Note: Descriptions are shown in the official language in which they were submitted.
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Description
Nickel Base Superalloy
Technical Field
This invention relates to the field of nickel base
superalloys which have ~oth exceptional resistance to
oxidation and exceptional high temperature mechanical
properties.
Background Art
Previous investigators have worked with alloys
based on the Ni-Al-Mo system. This work is typified
by U.S. Patents Nos' 2,542,962, 3,655,462 and 3,933,483
which are assigned to the present assignee.
U.S. Patent ~o. 3,904,403 suggests th~ addition
of .1-3 atomic percent (total) of one or more elements
from a group which includes Cr, Ta, and W to the Ni-
Al-Mo type of alloys.
Disclosure of Invention
A class of nickel base superalloys is provided
with substantially enhanced oxidation resistance through
the addition of coordinated quantikies of Cr, Ta and Y.
Improved oxidation behavior is obtained without signifi-
cant detriment to mechanical properties.
The broad composition range is 5.8-7.~/o Al,
8-12% Mo, 4-~O W, 2-4% Cr, 1-2% Ta, 0-.3% Hf, .01-.1%
Y, balance essentially nickel. A preferred range is
6.3-7.3% Al,
8.5~11.5~ Mo, 5-7~ W, 2.5-3.5% Cr, 1-2% Ta, .05-.2% PAf,
.01-.~7% Y.
Alloys within these ranges may be fabricated into
useful articles using powder metallurgy techniques or may
be cast to size ~ then heat treated.
Accordingly, it is an object of this invention-~o
provide high strength oxidation resistant nickel base
superallvys.
The foregoing and other objPcts, features and
advantages of the present invention will become more
apparent f~om the following description of preferred
embodiments and accompanying drawings.
Brief Description of Drawings
Fig. 1 illustrates the effect of varying the yttrium
level on oxidation behavior.
Figs. 2A, 2B and 2C are scanning electron micrographs
illustrating the oxide morphology o~tained with various
yttrium levels.
Fig. 3 illustrates the effect of varying the chromium
level vn oxidation behavior at 2000F.
Fig. 4 illustrates the effect of varying the chromium
level on oxidation ~ehavior at 2100F.
Fig. 5 illustrates the stress rupture behavior of
several alloys.
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Best Mode For Carrying Out The Invention
The present invention relates to a nickel base
superalloy having a specific and narrow composition
range which provides an exceptional combination of
oxidation resistance and high temperature mechanical
properties.
The broad and preferred composition ranges are set
forth in Tables 1 and 2. The Tables are in weight
percent as are all other percentage values in this
application unless otherwise specified. Table 1 also
contains the equivalent values in atomic percent.
The particular combination of the Ni-Al-Mo constituents
is similar in some respects to that described in U.S.
Patent Nos. 2,542,962, 3,655,462, 3,904,403 and
3,933,483. Ni-Al-Mo alloys are known to have exceptio-
nal mechanical properties, however heretofore, their
surface stability and oxidation resistance have been
unpredictable and marginal for long term applications.
The heart of the present invention is the addition
of carefully coordinated quantities o~ Cr, Ta, Y and
optionally Hf to these ~i-Al-Mo alloys to dramatically
improve oxidation resistance while simultaneously main-
taining or improving mechanical properties.
Cr is added for oxidation resistance by promoting
the formation o~ an A1203 oxide rather than an oxide
based on NiO. For this purpose at least about 2% Cr
appears to be necessary~ Increasing the Cr level above
about 4% does not appear to provide substantial
improvements over those obtained with abou~ 3% Cr.
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Table 1
BROAD COMPOS ITION
Lc~w Hi gh
(wt %) (at g6)(wt %) (at 96)
Ni ~Bal) ~7~.20) (78.65) (66.1) (66.15)
~1 5.~ 12.8 7.8 17.3
~o 8.~ ~.712.0 7.8
W 4.0 1.~ 8.0 2.4
Cr 2.0 2.3 4.0 4.8
T~ 1.0 0,352.0 1.~
0.01 0.010.1 0.05
Hf 0.0 0.0 0.0 - 0.3
Table 2
PREFERRED COMPOSITION ~wt ~6 )
1~ Low ~igh
Ni Bal. Bal.
. 3 7. 3
MC) 8 . 5 11. 5
W 5.0 7.0
Cr 2.5 3
Ta 1.0 2.
Hf 0.0 0.2
0.01 0.7
. i
Since Cr concurrently reduces the mechanical properties,
Cr additions in excess of about ~ are undesirable.
Ta is added to stabilize the microstructure and Ta
in the levels indicated overcomes the mechanical
property deficit which results from the Cr additions.
Thus, the Cr and Ta levels are to a certain extent
related and optimum alloy performance will be obtained
by coordinating the Ta an2 Cr levels such that for high
Cr levels, high Ta levels are employed and for low
Cr levels, low Ta levels are employed.
At least one ~aterial selected from the group
consisting of Y and Y~f must also be added. Such elements
improve the adherence of the surfac~ oxide to superalloys,
thereby reducin~ spallation and mlnimized weight loss due
to oxidation~ It appears that .1 to .3 (total) weight
of these elements will perform the reguired function
with the preferred range being .02-.2 (total) and Y
preferably being present in an amount of at least
.01~.07%-
~isures 1, 2 and 3 will help to illustrate the
previously set forth element effects. The figures list
the allsy compositions tested and show the weisht change
during oxidation testins. It will be appreciated that
when an alloy oxidizes, it initially gains weight as
a result of the formation of an oxide layer. Sub
sequently, if this oxide layer spalls off, a weight loss
will result and the oxide layer will reform. Oxide
spallation and resultant weight loss are undesirable
since this results in the depletion of the oxide formins
3~ elements in the underlyin~ substrate. Oxide spallation
can proceed to the point where the allov is unable
to reform the desired pro.ective oxide layer to ,hat
what forms is a non-protective oxide laver. At
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this point, oxidation becomes increasingly rapid and
7mcontrolled and ev~ntually the s~ecimen will be
destroyed. As most alloys derive their oxidation
resistance from the form2tion of 2 protective oxide
layer, the desirable weight change behavior is an ini~ial
slight increase in weight indicating the formation of
a protective oxide layer followed by essentially no
weight change (or a ve~y slight increase).
The critical and unexpected result of yttri7~m addi-
tions are illustrated in Figure 1. This figure shows
tne weight loss experienced by several alloys with
differing yttxi7~m levels, after cyclic tQsting at
2200~ for 50 one hour cycles. It is apparent that
for the base alloy tested (10% Mo, 6.7% Al, 6% W,
3% Cr, 1.5% Ta, 1% Hf bal ~i) additions of from about
.01 to about .06% Y produce a remzrkable improvement in
oxidation behavior. Although it has been previously
observed ~hat Y can improve the oxidation performance of
coatings (U.S. Patents 3,676,085 and 3,75G,903) and
alloys (U.S. Patent 3,75~,303 ) it has never before to
our knowledge been shown that Y levels in excess of
about .1% were harmful.
The results shown in ~gure 1 may be explained throuyh
reference to Figures 2A, 2B, an~ 2C which are SEM photos
2i (at3,000 X) of the ~ e~ sw~ace of three samples.
The nominal`sample composition is that sho~ in Fig. 2.
Figure ~A is of a sample containing .1% Hf and less than
.002~ Y. Figure 2B is of a sample containing .1~ Hf and
.029% Y. Figure 2C is of a sample contzining .1% Hf
and .073% Y.
Figures 2A and 2C both show a rough irreaular oxide
morphclogy and sho-.7 evidence of oxide spalla~ion while
Fig. 2B shows ~vidence of an adherent oxide ~orpholosy.
7 _
Thus, Figures 1, ~A, 2B and 2C clearly show that a
limited critical amount of Y produces a substantial
improvement in oxidation behavior.
Figures 3 and 4 illustrate that a critical ch.romium
level is necessary for optimum oY.idation resistance.
Figure 3 shows the effect of varying Cr content on the
oxidation behavior of a base 2110y containing 10% Mo,
7.4% Al, 6~ W, 1.~% Ta, .1% Y bal Ni. It can be seen
that under the test conditions (500 one hour cycles of
furnace oxidation at 2000f) the desired m;n;m~l weight
change is obtained with Cr levels of about 3%.
Figure 4 makes the same point using cyclic oxidation
data generated at 2100F. The figure shows the change
in weight as a function of time in test. Four curves are
plotted for a base alloy containing 10% Mo, 6.6~ Al,
1.5% Ta, .1% Y ~al N~ (with varying Cr levels). The effect
o~ increasing the Cr is to rotate the curves up towards
the horizontal ~or zero weight change).
~igures 3 and 4 illustrate that a level of Cr of
about 3~ is necessary to provide good oxidation behavior
in this class o~ alloys.
The mechanical properties of the Al-Mo alloys have
been shown in prior work to be superior, in most respects,
to those of conventional superalloys. The present
invention, balanced additions of Cr, Ta, Y and/or Hf
achieve substantially improved oxidation behavior in
combination with mechanical properties which are at least
equi~alent and in some cases superior to the properties
- of the baseline Al-Mo~Ni alloys. This is a marked
contrast to typical alloys in ~hich impro~ement of one
property is invariably accompanied by a decrease in
other prope~ties.
Figure 5 is a stress rupture plot ~or several
alloys including the previously described l~R-M200
conventional superalloy and an alloy falling within
the scope of the present invention. The data in
~igure 5 is for stress ru~ture properties o the
various compositions tested in single crystal form in
the <111> orientation~ As can be seen in the figure,
the modified Ni-Al-Mo composition has an improved
stress rupture life when compared with the other alloys
tested. It appears that the modified alloy has about
a 190~ te~perature improvement when compared with the
conventional superalloys. This means that under
eguivalent conditions of stress, the invention alloy
could be operated at 190~ higher temperature and still
achieve the same part life. This high temperature could
be the result of the higher engine operating temperature
or reduced flow of cooling air if the engine temperature
were unchanged. Both of th se alternatives give enhanced
economy. Another possibility is to maintain the operating
conditions including temperature at the same level and
obtain a substantially increased part life. Finally,
one could maintain the same temperature, but by increasing
the operating stress obtain increased performance for
the same fuel consumption and part life.
The pxeviously described compositions may be used
in cast single crystal form or alternately, can be fabri-
cated into parts using po-~der metallurgical techniques
followed by direction2lly recrystallization to achieve
an aligned grain structure ~hich in the limiting case
3Q may be a single crystal.
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II1 the event that the cast single crystal route is
pursued, it is necessary that the cast part be
homogenized and heat treated. If the part is to be
fabricated throuyh the powder metallurgical approach,
the composition may be formed into powder by several
techniques although a technique resulting in a r~pid
solidi~ication rate is desirable because of the enhanced
homogeneity which results. Such a process is described
in U.S. Patent Nos. 4,025,249, 4,053,264 and 4,078,873.
The resultant powder is then consolidated and directio-
nally recrystallized to produce the desired structure.
Directional recrystallization is descrihed in U.S.
Patent No. 3,975,219.
The resultant products find particular utility in
gas turbine engines_ If the casting approach is pursued,
a casting may be produced directly to the desirea size.
~owever, if the powder metallurgical approach is
pursued, the blade fabrication technique described in
U.S. Patent No. 3,872,563 may be advantageously
followed in order to arrive at a blade having the maxi-
mum cooliny capability. Although the compositions
described herein are exceptionally oxidation resistant
they will undoubtedly be used in coated form and such
coatings may comprise the aluminide coating or the
MCrAlY type overlay coatings.
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Although this invention has been shown and
described with respect to a preferred embodiment, it
will be understood by those skilled in .he art that
various changes in form and detail ther20f may be made
without departing from the spirit and scope of the
claimed invention.
