U.S. patent application number 12/017101 was filed with the patent office on 2009-07-23 for superalloy compositions with improved oxidation performance and gas turbine components made therefrom.
This patent application is currently assigned to HONEYWELL INTERNATIONAL, INC.. Invention is credited to Yiping Hu.
Application Number | 20090185944 12/017101 |
Document ID | / |
Family ID | 40876640 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090185944 |
Kind Code |
A1 |
Hu; Yiping |
July 23, 2009 |
SUPERALLOY COMPOSITIONS WITH IMPROVED OXIDATION PERFORMANCE AND GAS
TURBINE COMPONENTS MADE THEREFROM
Abstract
Single crystal superalloy compositions and components made from
such compositions are provided. One composition consists
essentially of, in weight percent, from about 4 to about 7 percent
chromium; from about 8 to about 12 percent cobalt; from about 1 to
about 2.5 percent molybdenum; from about 3 to about 6 percent
tungsten; from about 2 to about 4 percent rhenium; from about 5 to
about 7 percent aluminum; from about 0 to about 1.5 percent
titanium; from about 6 to about 10 percent tantalum; from about
0.08 to about 1.2 percent hafnium; no more than about 0.0002
percent sulfur; no more than about 0.007 percent zirconium; and the
balance nickel.
Inventors: |
Hu; Yiping; (Greer,
SC) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.;PATENT SERVICES
101 COLUMBIA ROAD, P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
HONEYWELL INTERNATIONAL,
INC.
Morristown
NJ
|
Family ID: |
40876640 |
Appl. No.: |
12/017101 |
Filed: |
January 21, 2008 |
Current U.S.
Class: |
420/448 ;
420/445 |
Current CPC
Class: |
C22C 19/057
20130101 |
Class at
Publication: |
420/448 ;
420/445 |
International
Class: |
C22C 19/05 20060101
C22C019/05 |
Claims
1. A single crystal nickel-based superalloy composition consisting
essentially of, in weight percent: from about 4 to about 7 percent
chromium; from about 8 to about 12 percent cobalt; from about 1 to
about 2.5 percent molybdenum; from about 3 to about 6 percent
tungsten; from about 2 to about 4 percent rhenium; from about 5 to
about 7 percent aluminum; from about 0 to about 1.5 percent
titanium; from about 6 to about 10 percent tantalum; from about
0.08 to about 1.2 percent hafnium; no more than about 0.0002
percent sulfur; no more than about 0.007 percent zirconium; and the
balance nickel.
2. The single crystal nickel-based superalloy composition of claim
1, further consisting essentially of (in weight percent) from about
0.001 to about 0.015 weight percent of one selected from the group
consisting of yttrium, lanthanum, cerium, and a combination
thereof.
3. The single crystal nickel-based superalloy composition of claim
2, further consisting essentially of (in weight percent) from about
0.03 to about 0.10 percent carbon and from about 0.003 to about
0.006 percent boron.
4. The single crystal nickel-based superalloy composition of claim
1, wherein a sum of the molybdenum, tungsten, and rhenium is from
about 8.4 to about 10.4 weight percent.
5. The single crystal nickel-based superalloy composition of claim
1, wherein a sum of aluminum, titanium, and tantalum is about 13.8
to about 15.7 weight percent.
6. The single crystal nickel-based superalloy composition of claim
1, wherein the percentage of hafnium is from about 0.08 to about
0.12 weight percent.
7. The single crystal nickel-based superalloy composition of claim
1, wherein the percentage of hafnium is from about 0.15 to about
1.2 weight percent.
8. A single crystal nickel-based superalloy component fabricated of
a single crystal composition consisting essentially of (in weight
percent): from about 4 to about 7 percent chromium; from about 8 to
about 12 percent cobalt; from about 1 to about 2.5 percent
molybdenum; from about 3 to about 6 percent tungsten; from about 2
to about 4 percent rhenium; from about 5 to about 7 percent
aluminum; from about 0 to about 1.5 percent titanium; from about 6
to about 10 percent tantalum; from about 0.08 to about 1.2 percent
hafnium; no more than about 0.0002 percent sulfur; no more than
about 0.007 percent zirconium; and the balance nickel.
9. The single crystal nickel-based superalloy component of claim 8,
wherein the single crystal nickel-based superalloy component is a
turbine blade.
10. The single crystal nickel-based superalloy component of claim
8, wherein the single crystal nickel-based superalloy component is
a vane.
11. The single crystal nickel-based superalloy component of claim
8, further consisting essentially of (in weight percent) from about
0.001 to about 0.015 weight percent of one selected from the group
consisting of yttrium, lanthanum, cerium, and a combination
thereof.
12. The single crystal nickel-based superalloy component of claim
10, further consisting essentially of (in weight percent) from
about 0.03 to about 0.10 percent carbon and from about 0.003 to
about 0.006 percent boron.
13. The single crystal nickel-based superalloy component of claim
8, wherein the percentage of hafnium is from about 0.08 to about
0.12 percent.
14. The single crystal nickel-based superalloy component of claim
8, wherein the percentage of hafnium is from about 0.15 to about
1.2 percent.
15. A process for preparing a single crystal nickel-based
superalloy component, the method comprising the steps of: providing
an alloy comprising (in weight percent): from about 4 to about 7
percent chromium; from about 8 to about 12 percent cobalt; from
about 1 to about 2.5 percent molybdenum; from about 3 to about 6
percent tungsten; from about 2 to about 4 percent rhenium; from
about 5 to about 7 percent aluminum; from about 0 to about 1.5
percent titanium; from about 6 to about 10 percent tantalum; from
about 0.08 to about 1.2 percent hafnium; no more than about 0.0002
percent sulfur; no more than about 0.007 percent zirconium; and the
balance nickel; and fabricating a single crystal component from the
alloy.
16. The process of claim 15, wherein the step of providing an alloy
comprises the step of providing an alloy further comprising (in
weight percent) from about 0.001 to about 0.015 weight percent of
one selected from the group consisting of yttrium, lanthanum,
cerium, and a combination thereof.
17. The process of claim 16, wherein the step of providing an alloy
comprises the step of providing an alloy further comprising (in
weight percent) from about 0.03 to about 0.10 percent carbon and
from about 0.003 to about 0.006 percent boron.
18. The process of claim 15, wherein the step of fabricating a
single crystal component from the alloy comprises the step of
fabricating a turbine blade and a vane from the alloy.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to metallic
materials for gas turbine engine applications, and more
particularly relates to superalloy compositions with improved
oxidation performance and components of gas turbine engines made
therefrom.
BACKGROUND OF THE INVENTION
[0002] In an attempt to increase the efficiencies and performance
of contemporary gas turbine engines generally, engineers have
progressively pushed the engine environment to more extreme
operating conditions. The harsh operating conditions of high
temperature and pressure that are now frequently specified place
increased demands on engine component-manufacturing technologies
and new materials. Indeed the gradual improvement in engine design
has come about in part due to the increased strength and durability
of new materials that can withstand the operating conditions
present in the modern gas turbine engine.
[0003] Turbine airfoils are key engine components that directly
experience severe engine operation conditions. Turbine blades of
turbine airfoils are thus designed and manufactured to perform
under repeated cycles of high stress and high temperature. Turbine
blades used in modern gas turbine engines are frequently cast from
a class of materials known as precipitation-strengthening
superalloys. These superalloys include nickel-, cobalt-, and
iron-based alloys. In the cast form, turbine blades made from these
superalloys include many desirable elevated-temperature properties
such as elevated-temperature strength and good environment
resistance. Advantageously, the strength displayed by this type of
material remains even under stressful conditions, such as high
temperature and high pressure, that are experienced during engine
operation. The precipitation-strengthening superalloys are thus a
preferred material for the manufacturing of turbine blades and
vanes.
[0004] However, while the superalloys exhibit superior mechanical
properties under high temperature and pressure conditions, they are
subject to oxidation and corrosion attack by chemical degradation.
The gases at high temperature and pressure in the turbine engine
can lead to oxidation of the exposed superalloy substrates.
High-pressure turbine (HPT) blades, that is, those turbine blades
at the high pressure stages following the combustion stage of a gas
turbine engine, are particularly subject to this kind of oxidation
attack and erosion, particularly at the blade tip areas. Blade tips
are also potential wear points. Oxidation is undesirable because it
can lead to the gradual erosion of blade tip material, which
affects the dimensional characteristic of the blade and its
physical integrity. In general, eroded blade tips negatively affect
engine performance.
[0005] Thermal barrier coatings (TBCs) may be used to protect the
superalloy components of a gas turbine engine that are subjected to
extremely high temperatures. Typical TBCs include those formed of
yttria stabilized zirconia (YSZ) and yttria stabilized zirconia
doped with other oxides such as Gd.sub.2O.sub.3, TiO.sub.2, and the
like. An effective TBC has a low thermal conductivity and strongly
adheres to an underlying bond coating, to which it is bonded under
contemplated use conditions. To extend the service life of a TBC,
an environment-resistant bond coating is commonly employed. Bond
coatings typically are in the form of overlay coatings such as
MCrAlX, where M is a metallic element such as nickel, cobalt,
and/or a combination of both nickel and cobalt, and X is yttrium or
other reactive and metallic elements. A bond coating also can be a
diffusion coating such as a simple aluminide coating, a reactive
element-modified aluminide coating, a platinum-modified aluminide
coating or a reactive element-modified platinum aluminide coating.
During exposure of TBCs to high temperature, such as during
ordinary service use thereof, bond coatings of the type described
above oxidize first to form a thermally grown oxide (TGO) that
protects the underlying structure from further catastrophic
oxidation. However, if the TGO layer grows too quickly and/or too
thickly, adherence of the TBC to the bond coating can be
compromised, and cracks between the TBC and the TGO as well as
between the TGO and the bond coating form. This causes the TBC to
prematurally spall off, thus decreasing the service life of the
superalloy component.
[0006] Accordingly, it is desirable to provide an improved
superalloy composition for gas turbine engine applications wherein
the superalloy has improved oxidation performance. In addition, it
is desirable to provide an improved superalloy that reacts with the
bond coating to improve the life of both the bond coating and the
overlying TBC. Furthermore, other desirable features and
characteristics of the present invention will become apparent from
the subsequent detailed description of the invention and the
appended claims, taken in conjunction with the accompanying
drawings and this background of the invention.
BRIEF SUMMARY OF THE INVENTION
[0007] In accordance with an exemplary embodiment of the present
invention, a single crystal nickel-based superalloy composition is
provided. The single crystal nickel-based superalloy composition
consists essentially of, in weight percent, from about 4 to about 7
percent chromium; from about 8 to about 12 percent cobalt; from
about 1 to about 2.5 percent molybdenum; from about 3 to about 6
percent tungsten; from about 2 to about 4 percent rhenium; from
about 5 to about 7 percent aluminum; from about 0 to about 1.5
percent titanium; from about 6 to about 10 percent tantalum; from
about 0.08 to about 1.2 percent hafnium; no more than about 0.0002
percent sulfur; no more than about 0.007 percent zirconium; and the
balance nickel.
[0008] In accordance with an exemplary embodiment of the present
invention, a single crystal nickel-based superalloy component is
provided. The single crystal nickel-based superalloy component is
fabricated of a single crystal composition consisting essentially
of (in weight percent) from about 4 to about 7 percent chromium;
from about 8 to about 12 percent cobalt; from about 1 to about 2.5
percent molybdenum; from about 3 to about 6 percent tungsten; from
about 2 to about 4 percent rhenium; from about 5 to about 7 percent
aluminum; from about 0 to about 1.5 percent titanium; from about 6
to about 10 percent tantalum; from about 0.08 to about 1.2 percent
hafnium; no more than about 0.0002 percent sulfur; no more than
about 0.007 percent zirconium; and the balance nickel.
[0009] In accordance with an exemplary embodiment of the present
invention, a process for preparing a single crystal nickel-based
superalloy component is provided. The method comprises the steps of
providing an alloy comprising (in weight percent) from about 4 to
about 7 percent chromium; from about 8 to about 12 percent cobalt;
from about 1 to about 2.5 percent molybdenum; from about 3 to about
6 percent tungsten; from about 2 to about 4 percent rhenium; from
about 5 to about 7 percent aluminum; from about 0 to about 1.5
percent titanium; from about 6 to about 10 percent tantalum; from
about 0.08 to about 1.2 percent hafnium; no more than about 0.0002
percent sulfur; no more than about 0.007 percent zirconium; and the
balance nickel; and fabricating a single crystal component from the
alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention will hereinafter be described in
conjunction with the following drawing figures, wherein like
numerals denote like elements, and wherein:
[0011] FIG. 1 is a perspective view of a gas turbine blade; and
[0012] FIG. 2 is a cross-sectional view of a protective coating
system formed on a component of a gas turbine engine.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The following detailed description of the invention is
merely exemplary in nature and is not intended to limit the
invention or the application and uses of the invention.
Furthermore, there is no intention to be bound by any theory
presented in the preceding background of the invention or the
following detailed description of the invention.
[0014] The present invention includes an improved superalloy for a
variety of substrates, including industrial gas turbine and
aero-engine components. In one exemplary embodiment, the superalloy
has a composition that results in improved resistance to oxidation
compared to conventional superalloy compositions. In another
exemplary embodiment, the reactive elements of the superalloy, such
as hafnium and yttrium, diffuse into the bond coating to promote
adherence of the TGO to the bond coating and improve its
performance and hence the performance of an overlying thermal
barrier coating.
[0015] As is shown in the figures for the purposes of illustration,
the exemplary embodiments of the present invention are embodied in
a single crystal nickel-based superalloy component such as, for
example, a turbine blade or vane used in a gas turbine engine. FIG.
1 illustrates a turbine blade 150 that is exemplary of the types of
components or substrates that are used in turbine engines, although
turbine blades commonly have different shapes, dimensions and sizes
depending on gas turbine engine models and applications. The
illustrated blade 150 has an airfoil portion 152 including a
pressure surface 153, an attachment or root portion 154, a leading
edge 158 including a blade tip 155, and a platform 156. The blade
150 may be formed with a non-illustrated outer shroud attached to
the tip 155. The blade 150 may have non-illustrated internal
air-cooling passages that remove heat from the turbine airfoil.
After the internal air has absorbed heat from the superalloy blade,
the air is discharged into a combustion gas flow path through
passages 159 in the airfoil wall.
[0016] FIG. 2 is a cross-sectional view of a portion of a component
10 upon which is disposed a protective coating system 12 fabricated
in accordance with an exemplary embodiment of the present
invention. The component 10 may be, for example, a turbine airfoil
such as turbine blade 150 of FIG. 1. The protective coating system
12 overlies the component 10 and any intermediate layers, and is
formed of a bond coating 14, a thermal barrier coating 18, and any
intermediate layers therebetween, such as a thermally grown oxide
(TGO) 20. In one exemplary embodiment, the bond coating 14 is a
diffusion aluminide coating that is formed by depositing an
aluminum layer over the component 10, and by interdiffusing the
aluminum layer with the superalloy substrate. In one embodiment,
the bond coating is a simple diffusion aluminide. In another
embodiment, the bond coating is a more complex diffusion aluminide
that includes other metallic layers. In one embodiment, the other
metallic layer is a platinum layer. In another embodiment, the
other metallic layer is a hafnium and/or a zirconium layer. In yet
another embodiment, the other metallic layer is a co-deposited
hafnium, zirconium, and platinum layer. In another exemplary
embodiment, the bond coating 14 is an overlay coating known as an
MCrAlX coating, wherein M is cobalt, nickel, or combinations
thereof. The X is hafnium, zirconium, yttrium, tantalum, rhenium,
ruthenium, palladium, platinum, silicon, or combinations thereof.
Some examples of MCrAlX compositions include NiCoCrAlY and
CoNiCrAlY. In another exemplary embodiment, the bond coating 14 is
a combination of two types of bond coatings, a diffusion aluminide
coating formed on an MCrAlX coating. Thermal barrier coating 18 may
comprise, for example, a partially stabilized zirconia-based
thermal barrier coating, such as yttria stabilized zirconia
(YSZ).
[0017] The component of the present invention, such as the turbine
blade 150, is necessarily fabricated as a single crystal of
superalloy, at least in the section comprising the airfoil 152. As
used herein, a single crystal superalloy component is one in which
substantially the entire component has a single crystallographic
orientation through the load bearing portions, without the presence
of high angle boundaries. Low angle boundaries, such as tilt or
twist boundaries, are permitted within such a single crystal
component, but are preferably not present. However, such low angle
boundaries are often present after solidification and formation of
the single crystal superalloy component, or after some deformation
of the component during creep or other deformation process.
[0018] Other minor irregularities are also permitted within the
scope of the term "single crystal." For example, small areas of
high angle boundaries may be formed directly adjacent the coating
during the diffusional interaction of the coating and the
component, and during thermal cycling of the component. Small areas
of high angle boundaries may also be formed in the root portion
154, particularly adjacent the contact surfaces with turbine wheel.
Such minor amounts of deviation from a perfect single crystal,
which are found in normal commercial production operations and use
of the components, are within the scope of the term "single
crystal" as used herein.
[0019] The completed article typically primarily comprises two
phases, a precipitate formed within a matrix. The microstructure
may also contain minor amounts of a eutectic region formed during
solidification of the component and not dissolved during subsequent
heat treatment procedures. As with the presence of low angle
boundaries, small volume fractions of eutectic phase are tolerated
within a single crystal material.
[0020] The single crystal superalloy components can be manufactured
directly through vacuum-induction melting and casting processes.
Any fabrication technique which produces a substantially single
crystal article is operable in conjunction with the present
invention. The preferred technique, used to produce the single
crystal components described in the examples herein, is the thermal
gradient solidification method. Molten metal of the desired
composition is poured into a heat resistant ceramic mold having
essentially the desired shape of the final fabricated component.
The mold and metal contained therein are placed within a furnace,
induction heating coil, or other heating device to melt the metal,
and the mold and molten metal are gradually cooled in a temperature
gradient. In this process, metal adjacent the cooler end of the
mold solidifies first, and the interface between the solidified and
liquid metal gradually moves through the metal as cooling
continues. Such gradient solidification can be accomplished by
placing a chill block adjacent one end of the mold and then turning
off the heat source, allowing the mold and molten metal to cool and
solidify in a controlled desirable temperature gradient.
Alternatively, the mold and molten metal can be gradually withdrawn
from the heat source.
[0021] It is found that certain crystallographic orientations such
as <001> grow to the exclusion of others during such a
thermal gradient solidification process, so that a single grain
becomes dominant throughout the article. Techniques have been
developed to promote the formation of the single crystal
orientation rapidly, so that substantially all the article has the
same single crystal orientation. Such techniques include seeding
whereby an oriented single crystal starting material is positioned
adjacent the metal first solidified, so that the metal initially
develops that orientation. Another approach is a geometrical
selection process.
[0022] All other techniques for forming a single crystal are
acceptable for use in conjunction with the present invention. For
example, the liquid metal cooling process or cast may be used to
fabricate single crystal turbine components. In this process, a
nickel-based superalloy is melted and poured into a ceramic mold
placed inside a multi-zone heater. For solidification, the cast
components are immersed at a constant rate into a liquid tin
bath.
[0023] In accordance with an exemplary embodiment of the present
invention, the single crystal superalloy has a composition in
weight percent consisting essentially of from about 4 to about 7
percent chromium, from about 8 to about 12 percent cobalt, from
about 1 to about 2.5 percent molybdenum, from about 3 to about 6
percent tungsten, from about 2 to about 4 percent rhenium, from
about 5 to about 7 percent aluminum, from about 0 to about 1.5
percent titanium, from about 6 to about 10 percent tantalum, from
about 0.08 to about 1.2 percent hafnium, no more than 0.0002
percent sulfur, no more than 0.007 zirconium, and balance in nickel
totaling 100 percent. Further, in such composition the sum of the
molybdenum plus tungsten plus rhenium is from about 8.4 to about
10.4 percent, and the sum of aluminum plus titanium plus tantalum
is about 13.8 to about 15.7 percent. In another exemplary
embodiment, the composition further includes from about 0.001 to
about 0.015 percent yttrium, lanthanum, cerium or a combination
thereof. In a further exemplary embodiment of the present
invention, the composition also may comprise about 0.03 to about
0.10 percent carbon and about 0.003 to about 0.006 percent
boron.
[0024] These alloying elements are selected to achieve a
cooperative optimization of the physical and chemical as well as
mechanical properties of the completed component, and to optimize
the retention of such properties during the operating lifetime of
the component. A consideration in the selection of the alloying
ingredients is the attainment of creep strength and phase stability
as well as environmental resistance, and to achieve these goals the
strengthening mechanisms of the single crystal component must be
optimized. The preferred microstructure of the component, after
heat treatment, is an array of gamma prime precipitates in a
matrix. The matrix is nickel which has been strengthened by the
addition of various solid-solution strengthening elements, and is
termed gamma phase. Most elements have at least some solid
solubility in nickel, but molybdenum, tungsten and rhenium have
been found to be potent solid-solution strengtheners which do not
have significant detrimental effects on other properties when used
in controlled amounts, and in fact can promote the attainment of
desirable properties. Molybdenum is present in an amount of from
about 1 to about 2.5 weight percent, tungsten is present in an
amount of from about 3 to about 6 weight percent, and rhenium is
present in an amount of from about 2 to about 4 weight percent. The
sum of these solid-solution strengthening elements should be from
about 8.4 to about 10.4 percent. If too low a level of these
alloying elements is used, the strength of the matrix is low. If
excessively high levels are used, other properties such as hot
corrosion resistance and oxidation resistance are reduced.
[0025] Rhenium has an additional benefit of refining the size of
the precipitates, which contributes to improved strength of the
gamma matrix. Rhenium also improves the creep strength of the gamma
matrix and retards the rate of coarsening of the precipitates,
during extended elevated temperature exposure.
[0026] In addition to solid-solution strengthening, the strength of
the single crystal article is promoted by precipitation hardening
due to the presence of the precipitates in the matrix. The
precipitates are formed as compounds of nickel, aluminum, titanium
and tantalum, the compound being known as gamma-prime phase and
having a composition conventionally represented as
Ni.sub.3(Al,Ti,Ta). It is desirable that the volume fraction of the
gamma-prime phase be maintained at a high level, preferably in the
range of from about 65 to about 70 volume percent.
[0027] To achieve this quantity of the gamma-prime phase, aluminum
is present in an amount of from about 5 to about 7 weight percent,
titanium is present in an amount of from about 0 to about 1.5
weight percent, and tantalum is present in an amount of from about
6 to about 10 weight percent. If lower levels of these gamma-prime
forming elements are utilized, the volume fraction of gamma-prime
precipitate is low, with the result that the tensile and creep
strengths are reduced below acceptable levels. If too high levels
are used, the volume fraction of eutectic gamma-prime is
excessively high. Since the eutectic gamma-prime is highly alloyed
with refractory elements, the alloy becomes less responsive to a
solution heat treat that dissolves all or most of the eutectic
gamma-prime. Hence, the full potential strength of the alloy as a
single crystal article cannot be realized. The tantalum content of
the alloy improves the rupture life of the alloy because the high
tantalum level also is effective in maintaining the desired volume
fraction of the gamma-prime precipitate.
[0028] The sum of the aluminum plus titanium plus tantalum
percentages is maintained in the range of from about 13.8 to about
15.7 weight percent. Lower levels result in the insufficient
availability of gamma-prime forming elements, a low volume fraction
of gamma-prime precipitate phase, and corresponding low strengths.
Excessively high amounts of these three gamma-prime forming
elements result in the formation of topologically closepacked
phases (TCP) such as P and R, and sigma phases, a brittle,
undesirable precipitated constituent that may be formed during
subsequent elevated temperature exposure of the article. It has
been found that the simultaneous limitation of molybdenum plus
tungsten plus rhenium to the range of from about 8.4 to about 10.4
percent, and the limitation of the sum of aluminum plus titanium
plus tantalum to about 13.8 to about 15.7 percent, results in an
optimum combination of strength of the article in creep loading,
and chemical stability of the article to the formation of the
undesirable sigma phase during extended elevated temperature
exposure.
[0029] Chromium is present in the alloy in the amount of from about
4 to about 7 weight percent. The chromium promotes environmental
resistance of the alloy to hot corrosion in the sulfur-containing
hot gas of the gas turbine and to oxidation damage. Such inherent
resistance to environmental damage is desirable in the article,
even though it may be coated with a protective coating. Too low of
a level of chromium results in insufficient protection against
environmental attack, while too high of a level of chromium tends
to promote formation of the undesirable brittle sigma phase.
[0030] Cobalt is present in the alloy in the amount of from about 8
to about 12 percent. The addition of elemental cobalt increases
solubility of gamma matrix, thus inhibiting the formation of TCP
phases like sigma phase-containing refractory elements, thus
allowing these elements to be present for the reasons previously
discussed. A too low cobalt level has an insufficient inhibiting
effect, while a too high cobalt level increases undesirably the
solubility of the gamma-prime precipitates in the gamma matrix,
reduces solvus temperature of gamma prime, and reduces oxidation
resistance. Such increased solubility tends to reduce the volume
fraction of the gamma-prime precipitates, thereby decreasing the
strength of the article. However, it is found that, in combination
with the ranges of the other alloying elements, the relatively
higher level of cobalt in the exemplary embodiments of this
invention is not detrimental in the present articles.
[0031] In one exemplary embodiment, hafnium is present in an amount
of from about 0.08 to about 1.2 weight percent. In another
exemplary embodiment, hafnium is present in an amount of from about
0.08 to about 0.12 weight percent. In yet another exemplary
embodiment, hafnium is present in an amount of from about 0.15 to
about 1.2 weight percent. Hafnium promotes resistance to
environmental damage by oxidation. In addition, hafnium can diffuse
into the bond coating 14 illustrated in FIG. 2 and slow the growth
rate of the TGO on the bond coating, which can dramatically improve
the bond coating performance. Thus, in combination with the other
alloying elements in the indicated ranges, the presence of hafnium
promotes optimized alloy and thermal barrier coating
performance.
[0032] In addition, exemplary embodiments of the present invention
minimize the amount of elemental sulfur within the composition to
no more than 0.0002 percent. A high level of sulfur tends to
migrate to the free surfaces of the component, thereby dramatically
decreasing oxidation and corrosion performance of the superalloy
and the coating. In one exemplary embodiment, the composition also
includes yttria, lanthanum, cerium or a combination thereof, which
react with the sulfur to form stable compounds that minimize the
amount of sulfur that can migrate to the free surface of the
component. Accordingly, by minimizing the amount of sulfur in the
composition and, optionally, binding the sulfur to some reactive
elements that prevent migration of the sulfur, the oxidation
properties of the composition are greatly improved.
[0033] In another exemplary embodiment, the composition includes
from about 0.03 to about 0.10 percent carbon and from about 0.003
to about 0.006 percent boron. Typically, carbon and boron are
absent from single crystal superalloy compositions. However, in the
exemplary embodiments of the present invention, carbon and boron
are present in the disclosed amounts to accommodate both high angle
and low angle boundaries.
[0034] The selection of the ranges of the alloying elements also
leads to an improved ability to heat treat the cast single crystal
articles. In the process for preparing a single crystal component
of the invention, an alloy of the desired composition is formed and
then a single crystal is prepared from the alloy composition.
Without further processing, the microstructure of the resulting
single crystal contains gamma-prime precipitates (referred to as
cooling gamma-prime) having a variety of sizes. A further solution
treat and age heat treatment procedure is performed, wherein the
material in the gamma-prime precipitate phase is dissolved into the
gamma matrix in solution heat treatment, and then reprecipitated in
an aging treatment conducted at a lower temperature.
[0035] To place the gamma-prime phase into solution, the single
crystal piece must be heated to a temperature which is greater than
the solvus temperature of the gamma-prime phase, but less than the
melting temperature of the alloy. The melting temperature, termed
the solidus for a composition which melts over a temperature range,
should be sufficiently greater than the solvus temperature so that
the single crystal piece may be heated and maintained within the
temperature range between the solvus and the solidus for a time
sufficiently long to dissolve the gamma-prime precipitate phase
into the gamma matrix. The solidus temperature is typically about
2,400.degree. F. (about 1315.56.degree. C.) to 2,450.degree. F.
(about 1343.33.degree. C.) and accurate control to within a few
degrees in commercial heat treating equipment is simply not
possible. It is therefore preferred that the solidus be at least
about 15.degree. F. (about 8.3.degree. C.) greater than the solvus
temperature for the gamma-prime precipitates.
[0036] Both the solvus temperature and solidus temperature are
altered by changes in the amounts of the elements contained within
the alloy. Generally, greater amounts of the alloying elements
reduce the solidus temperature and cause it to approach the solvus
temperature, thereby making commercial heat treatment procedures
impractical to perform. The composition of the present alloy has
been selected with this consideration in mind, and the optimized
levels of molybdenum, titanium, carbon, boron, zirconium, and
tungsten all reduce the depression of the solidus temperature,
largely without detrimental effects on other properties.
[0037] The "heat treatment window" or difference between the
gamma-prime solvus and the alloy solidus temperatures preferably is
at least at 15.degree. F. (about 8.3.degree. C.), and more
preferably is greater than about 50.degree. F. (about 27.8.degree.
C.). In a preferred heat treatment of the cast single crystal
components, the components are solution heat treated at a
temperature of about 2,415.degree. F. (about 1323.89 C) for a
period of about three hours, to dissolve the gamma-prime
precipitate phase formed during solidification, into the gamma
matrix. The solution heat treatment may be accomplished at any
temperature within the heat treatment window between the
gamma-prime solvus and the solidus temperatures. Greater
temperatures allow shorter heat treatment times. However, the heat
treatment temperature is not typically pushed to a maximum level,
to allow a margin of error in the heat treatment equipment. After
the heat treating process is completed, the solution heat treated
single crystal components are rapidly cooled to supersaturate the
matrix with the gamma-prime forming elements. A fast argon fan cool
to a temperature of less than about 1,000.degree. F. (about
537.78.degree. C.) has been found sufficient to achieve the
necessary supersaturation. Excessively high cooling rates may not
be achieved in all commercial heat treat furnaces, while
excessively low cooling rates would not provide the necessary
supersaturation.
[0038] Following the solution heat treatment and supersaturation
cooling, the solution heat treated single crystal articles are aged
to precipitate the gamma-prime precipitates within the single
crystal gamma matrix. The aging heat treatment can be combined with
the coating treatment. As noted previously, gas turbine components
are typically coated with a corrosion- and oxidation-resistant
coating and thermal barrier coating prior to use. During the
coating procedure, the component being coated is heated to elevated
temperature. A typical coating treatment requires heating the
component to a temperature of about 1,950.degree. F. (about
1065.56.degree. C.) for about four hours. This heat treatment
causes some precipitation of the gamma-prime phase within the gamma
matrix, thus accomplishing in part the aging heat treatment. The
aging heat treatment may be completed by a further elevated
temperature exposure, separate from the coating procedure. A
sufficient additional aging heat treatment is accomplished at a
temperature of about 1,600.degree. F. (about 871.11 .degree. C.)
for a time of from about four to about twenty hours, following the
heat treatment at 1,950.degree. F. (about 1065.56.degree. C.) for
four hours. The aging heat treatment is not limited to this
preferred heat treatment sequence, but instead may be accomplished
by any acceptable approach which precipitates the desired volume
fraction of gamma-prime particles within the gamma matrix, the
precipitation occurring from the supersaturated heat treated single
crystal matrix.
[0039] The microstructure of the as-solidified single crystals
includes irregular gamma-prime particles and regions of gamma-prime
eutectic phase. The solution heat treatment dissolves the irregular
gamma-prime particles and most or all the gamma-prime eutectic
constituent into the gamma matrix. The subsequent aging treatment
precipitates an array of gamma-prime precipitates having a
generally cuboidal shape and somewhat uniform size. The gamma-prime
precipitates vary from about 0.3 to about 0.6 micrometers in
size.
[0040] The following examples are presented to illustrate aspects
and features of various embodiments of the present invention, and
are not to be taken as limiting the invention in any respect.
EXAMPLE 1
In Weight Percent
TABLE-US-00001 [0041] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 8.0-9.0 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.08-0.12 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others La: 0.001-0.013 Ni
Balance
EXAMPLE 2
In Weight Percent
TABLE-US-00002 [0042] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 8.0-9.0 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.08-0.12 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others Y: 0.001-0.013 Ni
Balance
EXAMPLE 3
In Weight Percent
TABLE-US-00003 [0043] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 8.0-9.0 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.08-0.12 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others La and Y (combined weight
percent): 0.001-0.013; C: 0.03-0.06; B: 0.004-0.006 Ni Balance
EXAMPLE 4
In Weight Percent
TABLE-US-00004 [0044] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 8.0-9.0 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.08-0.12 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others La and Y (combined weight
percent): 0.001-0.013 Ni Balance
EXAMPLE 5
In Weight Percent
TABLE-US-00005 [0045] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 7.3-8.5 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.15-1.2 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others Y: 0.001-0.013; C:
0.03-0.09; B: 0.004-0.006 Ni Balance
EXAMPLE 6
In Weight Percent
TABLE-US-00006 [0046] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 7.3-8.5 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.15-1.2 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others La and Y (combined weight
percent): 0.001-0.013 Ni Balance
EXAMPLE 7
In Weight Percent
TABLE-US-00007 [0047] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 7.3-8.5 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.15-1.2 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others La and Y (combined weight
percent): 0.001-0.013; C: 0.03-0.09; B: 0.004-0.006 Ni Balance
EXAMPLE 8
In Weight Percent
TABLE-US-00008 [0048] Co 9.8-10.2 Cr 5.2-5.4 Mo 1.6-1.8 W 4.8-5.2
Re 2.8-3.2 Ta 7.3-8.5 Al 5.0-5.4 Ti 0.9-1.1 Hf 0.15-1.2 S
.ltoreq.0.0002 Zr .ltoreq.0.007 Others C: 0.03-0.09; B: 0.004-0.006
Ni Balance
EXAMPLE 9
In Weight Percent
TABLE-US-00009 [0049] Co 9.3-9.8 Cr 6.3-6.7 Mo 1.6-2.0 W 5.3-5.7 Re
2.8-3.2 Ta 6.8-7.2 Al 6.1-6.4 Hf 0.13-0.17 S .ltoreq.0.0002 Zr
.ltoreq.0.007 Others La and Y (combined weight percent):
0.001-0.013; C: 0.03-0.07; B: 0.004-0.006 Ni Balance
[0050] Accordingly, single crystal nickel-based superalloy
compositions for hot-section components of gas turbine engines,
such as gas turbine blades and vanes, have been provided. The
compositions provide such components with mechanical, phase
stability, and environmental properties superior to those of prior
art superalloy materials. In addition, the compositions provide
such components with improved oxidation resistance. While at least
one exemplary embodiment has been presented in the foregoing
detailed description of the invention, it should be appreciated
that a vast number of variations exist. It should also be
appreciated that the exemplary embodiment or exemplary embodiments
are only examples, and are not intended to limit the scope,
applicability, or configuration of the invention in any way.
Rather, the foregoing detailed description will provide those
skilled in the art with a convenient road map for implementing an
exemplary embodiment of the invention, it being understood that
various changes may be made in the function and arrangement of
elements described in an exemplary embodiment without departing
from the scope of the invention as set forth in the appended claims
and their legal equivalents.
* * * * *