Ti-Al-Nb合金

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Intermetallics 3 (1995) 351-363
01995 Elsevier Science Limited
Printed in Great Britain. All rights reserved
0966-979595$$09.50
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.
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Eflects of Nb addition on oxidation behavior of TiAl
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