A-286 bolts, Inconel 718, A1 7039-T63 and
Ti-SAI-2.5Sn EL1) were irradiated in liquid nitrogen (140°R) to neutron fluences
between 1017 and 1019 nvt (E > 1.0 Mev). After irradiation, tensile properties were
obtained in liquid nitrogen without permitting any warmup except for some specimens
which were annealed at 540°R. The usual trend of radiation damage typical for
materials irradiated at and above room temperature was observed; e.g., increase in
strength and decrease in ductility. However, the damage at 140°R was greater because
this temperature prevented the annealing of radiation-induced defects which occurs
above 140°R. The more significant conclusions from these tests are:
(i) The threshold for measurable mechanical property damage in these
materials at 140°R is between 1016 and 1017 nvt (E > 1.0 Mev).
(2) Room temperature annealing of tensile properties was performed
prior to testing some of the alloys. AI 7039-T63 recovers about
half the damage incurred at the highes_ exposure while the other
a_loys recover a smaller amount. Previous similar studies of
A t 2219 showed almost complete recovery after annealing at room
temperature. Aluminum components in the NERVA engine will be at
temperatures above 140°R during its coast periods. Additional
pos_radiation annealing studies are required to establish design
properties after thermal cycling to various temperatures.
The high reliability requirements for the NERVA
engine make it imperative that candidate material
properties be determined _n the particular
temperature, nuclear radiation or other possible
detrimental environment to which the material may
be subjected. This test involves the effects of
fast neutron fluences up to 1019 nvt on the
embrittlement of several alloys (Hastelloy X,
*The Nuclear Engine for Rocket Vehicle Application
Program (NERVA) is administered by the Space
Nuclear Systems Office (SNSO), a joint office of
the U.S. Atomic Energy Commission and the
National Aeronautics and Space Administration.
AeroJet Nuclear Systems Company, as prime con-
tractor for the engine system, and Westinghouse
Electric Corporation, as subcontractor for the
nuclear subsystem, are developing a nuclear
propulsion system for space application.
(i) Supervisor, Materials and Processes
(2) Supervisor, Radiation Effects
(3) Manager, Materials and Processes
AISI 347, A-286 bolts, Inconel 718, AI 7039-T63,
and Ti-5AI-2.SSn EL1). Previous investigations in
this program have shown that the nuclear radiation
exposure of structural materials in the NERVA
engine can impose limitations on design if
ductility losses are severe. Quantitative data
were obtained on engineering alloys irradiated at
cryogenic temperatures at higher fast neutron
fluences than in previous tests. The alloys were
irradiated** in liquid nitrogen (140=R) and
tensile tested at 140°R without an intermediate
warmup except for selected specimens which were
annealed at 5400R. The test data for each
alloy are presented in tables and engineering
**This irradiation test was performed at the
Ground Test Reactor of the Nuclear Aerospace
Research Facility, General Dynamics, Fort Worth.
These results are part of a larger test
designated GTR-20C.
603
https://ntrs.nasa.gov/search.jsp?R=19720010029 2020-03-17T03:45:05+00:00Z
judgementsaremadeof whetherthedesignclassi-
fication for thesematerialsshouldbe "brittle"
or "ductile"* in theanticipatedNERVAnuclear
environment.
Hastelloy X (As-Brazed) Tensile Data (Table i)
This material was a plate forging made from a low
aluminum (0.012%) heat. It was subjected to three
thermal cycles to about 1950°F and slow cooled
simulating the NERVA nozzle processing heat treat-
ment. The maximum exposure of 5 x 1018 nvt > 1.0
Mev is lower than the i x 1019 nvt anticipated
for NERVA lO-hour service. However, the data
shown in Table i indicate that extrapolation of
properties to 1019 nvt can be made with confidence.
The elongation decrease from 28% to 14% is
significant, but the material is still considered
ductile at 140°R after 1019 nvt > 1.0 Mev neutron
fluence. The relatively large (104%) increase in
tensile yield strength suggests this is a good
material to use for more basic studies of radia-
tion damage. It has been chosen for studies
which will make quantitative investigations of
the effects of irradiation temperature and
irradiation atmosphere (GH 2 versus LN2). The
annealing studies have shown less damage
recovery at 540°R than was anticipated from
earlier studies made on materials irradiated to
i x 1018 nvt.
AISI 347 Forging (As-Brazed) Tensile Data (Table 2)
This material was obtained from the NERVA
technology nozzle S/N 033. The manufacturing
processes included three thermal cycles to
about 1800°F followed by a slow cool which
*The allowable stress for a "brittle" material may
be less than the tensile yield strength depending
upon the maximum flaw size which could be present
and is determined by linear elastic fracture
mechanics data and analysis. The allowable
stress for a "ductile" material can be greater
than the tensile yield strength at points of high
stress concentration. A more detailed specifica-
tion of the "ductile" or "brittle" classification
as used here may be found in SNPO-C-I, "NERVA
Program Structural Design Requirements".
See Appendix for the guidelines used in this
program for establishing tentative ductile-
brittle classifications.
slightly degraded the ductility at cryogenic
temperatures; however, the unirradiated
elongation of 52% is excellent, and it is con-
sidered a tough, ductile material. Its response
to cryogenic irradiation is the most unusual in
this group of alloys. After a fluence of 4.8 x
1018 nvt > 1.0 Mev, the tensile yield stress
increased from 45 to 122 ksi (170%), but the
elongation, area reduction and ultimate strength
were not significantly changed. This alloy
undergoes a strain-induced martensitic transforma-
tion during cryogenic tensile testing. The major
part of the strain hardening is attributed to
this phase transformation which appears to be
insensitive to prior irradiation. AISI 347 is
judged to remain ductile at cryogenic temperatures
after a neutron fluence of 1019 nvt > 1.0 Mev.
A286 Technology Bolts Tensile Data (Table 3)
These bolts designed for the NERVA technology
engines were cold-reduced and aged by proprietary
processes to have a room temperature minimum yield
and ultimate strengths of 180 and 200 ksi,
respectively. The average elongation decreased
from 16 to i0 percent after a neutron fluence of
6.5 x 1018 nvt; however, the area reduction
remained unchanged at about 35%. This excellent
area reduction implies good fracture toughness.
These bolts are ductile at 140°R after a neutron
fluence of 1 x _19 .............
• u LLVL _ _.U L'_V.
Inconel 718 Tensile Data (Table 4)
These specimens from a large pancake forging after
irradiation at 140°R to 4.4 x 1018 nvt > 1.0 Mev
decreased their 140°R elongation from 23 to 12%,
however, the area reduction did not decrease,
remaining at about 30%. Inconel 718 is ductile
in this cryogenic nuclear environment. A greater
fraction of the damage annealed out in i00
minutes at 540°R than the other alloys except
for aluminum alloy 7039-T63.
Alloy 7039-T63 Tensile Data (Table 5)
This material was obtained from a ring forging
which was processed similarly to that anticipated
604
for theNERVApressurevessel. Theaverage
elongationat 140°Rdecreasedfrom12to 5%after
aneutronfluenceof 5.5x 1018nvt > 1.0Mevat
140°R.Thismaterialis tentativelyclassified
brittle udertheseconditionsbasedonlow
elongation;however,whenfracturetoughness
dataarecompleted,thejudgementof "ductile or
brittle" accordingto SNPO-C-Icanbemadewith
moreconfidence.After annealingfor i0 minutes
andlongerat 540°R,theelongationdamagealmost
completelyrecovered,but 50%of the increasein
yield strengthremained.Previousannealing
studiesin AA2219-T6hadshownalmost100%of
theradiationdamagein all propertiesannealed
outafter a 540°Ranneal. Thus,it appearsthat
whiledamageannealingbehavioris similar in
aluminumalloys, it is not identical andextrapo-
lation betweenalloy familiesmustbemadewith
caution.
Ti-5AI-2.5Sn ELI Tensile Data (Table 6)
These tensile specimens were fabricated from a
large forging (17 in. diameter by i0 in.). The
average 140°R elongation decreased from i0 to 4%
after a neutron fluence of 4.9 x 1018 nvt > 1.0
Mev. This material is Judged to be brittle under
these conditions based on low elongation. The
NERVA application for this material involves a
less severe exposure (i x 1017 nvt > 1.0 Mev) than
the maximum used in this test. For NERVA applica-
tions at 1017 nvt, this titanium alloy is judged
to be ductile. This judgement is supported by
preliminary fracture toughness tests which show
no decrease in 140°R plane-strain fracture tough-
ness, Klc, after irradiation to 3 x 1017 nvt.
CONCLUSIONS
i. A neutron fluence of 5 x 1018 nvt > 1.0 Mev
at 140°R does not embrittle Hastelloy X, AISI 347
or A286 (cold-worked plus aged bolts) sufficiently
to change their design classifications of
-ductile.
10182. The same neutron fluence (5 x nvt > 1.0
Mev) at 140°R degrades the ductility of A1
7039-T63 and Ti-5AI-2.5Sn enough to require a
brittle classification and different design
criteria for this neutron fluence at 140°R. A
lower neutron fluence of 1017 nvt does not degrade
the ductility of these two alloys significantly.
3. Annealing experiments at 540°R following
irradiation at 1400R show various amounts of
improvement in these alloys of the radiation
damage to ductility. A1 7039-T63 completely
recovers its ductility loss after annealing at
540°R.
6O5
TABLEi
EFFECTOFFASTNEUTRONIRRADIATION
AT140°RONTHETENSILEPROPERTIES
OFLOWAL(0.012%)HASTELLOYX (AS-BRAZED)
FORGINGAT140°R
FLUENCE
NVT
> i MEV
ANNEAL 0.2% YIELD ULTIMATE ELONGATION AREA
TIME AT STRENGTH, STRENGTH, CHART, REDUCTION,
540°R KSI KSI % %
MIN. AVG STD DEV AVG STD DEV AVG STD DEV AVG STD DEV
Control 0 71.2
5.4 x 1017 0 101.6
1.2 x 1018 0 113.8
5.0 x 1018 0 144.8
4.7 x 1018 i0 127.8
5.2 x 1018 i00 126.6
4.9 x 1018 i000 124.4
0.4 135.2 1.5 28.0 i.i 23.9 2.0
1.4 142.1 1.4 20.3 1.5 20.3 2.4
1.0 150.7 2.0 19.0 1.3 19.7 1.2
0.9 170.5 3.3 14.0 0.2 14.7 2.0
2.6 162.3 1.8 16.7 i.i 18.0 1.2
0.8 162.9 0.3 15.3 1.2 17.7 2.6
2.8 161.7 2.8 17.8 1.2 17.5 2.1
NO.
SPECIMENS
Control 0
6.2 x 1017 0
1.2 x 1018 0
5.8 x 1018 0
5.2 x 1018 i0
4.7 x 1018 i00
5.0 x 1018 i000
TABLE 2
EFFECT OF FAST NEUTRON IRRADIATION
AT 140°R ON THE TENSILE PROPERTIES
OF AISI 347 (FORGING FROM NOZZLE S/N 033) AT 140°R
45.4 2.0 176.8 1.5 51.6 6.0 46.0 13.9
68.0 0.8 177.2 i.i 50.2 5.1 43.6 ii.i
86.0 4.4 178.6 0.8 50.0 4.7 40.6 9.9
121.9 2.2 179.4 1.3 52.4 3.3 49.0 11.5
106.9 6.2 180.2 0.0 51.0 3.1 50.2 7.3
109.8 1.4 183.1 4.2 51.7 2.6 43.9 10.8
107.3 2.7 179.8 0.6 48.8 4.7 51.5 10.7
Control 0
4.2 x 1017 0
1.0 x 1018 0
6.5 x 1018 0
6.5 x 1018 i00
6.6 x 1018 i000
TABLE 3
EFFECT OF FAST NEUTRON IRRADIATION
AT 140°R ON THE TENSILE PROPERTIES
OF A-286 BOLTS (NERVA TECHNOLOGY) AT 140°R
222.9 1.6 256.3 2.8 15.8 1.0 34.8 0.5
223.0 4.0 258.9 3.8 15.5 1.7 33.9 1.5
240.0 3.6 259.3 1.7 13.3 1.5 34.0 1.3
250.4 7.2 261.5 3.9 i0.i 0.5 36.8 1.2
246.7 1.2 256.4 0.5 10.8 0.8 36.9 0.0
243.3 1.9 254.8 0.5 ii.i 0.8 36.1 2.3
4
4
4
3
3
3
606
TABLE4
EFFECTOFFASTNEUTRONIRRADIATION
AT140°RONTHETENSILEPROPERTIES
OFINCONEL718(PANCAKEFORGING)AT140°R
FLUENCE
NVT
> 1 MEV
Control
2.9 x 1017
4.3 x 1018
4.1 x 1018
ANNEAL 0.2% YIELD ULTIMATE ELONGATION
TIME AT STRENGTH, STRENGTH, CHART,
540°R KSI KSI %
MIN. AVG STD DEV AVG STD DEV AVG STD DEV
197.1 1.5 244.4 0.7 22.9 1.5
206.4 2.2 244.6 2.2 19.4 3.9
233.6 1.0 248.6 1.7 12.3 2.6
214.7 1.8 241.6 1.5 19.9 1.8
0
0
0
I00
AREA
REDUCTION,
%
AVG STD DEV
NO.
SPECIMENS
33.5 3.4
28.1 7.2
30.6 3.1
28.6 3.5
TABLE 5
EFFECT OF FAST NEUTRON IRRADIATION
AT 140°R ON THE TENSILE PROPERTIES
OF AA 7039-T63 (RING FORGING) AT 140°R
FLUENCE
NVT
> IMEV
Control
4.2 x 1017
8.6 x 1017
8.5 x 1017
5.5 x 1018
5.9 x 1018
5.9 x 1018
5.6 x 1018
6.0 x 1018
ANNEAL 0.2% YIELD
TIME & STRENGTH,
TEMP. KSI
MIN., °R AVG STD DEV
0 76.5 1.8
0 85.0 i.i
0 85.8 i.i
i000, 540 79.1 1.6
0 94.9 1.6
i000, 340 89.9 0.3
i0, 540 85.0 1.2
i00, 540 85.5 0.8
i000, 540 83.1 0.9
ULTIMATE ELON GAT ION
STRENGTH, CHART,
KSI %
AVG STD DEV AVG STD DEV
91.2 1.3 12.4 2.0
92.1 1.0 11.7 0.2
91.5 1.0 ii.0 0.8
92.0 1.3 13.2 i.i
95.0 1.5 4.9 0.4
89.9 0.3 9.0 1.2
90.4 i.i 11.3 0.7
90.6 0.8 12.2 1.6
92.1 0.6 11.3 i.i
AREA
REDUCTION,
%
AVG STD DEV
NO.
SPECIMENS
19.6 1.2
20.6 2.1
22.2 2.7
20.2 2.8
27.5 2.9
26.9 1.9
23.6 2 .i
22.3 2.7
19 .I 4.8
TABLE 6
EFFECT OF FAST NEUTRON IRRADIATION
AT 140°R ON THE TENSILE PROPERTIES
OF A LARGE Ti-5AI-2.5Sn (ELI) FORGING AT 140°R
ANNEAL 0.2% YIELD
FLUENCE TIME AT STRENGTH,
NVT 540°R KSI
> 1 MEV MIN. AVG STD DEV
Control 0 175.6 1.5
2.3 x 1017 0 182.4 1.8
8.2 x 1017 0 187.7 1.5
4.9 x 1018 0 199.1 3.3
5.2 x 1018 i00 192.7 1.9
ULTIMATE ELONGATION
STRENGTH, CHART,
KSI %
AVG STD DEV AVG STD DEV
185.8 2. i i0.4 2.3
189.5 i.i 9.1 0.9
195.5 1.7 7.7 i.i
204.4 3.1 4.1 0.5
200.0 1.0 6.3 0.4
AREA
REDUCT ION,
%
AVG STD DEV
29.9 3.2
30.4 2.6
29.5 5.0
23.2 3.4
27.0 1.5
NO.
SPECIMENS
4
4
4
4
4
607
DUCTILE-BRITTLE CLASSIFICATION
Tentative ductile-brittle classifications of
reactor and engine system candidate materials
have been established for design utilization,
applicable to the temperature regime anticipated
for NERVA operation. Factors such as form,
condition, and direction of the material, as
well as exposure to irradiation, gaseous and
liquid hydrogen, and cryogenic temperature
environments, are considered for each
classification.
"Ductile" classification is based upon conser-
vative judgement where data are available from
NERVA experience or literature to substantiate
the decision for all anticipated NERVA environ-
ments. "Brittle" classification is also based
upon conservative judgement for the established
brittle materials and, in addition, includes all
materials with marginal or suspect ductility
that could possibly be upgraded to "ductile"
by further testing and/or experience.
Judgement is exercised by perusal of all
pertinent information available and includes
(but is not limited to) prior NERVA experience;
available tensile ductility and notch tensile
data; available fracture toughness data; and
professional metallurgical expertise. This
expertise is especially applied for anticipated
deleterious effects of irradiation damage,
high-pressure gaseous hydrogen embrittlement,
cryogenic temperatures, and their combined
synergistic effects.
APPENDIX*
The classifications for engine materials and
reactor materials apply as the required design
criterion in accordance with Paragraph 3.9 of
the governing Structural Specification, SNPO-C-I.
However, both the classifications and the
associated temperature ranges are subject to
change as new test data become available.
Statistical fracture toughness testing would be
required for all materials utilized for design
which are classified "brittle".
*_NSC Report 2275, "Materials Properties Data
Book," 15 November 1970 Supplement
608


Effect of cryogenic irradiation on NERVA structural alloys

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