Ti-Al-Nb合金
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Intermetallics 3 (1995) 351-363
01995
Elsevier Science Limited
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ELSEVIER
Effects of Nb addition on
oxidation
behavior of TiAl
M. Yoshihara* &
K. Miura
Department of Mechanical Engineering
and Materials Science, Yokohama National
University, 156 Tokiwadai,
Hodogaya-ku,
Yokohama 240, Japan
(Received 1 September
1994; accepted 19 October 1994)
Improvement of
oxidation behavior of TiAl due to Nb addition has
been inves-
tigated. For TiAl fixed at
SOat%Al, the optimum content of Nb was found to be
in the vicinity of IOat%. The effect of Nb on
rutile growth has been studied for
Ti-Nb
alloys in order to estimate the role of Nb in
TiAl. For Ti-Nb alloys, rutile
growth was
remarkably suppressed when Ti was substituted by
I-lOat% of Nb.
However, higher Nb content
yielded TiNbz07 formation and resulted in larger
mass gain due to oxidation. It is difficult to
interpret the best oxidation behavior
found in
TiAl containing lOat%Nb only in terms of rutile
growth suppression,
because Nb content of this
alloy is supposed to be large enough to form
TiNb20T. It is suggested that an external
alumina scale formed in the initial stage
of
oxidation serves to suppress further oxidation of
the alloy.
Key words:
high temperature
oxidation, TiAl, Nb addition, rutile growth, Ti-Nb
alloy.
1 INTRODUCTION
The
intermetallic compound TiAl is expected to be
a new lightweight heat-resisting material in
such
applications as aerospace and automobile
engine
components.’ Unfortunately, its
oxidation resis-
tance at high temperatures is
considerably inferior
to that of conventional
superalloys. Improvement
of oxidation
resistance and understanding of its
mechanisms
are important in practical applications
of
TiAl. In view of third element addition, niobium
and several other elements have been reported
to
be effective in improving the oxidation
resistance of
TiA1.2-9
The poor oxidation
resistance of TiAl has been
attributed to the
deficiency of external alumina
scales and
undesired rapid growth of rutile. The
criterion for the transition from internal to
external
oxidation has been given by Wagner.”
According
to his model, a failure to form
external alumina
scales on TiAl results from
the high oxygen solu-
bility (No), high oxygen
diffusivity (Do) and low
aluminium diffusivity
(D,&. Therefore, one approach
for improvement
is to modify these factors. The
formation of
external alumina scales on Ti-Al
l
To whom
correspondence should be addressed.
357
alloys containing large amounts of chromium or
vanadium has been achieved by increasing DA1
and
simultaneously decreasing No and Do
through
retention of &phase.”
However,
this approach
appears difficult for y-TiAl
because of lower DA1 in
y-phase than in
P-phase.
The second approach for improvement
is the
suppression of rutile growth that
accounts for
most of the mass gain of TiAl due
to oxidation.
Improvement of oxidation
resistance in phosphorus-
doped TiAl has been
interpreted in this argument.6
The rutile
grows mainly by oxygen diffusionI
through
oxide scales via a vacancy mechanism.
Since
the oxygen vacancies are regarded as major
defects in rutile,13 a dopant element with a
higher
valence than titanium is expected to
decrease the
oxygen vacancy concentration
owing to the elec-
troneutrality in the oxide,
and thus to suppress
rutile growth. Niobium is
supposed to function as
such a dopant element,
because it is a Va group
element and is
presumed to have a higher valence
than
titanium. In fact, it has been reported that the
addition of a small amount of niobium
decreases
the weight gain of titanium during
oxidation.14
Hence, improvement of oxidation
resistance of
TiAl due to niobium addition may
possibly be
explained in terms of rutile
growth suppression.3T9
358
M.
Yoshihara, K. Miura
There is no direct
evidence, however, if the
improved oxidation
resistance by niobium addition
can be
attributed entirely to suppression of rutile
growth.
The purpose of this paper is to
investigate the
oxidation behavior of y-TiAl
containing varying
amounts of niobium up to
20at%, and to discuss
the extent to which the
rutile growth suppression
contributes to the
improvement of oxidation resis-
tance. The
oxidation of Ti-Nb binary alloys with
niobium
content up to 30at% has also been studied
in
order to evaluate directly the effect of niobium
on rutile growth.
2 EXPERIMENTAL PROCEDURE
The TiAl alloys used in the present study had
a
fixed aluminium content at 50% and varying
nio-
bium content up to 20% (all in at%).
According to
a Ti-Al-Nb phase diagram,15 these
alloys are well
inside the y-phase field. They
are designated as
SOAl-Nb alloys hereafter. In
addition to Ti-Al-Nb
ternary alloys, Ti-Nb
binary alloys containing up
to 30% niobium
were also made. The ingots were
prepared by
non-consumable electrode arc melting
in an
argon gas atmosphere. The 50Al-
Nb and Ti-Nb
alloys were homogenized in
vacuum at 1000°C
for 168 and 50 h, respectively.
The specimens
for oxidation tests, about 10 x 5 x
1.5 mm3 in
size, were cut out from the ingots,
polished
with emery papers up to #lOOO, and
cleaned
ultrasonically in acetone to remove grease.
The cyclic oxidation tests were carried out in
static air in the temperature range between
900 and
1000°C. The specimens were put in a
furnace pre-
heated to the test temperature,
held for 5 h in each
cycle, and then cooled
down to room temperature
in air. The mass gain
due to oxidation was mea-
sured including
spalled oxide scales. The iso-
thermal
oxidation tests were performed for
observing
the scales on SOAl-Nb alloys. The iso-
thermal
oxidation was also employed to examine
the
effect of niobium on rutile growth for Ti and
Ti-Nb alloys, and to examine kinetics of
rutile
growth for the alloys with zirconia
powder mar-
kers.
The metallographic
examinations were per-
formed by X-ray
diffraction (XRD) and an elec-
tron probe
microanalyzer (EPMA). Energy
dispersive X-ray
spectroscopy (EDX) was also used
to examine
the oxides in detail. The specimens for
EDX
analysis were fine oxide particles prepared by
crushing the flaked-off oxide scales.
10;
0 9ooc
A 9507-Z
v
1oooc
A
3
M
vV
9
.r(
c
10:
- v
8
V
V
cl
1
tA
A
V
0
0 A
10
A
0
08
I I
I
I
I
5 10 15 20
Nb content (at%)
Fig. 1.
Mass gain of
SOAl-Nb alloys after 100 h of cumulative
5 h
cyclic oxidation.
3 RESULTS
Figure 1 shows
the mass gain of SOAl-Nb alloys
after 100 h of
cumulative 5 h cyclic oxidation. The
mass gain
due to oxidation decreased significantly
with
increasing Nb content up to about lo%,
indicating the improved oxidation resistance
of
TiAl. An excess quantity of Nb reduced the
extent
of the improvement, and the optimum
content of
Nb was found to be in the vicinity
of 10%. This
agrees well with the observations
by Chen
et aL8
Figure 2 shows typical
examples of the oxide
scales formed on SOAl-Nb
alloys after isothermal
oxidation at 900°C for
25 h. The morphology of
the oxide scales
containing a small amount of Nb
was similar to
that formed on binary TiAl. The
oxide scales
became thinner and more adherent to
the matrix
with increasing Nb content up to 10%.
The
scales formed on SOAl-1ONb were so thin that
detailed examinations of the scale structure
were
difficult. The scales formed on the
alloys containing
more than 15% Nb were
relatively thick and
showed a tendency to
spa11 off. The scales formed
on 50Al-20Nb
contained many cracks and were
comparable in
thickness to those formed on binary
TiAl.
The oxides formed on the alloys containing up
to 10% Nb were identified as a-alumina and
rutile
by XRD, being identical to those formed
on binary
TiAl. For the alloys containing more
than 15%
Nb, additional peaks were detected.
They are due
Eflects of Nb addition on
oxidation behavior of TiAl
359
Oxide
scale
Ti,Al
-
Matrix
Fig. 2.
Oxide scales formed during isothermal oxidation at
900°C for 25 h: (a) TiAl (SOAl); (b) 50A142Nb;
(d) SOAl-1ONb; (e) 50Al-20Nb.
(c)
50A1-2Nb;
to TiNb207, judging from the
structural data by
Wadsleyi6
and a
TiOTNb205 phase diagram,17
though a small
amount of Nb205 may be present.
Formation of
nitrides was not confirmed. The
formation of
TiNb207 results in relatively large
mass gain
of the alloys, as shown in Fig. 1. EPMA
analysis for 50Al-20Nb showed an almost
uniform
distribution of Al, Ti and Nb
throughout the oxide
scales, so that the oxide
scales formed on 50Al-
20Nb should be a
mixture of fine alumina, rutile
and TiNb207.
EDX analysis showed that the oxide scales were
composed of two types of oxides both in
50Al-5Nb
and 50Al-20Nb; alumina and Ti-rich
oxide (rutile
or TiNbZ07). Nb was found only
in the T&rich
oxides in the form of a cation
with valence 5, while
it was not detected in
alumina. The average ratio of
Nb5+ to Ti4+ in
the oxide was very close to the Nb
93
23
3
aoo
0
0
@lo -
0
I I
I
10 20 30
Nb content (at%)
Fig. 3.
Mass gain of Ti-Nb alloys isothermally oxidized at
900°C for 5 h.
to Ti ratio of the alloys.
On the basis of the Ti02-
Nb205 phase diagrami
and EDX results, it is
concluded that Nb’+
substitutes for Ti4+ in rutile.
Figure 3 shows
the mass gain of Ti-Nb alloys
oxidized at
900°C for 5 h. The mass gain due to
oxidation
of the alloys containing 1 - 10% Nb was
remarkably smaller than that of Ti. When Nb
content exceeded 15%, the mass gain rather
started
to increase. While only rutile was
identified by
XRD for the alloys containing Nb
less than lo%,
TiNb207 was detected in
addition to rutile for the
composition over
15%Nb. It is apparent from Fig.
3 that TiNb207
grows faster than rutile. These
observations
indicate that rutile growth is remark-
ably
suppressed by Nb addition, and that TiNb207
is
formed when Nb is added beyond the solubility
limit of Nb205 in TiOz.
Although the
accurate solubility of Nb in rutile
has not
been well established, the present study
enables us to make a rough estimate of the
solubi-
lity. According to the Ti02-Nb205
phase dia-
gram, l7 the region of rutile solid
solution extends
to 18mol% Nbz05 at 1470°C and
the solubility
decreases as the temperature
decreases. From the
present experimental
results for Ti-Nb alloys, the
maximum ratio of
Nb to Ti in rutile is found to be
slightly
less than 1585 at 9OO”C, or the solubility
of
Nb205 in rutile at 900°C being slightly less than
8mol%.
Figure 4 shows typical examples of
the oxide
scales formed on Ti and Ti-Nb alloys
at 900°C for
2 h. A large crack between the
oxide scale and the
matrix observed in Ti was
induced during sample
preparation for EPMA.
The zirconia markers were
found on the surface
of the oxide scales, and were
identified in
Fig. 4 as particles of bright image in
360
44. Yoshihara, K. Miura
Marker
Oxide
scale
Crack
Matrix
Fig. 4.
Oxide scales formed
during isothermal oxidation at 900°C for 2 h: (a)
Ti; (b) Ti-1Nb; (c) Ti-1ONb; (d) Ti-30Nb.
both
Ti and Ti-Nb alloys except in Ti-1Nb. These
observations indicate that the rutile growth
is gov-
erned by inward oxygen diffusion, in
agreement
with the previous investigation.12
The same argu-
ment stands even if Nb
substitutes for Ti in rutile.
It should be
noted that Zr was detected only within
marker
particles according to EPMA and EDX
analysis,
despite the fact that a Ti02-Zr02 phase
diagram shows the solubility of Zr02 in
Ti02.i8
This means that Zr scarcely dissolves
into rutile or
the matrix during the marker
experiments and that
zirconia powder functions
properly as markers. It
should also be noted
that zirconia is a stable oxide
and its
dissociation pressure is very 10w.i~ In any
event, rutile growth was not affected by the
markers.
Figure 5 shows the scale thickness
estimated
from the microscopic observations of
the Ti-Nb
alloys oxidized at 900°C for 5 h. It
can be seen
Nb content
Fig. 5.
(at%)
Oxide scale thickness formed on Ti-Nb alloys
iso-
thermally oxidized at 900°C for 5 h.
from Figs 4 and 5 that the oxide scales formed
on
Ti were thick and contained many cracks and
por-
osities. The scales formed on Ti-Nb
alloys with Nb
content of 1 - 10% were thinner
and denser, but
they still contained some
porosities. The scales of
Ti-15Nb grew
abruptly after a certain incubation
period.
These observations further support the
results
of the mass gain measurements.
Comparing the
observed thickness of the scales
with that
calculated from the weight change during
oxidation, the observed value turned out to be
smaller than the calculated. This suggests
that the
mass gain during oxidation is caused
not only by
scale growth but also by oxygen
dissolution into
the matrix. The solubility of
oxygen in the matrix
may be roughly estimated
from the difference
between observed and
calculated scale thickness.
The oxygen
solubility so calculated was found to
be
smaller in Ti-Nb alloys than in Ti. A major
inaccuracy arises from the scale thickness
mea-
surements because of the presence of
cracks and
porosities in the scale.
Nevertheless, such calcula-
tions imply that
the dissolution of oxygen into the
matrix may
be suppressed by Nb addition. Ti and
Ti-Nb
alloys are in a b-phase region at the
experimental temperature of 900”C.20 However,
Ti
transforms to a-phase at 885”C, just below
the
experimental temperature, and the
transformation
temperature decreases with
increasing Nb content.
As the solubility of
oxygen is much higher in a-Ti
than in 3-Ti,21
the observed
difference of oxygen
solubility between Ti and Ti-Nb may be
influenced
to some extent by the phase
transformation of the
alloys during cooling.
In addition, the morphology
of the scale may
also affect the results. Further
investigations are necessary.
Effects of Nb addition on oxidation
behavior of TiAl
361
4 DISCUSSION
It
is apparent from experimental results that Nb
addition suppresses rutile growth. Since it
has been
pointed out that the oxidation
resistance of TiAl is
expected to be improved
either by suppression of
rutile growth3,9
or by enhancement of external
alumina
layer formation,3’7’8 the contribution of
rutile growth suppression will be discussed in
detail.
Rutile is known as a non-
stoichiometric com-
pound and is often
expressed as TiOz_x. According
to Kofstad,i3
the defect structure in rutile involves
both oxygen vacancies and tri- and tetra-
valent
interstitial Ti cations:
oxygen
vacancies pre-
dominate at low temperatures
and high oxygen
pressures, while the
interstitial cations predominate
at high
temperatures and low oxygen pressures.
Therefore, the major defects in rutile formed
dur-
ing oxidation in static air at 900-1000°C
are pre-
sumed to be oxygen vacancies that
play a role in
oxygen diffusion. This is
consistent with the results
of marker
experiments, which showed that the
rutile
growth is governed by inward oxygen diffu-
sion.
The concentration of the defects in
rutile is
affected by impurities, as has been
pointed out
previously. 12,i3 Under the
assumption that the
defects in rutile are
entirely the doubly charged
oxygen vacancies,
the effects of impurities can be
estimated as
follows. When the foreign metal
cations with
higher valence than Ti occupy normal
Ti-sites,
the electroneutrality condition is given by
(e’) = (M&i) + 2( Vi)
where (M&i) denotes
concentration of Me5+ for-
eign cation on a
normal Ti-site, (Vi) an oxygen
vacancy and
(e’) an electron. The left-hand side of
the
equation represents the total negative charges
and the right-hand side the total positive
charges.
The substitution of two foreign metal
cations with
valence 5 accordingly reduces one
oxygen vacancy
and suppresses the rutile
growth, assuming that the
concentration of
electrons is constant. Since the
EDX results
indicate that Nb’+ substitutes for
Ti4+ in
rutile, suppression
of rutile growth due to
Nb addition can be interpreted qualitatively
in
terms of the reduced oxygen vacancy
concentra-
tion.
According to a Ti-Ti02
phase diagram,21 con-
centration of oxygen
vacancy, X, is about 0.02.
Hence, according to
the simplified model described
above, rutile
formed on Ti-Nb alloy with Nb con-
centration
of 0.04 (Ti4Nb) should be free from
oxygen
vacancy, and its growth rate is expected to
be
at the minimum. The experimental results show
that the growth of rutile is considerably
suppressed
in Ti-Nb alloys with Nb content
ranging from 1 to
10%. The extended region of
Nb content as com-
pared with the theoretical
estimation is not sur-
prising because of the
over-simplification of the
model. The
diffusivity of oxygen in t-utile, however,
may
also be affected by other factors such as size
difference between cations. In the present
case, the
size effect appears to be trivial;
ion radius being
O-064 nm for Ti4+ and O-069
nm for Nb5+.
Let us now discuss the oxidation
behavior of
SOAl-Nb alloys in the viewpoint of
rutile growth
suppression confirmed in Ti-Nb
alloys. As Nb
substitutes preferentially for
Ti in TiA1,22 Nb con-
tent in SOAl-Nb alloys
is equivalent to twice the
Nb content in Ti-Nb
alloys, noting the Nb to Ti
ratio in the
alloy. If the rutile growth suppression is
the
primary factor for the improvement of oxida-
tion resistance of TiAl, the oxidation
resistance of
SOAl-Nb alloys should be
improved for Nb con-
tent of the range from
0.5 to 5%, because this
range is equivalent to
Nb content from 1 to 10% in
Ti-Nb alloys. Let
us next compare these estimates
with the
observed oxidation behavior of SOAl-Nb
alloys.
The mass gain due to oxidation decreased
with
increasing Nb content up to lo%, or equiva-
lent to Nb content up to 20% in Ti-Nb alloy.
Hence, there exists a discrepancy between the
esti-
mated and experimental results about the
range of
Nb concentration for the improved
oxidation
resistance. For 50Al-1ONb which
showed the best
oxidation resistance, the
scale was thin, and
TiNb207 was not detected
by XRD in spite of Nb
content being considered
to be large enough for
TiNb207 formation.
These facts suggest that it is
difficult to
interpret the improvement of oxidation
behavior of TiAl due to Nb addition only in
terms
of rutile growth suppression. The
contribution of
the rutile growth suppression
is limited to a certain
extent.
The
improved oxidation resistance can also be
attained by enhanced formation of external
alu-
mina scale. Provided that Nb promotes an
external
alumina scale formation, higher Nb
content will
result in greater effects in
improving the oxidation
resistance in TiAl.
This view agrees with the
experimental
results, as is seen in Fig. 1. When an
external alumina scale is formed on the alloy
in the
initial stage of oxidation, further
oxidation will be
suppressed by lower
diffusivity of the constituent
species in
protective alumina scale. If Nb content is
362
M. Yoshihara, K. Miura
slightly higher than the solubility in rutile,
such as
the case of SOAl-lONb, TiNb207
probably forms
during the very initial stage
of oxidation, but its
amount is expected to be
very small. However,
when Nb content in SOAl-
Nb alloys increases to
over 15%, considerable
amount of TiNbz07 for-
mation cannot be
avoided, resulting in deteriora-
tion in the
quality of alumina scale.
Enhancement of an
external alumina scale for-
mation due to Nb
addition can be explained either
by Wagner
model” or by the variation of activities
of Ti
and Al in the Ti-Al system.3’7 By the same
argument for Ti-Nb alloys in which the
solubility
of oxygen is reduced by Nb
addition, it is possible
that N, in TiAl
decreases by Nb addition. Unfor-
tunately,
there are no data available for the factors
such as D,, DA1 or N, in y-TiAl, and hence
further
discussion based on Wagner’s model is
mean-
ingless. Concerning the variation of
activity ratio,
Choudhury
et cd3
have
suggested that enhanced
alumina formation in
the Nb-containing alloys is
attributed to the
increased Al activity by quoting
the activity
data available at much higher tem-
peratures.
Becker
et al.7
have discussed the poss-
ibility for improvement of oxidation
resistance of
TiAl due to the activity
variation of Al and Ti in
TiAl, but have
pointed out that the activity ratio
(ariaA,)
is not altered significantly by Nb addition
either in arphase or in y-phase. Because of
lack of
necessary data, further discussion is
difficult con-
cerning the mechanism of
enhancement of the
external alumina scale
formation on TiAl by Nb
addition.
5
CONCLUSIONS
The effect of niobium addition up
to 20at% on
oxidation behavior of y-TiAl with
aluminium con-
tent fixed at 50at% was
investigated in the tem-
perature range
between 900 and 1000°C in air. The
effect of
niobium on the growth rate of rutile was
also
studied for Ti-Nb alloys with niobium content
up to 30at% in order to better understand the
role
of Nb. Results are summarized as follows:
1. The oxidation behavior of y-TiAl is sig-
nificantly improved by the Nb addition. The
optimum content of Nb is in the vicinity of
lOat%, and the oxides formed on the alloy
are alumina and rutile. When the Nb content
is higher than about 15at%, TiNbz07 is
formed resulting in reduced effects.
2. Nb
substitutes for Ti in rutile as a cation with
valence 5, while Nb is not found in alumina.
3.
The growth rate of rutile in Ti-Nb
alloys is
remarkably reduced by Nb
substitution of 1 -
lOat% for Ti. When the Nb
content exceeds
the solubility limit in
rutile, TiNbz07 is formed
resulting in larger
mass gain of the alloy. The
solubility of
NbzOs in TiOz at 900°C is esti-
mated to be
slightly less than 8mol%.
4.
Suppression
of rutile growth partially con-
tributes to
the improvement of oxidation
resistance of
TiAl.
ACKNOWLEDGEMENT
The present work was
supported in part by a
Grant-in-Aid for
Scientific Research on Priority
Areas on
Intermetallic Compounds as New High
Temperature Structural Materials given by the
Ministry of Education, Science and Culture,
Japan.
REFERENCES
1.
Kim, Y-W., in
Proc. 3rd Japan Znt. SAMPE Symp. on
Advanced Materials,
eds T. Kishi, N.
Takeda & Y.
Kagawa, Japan Chapter of SAMPE,
Tokyo, Japan, 1993,
p. 1310.
2.
McAndrew, J. B. & Kessler, H. D.,
J.
of
Metals,
8 (1956)
1348.
3.
Choudhury, N. S., Graham, H. C. & Hinze, J.
W., in
Proc. Symp. on Properties of High
Temperature Alloys,
eds
Z. A. Foroulis &
F. S. Petit, Electrochemical Society,
Princeton, NJ, USA, 1976, p. 668.
4.
Kasahara, K., Hashimoto, K., Doi, H. &
Tsujimoto, T.,
J. Japan Inst. Metals
(in
Japanese), 54 (1990) 948.
5.
Anada, H. &
Shida Y., in
Proc. Znt. Symp. on Zntermetallic
Compounds (JZMZSd),
ed. 0. Izumi, Japan
Inst. Metals,
Sendai, Japan, 1991, p. 731.
6. Ikematsu, Y., Hanamura, T., Morikawa, H.,
Tanino, M.
& Takamura, J., in
Proc. Znt.
Symp. on Zntermetallic
Compounds (JZMZSd),
ed. 0. Izumi, Japan Inst. Metals,
Sendai,
Japan, 1991, p.191.
7.
Becker, S., Rahmel,
A., Schorr, M., & Schtitze, M.,
Oxi-
dation of Metals, 38
(1992)
425.
8. Chen, G., Sun, Z. & Zhou, X.,
Mater.
Sci.
Engng, AI52
(1992) 597.
9.
Yoshihara, M., Imamura, N., Kobayashi, E., Miura,
K.,
Mishima, Y., Suzuki, T., Tanaka, R.,
J. Japan Inst.
Metals
(in Japanese),
57 (1993) 574.
10.
Wagner, C., Z.
Elektrochem., 63
(1959) 772.
11.
Perkins, R. A., Chiang, K. T. & Meier, G. H.,
Scripta
Metall., 21 (1987) 1505.
12.
Hauffe, K., in
Oxidation of Metals,
Plenum
Press, New
York, USA, 1965, p. 217.
13.
Kofstad, P.,
J. Less-Common Metals, 13 (1967)
635.
14. Chen, Y. S. & Rosa, C. J.,
Oxidation of Metals, 14 (1980)
147.
15.
Perepezko, J. H., Chang, Y. A.,
Seizman, L. E., Lin, J. C.,
Bonda, N. R.,
Jewett, T. J. & Mishurda, J. C., in
Proc.
Symp. on High Temperature Aluminides and
Zntermetallics,
eds S. H. Whang, C. T. Lium D.
P. Pope & J. 0. Steigler,
TMS, Warrendale, PA,
USA, 1990, p. 19.
16. Wadsley, A. D.,
Acta
Cryst.,
14 (1961) 660.
17. Roth, R. S.,
Prog. Solid State Chem., 13
(1980) 159.
Eflects of Nb addition on oxidation
behavior of TiAl
363
18. Shevchenko, A.
V., Lopato, L. M., Maister, I. M. &
Gorbunov,
0. S.,
Zh. Neorg. Khim., 25 (1980) 2469; Russ.
J.
Znorg. Chem.
(Engl. Transl.), 25
(1980) 1379.
19. Elliot, J. F. & Gleiser, M.,
in
Thermochemistry for Steel-
making,
Vol. I, Addison-Wesley, Massachusetts, USA,
1960.
20. Hansen, M. & Anderko, K., in
Constitution of Binary
Alloys,
2nd
edition, McGraw-Hill, New York, USA, 1958.
21.
Wahlbeck, P. G. & Gilles, P. W., .J.
.4m.
Ceram. Sot., 49
(1966)
181.
22. Chen,
G. L., Wang, J. G., Sun, Z. Q. & Ye, H. Q.,
Znter-
metallics, 2
(1994) 3 1.