U.S. patent application number 10/229741 was filed with the patent office on 2004-03-04 for reduced-tantalum superalloy composition of matter and article made therefrom, and method for selecting a reduced-tantalum superalloy.
Invention is credited to O'Hara, Kevin Swayne, Ross, Earl Warren, Walston, William Scott.
Application Number | 20040042927 10/229741 |
Document ID | / |
Family ID | 31495359 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040042927 |
Kind Code |
A1 |
O'Hara, Kevin Swayne ; et
al. |
March 4, 2004 |
Reduced-tantalum superalloy composition of matter and article made
therefrom, and method for selecting a reduced-tantalum
superalloy
Abstract
A superalloy article has a composition consisting essentially
of, in weight percent, from about 4 to about 12 percent cobalt,
from about 3.5 to about 7 percent tungsten, from about 2 to about 9
percent chromium, from about 0.5 to about 4.5 percent tantalum,
from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5
percent rhenium, from about 0.1 to about 1.2 percent titanium, from
0 to about 3 percent molybdenum, from 0 to about 3 percent
ruthenium, from about 0.5 to about 2 percent columbium, about 0.01
percent maximum boron, about 0.07 percent maximum carbon, from
about 0.3 to about 1 percent hafnium, about 0.01 percent maximum
zirconium, about 0.03 percent maximum yttrium, from 0 to about 0.5
percent vanadium, about 0.01 percent maximum cerium, and about 0.01
percent maximum lanthanum, balance nickel and impurity elements.
The article is preferably substantially a single crystal or
oriented polycrystal in a shape such as a gas turbine blade.
Inventors: |
O'Hara, Kevin Swayne;
(Boxford, MA) ; Walston, William Scott; (Mason,
OH) ; Ross, Earl Warren; (Cincinnati, OH) |
Correspondence
Address: |
MCNEES, WALLACE & NURICK
100 PINE STREET
BOX 1166
HARRISBURG
PA
17108
US
|
Family ID: |
31495359 |
Appl. No.: |
10/229741 |
Filed: |
August 27, 2002 |
Current U.S.
Class: |
420/444 |
Current CPC
Class: |
C22C 19/057
20130101 |
Class at
Publication: |
420/444 |
International
Class: |
C22C 019/05 |
Claims
What is claimed is:
1. An article comprising a composition consisting essentially of,
in weight percent, from about 4 to about 12 percent cobalt, from
about 3.5 to about 7 percent tungsten, from about 2 to about 9
percent chromium, from about 0.5 to about 4.5 percent tantalum,
from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5
percent rhenium, from about 0.1 to about 1.2 percent titanium, from
0 to about 3 percent molybdenum, from 0 to about 3 percent
ruthenium, from about 0.5 to about 2 percent columbium, about 0.01
percent maximum boron, about 0.07 percent maximum carbon, from
about 0.3 to about 1 percent hafnium, about 0.01 percent maximum
zirconium, about 0.03 percent maximum yttrium, from 0 to about 0.5
percent vanadium, about 0.01 percent maximum cerium, and about 0.01
percent maximum lanthanum, balance nickel and impurity
elements.
2. The article of claim 1, wherein the article includes from about
3.0 to about 4.0 percent tantalum.
3. The article of claim 1, wherein the article includes from about
3.0 to about 4.0 percent tantalum, from about 0.2 to about 0.4
percent titanium, from about 0.5 to about 0.7 percent hafnium, and
from about 1 to about 2 percent columbium.
4. The article of claim 1, wherein the article includes from about
6 to about 12 percent cobalt, from about 4.5 to about 6.5 percent
tungsten, from about 5.5 to about 6.5 percent chromium, from about
3.0 to about 4 percent tantalum, from about 5.8 to about 6.3
percent aluminum, from about 2.8 to about 3.5 percent rhenium, from
about 0.2 to about 0.4 percent titanium, from about 1.3 to about
1.7 percent molybdenum, from about 0.5 to about 0.7 percent
hafnium, and from about 1 to about 2 percent columbium.
5. The article of claim 1, wherein the article includes from about
7 to about 10 percent cobalt, from about 6 to about 6.3 percent
tungsten, about 6 percent chromium, from about 3.1. to about 3.5
percent tantalum, from about 5.9 to about 6.3 percent aluminum,
about 0.3 percent titanium, about 0.6 percent hafnium, about 3
percent rhenium, about 1.5 percent molybdenum, and about 1.5
percent columbium.
6. The article of claim 1, wherein the article is substantially a
single crystal.
7. The article of claim 1, wherein the article is a directionally
oriented polycrystal.
8. The article of claim 1, wherein the article is shaped as a
component of a gas turbine engine.
9. The article of claim 1, wherein the article is shaped as a gas
turbine blade.
10. A composition of matter consisting essentially of, in weight
percent, from about 4 to about 12 percent cobalt, from about 3.5 to
about 7 percent tungsten, from about 2 to about 9 percent chromium,
from about 0.5 to about 4.5 percent tantalum, from about 5.5 to
about 7.5 percent aluminum, from 0 to about 5.5 percent rhenium,
from about 0.1 to about 1.2 percent titanium, from 0 to about 3
percent molybdenum, from 0 to about 3 percent ruthenium, from about
0.5 to about 2 percent columbium, about 0.01 percent maximum boron,
about 0.07 percent maximum carbon, from about 0.3 to about 1
percent hafnium, about 0.01 percent maximum zirconium, about 0.03
percent maximum yttrium, from 0 to about 0.5 percent vanadium,
about 0.01 percent maximum cerium, and about 0.01 percent maximum
lanthanum, balance nickel and impurity elements.
11. The composition of matter of claim 10, wherein the composition
of matter includes from about 3.0 to about 4.0 percent
tantalum.
12. The composition of matter of claim 10, wherein the composition
of matter includes from about 3.0 to about 4.0 percent tantalum,
from about 0.2 to about 0.4 percent titanium, from about 0.5 to
about 0.7 percent hafnium, and from about 1 to about 2 percent
columbium.
13. The composition of matter of claim 10, wherein the composition
of matter includes from about 6 to about 12 percent cobalt, from
about 4.5 to about 6.5 percent tungsten, from about 5.5 to about
6.5 percent chromium, from about 3.0 to about 4 percent tantalum,
from about 5.8 to about 6.3 percent aluminum, from about 2.8 to
about 3.5 percent rhenium, from about 0.2 to about 0.4 percent
titanium, from about 1.3 to about 1.7 percent molybdenum, from
about 0.5 to about 0.7 percent hafnium, and from about 1 to about 2
percent columbium.
14. The composition of matter of claim 10, wherein the composition
of matter includes from about 7 to about 10 percent cobalt, from
about 6 to about 6.3 percent tungsten, about 6 percent chromium,
from about 3.1. to about 3.5 percent tantalum, from about 5.9 to
about 6.3 percent aluminum, about 0.3 percent titanium, about 0.6
percent hafnium, about 3 percent rhenium, about 1.5 percent
molybdenum, and about 1.5 percent columbium.
15. A method for selecting a reduced-cost nickel-base superalloy,
the method comprising the steps of identifying a baseline
nickel-base superalloy having a nominal composition, in weight
percent, comprising a baseline tantalum content of more than about
5 weight percent tantalum, and a baseline sum (hafnium content plus
columbium content plus titanium content plus tungsten content), in
weight percent, selecting a modified nickel-base superalloy having
a nominal composition, in weight percent, comprising a modified
tantalum content at least 1.5 weight percent less than the baseline
tantalum content, and a modified baseline sum of (modified hafnium
content plus modified columbium content plus modified titanium
content plus modified tungsten content) at least 1.5 weight percent
greater than the baseline sum.
16. The method of claim 15, wherein the step of selecting includes
the step of selecting an absolute value of (the modified baseline
sum minus the baseline sum) to be at least as great as the absolute
value of (the modified tantalum content minus the baseline tantalum
content).
17. The method of claim 15, wherein the step of selecting includes
the step of selecting the modified nickel-base superalloy to have a
nonzero modified hafnium content, a nonzero modified columbium
content, a nonzero modified titanium content, and a nonzero
modified tungsten content.
18. The method of claim 15, wherein the sum of the modified
tungsten content plus a modified molybdenum content in the modified
nickel-base superalloy is at least about 6.5 weight percent.
19. A method for selecting a reduced-cost nickel-base superalloy,
the method comprising the steps of identifying a baseline
nickel-base superalloy having a nominal composition, in weight
percent, comprising a baseline tantalum content of more than about
5 weight percent tantalum, and a baseline sum (baseline hafnium
content plus baseline columbium content plus baseline titanium
content plus baseline tungsten content), in weight percent,
selecting a modified nickel-base superalloy having a nominal
composition, in weight percent, comprising a modified tantalum
content at least 1.5 weight percent less than the baseline tantalum
content, and a modified baseline sum of (modified hafnium content
plus modified columbium content plus modified titanium content plus
modified tungsten content) at least 1.5 weight percent greater than
the baseline sum, wherein an absolute value of (the modified
baseline sum minus the baseline sum) is at least as great as the
absolute value of (the modified tantalum content minus the baseline
tantalum content), wherein the modified nickel-base superalloy has
a nonzero modified hafnium content, a nonzero modified columbium
content, a nonzero modified titanium content, and a nonzero
modified tungsten content, and wherein the sum of the modified
tungsten content plus a modified molybdenum content in the modified
nickel-base superalloy is at least about 6.5 weight percent.
Description
[0001] This invention relates to a composition of matter suitable
for use in aggressive, high-temperature gas turbine environments,
and articles made therefrom.
BACKGROUND OF THE INVENTION
[0002] Nickel-base superalloys are alloys having more nickel than
any other element, and containing a group of elements that produce
gamma-prime and related precipitates during an appropriate heat
treatment. Nickel-base superalloys are the currently preferred
alloy choice for making the components of aircraft-gas turbine
engines that are exposed to the highest temperatures. Examples
include turbine blades, turbine vanes, some shafts, some rotors,
interstage seals, and many high-temperature stationary gas-path
components.
[0003] The nickel-base superalloys must exhibit acceptable
mechanical properties at both low and high temperatures, such as
good strength, good fatigue resistance, low creep rates, sufficient
ductility, and acceptable density. They must also have good
corrosion and oxidation resistance in the harsh combustion-gas
environment. Further, the superalloys must have good stability in
both extended exposure at elevated temperature and cyclic heating
and cooling patterns. These properties are achieved through the
careful selection of the alloying elements and the processing of
the material. A number of superalloy compositions have been
developed to supply the appropriate combinations of these
properties for various applications in the gas turbine
environment.
[0004] Additionally, the cost of the superalloy material is a
consideration. While achieving the required properties is of
paramount concern, the manufacture and sales of gas turbine engines
is a competitive business. Some of the elements used in the
nickel-base superalloys are rather exotic in nature and costly, and
therefore their presence and amount is an important factor in the
cost of the gas turbine engine. Further, some elements are subject
to periodic shortages wherein the price becomes almost
prohibitively high.
[0005] In the work leading to the present invention, the inventors
recognized that one such element that is used in advanced
nickel-base superalloys is tantalum. An important nickel-base
superalloy used in gas turbine blades and other applications,
Rene.TM. N5, contains a nominal 6.2 weight percent tantalum, and
other nickel-base superalloys contain 5 percent or more of
tantalum. In the last several years, the price of tantalum of the
quality required for use in nickelbase superalloys increased from
about $100 per pound to $475 per pound, with some projections of
even higher price based on worldwide shortages. Other economic
forces have temporarily lowered the price, but there is a future
potential for comparable price increases and shortages. The high
prices and potential shortages thus threaten the continued economic
viability and availability of articles made of such materials.
[0006] There is accordingly a need for improved nickel-base
superalloys with properties comparable with existing high-tantalum
nickel-base superalloys such as Rene.TM. N5, but which are not as
dependent upon the use of high percentages of tantalum. The present
invention fulfills this need, and further provides related
advantages.
SUMMARY OF THE INVENTION
[0007] The present invention provides a nickel-base superalloy and
articles made from the nickel-base superalloy, and an approach for
selecting and designing nickel-base superalloys. The nickel-base
superalloy contains a reduced nominal tantalum content as compared
with higher-tantalum alloys, and with corresponding modifications
of other alloying elements to provide the comparable performance of
the higher-tantalum alloys.
[0008] An article comprises a composition consists essentially of,
in weight percent, from about 4 to about 12 percent cobalt, from
about 3.5 to about 7 percent tungsten, from about 2 to about 9
percent chromium, from about 0.5 to about 4.5 percent tantalum,
from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5
percent rhenium, from about 0.1 to about 1.2 percent titanium, from
0 to about 3 percent molybdenum, from 0 to about 3 percent
ruthenium, from about 0.5 to about 2 percent columbium, about 0.01
percent maximum boron, about 0.07 percent maximum carbon, from
about 0.3 to about 1 percent hafnium, about 0.01 percent maximum
zirconium, about 0.03 percent maximum yttrium, from 0 to about 0.5
percent vanadium, about 0.01 percent maximum cerium, and about 0.01
percent maximum lanthanum, balance nickel and impurity elements.
Most preferably, the article includes from about 3.0 to about 4.0
percent tantalum.
[0009] In one preferred form, the article includes from about 3.0
to about 4.0 percent tantalum, from about 0.2 to about 0.4 percent
titanium, from about 0.5 to about 0.7 percent hafnium, and from
about 1 to about 2 percent columbium. In another preferred form,
the article includes from about 6 to about 12 percent cobalt, from
about 4.5 to about 6.5 percent tungsten, from about 5.5 to about
6.5 percent chromium, from about 3.0 to about 4 percent tantalum,
from about 5.8 to about 6.3 percent aluminum, from about 2.8 to
about 3.5 percent rhenium, from about 0.2 to about 0.4 percent
titanium, from about 1.3 to about 1.7 percent molybdenum, from
about 0.5 to about 0.7 percent hafnium, and from about 1 to about 2
percent columbium. In a most-preferred form, the article includes
from about 7 to about 10 percent cobalt, from about 6 to about 6.3
percent tungsten, about 6 percent chromium, from about 3.1. to
about 3.5 percent tantalum, from about 5.9 to about 6.3 percent
aluminum, about 0.3 percent titanium, about 0.6 percent hafnium,
about 3 percent rhenium, about 1.5 percent molybdenum, and about
1.5 percent columbium.
[0010] Desirably, the article is substantially a single crystal or
a directionally oriented polycrystal produced by directional
solidification. It is preferably shaped as a component of a gas
turbine engine, such as a gas turbine blade.
[0011] A related composition of matter consists essentially of, in
weight percent, from about 4 to about 12 percent cobalt, from about
3.5 to about 7 percent tungsten, from about 2 to about 9 percent
chromium, from about 0.5 to about 4.5 percent tantalum, from about
5.5 to about 7.5 percent aluminum, from 0 to about 5.5 percent
rhenium, from about 0.1 to about 1.2 percent titanium, from 0 to
about 3 percent molybdenum, from 0 to about 3 percent ruthenium,
from about 0.5 to about 2 percent columbium, about 0.01 percent
maximum boron, about 0.07 percent maximum carbon, from about 0.3 to
about 1 percent hafnium, about 0.01 percent maximum zirconium,
about 0.03 percent maximum yttrium, from 0 to about 0.5 percent
vanadium, about 0.01 percent maximum cerium, and about 0.01 percent
maximum lanthanum, balance nickel and impurity elements. Preferred
and most-preferred compositions as discussed elsewhere herein are
applicable to the composition of matter.
[0012] The present invention also provides an approach for
extending the principles used to develop the above-described
composition to the modification of other nickel-superalloys to
reduce their tantalum contents. A method for selecting a
reduced-cost nickel-base superalloy comprises the steps of
identifying a baseline nickel-base superalloy having a nominal
composition, in weight percent, comprising a baseline tantalum
content of more than about 5 weight percent tantalum, and a
baseline sum (baseline hafnium content plus baseline columbium
content plus baseline titanium content plus baseline tungsten
content), in weight percent. (Calculated quantities are enclosed in
parentheses for clarity herein.) The method further includes
selecting a modified nickel-base superalloy having a nominal
composition, in weight percent, comprising a modified tantalum
content at least 1.5 weight percent less than the baseline tantalum
content, and a modified baseline sum of (modified hafnium content
plus modified columbium content plus modified titanium content plus
modified tungsten content) at least 1.5 weight percent greater than
the baseline sum.
[0013] That is, the reduction in tantalum content for cost reasons
must be compensated for by increasing the sum of hafnium,
columbium, titanium, and tungsten. Preferably, the increase in the
sum of hafnium, columbium, titanium, and tungsten is at least as
great as the decrease in the tantalum content. That is, the
absolute value of (the modified baseline sum minus the baseline
sum) is preferably at least as great as the absolute value of (the
modified tantalum content minus the baseline tantalum content). It
is also preferred that the modified nickel-base superalloy have a
nonzero modified hafnium content, a nonzero modified columbium
content, a nonzero modified titanium content, and a nonzero
modified tungsten content. Desirably, the sum of the modified
tungsten content plus a modified molybdenum content is at least
about 6.5 weight percent, for most modified nickel-base
superalloys.
[0014] Commercial baseline nickel-base superalloys such as PWA
1484, Rene.TM. 142, and the CMSX alloys such as CMSX-4 and CMSX-10
may be modified according to these principles to reduce their
tantalum contents while maintaining acceptable properties.
[0015] The present article, especially in its preferred and
most-preferred forms, exhibits performance comparable with that of
higher-tantalum alloys, but with a reduced tantalum content, and
consequently a reduced cost. The cost savings becomes highly
significant when tantalum prices exceed several hundred dollars per
pound, as has been the case recently and which may occur again in
the future. Other features and advantages of the present invention
will be apparent from the following more detailed description of
the preferred embodiment, taken in conjunction with the
accompanying drawings, which illustrate, by way of example, the
principles of the invention. The scope of the invention is not,
however, limited to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a perspective view of a gas turbine blade;
[0017] FIG. 2 is a block flow diagram of a method for fabricating
the article of FIG. 1; and
[0018] FIG. 3 is a bar chart of oxidation weight loss in cyclic
oxidation tests, for the tested alloys.
DETAILED DESCRIPTION OF THE INVENTION
[0019] FIG. 1 depicts a component article 20 of a gas turbine
engine, illustrated as a gas turbine blade 22. The gas turbine
blade 22 includes an airfoil 24, an attachment 26 in the form of a
dovetail to attach the gas turbine blade 22 to a turbine disk (not
shown), and a laterally extending platform 28 intermediate the
airfoil 24 and the attachment 26. In one preferred embodiment, the
component article 20 is substantially a single crystal. That is,
the component article 20 is at least about 80 percent by volume,
and more preferably at least about 95 percent by volume, a single
grain with a single crystallographic orientation. There may be
minor volume fractions of other crystallographic orientations and
also regions separated by low-angle boundaries. The single-crystal
structure is prepared by the directional solidification of an alloy
composition as discussed herein, usually from a seed or other
structure which induces the growth of the single crystal and single
grain orientation. In another preferred embodiment, the component
article 20 is a directionally oriented polycrystal, in which there
are at least several grains all with a commonly oriented preferred
growth direction. The directionally oriented polycrystal is
produced by directional solidification, typically without a
seed.
[0020] The use of the alloy composition discussed herein is not
limited to the gas turbine blade 22, and it may be employed in
other articles such as gas turbine vanes, or articles that are not
to be used in gas turbine engines.
[0021] FIG. 2 is a block-flow diagram of a preferred approach for
practicing the invention. The alloying elements that form the
nickel-base alloy are provided in the proper proportions and melted
together to form the molten alloy, step 40. In a specific form, the
alloy has a composition consisting essentially of, in weight
percent, from about 4 to about 12 percent cobalt, from about 3.5 to
about 7 percent tungsten, from about 2 to about 9 percent chromium,
from about 0.5 to about 4.5 percent tantalum, from about 5.5 to
about 7.5 percent aluminum, from 0 to about 3.5 percent rhenium,
from about 0.1 to about 1.2 percent titanium, from 0 to about 3
percent molybdenum, from 0 to about 3 percent ruthenium, from about
0.5 to about 2 percent columbium, about 0.01 percent maximum boron,
about 0.07 percent maximum carbon, from about 0.3 to about 1
percent hafnium, about 0.01 percent maximum zirconium, about 0.03
percent maximum yttrium, from 0 to about 0.5 percent vanadium,
about 0.01 percent maximum cerium, and about 0.01 percent maximum
lanthanum, balance nickel and impurity elements. (All compositional
percentages herein are stated in weight percent, unless indicated
to the contrary.)
[0022] Preferably, the alloy includes from about 3.0 to about 4.0
percent tantalum. In one preferred form, the article includes from
about 3.0 to about 4.0 percent tantalum, from about 0.2 to about
0.4 percent titanium, from about 0.5 to about 0.7 percent hafnium,
and from about 1 to about 2 percent columbium. In another preferred
form, the article includes from about 6 to about 12 percent cobalt,
from about 4.5 to about 6.5 percent tungsten, from about 5.5 to
about 6.5 percent chromium, from about 3.0 to about 4 percent
tantalum, from about 5.8 to about 6.3 percent aluminum, from about
2.8 to about 3.5 percent rhenium, from about 0.2 to about 0.4
percent titanium, from about 1.3 to about 1.7 percent molybdenum,
from about 0.5 to about 0.7 percent hafnium, and from about 1 to
about 2 percent columbium. In a most-preferred form, the article
includes from about 7 to about 10 percent cobalt, from about 6 to
about 6.3 percent tungsten, about 6 percent chromium, from about
3.1. to about 3.5 percent tantalum, from about 5.9 to about 6.3
percent aluminum, about 0.3 percent titanium, about 0.6 percent
hafnium, about 3 percent rhenium, about 1.5 percent molybdenum, and
about 1.5 percent columbium.
[0023] Most high performance superalloys for the most-demanding
applications contain at least about 5-6 weight percent tantalum,
and in some cases considerably more tantalum. The present invention
desirably reduces the tantalum content of the alloy of the
invention to no more than about half the initial amount in the
baseline nickel-base superalloy, and typically less than about 4
weight percent. The resulting superalloy, with adjustments of other
alloying proportions, has acceptable performance and also a cost
which is significantly less than that of comparable superalloys, an
important consideration at times when the cost of tantalum is
high.
[0024] The tantalum in the gamma-prime hardened superalloy is an
important ingredient because tantalum is a heavy refractory which
can replace aluminum in the Ni.sub.3Al-based gamma-prime
strengthening phase. The tantalum has a secondary effect of
improved castability with respect to grain defects by balancing out
the density differences between the first and last liquid to
solidify (i.e., between the dendrite core and the interdendritic
regions). The presence of tantalum also does not have a negative
effect on environmental resistance in respect to oxidation and hot
corrosion, unlike other refractory metals such as molybdenum and
tungsten. Thus, reducing the tantalum content below about 5 weight
percent, while retaining strength and environmental-resistance
properties is challenging.
[0025] In the present approach, tantalum may be replaced by
columbium and/or hafnium and/or titanium and/or tungsten on the
gamma-prime aluminum sites. The tungsten partitions to both the
gamma phase and to the gamma prime phase, so that it aids in
increasing the strength of the gamma prime phase as well as the
gamma matrix. However, an excessively large increase in the
tungsten content tends to lead to phase instability in the alloy
over long exposure to elevated temperature, and therefore the
tungsten content is limited.
[0026] The present alloy contains from about 0.5 to about 4.5
percent tantalum, more preferably from about 3 to about 4 percent,
and most preferably from about 3.1 to about 3.5 percent. If the
tantalum content is less than about 0.5 percent, the alloy has
insufficient strength. If the tantalum content is less than about
2.5 percent, the strength is unsatisfactory for many applications,
and the article is prone to casting defects. If the tantalum
content is more than about 4.5 percent, the alloy cost becomes
prohibitive as the cost of tantalum increases. Also, a tantalum
content of greater than about 4 percent, with the present levels of
columbium, titanium, and hafnium, produces a nickel-base superalloy
with too much gamma prime phase and resulting instability.
[0027] The alloy contains from about 0.1 to about 1.2 percent
titanium. Titanium is a potent gamma prime hardener, and at least
about 0.1 percent must be present in order to compensate for the
reduced tantalum content. The optional titanium addition
substitutes for aluminum and tantalum in the gamma prime phase,
improving the strength. However, higher levels of titanium
adversely affect oxidation resistance.
[0028] The alloy contains from about 0.3 to about 1 percent
hafnium. Hafnium improves the oxidation and hot corrosion
resistance of coated alloys, but can degrade the corrosion
resistance of uncoated alloys. Hafnium also improves the life of
thermal barrier coatings, where used. Experience with other alloys
has shown that hafnium contents on the order of 0.75 percent are
satisfactory. However, when the hafnium content exceeds about 1
percent, the stress rupture properties are reduced and the
incipient melting temperature is reduced.
[0029] The alloy contains from about 0.5 to about 2 percent
columbium (also sometimes termed "niobium"), which substitutes for
tantalum in the gamma prime phase. Lesser amounts result in
insufficient amounts and strength of the gamma prime phase. Greater
amounts excessively reduce the gamma-prime solvus temperature and
reduce oxidation resistance.
[0030] The alloy contains from about 4 to about 12 percent cobalt.
Lesser amounts result in reduced alloy stability. Greater amounts
reduce the gamma prime solvus temperature and thus the
high-temperature strength, and impair the oxidation resistance.
[0031] The alloy contains from about from about 3.5 to about 7
percent tungsten. Lesser amounts unacceptably decrease the strength
of the superalloy, and greater amounts produce instability with
respect to TCP (topologically close packed) phase formation.
[0032] The alloy contains from about 2 to about 9 percent chromium.
Lesser amounts reduce hot corrosion resistance while greater
amounts lead to phase instability and poor cyclic oxidation
resistance.
[0033] The alloy contains from about 5.5 to about 7.5 percent
aluminum. Lesser amounts reduce strength due to a reduction in the
gamma prime phase. Greater amounts produce instability with respect
to TCP phase formation and incipient melting problems during alloy
heat treatment.
[0034] The alloy contains from 0 to about 5.5 percent rhenium, more
preferably from 0 to about 3.5 percent rhenium, even more
preferably from about 2.8 to about 3.5 percent rhenium, and most
preferably about 3 percent rhenium. Greater amounts produce alloy
instability with respect to TCP phase formation.
[0035] The alloy contains from 0 to about 3 percent ruthenium.
Greater amounts reduce oxidation resistance and do not improve
alloy stability.
[0036] The alloy contains about 0.01 percent maximum boron,
preferably about 0.006 percent maximum boron. Greater amounts cause
incipient melting problems during alloy heat treatment.
[0037] The alloy contains about 0.07 percent maximum carbon. The
carbon is a deoxidizer to reduce inclusions. Greater amounts sap
the strength of the superalloy by chemically combining to form
carbides of hardening elements. The carbides also serve as the
sites for fatigue failure initiation.
[0038] The alloy contains about 0.01 percent maximum zirconium.
Greater amounts cause incipient melting problems during alloy heat
treatment.
[0039] The alloy contains about 0.03 percent maximum yttrium.
Greater amounts promote undesirable mold-metal reaction at the
casting surface and increase the inclusion content of the cast
article.
[0040] The alloy contains from 0 to about 0.5 percent vanadium.
Greater amounts reduce the hot corrosion resistance of the
alloy.
[0041] The alloy contains about 0.01 percent maximum cerium and
about 0.01 percent maximum lanthanum. Greater amounts of either of
these elements promote an undesirable mold-metal reaction at the
casting surface and increase the inclusion content of the
component.
[0042] The alloy preferably contains about 0.1 percent maximum
silicon. Silicon in such minor amounts may aid oxidation
resistance.
[0043] The alloy preferably contains about 0.04 percent maximum
magnesium and about 0.01 percent maximum calcium as de-oxidizers.
These elements in small quantities may also improve the oxidation
resistance.
[0044] The balance of the alloy is nickel and impurity elements.
The nickel content is preferably in the range of from about 61 to
about 64 weight percent.
[0045] Studies and calculations were performed to establish limits
for the various elements. The following Table I sets for the
compositions of alloys actually melted. Alloys E1-E18 are alloys
within the scope of the present invention, and alloy RN5 is
commercial Rene.TM. N5 alloy, which is not within the scope of the
invention.
1TABLE 1 No. Al Ta Cr W Cb Co Ti Hf Y Ni E1 6.25 3.5 6 5 1 10 0
0.15 0.015 63.5 E2 6.25 3.5 6 6 1 10 0 0.15 0.015 62.5 E3 6.25 3.5
6 5 1.5 10 0 0.15 0.015 63.0 E4 6.25 3.5 6 6 1.5 10 0 0.15 0.015
62.0 E5 6.25 3.5 6 5 1 10 0 0.6 0.015 63.1 E6 6.25 3.5 6 6 1 10 0
0.6 0.015 62.1 E7 6.25 3.5 6 5 1.5 10 0 0.6 0.015 62.6 E8 6.25 3.5
6 6 1.5 10 0 0.6 0.015 61.6 E9 6.25 3.5 6 6 1 10 0.3 0.15 0.015
62.2 E10 6.25 3.5 6 5 1.5 10 0.3 0.15 0.015 62.7 E11 6.25 3.5 6 5 1
10 0.3 0.6 0.015 62.8 E12 6.25 3.5 6 6 1.5 10 0.3 0.6 0.015 61.3
E13 6.22 3.5 6 6.5 1.5 10 0 0.15 0.015 61.5 E14 6.22 3.5 6 6.5 1.0
10 0 0.6 0.015 61.5 E15 6.25 4.0 6 5.5 1.3 10 0 0.15 0.015 62.1 E16
6.60 3.5 6 5.5 1.0 10 0.3 0.15 0.015 62.4 E17 6.20 3.5 7 5 1.5 10
0.3 0.15 0.015 61.8 E18 6.20 3.5 7 5 2.0 10 0.3 0.15 0.015 61.3 RN5
6.2 6.5 7 5 0 7.5 0 0.15 0 63.1
[0046] For these alloys, in all cases the Mo content was 1.5 weight
percent, the Re content was 3 weight percent, the Ru content was 0,
and the carbon content was 0.05 weight percent.
[0047] Compositional and property computed values for the alloys
are set forth in Table II. The value of .DELTA.Ta is the change in
tantalum content for the indicated alloy as compared with RN5. The
value of .DELTA.(Ti+Hf+Cb+W) is the change in the computed sum for
the indicated alloy as compared with RN5. The value of (W+Mo) is
the numerical sum of these two elements.
2TABLE II No. .DELTA.Ta .DELTA.(Ti + Hf + Cb + W) (W + Mo) Density
E1 -3.0 1.0 6.5 0.309 E2 -3.0 2.0 7.5 0.311 E3 -3.0 1.5 6.5 0.310
E4 -3.0 2.5 7.5 0.311 E5 -3.0 1.5 6.5 0.309 E6 -3.0 2.5 7.5 0.311
E7 -3.0 2.0 6.5 0.310 E8 -3.0 3.0 7.5 0.311 E9 -3.0 2.3 7.5 0.310
E10 -3.0 1.8 6.5 0.309 E11 -3.0 1.8 6.5 0.309 E12 -3.0 3.3 7.5
0.311 E13 -3.0 3.0 8.0 0.313 E14 -3.0 2.9 8.0 0.312 E15 -2.5 1.8
7.0 0.311 E16 -3.0 1.8 7.0 0.308 E17 -3.0 1.8 6.5 0.309 E18 -3.0
2.3 6.5 0.309 RN5 0 0 6.5 0.312
[0048] Creep Rupture tests were performed for these alloys. The
temperatures, times, and number of hours to failure are shown in
Table III:
3TABLE III 2100.degree. F., 2000.degree. F., No. 11 ksi 18 ksi
1800.degree. F., 35 ksi 1600.degree. F., 75 ksi E1 20.0 32.7 69.8
40.8 E2 47.3 114.4 118.1 123.9 E3 40.9 61.1 114.6 95.9 E4 67.6
100.9 147.1 162.3 E5 23.7 42.1 76.9 48.4 E6 34.1 68.1 99.7 79.1 E7
52.5 68.6 120.6 95.3 E8 75.1 97.3 120.4 139.5 E9 42.7 80.4 105.2
102.1 E10 34.3 53.3 88.6 40.0 E11 61.6 58.2 106.0 81.8 E12 217.3
165.9 132.9 205.9 E13 42.7 83.7 123.5 100.2 E14 46.7 87.9 102.9
75.0 E15 61.9 82.7 130.5 211.8 E16 180.9 98/76.5 127.5 174.4 E17
33.0 62.5 96.5 93.5 E18 86.3 85.6 118.5 174.7 RN5 104.3 147 150.5
182.2
[0049] The approach taken in the alloy development was to replace
tantalum with columbium and/or hafnium and/or titanium and/or
tungsten on gamma prime aluminum sites, and to provide tungsten for
additional gamma solid solution strengthening. A slight chromium
reduction was made to offset the tungsten increase to maintain
alloy stability. The alloy compositions set forth in Table I were
evaluated. The compositions E1-E12 represent two designed
experiments, to establish the effects of tungsten, columbium,
hafnium, and titanium modifications. Alloys E1-E8 are a full
factorial in tungsten, columbium, and hafnium, while alloys E1, E4,
E6-7, and E9-12 are a 2.sup.4+1.sub.IV experiment to economically
understand the effects of titanium modifications. Alloys E8 and E12
have the same tungsten, columbium, and hafnium contents, but alloy
E12 has 0.3 percent titanium, with the result that the mechanical
performance of alloy E12 is substantially improved over that of
alloy E8.
[0050] Small lab scale heats were vacuum melted for each
composition. The melts were subsequently directionally solidified
into columnar grained specimens to form directionally oriented
polycrystals and tested in the longitudinal direction. Because the
grain boundaries are parallel to the stress direction in the
testing, the effect of the grain boundaries is minor. Alloy RN5 has
the nominal composition of Rene.TM. N5 alloy.
[0051] Based upon the testing, composition E12 was selected as the
preferred alloy composition, and acceptable variations were defined
for specific applications as set forth above. Some specific alloys
of interest include:
4 TABLE IV Alloy No. Al Ta W Co Hf Y-1716 6.25 3.5 6.0 10 0.60
Y-1717 6.25 3.5 6.0 7.5 0.60 Y-1718 6.20 3.25 6.25 10 0.50
[0052] For these alloys, in all cases the Cr content was 6.0 weight
percent, the Mo content was 1.5 weight percent, the Re content was
3 weight percent, the columbium content was 1.5 percent, the carbon
content was 0.03 weight percent, and the boron content was 0.004
weight percent. Three hundred pound heats of each of these alloys
of Table IV were prepared for evaluation.
[0053] Cyclic oxidation tests, with 20 cycles per hour for 103
hours to 2200.degree. F. and in a Mach 1.0 gas flow were performed
and the weight losses measured. The results are illustrated in FIG.
3.
[0054] Returning to the discussion of FIG. 2, the melted alloy is
solidified to form an article, step 42. The solidification may be
of any operable type, such as a multidirectional heat flow to
produce an unoriented polycrystalline article, a substantially
uniaxial directional solidification to produce a directionally
oriented polycrystalline article, or a uniaxial solidification with
a seed, constriction, or other approach to producing a
substantially single crystal article.
[0055] The solidified article may optionally be post processed,
step 44, by any operable approach. Post processing may include, for
example, cleaning, coating, grinding, machining, and the like.
[0056] The approach just described has defined a low-tantalum
modification of the baseline Rene.TM. N5 nickel-base superalloy.
Low-tantalum modifications of other baseline nickel-base
superalloys may be made using the same principles. In one approach,
a reduced-cost nickel-base superalloy is selected by first
identifying a baseline nickel-base superalloy having a nominal
composition, in weight percent, comprising a baseline tantalum
content of more than about 5 weight percent tantalum, and a
baseline sum (baseline hafnium content plus baseline columbium
content plus baseline titanium content plus baseline tungsten
content), in weight percent. A number of baseline nickel-base
superalloys are candidates for the application of the present
approach, because of their high tantalum contents. Examples of such
commercial baseline nickel-base superalloys include PWA 1484
(nominally 8.7 percent tantalum), Rene.TM. 142 (nominally 6.35
percent tantalum), and the CMSX alloys such as CMSX-4 (nominally
6.5 percent tantalum) and CMSX-10 (nominally 7.5 percent tantalum)
may be modified according to these principles to reduce their
tantalum contents while maintaining acceptable properties.
[0057] A modified nickel-base superalloy is selected having a
nominal composition, in weight percent, comprising a modified
tantalum content at least 1.5 weight percent less than the baseline
tantalum content, and a modified baseline sum of (modified hafnium
content plus modified columbium content plus modified titanium
content plus modified tungsten content) at least 1.5 weight percent
greater than the baseline sum. It is preferred that the increase in
the sum of hafnium, columbium, titanium, and tungsten contents be
at least as great as the decrease in the tantalum content. It is
also preferred that the modified nickel-base superalloy has a
nonzero modified hafnium content, a nonzero modified columbium
content, a nonzero modified titanium content, and a nonzero
modified tungsten content; that is, all of these elements should be
present in nonzero amounts. The data also shows that the sum of the
modified tungsten content plus a modified molybdenum content in the
modified nickel-base superalloy should be at least about 6.5 weight
percent.
[0058] Other features and advantages of the present invention will
be apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. The scope of the invention is not, however, limited
to this preferred embodiment.
* * * * *