A study is being conducted at NASA Lewis Research Center to determine the feasibility of using a carbide particle strengthened Nb-1 percent Zr base alloy to meet the anticipated temperature and creep resistance requirements of proposed near term space power systems. In order to provide information to aid in the determination of the suitability of the PWC-11 alloy as an alternative to Nb-1 percent Zr in space power systems this study investigated: (1) the long-time high-vacuum creep behavior of the PWC-11 material and the Nb-1 percent Zr alloy, (2) the effect of prior stress-free thermal aging on this creep behavior, (3) the effect of electron beam (EB) welding on this creep behavior, and (4) the stability of creep strengthening carbide particles

Titran, Robert H.

English

NASA Technical Reports Server

DOE/NASA/16310-13
NASA TM-102390
Creep Strength of Niobium Alloys,
Nb-1%Zr and PWC-11
Robert H. Titran
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Work performed for
U.S. DEPARTMENT OF ENERGY
Nuclear Energy
Reactor Systems Development and Technology
Washington, D.C. 20545
Under Interagency Agreement DE-AI03-86SF16310
Prepared for the
7th Symposium on Space Nuclear Power Systems
sponsored by the University of New Mexico
Albuquerque, New Mexico, January 7-11, 1990
https://ntrs.nasa.gov/search.jsp?R=19900002538 2020-03-20T00:48:38+00:00Z

CREEP STRENGTH OF NIOBIUM ALLOYS, Nb-l%Zr AND PNC-II
Robert H. Titran
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, OH 44135
INTRODUCTION
Space power requirements for future civilian and military missions will
range from multikilowatts to megawatts of electricity. Nuclear reactor power
systems have been identified as the primary power source to meet these high
levels of electrical demand. Advanced materials will play a major role in
meeting the stringent size and performance requirements of space nuclear power
systems. The requirements for such a nuclear power system, which presently in-
clude a service life of greater than 7 years at a temperature greater than
1350 K, dictate the use of refractory metals (Cooper !984). The alloy Nb-l%Zr
has been suggested for use in space power applications where resistance to liq-
uid alkali metal corrosion at temperatures near II00 K was the primary concern
(Lane and Ault 1965 and Buckman 1984). Although current designs of space nu-
clear power systems for ground demonstration specify the Nb-l%Zr alloy for re-
actor, heat pipe and power components (Kruger et a1. 1987), future flight
applicatlons will need niobium base alloys with greater high temperature
strength and increased creep resistance to provide additional design margins
(Dokko et al. 1984).
A study is being conducted at NASA Lewis Research Center to determine the
feasibility of using a carbide particle strengthened Nb-l%Zr base alloy,
to meet the anticipated temperature and creep resistance requirements of pro-
posed near term space power systems (Titran et al. 1987). In order to provide
information to aid in determination of the suitability of the PHC-II alloy
(DelGrosso et at. 1965) as an alternative to Nb-l%Zr in space power systems
this study investigated: (I) the long-time high-vacuum creep behavior of the
PNC-II material and the Nb-l%Zr a11oy; (2) the effect of prior stress-free
thermal aging on creep behavior; (3) the effect of electron beam (EB) welding
on creep behavior, and (4) the stability of creep strengthening carbide
particles.
MATERIAL AND PROCEDURE
The PNC-11 alloy used in this study was received from the Oak Ridge National
Laboratory (ORNL) in the form of l-mm thick machined tensile creep specimens.
The material contained 0.90 wt % ZF and 0.063 wt % C, slightly below the PNC-II
alloy minimum specification of 0.075 wt % C. Unfortunately, a detailed proc-
essing history of this heat is not available. All material was annealed in
high-vacuum for I h at ]755 K plus 2 h at 1475 K as 0er the manufacturer's
recommendations (De]Grosso et a1. 1967) prior to any aging, c_eep testing, or
welding. Following the anneal, various creep and creep-rupture specimens were
subjected to one or both of the following treatments: <I) single pass, full
penetration EB welding across the width of the test specimen, perpendicular to
the tensile axis; and (2) aging for I000 h at 1350 K in high vacuum. The
Nb-l%Zr alloy was received from ORNL in the form of cold worked l-mm sheet.
Machined tensile creep specimens were annealed in high vacuum for, (I) 1 h at
1475 K and (2) 1 h at 1755 K plus 2 h at 1475 K prior to creep testing. The
results of chemical analyses following the high vacuumannealing are given in
Table I.
RESULTS
Typical microstructures of the annealed base metal and the EB weld area for
the PWC-II material are shown in Figure I. The base metal area shows that the
annealed condition has an average grain size of 25 pm measured by the circle-
intercept method, with an aspect ratio of approximately 5:1 orientated in the
sheet rolling direction. The EB weldment has a highly oriented grain struc-
ture with a grain size ranging from about 45 pm up to over 200 pm. The
annealed condition has an extensive amount of second phase precipitated in the
grain boundaries and matrix. The EB weld zone also shows extensive second
phase precipitation; however, the particles appear to be finer and form cell-
like domains within the grains. The Nb-1%Zr annealed for I h at 1475 K had an
average grain size of 20 pm with a fine dispersion of ZrO 2 particles. The
material annealed 1 h at 1755 K + 2 h 1475 K is expected tO have an average
grain size of about 80 pm based upon results of I h annealing studies conducted
at 1600, 1700 and 1800 K. Constant load creep tests were conducted in the
internally loaded, high vacuum (10 -7 MPa) creep chambers described by Hall and
Titran (1966). Creep tests were run at 1350 K to assess the effect of carbon
content, grain size, thermal aging and EB welding. Figure 2 shows the creep
curves for the annealed Nb-l%Zr and the PWC-II material tested at an applied
stress of I0 MPa. The fine grain (20 pm) Nb-l%Zr required about 3500 h to
achieve I percent strain whereas the coarse grain (-80 pm) Nb-l%Zr required
11 200 h. The coarse grained Nb-1%Zr which had the identical anneal as that
recommended for the PNC-II material achieved 2 percent strain in 18 000 h and
3 percent in 24 100 h. The PNC-II material, in both the recommended annealed
condition and the annealed plus I000 h at 1350 K thermal aged condition, it
has exhibited essentially zero creep strain after 28 000 h at 1350 K and an
applied stress of I0 MPa.
TABLE I Chemical Analyses of the Nb-l%Zr
Alloy and the PNC-II Material after
High Vacuum Annealling prior to
Creep Testing.
Zirconium
Carbon
Oxygen
Nitrogen
Hydrogen
Niobium
Nb-Zr-C Nb-1%Zr
0.90 wt %
630 ppm wt
80 ppm wt
53 ppm wt
11 ppm wt
Balance
1.1 wt %
16 ppm wt
170 ppm wt
41 ppm wt
0.4 ppm wt
Balance
FACE
I MM
ROOT
(a) CROSS-SECTION OF SINGLE-PASS FULL PENETRATION WELb.
(I))UNAFFECTED BASE METAL. (c) WELD FUSION ZONE.
FIGURE I. Transve_e ElectronBeam Weldment o!the PWO-I I Materialinthe
As-welded ConditionpriortoSudace Grindingand Creep Testing.
Materialvacuum annealed I h at1755 K plus2 h at1475 K priorto
welding.
Nb-1%Zr
I(475K -
20 pM gs
Nb-1%Zr
1755 K I h +
Ih75 K - 2 h
80 OM gS
PWC-II (2 TESTS)
(I) 1755 K - I h ÷ Iq75 K - 2hAND (2)
AGED 1000h - 1350 K
25 _M gS
qO00 8000 I? 000 16 000 20 000 24 000 28 000
TIME, h
FIGURE 2. Creep Curves lor Nb-l°/oZr and PWC-11 Material
Annealed at Indicated Conditions and tested at
1350 Kand 10 MPa.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
Creep tests on the EB welded PNC-11 material were conducted in high vacuum
(10 -7 Pa) at 1350 K and 40 MPa to assess the effect of welding on creep
strength. The creep curves, Figure 3, show that a sample in the annealed con-
dition required about 3200 h to achieve l percent strain. A similarly treated
sample with an EB weldment required 2200 h to reach I percent strain. Moore
et al. (1986) conducted short-time creep-rupture tests to further characterize
the effect of EB welding on the PNC-ll material. Figure 4 shows a typical
PNC-11 material creep-rupture specimen with a transverse EB weld. In all
creep-rupture tests, failure occurred in the unaffected base metal, which is a
clear demonstration that the weld region was stronger. The cause for the lower
creep strength in welded specimens is not believed to be related to welding
effects per se. Since only single test were conducted for each condition and
base metal properties can vary (Titran et al. 1986), it is difficult to make
conclusions at a11.
1.2
• 1.0
=__ .8
_ .q
_-_ .2
ANNEALED FOR I H AT 1755 K ÷ 2 H AT Iq75 K
ANNEALED + EB WELDED
;_ I I I I I I l
500 I000 1500 2000 2500 3000 3500
CREEP TIME, HR
FIGURE 3. Creep Curves for PWC-I I Material with and without
a Transverse EB Weld showing the Time to reach
I Percent Creep Strain, Samples were tested at
1350 K with an applied stress of 40 MPa.
F WELD
FIGURE 4. Photomicrograph of a Typical EB Welded PWC-11 Material
Specimen after Creep-Rupture Testing showing that Failure
occurred in the Base Metal Material.
The high temperature (>0.5 Tm) creep strength of PNC-II, relative to the
order of magnitude lower carbon content Nb-l%Zr alloy, has been attributed to
the presence of very flne precipitates of (Nb,Zr)2C and/or (Nb,Zr)C ranging in
size from 1 to 10 pm in diameter (PWAC 1965). Results of metallographic analy-
sis of several samples annealed and aged with and without an applied stress
confirmed the stability of the strengthening carbide particle (Grobstein
et al. 1986). The concern about overaging of the precipitate particles during
high-temperature exposure for long times was addressed by microstructural char-
acterization of the precipitate (complex carbide) morphology. Several tech-
niques were used including light microscopy, scanning and transmission electron
microscopy, x-ray diffraction, and chemical analysis of extracted particles.
Table 2 summarizes the results of this study. In the as-rolled condition, the
precipitates were relatively coarse, I- to lO-pm in diameter and were found to
be hcp Nb2C. After an initial heat treatment of 1 h at 1755 K and 2 h at
1475 K, a different finer precipitate formed. These particles were 0.05 to
0.I wm in diameter and were determined to be fcc (Zr,Nb)C with the Zr/Nb ratio
approximately 70:30. After approximately 5000 h at 1350 K (0.5 Tm), these fine
precipitates almost doubled in size, but dld not "overage." The stability and
effectiveness of this fcc(Zr,Nb)C precipitate in pinning dislocations and grain
boundaries, thus resisting plastic deformation is self-evident from the two
28 O00-h creep tests at 1350 K and I0 MPa (Figure 2).
Multiple llnear regression analysis of the time to achieve l percent creep
strain was performed on the high vacuum (10 -7 Pa) creep data for both Nb-l%Zr
(Horak 1989) and the PWC-II (Titran 1986) material. The Orr-Sherby-Dorh param-
eter (Orr et al. 1954) was determined and the results used for extrapolation of
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
TABLE2 Characterization of Precipitates in the PNC-II Material
(Nb-I%Zr-O.06%C).
Size
Structure
Composition
As-rolled
1 to I0 _m
HCP
Nb2C
After initial
heat treatment
0.05 to 0.I pm
FCC
(Zr,Nb)C
After long-term
high temperature
exposure
0.I to 0.15 Nm
FCC
(ZF,Nb)C
24--
_ 18--
DESIGN
TEMPERATURE
RANGE
I I
i l C-ll
%C)
F COARSE
/ GRAIN
II
ILFINE GRAIN
DESIGN
STRESS
RANGE
1300 1350 lqO0 1450
TEMPERATURE, K
FIGURE 5. Temperature and Creep Strength Potential
ol the PWC-11 Material compared to
Nb-1%Zr AlLoy.
the vacuum creep data to 7 years (an extrapolation of approximately one order
of magnitude in time). For the PWC-II material, a projected nominal stress of
20 MPa would be required to achieve l percent creep strain in 7 years at
1350 K. For the Nb-l_Zr alloy, the projected 7 year 1350 K nominal creep
strength is 4 MPa for the fine grain size and about 6 MPa for the coarse grain
size as shown in Figure 5.
SUMMARYOF RESULTS
Based upon the preliminary studies of the high vacuum (10-7 Pa) creep and
creep-rupture behavior of the Nb-l%Zr alloy and PNC-II material at 1350 and
1400 K, the following observations could be drawn"
I. The PWC-ll material in the annealed state (l h at 1755 K plus 2 h at
1475 K) is more creep resistant than the similarly annealed Nb-lZr alloy
by at least a factor of three in app]ied stress.
2. Aging the PWC-II material for 1000 h at 1350 K prior to low stress (tO MPa)
creep testing does not appear to affect the long term (>28 000 h) strength.
3. The electron-beam weld region was stronger than the base metal for speci-
mens creep-rupture tested at 1350 and 1400 K.
4. The (70% ZrC - 30% NbC) fcc monocarbide was the only extracted phase
identified after high temperature exposure of the PNC-II material and does
not appear to overage.
Acknowledgments
This research was performed by the NASA Lewis Research Center a part of the
NASA Civil Space Technology Initiative Program.
REFERENCES
Buckman, R.W., Jr. (1984) "A Review of Tantalum and Niobium Alloy Production,"
in Refractory Alloy Technology for Space Nuclear Power Applications, R.H.
Cooper Jr. and E.E. Hoffman, eds., CONF-8308130, DOE, pp. 86-97.
Cooper, R.H., Jr. (1984) "Potential Refractory A11oy Requirements for Space
Nuclear Power Applications," in Refractory A11oy Technology for Space
Nuclear Power Applications, R.H. Cooper Jr. and E.E. Hoffman, eds.,
CONF-8308130, DOE, pp. 14-17.
DelGrosso, E.J., C.E. Carlson, and J.J. Kaminsky, (1967) "Development of
Niobium-Zirconium-Carbon Alloys," J. Less Common Metals, 12: 173-201.
Dokko, N., et a1. (1984) Applications for Composites and Beryllium in the
SP-IO0 Power System, JPL D-1948, Jet Propulsion Laboratory, Pasadena, CA.
Grobstein, T.L. and R.H. Titran, (1986) Characterization of Precipitates in a
Nb-Zr-C Alloy, DOE/NASA/16310-6, NASA TM-I00848.
Hall, R.W. and R.H. Titran, (1966) "Creep Properties of Columbium Alloys in
Very High Vacuum," in Refractor V Metals and Alloys III, R.I. Jaffee, ed.,
Gordon and Breach Science Publishers, New York, pp. 885-900.
Horak, J.A., (1989) Second Interim Report on the Mechanical Properties of
Nb-IZr, ORNL/M-258, Oak Ridge National Laboratory, Oak Ridge, TN.
Kruger, G.C., S. Vaidyanathan, N. Deane, R. Protsik, and R.E. Murata, (1989)
"SP-IO0 Reactor Design," in 22nd Intersociety Energy Conversion Engineering
Conference, Vo1. I, AIAA, New York, pp. 419-423.
Lane, J.R. and G.M. Ault, (1965) "The Refractory Metals Sheet Rolling
Program," Met. Eng. Q., 5 (3): 23-29.
Moore, T.J., R.H. Titran, and T.L. Grobstein, (1986) The Effect of Electron
Beam Welding on the CreeD-Rupture Properties of Nb-Zr-Cr Alloy,
DOE/NASA/16310-1 NASA TM-88892.
Orr, R.L., O.D. Sherby, and J.E. Dorn, (1954) "Correlations of Rupture Data
for Metals at Elevated Temperatures," Trans. ASM, 46: 113-128.
PNAC, Advanced Materials Program for November and December 1964, PNAC-IOI8,
Pratt and Nhitney Aircraft Corp., Middletown, CT, !965.
Titran, R.H., T.J. Moore, and T.L. Grobstein, (1986) Creep Properties of
PNC-II Base Metal and Neldments as Affected by Heat Treatment, NASA TM-88842.
Titran, R.H., T.J. Moore, and T.L. Grobstein, (1987) Creep Behavior of Niobium
Alloy PNC-II, DOE/NASA/16310-3, NASA TM-89834.

Report Documentation PageNational Aeronatltlcs and
Space Adrn_nlslrabon
1, Report No, NASA TM-102390 2. Government Accession No. 3. Recipient's Catalog No.
DOE/NASA/16310-13
5. Report Date
October 1989
4. Title and Subtitle
Creep Strength of Niobium Alloys, Nb-1%Zr and PWC-I 1
7. Author(s)
Robert H. Titran
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
U.S. Department of Energy
Reactor Systems Development and Technology
Washington, D.C. 20545
6. Performing Organization Code
8. Performing Organization Report No.
E-5102
10. Work Unit No.
586-01-11
11, Contract or Grant No.
13. Type of Report and Period Covered
Technical Mernorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Final Report. Prepared under Interagency Agreement DE-AI03-86SF16310. Prepared fi)r the 7th Symposium on
Space Nuclear Power Systems, sponsored by the University of New Mexico, Albuquerque, New Mexico, January
7-11, 1990.
16. Abstract
A study is being conducted at NASA Lewis Research Center to determine the feasibility of using a carbide
particle strengthened Nb-1%Zr base alloy to meet the anticipated temperature and creep resistance requirements
of proposed near term space power systems. In order to provide information to aid in the determination of the
suitability of the PWC-11 alloy as an alternative to Nb-1%Zr in space power systems this study investigated (1)
the long-time high-vacuum creep behavior of the PWC-I 1 material and the Nb-1%Zr alloy, (2) the effect of
prior stress-free thermal aging on this creep behavior, (3) the effect of electron beam (EB) welding on this creep
behavior, and (4) the stability of creep strengthening carbide particles.
17. Key Words (Suggested by Author(s))
Nb-] %Zr; PWC-I 1; Creep; Niobium alloys; Refractory
metals; Space power
18. Distribution Statement
Unclassified- Unlimited
Subject Category 26
DOE Category UC-25
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages
Unclassified Unclassified 10
NASAFORM1626OCT86 *For sale by the National Technical Information Service, Springfield, Virginia 22161
22. Price*
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Creep strength of niobium alloys, Nb-1%Zr and PWC-11

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