Available creep data for the refractory-metal alloys C-103 (Nb/10 percent Hf/1 percent Ti/0.7 percent Zr), ASTAR-811C (Ta/8 percent W/1 percent Re/0.7 percent Hf/0.025 percent C), W-5Re (W/5 percent Re), and W-25Re (W/25 percent Re) were correlated by the Orr-Sherby-Dorn method and extrapolated to 1 percent creep over 10 years. Useful life was specified to be 2 standard estimates of error below the mean surface through the data. Over the temperature range of 1200 to 1800 K, ASTAR-811C was found to be the strongest of these alloys. In particular, ASTAR-811C was found to have at 1800 K the same creep strength as W-25Re at 1420 K. The difference between these results and those of Horak and Booker likely devolves from the comparative lack of long-time data on tungsten alloys

English, Robert E.

English

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NASA Technical Memorandum 105228
Orr-Sherby-Dorn Creep Strengths of
the Refractory-Metal Alloys C-103,
ASTAR-811 C, W-5Re,
and W-25Re
Robert E. English
Lewis Research Center
Cleveland, Ohio
Prepared for the
Ninth Symposium on Space Nuclear Power Systems
sponsored by the University of New Mexico
Albuquerque, New Mexico, January 12-16, 1992
IV/\SA
https://ntrs.nasa.gov/search.jsp?R=19910022851 2020-03-17T14:55:42+00:00Z
ORR-SHERBY-DORN CREEP STRENGTHS OF THE REFRACTORY-METAL ALLOYS
C-103, ASTAR-811C, W-SRe, AND W-25Re
Robert E. English
National Aeronautics and Space Administration
Lewis Research Center
21000 Brookpark Road, MS AAC-2
Cleveland, OH 44135
(216) 977-7078
Abstract
Available creep data for the refractory-metal alloys C-103 (Nb-10%Hf-1%Ti-0.7%Zr), ASTAR-811C (Ta-
8%W-1%Re-0.7%Hf-0.025%C), W-5Re (W-5%Re), and W-25Re (W-25%Re) were correlated by the Orr-
Sherby-Dorn method and extrapolated to 1 percent creep over 10 years. In addition, useful life was specified
to be two standard estimates of error below the mean surface through the data. Over the temperature range
of 1200 to 1800 K, ASTAR-811C was found to be the strongest of these alloys. In particular, ASTAR-811C
was found to have at 1800 K the same creep strength as W-25Re at 1420 K. The difference between these
results and those of Horak and Booker (1990) likely devolves from the comparative lack of long-time data on
the tungsten alloys.
INTRODUCTION
The long powerplant lives (at least 7 years) necessary to enable use of nuclear space power systems pose
special problems for technologists. Inasmuch as nearly all tests of materials are for time periods substantially
less than the powerplant endurance actually sought, extrapolation from these shorter tests is naturally
required. IIerein the test data are treated in the following ways: Tl;e criterion for evaluation is 1-percent
creep, not rupture, and the service life sought is 10 years (87 660 hr). Ti ►e test data are correlated by the Orr-
Sherby-Dorn (OSD) method. The standard estimate of error (SEE) of the test data from the correlating
surface is computed, and the logarithm of the useful life is specified to be two SEES below the correlating
surface. On this basis, the following alloys are compared: C-103 (Nb-10%Hf-1%Ti-0.7%Zr), ASTAR-811C
(Ta-8%W-1%Re-0.7%Hf-0.025%C), W-511e (W-5%Re), and W-25Re (W-25%Re).
DATA ANALYSIS
For the Orr-Sherby-Dorn correlation,
t 1% = a s -m exp ^q]
T
where t l% = hours to 1 percent creep, s = stress (MPa), T = temperature (K), and a, m, and q are
constants selected to fit the data. Because of the limited data for the tungsten alloys, linear creep was
assumed, when necessary, in estimating their times to 1 percent creep. Linear regression was applied to the
logarithm (to the base 10) of Equation (1), that is,
log(t 1% ) = b - m log(s) + Q
T
(1)
(2)
where b = log(a) and Q = q log(e).
Analysis of 0-103
The 35 creep tests of C-103 (Titran and Klopp 1980) totaled 118 806 hr and spanned the temperature
range of 1100 to 1477 K. One data point was ignored inasmuch as that test did not continue to 1 percent
strain. For the OSD correlation, the correlation coefficient r 2 was 0.92.
Analysis of ASTAR-811C
Although the 98 long-term creep tests of ASTAR-811C (Klopp et al. 1980) continued for a total of
314 140 hr (35.8 years), 41 data were set aside in this analysis for the following reasons: For 29 tests, the
time to 1 percent creep was not measured. Annealing at temperatures as low as 1811 K was found to be
detrimental to creep strength. And the OSD correlation was improved by ignoring imposed stresses over
190 MPa. The remaining 57 data points spanned a total of 123 916 measured hours to 1 percent creep, total
test duration of 153 485 hr, and test temperatures of 1366 to 1811 K. For the OSD correlation, the
correlation coefficient r 2 was 0.76.
Analysis of W-5Re
A severely limited source of creep data on W-5Re was found in Table 13 in Horak and Booker (1990).
The 28 test durations totaled only 96 hr, and the longest was 89 hr. The test temperatures spanned 1673 to
2473 K. For the OSD correlation, the correlation coefficient r 2 was 0.84.
Analysis of W-25Re
The data sources on W-25Re were Conway and Flagella (1971, pp. 396 and 402) and Sheffler and Ebert
(1973, Table II-3). The 67 tests spanned the wide temperature range of 1144 to 3073 K. Although the tests
were continued to rupture, the test tunes totaled only 6447 hr, averaging less than 100 hr apiece; one other
test continued to 2686 hr without rupture. For the OSD correlation, the correlation coefficient r 2 was 0.96.
Results
For these four alloys, the parameters for substitution into Equation (2) are listed in Table 1. The SEE
for log(t l% ) is also listed.
TABLE 1. Statistical Results for
Orr-Sherby-Dorn Creep
Correlation of Four
Refractory-Metal Alloys.'
Alloy b m	 Q S.E.E.'
C-103 -5.944 3.077	 17 855 0.129
ASTAR-811C -4.989 3.567	 23 497 .290
W-5Re -4.405 4.109	 23 017 .407
W-25Re -2.963 2.726	 14 939 .339
'Refer to Equation (2).
h Standard estimate of error for log(t l a).
2
F
a 11
g 10
n
u^
CO 9
L
S 8
7
5.5
12
m
10
00
•_	 8
c
L
S 6
4
4.0
10
a g
6
+	 4
L
g 2
5.9	 6.3	 6.7	 7.1	 7.5
4.4	 4.8	 5.2	 5.6	 6.0
10.5
9.5
^ 8.5
m
0
C'' 7.5
6.5
5.5 1	 1	 1	 1	 1	 1	 1
	 1
6.6	 7.0	 7.4	 7.8	 8.2	 8.6	 9.0	 9.4
12 r	 -e
	
0'	 '
	
3	 4	 5	 6	 7	 8	 9
10,OOOfT
Figure 1.—OSD correlations of 1 % creep of 4 refractory-
metal alloys. (a) C-103, (b) ASTAR-811 C, (c) W-5Re,
(d) W-25Re.
0J
2.8
2.4
m 2.0
a
1.6
3
—0 12
S 
.8
.4
0 I	 I	 I	 I	 __1L	
-1'—
	 1
181200 1300	 1400	 1500 1600	 1700	 00
Temperature, K
Figure 2—Comparative creep strengths for 1 % creep in 10
years wtth 2 S. E E
Figure 1 shows for each alloy the measured data, the mean line through these data, and the line for which
log(t l% ) has been reduced by two SEES. Just as anticipated, the two-SEE allowance pretty well delineates
the lower bound of the test data, for, statistically, approximately 98 percent of the data should have strengths
above this line.
DISCUSSION OF RESULTS
The results of this OSD analysis were employed in order to determine the stresses producing 1 percent
creep over 10 years for each of these alloys, the allowable stresses being shown in Figure 2 and in Table 2.
In each determination of allowable stress, the log(t l%) (in Equation (2)) was decreased by two SEES from
the mean surface through the data. These four alloys have very similar trends of strength versus use
temperature, and their comparative strengths are about constant over the temperature range of 1200 to
1800 K. C-103 exhibits the low strengths characteristic of Nb alloys; even at 1200 K, its allowable stress
producing 1 percent creep in 10 years is only 16 MPa.
ASTAR-811C has superior creep strength over this entire range, but W-25Re has creep strength only
slightly better than C-103. For example, the long-time creep strength of ASTAR-811C at 1800 K (5.2 MPa)
is equal to that for W-25Re at 1420 K, a 380-K advantage. In addition, the derivative alloys ASTAR-1211C
(Tn-12 %W-1 %Re-0.7%Hf-0.025%C) and -1511C (Ta-15%W-1%Re-0.7%Hf-0.025%C) offer the possibility of
a further 200-K increase in operating temperature over ASTAR-811C at the same strength (Buckman and
Ammon 1990). W-511e, less highly alloyed than W-2511e, appears to have superior strength and rivals
TABLE 2. Allowable Stresses (NiPa) to Produce
1 Percent Creev ever 10 Years
Temperature,	 1200	 1300	 1400	 1500	 1600	 1700	 1800
K
C-103 16 7.0 3.3 1.8 1.0 0.62 0.40
ASTAR-811C (a) 130 57 28 15 8.5 5.2
W-513e 156 68 34 18 11 6.6 4.4
V47 -2513e 26 12 5.8 3 2 1.9 1.2 .79
'Outside the range of the correlation
4
8.4
7.8
7.20
S0
6.6
6.0
5.4
8.4
7.8
7.20
g0
6.6
6.0
5.4
0.6 0.8	 1.0	 1.2	 1.4	 1'.6	 1.8	 2.0
log (allowed stress, MPa)
Figure 3.-1 % creep of 3 alloys over 10 years. (a) Orr-Sherby-
Dorn and 2 S.E.E. (b) Horak's data and method; 2 S.EE
ASTAR-811C in this OSD correlation. I infer that the creep of either of these tungsten alloys over 10 years
is very uncertain because of the gross extrapolation required from its very limited, short-time creep tests,
especially so for W-511e. The range of creep strengths found for the W-Re family is, I judge, representative
of this family for Re contents ranging from 5 to 25 percent. With present knowledge, the ASTAR family thus
appears to be more promising than the W-Re family as a focus for a program on materials technology to
evolve adequate creep strength at high temperature.
For the three alloys ASTAR-811C, W-511e, and W-25Re, Figure 3 contrasts these OSD results with those
of Horak and Booker. Rather than using the straight lines in Figure 3(a), Horak and Booker elected to use
polynomials for their correlations, their data correlations being the basis for Figure 3(b). From their analysis
for a 7-years service period and with no allowance for scatter in the test data, they conclude (p. 39) that
'ASTAII-811C has the highest long-term creep strength of the six materials (they) evaluated over the
temperature range 1300 to 1650 K.' In contrast with this result, Figure 3(b) (for a 10-year5 service period
and with two-SEE allowance for scatter of the test data) shows ASTAR-811C and W-25Re to have equal
allowable stresses at about 1500 K. Overwhelming the modest differences between results from OSD and from
Ilorak-Booker are the scanty data base for the W-Re family and the gross extrapolation to 10-year duration
that that limited data base requires.
Admittedly, the specification of 1 percent creep over 10 years as the criterion for selection of materials
and operating temperatures is conservative, these materials deforming 20 percent or more in short-time tests
before rupturing. In addition. my election to reduce the logarithm of the useful life by two standard estimates
of error (SEE) of the test data from the correlating surface is a more severe allowance than is common in
assessment of candidate materials and nuclear power systems. In my view, confidence of success in developing
a nuclear powerplant that will operate for 10 years requires that its design and development be based on (1)
an extensive, long-time data base on material properties and on (2) just such a conservative approach to
design as I outline.
Acknowledgment
This work was performed at NASA's Lewis Research Center.
References
Buckman, R.W., Jr., and R.L. Ammon (1990) "Comparative Strength Properties of Tantalum Alloys
ASTAR-811C, ASTAR-1211C, and ASTAR-1511C,' in Proc. Seventh Symposium on Space Nuclear
Power Systems, CONF-900109, Vol. 1, M.S. El-Geuk and M.D. Hoover, eds., DOE 1990, pp. 154-157.
Conway, J.B., and P.N. Flagella (1971) Creep-Rupture Data for the Refractory Metals to High Temperatures,
Gordon and Breach, Science Publishers, Inc., New York, NY.
Horak, J.A., and M.K. Booker (1990) Evaluated Creep Data for Selected Refractory Metal Alloys with
Potential Multimegawatt Space Power Applications, ORNL/TM-11163, Oak Ridge National Laboratory,
Oak Ridge, TN.
Klopp, W.D., R.H. Titran, and K.D. Sheffler (1980) Long-Time Creep Behavior of the Tantalum Alloy Astar
811C, NASA TP-1691, NASA Lewis Research Center, Cleveland, OH.
Sheffler, K.D., and R.R. Ebert (1973) Generation of Long Time Creep Data on Refractor}' Alloys at Elevated
Temperatures, NASA-CR-134481, also TRW-ER-7648, TRW Materials Technology Laboratories,
Cleveland, OH.
Titran, R.H., and W.D. Klopp (1980) Long-Time Creep Behavior of the Niobium Alloy C-103, NASA
TP-1727, NASA Lewis Research Center, Cleveland, OH.
6
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
1991 Technical Memorandum
4. TITLE AND SUBTITLE S. FUNDING NUMBERS
Orr-Sherby-Dom Creep Strengths of the Refractory-Metal Alloys C-103,
ASTAR-811C, W-5Re, and W-25Re
WU - None6. AUTHOR(S)
Robert E. English
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) S. PERFORMING ORGANIZATION
REPORT NUMBER
National Aeronautics and Space Administration
Lewis Research Center E - 6551
Cleveland, Ohio 44135 - 3191
9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORINGJMONITORING
AGENCY REPORT NUMBER
National Aeronautics and Space Administration
Washington, D.C. 20546-000 1 NASA TM-105228
11. SUPPLEMENTARY NOTES
Prepared for the Ninth Symposium on Space Nuclear Power S ystems sponsored by the University of New Mexico,
Albuquerque, New Mexico, January 12 -16, 1992. Robert E. English, Distinguished Research Associate at NASA Lewis
Research Center. Responsible person, Robert E. English (216) 977 - 7078.
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Unclassified - Unlimited
Subject Category 20
13. ABSTRACT (Maximum 200 words)
Available creep data for the refractory-metal alloys C-103 (Nb-10%Hf-I %Ti-0.7%Zr), ASTAR-811C (Ta-8%W-1%Re-
0.7%Hf-0.025%C), W-5Re (W-5%Re), and W-25Re (W-25%Re) were correlated by the Orr-Sherby-Dorn method and
extrapolated to 1 % creep over 10 years. In addition, useful life was specified to be 2 standard estimates of error below
the mean surface through the data. Over the temperature range of 1200-1800 K, ASTAR-811C was found to be the
strongest of these alloys. In particular, ASTAR-811C was found to have at 1800 K the same creep strength as W-25Re at
1420 K. The difference between these results and those of Horak and Booker (1990) likely devolves from the compara-
tive lack of long-time data on the tungsten alloys.
14. SUBJECT TERMS 15. NUMBER OF PAGES
Nuclear power plants; Refractory metals; Refractory materials; Creep strength; 8
16. P^I^^ CODETantalum alloys; Tungsten alloys
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT
OF REPORT OF THIS PAGE OF ABSTRACT
Unclassified Unclassified Unclassified
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Orr-Sherby-Dorn creep strengths of the refractory-metal alloys C-103, ASTAR-811C, W-5Re, and W-25Re

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