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

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 include 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 liquid alkali metal corrosion at temperatures near II00 K was the primary concern

(Lane and Ault 1965 and Buckman 1984). Although current designs of space nuclear power systems for ground demonstration specify the Nb-l%Zr alloy for reactor, 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 proposed 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 processing 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 circleintercept method, with an aspect ratio of approximately 5:1 orientated in the

sheet rolling direction. The EB weldment has a highly oriented grain structure 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 celllike 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 condition 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 analysis 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 characterization of the precipitate (complex carbide) morphology. Several techniques 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 parameter (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 specimens 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.

National Aeronatltlcs and Report Documentation Page

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

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