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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
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ORNI2-1677
Corf- 651025-7**
1885
NOV 8
ORNI - AEC - OFFICIAL
EQUILIBRIUM SOLID SOLUTIONS OF NITROGEN IN
C6--1% Zr BETWEEN 1200–1800°C
H. Inouye, Metals and Ceramics Division,
Oak Ridge National Laboratory, P.O. BOX X, Oak Ridge, Tennessee 37831

RUMTIRSIOD FOR ANNOUNCEMENT
IN MUCLEAR SCIENCE ABSTRACTS
LEU AL NUTILE
The report was prepared as an account of Government sponsored work. Neither the United
Batu, kor the Commission, nor any person acting on behalf of the Commisslon:
A. Maker way warranty or representation, expressed or implied, with respect to the accu-
racy, completenou, or unahulness of the information contained in this report, or that the use
of any information, apparatus, method, or proces, disclosed in the report may not infringe
privately owed rights or
B. Assumes may liabilites with respeot to the use of, or for damages resulting from the
use of any information, apparatus, method, or procon disoloned in this roport.
As wood in the abovo, "persoo acting on behalf of the Commission" includes any on-
ployee or contructor of the Commission, or employs of such contractor, to the extent that
auch employs or contractor of the Commission, or employee of such contractor proparos,
dienominates, or provides accous to, any laformation pursuant to his employmont or contract
with the Commission, or his employment with such contractor.
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EQUILIBRIUM SOLID SOLUTIONS OF NITROGEN IN
C6-1% Zr BETWEEN 1200-1800°C*
.ORNI - AL - OFFICIAL
H. Inouye
Metals and Ceramics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
ABSTRACT
An ultrahigh vacuum technique capable of measuring sorption rates
as low as 4 x 10-3 ppm N/hr and pressures of 10-10 torr of nitrogen was
used to measure the thermodynamic properties of, solutions of nitrogen in
06–1% Zr between 1200-1800°C.
In the composition range 31–278 ppm N, the solutions obeyed
Sievert's law. An anomalous solution behavior that was dependent on
the Xy, the amount of nitrogen in solution, occurred between 1200-1400°C
and was attributed to an oxygen-nitrogen interaction. The partial molar
free energy expression for the dissolution of nitrogen in the normal
solutions was AF -51,500 + 20.74T.
INTRODUCTION
The property variations of a refractory metal dramatically reflect
the influence of the interstitials oxygen, carbon, nitrogen, and/or
hydrogen. Concentration differences in the parts per million range can
*Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
ORNL - AEC - OFFICIAL
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promote brittleness in the alloys near ambient temperatures and have a
ONNL-AEC - OFFICIAL
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significant effect on their mechanical properties at high temperatures.
Their many effects are obscured because refractory metals tend to
absorb or degas these interstitials when heated in vacuum atmospheres.
Measurements of their individual or combined effects are further com-
plicated because these elements may be in solution or precipitated as
additional phases in the alloy as a consequence of their slight solu-
bility. Thus, the properties of the alloy are influenced by the prior
heat treatment as well as the concentration of the interstitials.
Clearly property control in refractory metals reduces to a problem of
controlling the interstitial content and the morphology of the precipi-
tated phases.
An objective of this study was to determine the N2/06-1% Zr
equilibrium between 1200–1800°C to establish the conditions of thermal
stability and the phase relationships in this system. A secondary
objective was to develop a technique for accurately measuring very
small concentrations of nitrogen in the alloy and the measurement of:
the equilibrium nitrogen pressure.
EXPERIMENTAL
A new technique was used to determine the I
equilibria.
This method depends upon measuring the rate of nitrogen flowing into and
out of a reaction vessel under a pressure gradient by means of E
Q = F2(P2 - P2) - F2(P2 - P3)
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P1, P2, P3 = nitrogen pressures (torr) in the manifold, over the
sample, in the exhaust, respectively.
F1 and F2 = conductances of leak valves (cm/sec)
Q = sorption rate (torr.cm /sec)*
The quantity F1(P1 – P2) is the flow rate of nitrogen into the reaction
vessel containing a heated sample and F2(P2 - P3) is the flow rate of
gas out. When Q is positive, the sample is sorbing gas at this rate since
the net flow of gas into the reaction vessel exceeds the rate of exhaust.
It also follows that when Q is negative, the sample is degassing and is
in equilibrium with the gas when Q = 0.
The various conditions of sorption, degassing, and equilibrium
were regulated as desired by adjusting the manifold pressure P, while
the conductances were fixed by the settings on the high vacuum leak
valves. The change in the nitrogen concentration, sx, was computed by
integrating the reaction rate Q by the reaction time. The dynamic flow
conditions maintained a low background pressure of interfering gases by
continuous pumping to about 1 x 10-9 torr. The arrangement of the ap-
paratus is schematically shown in Fig. 1. Except for the mass-
spectrometer, flanges, valves, and the electrodes, the apparatus was
constructed of Pyrex.
The conductances of Fi and F2 were calculated from the exhaust
rates of argon from the reaction vessel (1), and the ion-gages G1, G2,
and G3 were calibrated against each other with the mass spectrometer.
*Q x 1.205 x 10-3 = cm/sec at STP.
ORNL - AEC - OFFICIAL
ORNL - AEC - OFFICIAL
The nitrogen pressure over the sample (P2) was calculated from the spectra
of the peaks occurring between M/e 44 to M/e:2.* The measured nitrogen
pressure, P2, was corrected for thermal transpiration in accordance to
the method of Moore (2).
O*NI-AEC - OFFICIAL
The alloy sample was a ribbon 0.010 x 0.094 x 6.875 in. cut from a
cold-rolled section of an electron-beam melted ingot that analyzed
1.04% Zr, 120 ppm 0, 67 ppm N, 80 ppm C, and 13 ppm H. The filament
was formed into a "U" and the ends were spot welded to molybdenum con-
nectors, 3/4 in. lung * 0.007 * 0.085 in., to reduce the temperature
gradient in the filament to about 20°C, then on to 1/8-in.-diam
molybdenum electrodes. Temperatures were measured optically through
a magnetically shuttered port. An emissivity of 0.345 was used (3) and
a correction was applied to this temperature due to the absorbing charac-
teristics of the Pyrex sight port using a measurer absorbing constant of
3.5 x 10-6/0C (Ref. 4).
After bekeout of that part of the apparatus enclosed by the dashed
boundaries in Fig. 1 at 250°C, the filament was degassed at 1650°C until
the total pressure in the reaction vessel was about 6 x 10-9 torr.
Utilizing Eq. (1) the equilibrium nitrogen pressure was then determined
at a series of temperatures both on heating and cooling. Subsequent to
these measurements, an increment of nitrogen was added to the filament,
homogenized under equilibrium conditions at 1650°C, and the equilibrium
.. *M/e = mass-to-charge ratio. Nitrogen and co. peaks occur at“.
M/e = 28. The separation of the gases is accomplished from the ioniza-
tion and cracking patterns of CO, CO2, CH4, and Na.
ORNL - TEC - OFFICIAL
ORNI - AEC - OFFICIAL
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nitrogen pressures were measured again at a series of temperatures.*
s
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After several nitrogen additions, the total nitrogen content of the flla-
Termi
ment was determined by chemical analyses and compared with the calculated
additions.
*
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RESULTS
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The precision with which the amount of nitrogen could be added to
ULUU W
the filament is shown by four typical sorption rate curves in Fig. 2. . .
These data show that nitrogen sorption rates varying from 10-3 to 2 ppm/hr
could be obtained for the pressure range 2 x 10-10 to 1 x 10-? torr.
The calculated nitrogen content of the filament compared to that determined
by chemical analysis shown in Table 1 agree within 4 ppm in the 30 ppm
range and within 0.5 ppm at 100 ppm. Analysis also showed that the de-
gassed filament (88 hr at 1650°C) did not lose zirconium by vaporization
while the oxygen, nitrogéng and carbon contents were lower than the
starting composition. '
Figure 3 summarizes the pressure-temperature-composition relation-
ship in the N2/06-1% Zr system. The solid lines were drawn through the
experimental point at a slope of two at all temperatures. Between
100-200 ppm N and at 1350°C and below, the curves were drawn as dashed
lines because of the uncertainty in the data and to emphasize the devia-
tion of the data from the straight line.
*Equilibrium was assumed when a~o (ax ~ 1 ppm in 300 hr) and Pa
was constant.
ORNI - AEC - OFFICIAL
Equation (2) is the expression for año for the normal solutions
ORNI - AEC - OFFICIAL
OINE - AIC - OFFICIAL
SFO = -51,500 + 20.74T
(2)
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The reference state is the infinitely dilute solution in which Ax = No
(atom-fraction) and Px. 1s expressed in atmospheres. The activity
coefficients, for, of selected solutions are listed in Table 2 to 11lus-
trate the temperature-composition limits of the anomalous solutions.
Figure 4 shows the dependence of the equilibrium nitrogen pressure
on 1/T for six nitrogen concentrations in the alloy. Log Pri vs 1/T for
concentrations of 51, 67, and 100 ppm N'plot as, two straight lines
intersecting at about 1400°C while only a single straight line 18 ob-
tained when the nitrogen content exceeds 200 ppm. From the slopes of
the lines, sū values were calculated for each composition and are
tabulated in Table 3. The average value of sł was determined to be
-51.5 kcal/g-atom of nitrogen above 1400°C and when Xy, the amount of
nitrogen in solution was >200 ppm. Below 1400°C, sh was –34.2 kcal/g-atom
of nitrogen when the nitrogen content was 100 ppm or lower.
The equilibrium constant for the reaction 1/2 N2 + N (nitrogen ::
in solution) was calculated from the relationship* K = Xy/(Px) and
was found to be independent of the composition above 1400°C; however,
two values of K were obtained between 1200-1400°C as would be expected
from the values of að noted above. For the sake of clarity, the average
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PR. = torr, X = concentration, ppm.
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values for K for solutions containing between 31–100 ppm and for solu-
tions containing 205–278 ppm N are plotted versus 1/T in Fig. 5.
ORNE - AC - OR
DISCUESION
The coincidence of the experimental data with the solid lines
drawn at a slope of two in Fig. 3 shows that compositions in the range
31–278 ppm are solid solutions obeying Sievert's law, that 18,
X = K(Px). The deviation of the data shown in this figure as the
dashed curves initially suggested that concentrations in this range
were near the solubility limit since the temperature-composition-
pressure relationships were in the correct direction.
This early interpretation is unlikely because s normally changes
at a phase boundary. As shown in Fig. 4 and Table 3, was independent
of temperature at X > 200 ppm which are the concentrations that are
most likely to be in the two-phase region. Additional support for this
conclusion is based on the observation that sà for solutions containing
31 and >200 ppm N were the same within experimental scatter. The
nitrogen solubility in C6-0.86% Zr at 1400°C has been reported to be
about 5 at.% (-3750 ppm) (Ref. 5) and is in agreement with this analysis.
It appears therefore that the solutions above 1400°C and those con-
taining over 200 ppm n between 1200-1300°C exhibit the normal behavior
while the solutions with less than 100 ppm N exhibit an anomaly. This
is in spite of the solutions conforming to Sievert!.s law.
ORNI - AEC - OFFICIAL
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ORNI - AEC - OFFICIAL
ORNL - AEC - OFFICIAL
An analysis or other reported values for a may indicate why this
anomalous behavior is observed. Cost and Wert (3) determined that a
for columbium was 46.0 + 2 kcal/g-atom of nitrogen while de Lamotte et al.
(Ref.5) report 40.4 kcal/saotom of nitrogen. A more recent value of
(53.5 + 2) kcal has been reported for columbium by Pasternak (6). For
C6-0.86% Zr a value of 38.0 kcal/g-atom of nitrogen has been reported (5)
for a above 1600°C compared to 51.5 kcal/g-atom N in this investigation.
As shown in Table 4, the higher values of s for both columbium
and C6-1% Zr (53.5 and 51.5 kcal, respectively) were based on data ob-
tained in a baked ultrahigh vacuum system capable of attaining base
pressures approaching 10-10 torr while the lower A values were based
on measurements made in systems capable of ~ 10-5 to 10-6 torr. At
the latter pressures, both columbium and C6-1% Zr will be contaminated
with oxygen due to the residual gases in a vacuum system (7). Therefore,
the spread in ał above 1600°C of 7.5-13.5 kcal. may reflect the influence
of oxygen on the equilibrium nitrogen pressures for both columbium and
06-1% Zr. The observed difference for a below 1400°C in this investi-
gation was 17.3 kcal/g-atom nitrogen and couid also be due to the
influence of oxygen (67 ppm, see Table 1) in the alloy.
One mechanism by which the anomalous solution behavior in C6-1% Zr
may occur is indicated in Table 5. These data show that zirconium in
columbium sharply lowers Pro; or conversely, Pre over the alloy increases
in the absence of zirconium. Thus the removal of zirconium from solu-
tion (by association or combination with oxygen) could cause the observed
increase in Pot. below 1400°C, noted in Fig. 3. Since it appears that
the anomalous behavior also occurs in columbium, the removal of zirconium
from solution is not the only factor causing the anomaly.
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SUMMARY AND CONCLUSION
ORNL - AEC - OFFICIAL
An ultrahigh vacuum technique of high sensitivity was developeä
to measure the thermodynamic properties of solutions of nitrogen in
C6-1% Zr. In the composition range 31-278 ppm N, the solutions obey
Sievert's law.
An anomalous solution below 1400°C was attributed to an oxygen-
nitrogen interaction that appears to be dependent on Xy. This observa-
tion may explain in part why the mechanical properties of the alloy are
difficult to reproduce.
ACKNOWLEDGEMENT
The research described in this report was performed primarily by
S. H. Wheeler under the supervision of the author. The experimental
technique was inspired by the encouragement of R. E. Clausing.
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REFERENCES
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1. Dushman, S., Scientific Foundations of Vacuum Technique, p. 121,
John Wiley and Sons, Inc., New York, 1949.
2. Moore, B.C., J. Vac. Sci. Tech., 1(1), 10 (1964). .
3. Cost, J., and Wert, C. A., T.8A.M. Report No. 205, University
of Illinois (Dec. 1961).
4. Kostkowski, H. J. and Lee, R.D., Nat. Bur. Std. (U.s.) Monograph 41,
. (March 1962).
5. de Lamotte, E., Huang, Y. C., Altstetter, C. J., ML-TDR-64-134
(June 1964).
6. Pasternak, R. A., SRIA NO. 132 (Oct. 1964).
7. Inouye, H., ORNL-3674 (Sept. 1964).
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Table 1. Calculated vs Measured Nitrogen Content of Nb-1% Zra
Run
Total Nitrogen, ppm
Initial Nitrogen
in C6–1% Zr
(ppm)
Calculated
Nitrogen Added
(ppm)
Number
Calculated
Analyzed
32
69
100
-
100.50
28C
12.8
11.2
24
Analysis of degassed sample upon which reported data are based:
1.05 wt% Zr, 67 ppm 0, 40 ppm C, and 31 ppm N.
Average of 91 and 110 ppm.
Average of 27 and 29 ppm.
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ORNE - AEC - OFFICIAL
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Table 2. Activity Coefficient of Nitrogen in C6-1% Zr
Activity Coefficient, for
Temperature
(°C).
Kita)
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51 ppm N
other quimiother ry omi metodat mai bine crime there on there for interneto the
67 ppm N
100 ppm N
267 ppm N
1200
1223
2.42
2.34
1.86
1.04.
1250
terima o
789
474
1.88
1.53
1.03
1.05
1300
474
1.47
1.53
1.84
1.47
1.12
097
1.74
1.53
1.53
1.23
1350
263
1.17
1.02
1400
161
ordine Nations at the same as the laten
1.03
1.13
1.10
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11/25 "N2 in atmospheres, N, in atom-fraction.
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Table 3. Partial Molar Heat of Solution of
mismatchilarinas
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Witrogen in C1-1% Zr
Nominal YN
(ppm)
: A
(above 1400°C)
kcal/g-atom N
(below 1400°C)
kcal/g-atom N
49.2
46.7
52.7
33.4
33.4
52.7
1008
49.2
35.7
205
1
53.96
235
53.26
54.50
267
278
52.60
Average
.. 51.5 Average 1..2 34.2
Achemical analysis value.
valves for a between 1200–1800°C.
ORNL - AEC - OFFICIAL
ORNL - AEC - OFFICIAL
ORNIALC - OFFICIAL
Table 4. Summary of Values of s for Reaction
1/2 N2 (8) ► 1 (Cb and C5–1% Zr)
SĀ, kcal/g-atom N
Difference
Temperature
Metal
(°c)
.
in sł (c)
In ultrahigh
'Vacuum(a)
In High
Vacuum(b)
(kcal)
1100-1650
1525–2255
1600–2100
1600-2100
1400-1800
1200-1400
Cb
--53.5 1 2 (Ref. 6)
Cb
Cb
C6~0.86% Zr
C6–1.0% Zr -51.5(a)
C1-1.0% zi 134.2(a)
-46.0 (Ref. 3)
-40.4 (Ref. 5)
-38.0 (Ref. 5)
-7.5
-13.1
-13.5
Difference
Base pressures ~ 1 x 10-9 torr.
"Base pressures ~ 10-5 – 10-6 torr. .
Cvalues of sh in high vacuum subtracted from those of a in:
ultrahigh vacuum.
. This study.
is
Hier
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Table 5. Pressure-Temperature Composition
Relationships in Systems
N2/Cb and N2/Cb-1% Zr
Equilibrium Nitrogen
Temperature
Pressure, torr
(°c)
(ppm)
Columbium(a)
..^70
1200
1500
270
1650
~70
C1_1% zito)
5 x 10-10
3 x 10-8
3.8 x 10-7
1.6 x 10-9
6.6 x 10-7
6.6 x 10-6
1200
3 x 10-'(c)
2 x 10-6
1.6 x 10-5
3 x 10-8
2.3 x 10-5
2.* 10-4(c)
1 x 10-5
1500
1260
16260
1260
75
430(c)
1650
1600
1600
1 x 10-5
Pasternak, R. A., SRIA No. 132, (Oct. 31, 1964).
bThis study.
CEstimated.
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ORNL-AEC OFFICIAL
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1200°c]
0
400 800 1200 1600
'TIME (min) :
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TIME (min)

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ORNL-DWG 65-9072
1800°C
1750
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1650
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TEMPERATURE (°C)
1600 1400 1300
1200
10-5
1600
1550
4500
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SIEVERT'S LAW_
1450
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34 pprn
0 54 pprn
A 67 ppm
100 pprn
205 ppm
o 267 ppm
NITROGEN PRESSURE (torr)
NITROGEN PRESSURE (torr)
at 1250
1009
$ 1200
10-9
10-10
. 4.5
. 4.5
5.0
5.0
5.5
6.0
6.5
7.0
7.5
10,000/7.(K)
10000
10
100
1000
NITROGEN CONCENTRATION (ppm)
Fig. 4. Temperature Dependence of Equilibrium Nitrogen Pressure
lutions of Nitrogen in C6-1% Zr.
Isothermal Equilibrium for Solutions of Nitrogen in Cb
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ppm NITROGEN
2 (104)
CONCENTRATION
<100 ppm
Na CONCENTRATION
> 100 ppm
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4.5
5.0
6.5
7.0
7.5
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Fig. 5. Temperature Dependence of the Equilibrium Constant for
Solutions of Nitrogen
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