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ORNL-TM-2724.txt
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o ‘-
OAK RIDGE NATIONAL LABORATORY
operated by '
UNION CARBIDE CORPORATION
NUCLEAR DIVISION LTI
for the
U.S. ATOMIC ENERGY COMMISSION
ORNL- TM- 2724
L
C
Lt
&
COMPATIBILITY OF MOLYBDENUM-BASE ALLOY TZM WITH
LiF-BeF ,~ThF ,-UF , (68-20-11.7-0.3 mole %) ot 1100°C
J. W. Koger and A. P. Litman
P350n
v NOTICE This document contains information of o preliminary nature
and was prepared primarily for internal use at the Oak Ridge National
Laboratory. It is subject to revision or correction and therefore does
not represent a final report.
PISTRIBUTION OF THIS DOCUMENT 15 UNLIMITED
LEGAL NOTICE
This report was prepared as an account of Government sponsored work. Neither the United States,
nor the Commission, nor any person acting on behalf of the Commission:
A, Makes any warranty or representation, expressed or implied, with respect to the accuracy,
completeness, or usefulness of the informotion contoined in this report, or that the use of
any information, apporatus, method, or process disclosed in this report may not infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of
any information, apparatus, methed, or process disclosed in this report,
As used in the above, '“person acting on bshalf of the Commission’ includes any smployee or
contractor of the Commission, or employee of such contractor, to the extent that such employee
or contracter of the Commission, or employee of such contracter prepares, disseminates, or
provides access to, any informaticn pursuont to his employment or contract with the Commission,
or his employment with such contracter,
ORNL-TM-2724
Contract No. W-7405-eng-26
METALS AND CERAMICS DIVISION
; r——— LEGAL NOTICE ————
This report was prepared as an account of Government sponsoréd work. Neither the United
States, nor the Commiasion, nor any person acting on behaif of the Commission:
A, Makes any warranty or representation, expressed or implied, with respect to the accu-
racy, completeness, or usefulness of the information contained in thim report, or that the use
of any Information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damagea resulting from the
uee of any information, apparatus, method, or process disclosed in this report.
As used In the above, ‘‘person acting on behaif of the Commission® Includes any em-
ployee or contractor of the Commiesion, or employee of such contractor, to the extent that
suchk employee or contractor of the £ is8i or emp] of such t prepares,
disseminates, or provides access io, any information pursuant Lo his employment or contract
with the Commisaion, or his employment with such contractor,
COMPATIBILITY OF MOLYBDENUM-BASE ALLOY TZM WITH
LiF-BeF,-ThF,-UF, (68-20-11.7-0.3 mole %) at 1100°C
J. W, Koger and A. P. Litman
DECEMBER 1969
OAK RIDGE NATIONAL ILABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION
iii
CONTENTS
Abstract . . . . . . o o ..
Introduction .
Experimental Procedure . . . . . . « + « .+ .« .
Results and Discussion .
Salt Analysis . . . . . . . . . . .
Weight Changes . . . . . . . .« . + . . .
X~Ray Fluorescence and Microprobe Analysis
Microstructural Changes . . . . .
Recrystallization . . . . . . . . .
Strength . . . . . . . . . ..
Corrosion Regctions and Kinetics
Conclusions . . . . . . .
Acknowledgments . . . +« v v 4 v 0 e e e .. .
g
\OOQW%J-\mmNI\JI—'F’lO%;
=R
v
COMPATIBILITY OF MOLYBDENUM-BASE ALLOY TZM WITH
iF-BeF ,-ThF,-UF, (68-20-11.7-0.3 mole %) at 1100°C
J. W. Koger and A. P. Litman
ABSTRACT
The TZM alloy (Mo—0.5% Ti~0,08% Zr—0.02% C) showed
a very small amount of attack by the fused fluoride salt
(IiF-BeF,-ThF,-UF,, 68-20-11.7-0.3 mole %) at 1100°C for
1011 hr. Corrosion manifested itself as leaching of
titanium and possibly zirconium from the alloy. The TZM
alloy exposed to the salt partially recrystallized, while
that exposed to the vapor did not. This recrystallization
was attributed tc the removal of titanium and zirconium.
On the basis of this single test the magnitude and mecha-
nism of corrosion indicate no serious problems for long-
term use of TZM in the vacuum distillation procegsing
scheme for the Molten Salt Breeder Reactor. However, the
strength properties of the TZM alloy would approach those
of unalloyed molybdenum as salt exposure time increased;
this is not considered a problem now.
INTRODUCTION
The current success of the Molten Salt Reactor Experiment at ORNL
has stimulated work on a thermal Molten Salt Breeder Reactor (MSBR).!
One of the requirements for a successful MSBR system will be the con-
tinuous reconditioning of the fuel salt to remove unwanted fission prod-
ucts. A possibility under study for one step of the salt reprocéssing
2 Uranium would be stripped from the fuel salt,
is vacuum distillation.
and the remaining salt would be distilled at 1000°C and 2 torr. The
diluents of the fuel salt, lithium and beryllium fluorides, would distil
readily and leave behind the rare-earth and alkaline-earth fission
products. This process has been demonstrated in laboratory experiments
and with some radiocactive salt from the MSRE. 2
'M. W. Rosenthal et al., MSR Program Semiann. Progr. Rept. Aug. 31,
1968, ORNL-4344, pp. 53-108.
2J. R. Hightower and L. E. McNeese, MSR Program Semiann. Progr. Rept.
Aug. 31, 1968, ORNL-4344, pp. 306-308.
The strength and corrosion resistance required of a container
material for the high-temperature vacuum distillation step eliminate
most conventional alloys from consideration. Our preliminary survey
disclosed that certain refractory alloys, particularly molybdenum-base
materials, may be suitable for this special service. The alloy TZM
(Mo—0. 5% Ti—0.08% Zr—0.02% C) was selected for an initial experiment
because it is stronger and usually more fabricable than pure molybdenum.
Accordingly, the experiment reported here provides a preliminary test of
the compatibility of TZM alloy with a typical fertile-fissile salt
(LiF-BeF ,-ThF,-UF,, 68-20-11.7-0.3 mole %) at 1100°C. This salt is a
strong candidate for the single-fluid MSBR now being designed. No tests
were specifically conducted to determine the strength properties of TZM
alloy, but conditions caused by the exposure to the salt that could
affect the strength were noted.
EXPERIMENTAL PROCEDURE
The experimental system used for this study consisted of a simple
capsule fabricated of cold-worked TZM alloy, containing specimens of
the same alloy, and shown in Fig. 1. Note that the specimens were
located in the salt, at the salt-vapor interface, and in the wvapor. The
purified salt (60 g) was supplied by the Fluoride Processing Group of
the Reactor Chemistry Division. Purification involved sparging with an
HF-H, mixture at 600°C to remove oxides and sulfides and stripping with
H, at 700°C to remove metallic impurities. The loading operation, which
consists of introducing the fluoride salt into the capsule, welding the
test capsule, and sealing the outer Inconel protective container, was
carried out in an inert-gas atmosphere chamber containing argon purer
than 99.995%.
After being tested in the position shown in Fig. 1 for 1011 hr at
1100°C, the capsule was removed from the furnace, inverted to keep the
specimens out of the salt, and gquenched in liquid nitrogen to retain
high-temperature corrosion products. After test, weight changes of the
specimens were determined, the salt was analyzed for impurities, and the
specimens and capsules were analyzed by x-ray fluorescence and examined
metallographically.
-2
ORNI-DWG 69-10034
r-—————— — 1,625 in.
; bt 0.88 in.
Y
V/K/OJES—M.WALLINCONEL
+ v PROTECTIVE CONTAINER
fg
4 _— 0.040 -in, WALL TZM CAPSULE
--——0.004 -in. TANTALUM FOIL LINER
N N
NN \\\‘g- AL
| _—=—ARGON
e
e T e Ty e T S e T S
BN
.- LiF—BeF, -ThF, ~UF, SALT
(68-20 - 1.7 = 0.3 MOLE %)
——TZM SPECIMEN {0.30 in.x1.0in.x0.02in.)
LSy
TN
Fig. 1. Schematic Drawing of Corrosion Test Capsule Used to Study
Compatibility of TZM Alloy with a Fused Fluoride Salt.
RESULTS AND DISCUSSION
Salt Analysis
Concentrations of the constituents of the salt and its impurities
before and after test are given in Table 1. During the experiment
titanium, zirconium, and chromium concentrations in the salt increased
and that of iron decreased. The titanium and zirconium are intentional
alloying additions, but chromium is an unwanted impurity.
Weight Changes
The specimens exposed to the salt showed small (0.5 mg/cm®) weight
gains, and the one exposed to the vapor did not change weight measurably.
Table 1. Chemical Analysis of Fertile-Fissile Salt Exposed to
TZM Alloy Capsule for 1011 hr at 1100°C (2010°F)
Content, ppm Content , wt %
Constituent Constituent
Before After Before After
Mo < 5 < 10 Li 6.71 7.01
zr 37 134 Be 2.65 2.55
Ti vz 151 Th 43.1 42.6
Fe 80 38 U 1.75 1.93
Cr 20 o7 F 45,5 45,7
0 58 < 50
H,0 40 70
X-Ray Fluorescence and Microprobe Analysis
Table 2 gives the concentrations of the major elements in the TZM
alloy as determined by x-ray fluorescence before and after test. Iron
was found on the surface and probably caused a major portion of the
weight gains, but no quantitative value was obtained. Significantly,
the quantitative analysis shows a decrease in titanium concentration,
no significant change in zirconium concentration, and a corresponding
increase in the concentration of molybdenum after exposure to the salt.
Care must be taken in interpreting these results, since the sensitivity
of the fluorescence analysis is questionable at these low concentrations
and iron was deposited over the surface. The electron microprobe
analysis showed 0.3% Ti on the surface and 0.5% Ti in the matrix. The
zirconium content was about C.1% in all portions of the specimen. Any
changes at the level of 0.1% are beyond the limit of detection of the
instrument. However, these results agree reasonably with the increase in
concentration of certain alloying elements in the salt and are in accord
with the proposed corrosion mechanism(s). ({See Corrosion Reactions and
Kinetics. )
Microstructural Changes
Figure 2(a) shows the typical cold-worked structure of the specimens
and capsule before test. This figure is also typical of the specimen
LT
Table 2. Concentration of Alloying Elements in TZM Alloy Specimen
Before and After Exposure to a Fertile-Fissile Salt at 1100°C
for 1011 hr, as Determined by X-Ray Fluorescence Analysis®
Content, wt %
Sample Analyzed
Mo Zr Ti
Untested alloy 99. 4 0.08 0.5
Exposed specimens
in vapor 99.8 0.08 0.1
at interface 99, 87 0.09 0. 04
in salt 99,90 0.08 0.013
SThe analysis disclosed substantial iron on the alloy surface
after test, but iron was not considered in determining the quanti-
ties above.
exposed to the vapor, where no microstructural change occurred. An
unetched specimen, Fig. 2(b), exposed to the salt shows no attack at the
surface. The same specimen etched, Fig. 2(c), shows recrystallization
for a maximum depth of about 0.004 in. Examination of this specimen at
a lower magnification, Fig. 2(d), shows that both surfaces recrystallized
as the result of test. The inside capsule wall also recrystallized in
the same manner,
Recrystallization
In view of the microstructural and chemical changes induced in the
TZM alloy by this test, we compared reported recrystallization tempera-
tures for molybdenum and TZM alloy (Table 3). It is clear from the
above and from general metallurgical considerations that an increase in
annealing time from 1 hr to several thousand hours should lower the
recrystallization temperature of TZM alloy only 100 to 200°C. Moreover
the presence of as little as 0.01% of a foreign element in solid solution
can raise the recrystallization temperature as much as several hundred
3
degrees. Conversely, the removal of alloying constituents would free
’R. E. Reed-Hill, Physical Metallurgy Principles, Van Nostrand,
Princeton, N. J., 1964, p. 198.
Fig. 2. TZM Alloy Exposed to Fertile-Fissile Salt, LiF-BeF,-ThF;-UF,
(68-20-11.7-0.3 mole %) for 1011 hr at 1100°C. (a) Typical cold-worked
structure of capsule and specimens before test; also the structure of the
specimen exposed to the vapor during test. 500x. Etchant: H,0, Hy0;,
H,S804. (b) As-polished capsule and specimen exposed to salt. 500x.
(c) Capsule and specimen exposed to salt. 500x. Etchant: H,0, H,0,,
H»80,. (d) Specimen exposed to salt. 100x. Etchant: H0, Hy0», H>S0,.
e
4
Table 3. Recrystallization Behavior of Wrought, Stress-
Relieved, Unalloyed Molybdenum and TZM Alloy
Temperature Time Percent
Alloy (°c) (hr) Recrystallization Reference
Unalloyed Mo 1130 1 100 a
TZM 560 4400 0 o
TZM 1100 1 0 a
TZM 1160 4400 85 b
TZM 1250 44,00 100 b
TZM 1390 1 100 a
®p. A. Wilcox, p. 26 in Refractory Metal Alloys, Metallurgy and
Technology, ed. by I. Machlin, R. T. Begley, and E. D. Weisert, Plenum
Press; New York, 1968.
bD. H. Jansen, Fuels and Materials Development Program Quart.
Progr. Rept. Sept. 30, 1968, ORNL-4350, pp. 107-111, and private
communication.
the grain boundaries and allow them to move to form new grains. Thus,
the enhanced recrystallization (lower recrystallization temperature) in
the samples and capsule of this experiment is due primarily to the
removal of the titanium and possibly zirconium from the molybdenum
matrix. This is ffirther substantiated by the lack of recrystallization
in the samples exposed to the vapor, where the composition changed much
less.
The addition of carbon and one or more group IV-A elements to
molybdenum greatly increases the recrystallization temperature. Thus ,
carbon removal from the alloy should likewise change recrystallization
behavior. However, carbon analyses show no difference (about 0.035% C
in each) between exposed and unexposed TZM samples, so this effect is
very small or absent. Although carbon mass transport is common in
liquid metal systems, especially alkali metals, it is not considered a
problem in fused fluoride systems.
“W. H. Chang, A Study of the Influence of Heat Treatment on Micro-
structure and Properties of Refractory Alloys, ASD-TDR-62-211 (April 1962).
Strength
The molybdenum-base TZM alloy is about the best documented refrac-
tory alloy in which base metal strength is improved by precipitation
hardening. This alloy is strengthened by the formation of fine carbides
of titanium and zirconium as well as by cold working, and its ultimate
tensile strength is double or triple that of unalloyed molybdenum. The
100-hr rupture strength of TZM at 1100°C is also much greater than that
of molybdenum.” Although TZM is much more difficult to fabricate than
commercial alloys and many refractory alloys, it is usually much easier
to work than unalloyed molybdenum. Thus, as an engineering material TZM
has many advantages over molybdenum.
Comparing the strength and ductility of wrought, stress-relieved,
and recrystallized TZM, Wilcox et gl.é noted a significant increase in
yield and ultimate strengths due to working at test temperatures of 1200
to 1300°C. At 1550°C after recrystallization of the wrought sample
there was relatively little difference in the materials. However, at
1100°C, the temperature of our capsule test, Wilcox's recrystallized
alloy had much lower strength than the wrought alloy. Thus, the use of
a stress-relieved TZM alloy for conditions given in this experiment
should also be considered.
Although TZM would generally be favored over molybdenum for the
previous reasons, through the loss of its alloying elements (titanium
and zirconium) during exposure to the fused fluoride salt the composi-
tion and the strength properties of the cold-worked TZM approach those
of unalloyed recrystallized molybdenum. Although unalloyed molybdenum
or recrystallized TZM is weaker than the initial cold-worked material,
the strength of the exposed material would probably be ample for the
loads proposed in the MSBR vacuum distillation system. However, before
the depleted TZM is used, it should be tested to more carefully define
°T. E. Tietz and J. W. Wilson, Behavior and Properties of Refractory
Metals, Stanford University Press, California, 1965, pp. 156—205.
®B. A. Wilcox, A. Gilbert, and B. C. Allen, Intermediate Temperature
Ductility and Strength of Tungsten and Molybdenum TZM, AFML-TR-66-89
(April 1966).
the strength properties of the recrystallized material. An advantageous
trade-off with these mechanical property changes is, of course, that
pure molybdenum is more resistant than TZM to the fluoride salts of
interest to the MSRP. Thus, several benefits come from fabricating the
system with TZM while others accrue from the "conversion" of TZM to
molybdenum during the fluoride salt exposure.
Corrosion Reactions and Kinetics
In fluoride salt systems one of the major corrosion reactions 1is
the oxidation of one of the constituents of the container alloy by the
reduction of a less stable impurity metal fluoride initially in the salt,’
for example
Cr + FeF, — CrF, + Fe . (1)
The reduced metal substitutes for the oxidized metal on the container
material. This type of reaction apparently occurred in our experiment
involving the strong reducing agents titanium and zirconium:
Ti + FeF, -» TiF, + Fe , (2)
Zr + 2FeF» — ZrF, + 2Fe . (3)
The reported8 free energy changes for the reactions shown by
Egs. (1), (2), and (3) are strongly negative, and Egs. (2) and possibly
(3) seem to be indicated by the results reported above. As noted earlier,
the iron metal that formed in these reactions deposited in thin layers on
the container and specimens.
"W. R. Grimes, G. M. Watson, J. H. DeVan, and R. B. Evans,
"Radio-Tracer Techniques in the Study of Corrosion by Molten Fluorides,"
Pp. 559574 in Conference on the Use of Radioisotopes in the FPhysical
Sciences and Industry, September 6—17, 1960, Proceedings, Vol. IIT,
International Atomic Energy Agency, Vienna, 1962.
8A. Glassner, The Thermochemical Properties of the Oxides, Fluorides,
and Chlorides to 2500°K, ANI-5750 (1957).
10
Alternatively the fuel sglt corrosion reactions in which UF, is
reduced to UF3 also may have occurred to remove titanium and zirconium
from the alloy:
Ti + 2UF, — TiF, + 2UFs , (4)
Zr + 4UF4 -» Zr¥F, + 4UF3 . (5)
However, no data with which to determine the extent of these reactions
are available.
Assuming that the removal of the elements from the TZM was controlled
by solid~state diffusion, one can calculate from the increase of the
titanium and zirconium in the salt the apparent diffusion coefficients of
titanium and zirconium in the TZM glloy. From these one can estimate the
amount of those materials that would be removed at different times and
temperatures. In regard to the zirconium removal, we feel that the salt
analysis is correct and that the instruments involved in the fluorescence
and microprobe analyses are not sufficiently sensitive to measure the
movement of the zirconium.
The total amount of material, M, , that diffuses from the alloy held
under isothermal conditions with a zero surface concentration is given
9
by
M% = 2Co«/Dt7fl s (6)
where
Cop = the concentration of the diffusing element,
D = the diffusion coefficient, and
t = the time.
We calculated D = 1.2 x 1071? em?/sec for titanium in TZM and
2.9 x 10~!'! cm?/sec for zirconium in TZM at 1100°C. We did not calculate
for chromium removal, as its concentration fluctuated from sample to
sample and we could not assume that it was distributed homogeneously
through the alloy.
?J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford,
England, 1956, p. 11.
11
The expression
t ~X2/D , (7)
where X is the distance of composition change, is very useful in calcu-
lating approximately whether the composition has changed appreciably by
diffusion under a given set of circumstances. For example, we can calcu-
late the time required for appreciable removal — concentration between
the initial and ultimate concentrations — of the diffusing element at a
certain distance from the surface.
From the calculated diffusion coefficients and the experimental
time of 1011 hr, we find the depths of removal of titanium and zirconium,
respectively, are 0.0008 and 0.0040 in. Since the microstructures show
recrystallization for a distance of about 0.004 in., we may assume that
the calculated diffusion coefficient for zirconium may be more accurate
than that for titanium — that is, the salt analysis for the titanium may
be somewhat in error. Extrapclation of the calculated values shows that
it would require 4000 hr to recrystallize an additional 0.004 in. of
material. This illustrates the decrease of the corrosion rate with time
and the general usefulness of TZM alloy for MSR reprocessing service.
CONCLUSIONS
1. This test showed negligible corrosion of the TZM alloy by the
fused fluoride salt (ILiF-BeF,-ThF,-UF,, 68-20-11.7-0.3 mole %) at 1100°C
for over 1000 hr.
2. Corrosion manifested itself as leaching of titanium and possibly
zirconium from the alloy. FeF, initially present in the salt oxidized
the alloying elements to fluorides dissolved in the salt bath. The iron
metal resulting from the reaction deposited in thin layers on the speci-
mens and container. We found that Dy, and D, were 1.2 X 10-1? and
2.9 x 10-1* cm?/sec, respectively, at 1100°C in the alloy.
3. The TZM alloy exposed to the salt partially recrystallized,
while the TZM alloy simultaneously exposed to the vapor did not. This
recrystallization was attributed to the removal of titanium and zirconium.
12
4. On the basis of this single test, the magnitude and mechanism
of corrosion indicate no serious problems for long-term use of TZM alloy
in the MSBR vacuum distillation processing scheme. However, the strength
properties of the TZM alloy would approach those of unalloyed molybdenum
as salt exposure time increased.
ACKNOWLEDGMENTS
It is our pleasure to acknowledge the assistance of F. D. Harvey,
E. J. Lawrence, and J. B. Phillips with these experiments. We are also
indebted to J. R. DiStefano, W. O. Harms, and H. E. McCoy, Jr., for their
constructive review of the manuscript.
Thanks also go to H. R. Gaddis of the Metallography Group,
Harris Dunn and other members of the Analytical Chemistry Division, the
Graphic Arts Department, and the Metals and Ceramics Division Reports
Office for invaluable assistance.
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