Note: Descriptions are shown in the official language in which they were submitted.
13398t 1
High Strength Corrosion Resistant
Nickel Base Single Crystal Article
Technical Field
This invention relates to the field of nickel
base superalloy articles such as gas turbine compo-
nents which are both resistant to corrosion and are
capable of operating at high temperatures. This in-
vention also relates to the field of single crystal
superalloy articles.
Background Art
Increasing demands for efficiency in gas turbine
engines have resulted in d~m~nfl.s for materials capable
of withstanding more severe operating conditions. In
particular, increased temperature capabilities are
required for certain applications along with resistance
to corrosion.
U.S. patent 3,494,709 describes the fabrication
of gas turbine components in single crystal form for
improved performance. U.S. patent 4,116,723 describes
a heat treatment applicable to single crystal super-
alloy articles for the purpose of enhanced properties.
U.S. patent 3,619,182 assigned to International Nickel
Corporation, describes a moderate strength superalloy
having superior corrosion resistance.
~H-7499
~ 3398 1
Disclosure of Invention
Alloys in the composition range 9.5-14% Cr,
7-11% Co, 1-2.5% Mo, 3-6% W, 3-6% Ta, 3-4% Al, 3-5%
Ti balance essentially nickel are provided with im-
proved mechanical properties through fabrication ofthe alloy in single crystal form. The resultant
single crystal article is then preferably heat treated.
Heat treated single crystal alloys of this composition
display at least a 10X improvement in rupture life
in comparison to alloys of similar composition which
have been conventionally solidified to produce a
equiaxed polycrystalline structure.
Other features and advantages will be apparent
from the specification and claims which illustrate
an embodiment of the invention.
Best Mode For Carrying Out The Invention
This invention had its origin in a surprising
and unexpected observation. A series of commercial
superalloys were evaluated in three different forms.
Samples were prepared in polycrystalline (conventio-
nally cast) form, columnar grain (directionally soli-
dified) form and single crystal (directionally soli-
dified) form. The alloys tested were MAR-M200,
MAR-M247, IN 939 and IN 792 (trade marks). The first
two alloys are proprietary alloys produced under
license from the Martin Metals Corporation and the
latter two alloys are proprietary alloys supplied by
the International Nickel Corporation. The compo-
~33981 1
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sition of these alloys as tested is given in Table 1.
Through reference to Table 1 it can be seen that the
conventionally cast materials contained the grain
boundary strengtheners carbon, boron, and zirconium
as did all the columnar grain materials. Most of the
columnar grained samples also contained additions of
hafnium for improved transverse ductility. Most of
the single crystal samples did not contain any of
the elements, carbon, boron, zirconium or hafnium.
The cast alloy samples were heat treated as
described in Table 2 and the heat treatment described
therein are conventional heat treatments which are
typical of those which would likely be selected by
one skilled in the art.
These alloy samples were creep tested under
different conditions of load and temperature with
the results shown in Table 3. Table 3 shows the sur-
prising and unexpected benefits which derive from
the fabrication of the modified IN 792 alloy in single
crystal form. For the MAR-M200, ~R-M247
and IN 939 alloys the ratio of rupture life of the
single crystal samples to the conventional cast
samples averaged 4.1X, however for the case of the
IN 792, the ratio of single crystal rupture life to
25 conventionally cast rupture life was more than 17X
(average o~ 1600~F/50 ksi and 1800~F/27 ksi tests)
Tnis degree of improvement is surprising and un-
expected. It thus appears that the improvement (in
creep rupture life) obtained by fabricating (modified)
IN 792 in single crystal form is about 370~ greater
than the benefit one would predict based on the
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t33981 1
evidence of other superalloys. For purposes of de-
fining the invention results, it appears that a
minimum lOX improvement in creep rupture life will be
obtained.
A similar conclusion is reached when one con-
siders the time to 1% creep. In this case, based
on the two MAR M alloys, one would expect an average
improvement of 5.4X by changing the test sample
macrostructure from polycrystalline to single crystal
(in concert with minor composition and heat treatment
changes). In fact, making this change to the modified
IN 792 alloy results in an average benefit of about
12.6X. Again, this is a disproportionate and unexpec-
ted improvement and not predictable from the prior
knowledge in the art.
The substantial improvement in creep properties
is rendered more significant because the invention
composition is slightly less dense than the other
alloys evaluated. Further, the notable resistance
to corrosion exhibited by alloy IN 792 is fully main-
tained in this invention.
A heat treatment as described in U.S. 4,116,723
is preferred in order to obtain the maximum increase
in properties. Such a heat treatment involves solution-
ing of the gamma prime phase and homogenization of thecast structure at a temperature above the gamma prime
solvus (2250~F for the invention composition) followed
by one or more aging treatments at a lower temperature.
It should be understood that the invention is
not limited to the particular embodiments shown and
described herein, but that various changes and modifi-
cations may be made without departing from the spirit
and scope of this novel concept as defined by the
following claims.
