U.S. patent application number 12/187049 was filed with the patent office on 2010-02-11 for nickel-base superalloy, unidirectional-solidification process therefor, and castings formed therefrom.
This patent application is currently assigned to General Electric Company. Invention is credited to Ganjiang Feng, Shyh-Chin Huang, Jian Zheng.
Application Number | 20100034692 12/187049 |
Document ID | / |
Family ID | 41402441 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100034692 |
Kind Code |
A1 |
Feng; Ganjiang ; et
al. |
February 11, 2010 |
NICKEL-BASE SUPERALLOY, UNIDIRECTIONAL-SOLIDIFICATION PROCESS
THEREFOR, AND CASTINGS FORMED THEREFROM
Abstract
An alloy composition and method by which the incidence of
freckling can be reduced in castings produced with properties
similar to the Rene N5 nickel-base superalloy. The casting has a
unidirectional crystal structure and a composition consisting of,
by weight, 6% to 8% chromium, 6% to 9% cobalt, 0% to 2% molybdenum,
4% to 6% tungsten, 6.4% to 6.9% tantalum, 0% to 2% titanium, 5% to
7% aluminum, 2.7% to 3.0% rhenium, 0.3% to 0.7% haffiium, 0.04% to
0.08% carbon, 0.002% to 0.006% boron, 0% to 0.075% yttrium, 0.002%
to 0.004% zirconium, the balance being nickel and incidental
impurities.
Inventors: |
Feng; Ganjiang; (Greenville,
SC) ; Huang; Shyh-Chin; (Latham, NY) ; Zheng;
Jian; (Greenville, SC) |
Correspondence
Address: |
Hartman & Hartman, P.C.
552 E. 700 N.
Valparaiso
IN
46383
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
41402441 |
Appl. No.: |
12/187049 |
Filed: |
August 6, 2008 |
Current U.S.
Class: |
420/448 ;
148/555; 420/445 |
Current CPC
Class: |
B22D 27/04 20130101;
C22C 19/057 20130101; B22D 27/045 20130101 |
Class at
Publication: |
420/448 ;
420/445; 148/555 |
International
Class: |
C22C 19/05 20060101
C22C019/05; C22F 1/10 20060101 C22F001/10; B22D 30/00 20060101
B22D030/00 |
Claims
1. A nickel-base superalloy having a composition consisting of, by
weight: 6% to 8% chromium; 6% to 9% cobalt; 0% to 2% molybdenum; 4%
to 6% tungsten; 6.4% to 6.9% tantalum; 0% to 2% titanium; 5% to 7%
aluminum; 2.7% to 3.0% rhenium; 0.3% to 0.7% hafnium; 0.04% to
0.08% carbon; 0.002% to 0.006% boron; 0% to 0.075% yttrium; 0.002%
to 0.004% zirconium; the balance being nickel and incidental
impurities.
2. The nickel-base superalloy according to claim 1, wherein the
nickel-base superalloy is in the form of a casting.
3. The nickel-base superalloy according to claim 2, wherein the
casting is substantially free of freckle defects.
4. The nickel-base superalloy according to claim 2, wherein the
casting has a unidirectional crystal structure.
5. The nickel-base superalloy according to claim 4, wherein the
unidirectional crystal structure is a single crystal.
6. The nickel-base superalloy according to claim 4, wherein the
casting is a component for a gas turbine.
7. The nickel-base superalloy according to claim 1, wherein the
composition contains 6.5 to 6.8 weight percent tantalum.
8. The nickel-base superalloy according to claim 1, wherein the
composition contains about 6.7 weight percent tantalum.
9. The nickel-base superalloy according to claim 1, wherein the
composition contains 6.0 to 6.2 weight percent aluminum.
10. The nickel-base superalloy according to claim 1, wherein the
composition contains about 6.1 weight percent aluminum.
11. The nickel-base superalloy according to claim 1, wherein the
composition contains 2.75 to 2.9 weight percent rhenium.
12. The nickel-base superalloy according to claim 1, wherein the
composition contains about 2.8 weight percent rhenium.
13. The nickel-base superalloy according to claim 1, wherein the
composition contains 0.4 to 0.6 weight percent hafnium.
14. The nickel-base superalloy according to claim 1, wherein the
composition contains about 0.5 weight percent hafnium.
15. The nickel-base superalloy according to claim 1, wherein the
composition contains 0.002 to 0.003 weight percent zirconium.
16. The nickel-base superalloy according to claim 1, wherein the
composition contains about 0.0025 weight percent zirconium.
17. The nickel-base superalloy according to claim 1, wherein the
composition contains, by weight, less than 4.90% tungsten, more
than 6.44% tantalum, less than 6.20% aluminum, and less than 2.91%
rhenium.
18. (canceled)
19. A nickel-base superalloy in the form of a casting having a
unidirectional crystal structure that is substantially free of
freckle defects, the superalloy having a composition consisting of,
by weight: 6.75% to 7.25% chromium; 7% to 8% cobalt; 1.3% to 1.7%
molybdenum; 4.5% to 5.0% tungsten; 6.5% to 6.8% tantalum; 0% to
0.05% titanium; 6.0% to 6.2% aluminum; 2.75% to 2.90 rhenium; 0.4%
to 0.6% hafnium; 0.04% to 0.08% carbon; 0.003% to 0.005% boron; 0%
to 0.075% yttrium; 0.002% to 0.003% zirconium; the balance being
nickel and incidental impurities.
20. The nickel-base superalloy according to claim 19, wherein the
composition consists of, by weight: about 7% chromium; about 7.5%
cobalt; about 1.5% molybdenum; about 4.75% tungsten; about 6.7%
tantalum; about 0% to 0.02% titanium; about 6.1 % aluminum; about
2.8% rhenium; about 0.5% hafnium; about 0.06% carbon; about 0.004%
boron; 0% to 0.03% yttrium; about 0.0025% zirconium; the balance
being nickel and incidental impurities.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to nickel-base
superalloy compositions and processes for producing
directionally-solidified castings therefrom. More particularly, the
invention is directed to a nickel-base superalloy that is castable
as single-crystal articles suitable for use as components of gas
turbines and other high temperature applications.
[0002] Components of gas turbines, such as blades (buckets), vanes
(nozzles) and combustor components, are typically formed of nickel,
cobalt or iron-base superalloys characterized by desirable
mechanical properties at turbine operating temperatures. Because
the efficiency of a gas turbine is dependent on its operating
temperatures, there is an ongoing effort to develop components, and
particularly turbine buckets, nozzles, and combustor components,
that are capable of withstanding higher temperatures. As the
material requirements for gas turbine components have increased,
various processing methods and alloying constituents have been used
to enhance the mechanical, physical and environmental properties of
components formed from superalloys. For example, buckets, nozzles
and other components employed in demanding applications are often
cast by unidirectional casting techniques to have
directionally-solidified (DS) or single-crystal (SX)
microstructures, characterized by a crystal orientation or growth
direction in a selected direction to produce columnar
polycrystalline or single-crystal articles.
[0003] Mechanical properties of DS and SX articles depend in part
on the avoidance of high-angle grain boundaries, equiaxed grains,
and defects resulting from interdendrite segregation during the
directional solidification process. As an example, depending on the
particular chemistry of the superalloy, interdendrite segregation
can result in the formation of surface freckles, which form during
solidification as chains of very small equiaxed grains, with
typical chain lengths of about 0.25 inch (about 6 mm) up to a few
inches (about 50+ mm) in length. Such defects can reduce fatigue
life and act as grain initiators during the solidification process
that cause unacceptable off-axial grains. Casting process
parameters such as withdraw rate, cooling speed, and the
solid-liquid interface position appear to have an effect on freckle
formation. As such, prior efforts to eliminate freckle formation
have included process-related techniques; for example,
high-gradient casting technologies have been shown to have a strong
effect on freckle reduction as disclosed in commonly-assigned U.S.
Pat. No. 6,217,286 to Huang et al. The tendency for freckling is
also dependent on composition, an example being the level of
tantalum in the superalloy. Consequently, freckling has also been
addressed through careful control or modifications of the
superalloy composition, as reported in commonly-assigned U.S. Pat.
Nos. 5,151,249, 6,091,141 and 6,909,988.
[0004] The nickel-base superalloy commercially known as Rene N5,
disclosed in commonly-assigned U.S. Pat. No. 6,074,602, has a
composition of, by weight percent, about 5-10 chromium, about 5-10
cobalt, about 0-2 molybdenum, about 3-10 tungsten, about 3-8
tantalum, about 0-2 titanium, about 5-7 aluminum, about 0-6
rhenium, about 0-0.50 hafnium, about 0-0.07 carbon, about 0-0.015
boron, and about 0-0.075 yttrium, the balance being nickel and
incidental impurities. As with the formulation of other
superalloys, the composition of N5 is characterized by controlled
concentrations of certain critical alloying elements to achieve a
desired mix of properties. When cast as single-crystal components
for use in gas turbine applications, including high pressure
turbine buckets and nozzles, such properties include high
temperature creep strength, oxidation and corrosion resistance,
resistance to low and high cycle fatigue (LCF and HCF), and
single-crystal castability. While N5 performs extremely well in
applications within gas turbines, buckets and nozzles cast from N5
can exhibit external and internal freckle chains. Because excessive
freckling can necessitate scrappage of a casting, there is a desire
to reduce the incidence and level of freckling exhibited by the N5
alloy.
BRIEF DESCRIPTION OF THE INVENTION
[0005] The present invention provides an alloy composition and
method by which the incidence of freckling can be reduced in a
unidirectionally-solidified casting having properties similar to
unidirectionally-solidified castings produced from the N5
nickel-base superalloy.
[0006] According to a first aspect of the invention, a nickel-base
superalloy is provided that is particularly suitable for producing
castings with unidirectional crystal structures. The superalloy has
a composition consisting of, by weight, 6% to 8% chromium, 6% to 9%
cobalt, 0% to 2% molybdenum, 4% to 6% tungsten, 6.4% to 6.9%
tantalum, 0% to 2% titanium, 5% to 7% aluminum, 2.7% to 3.0%
rhenium, 0.3% to 0.7% hafnium, 0.04% to 0.08% carbon, 0.002% to
0.006% boron, 0% to 0.075% yttrium, 0.002% to 0.004% zirconium, the
balance being nickel and incidental impurities.
[0007] According to a second aspect of the invention, a method is
provided for producing the nickel-base superalloy as a
unidirectionally-solidified casting. The method generally entails
pouring a molten quantity of the composition into a heated zone of
a preheated mold comprising a main cavity having the shape of the
cast article, withdrawing the mold from the heated zone into a
cooling tank to directionally solidify the molten metal, and then
cooling the mold to produce the casting and a unidirectional
crystal structure.
[0008] According to a preferred aspect of the invention, the
unidirectional crystal structure of the casting is substantially
free of freckle defects. As used herein, the characterization of a
casting being substantially free of freckles is meant to indicate a
total combined length of all freckles of not more than 0.5 inch
(about 12 mm). A significant advantage of this invention is that,
though the composition of the casting contains the same alloying
constituents as N5 and the alloying levels ofthese constituents are
within or overlap the broadest ranges disclosed for N5, it has been
determined that the employment of particular levels and relative
proportions ofthese constituents are able to produce unexpected
superior results as compared to the N5 alloy processed under
identical conditions. In particular, the reduction in the incidence
of freckling is greater than was expected for composition
modifications alone, and the result is the absence of freckling
that would otherwise be an expected property for N5 under the same
processing conditions.
[0009] According to another preferred aspect of the invention, the
unidirectional crystal structure of the casting is a single crystal
with a preferred single crystal direction of <001>, though
crystalline structures having orientations other than <001>
are also within the scope of the invention. Finally, the alloy
composition and castings that can be produced in accordance with
the invention are well suited for components of a gas turbine, such
as buckets, nozzles, and other components of gas turbines.
[0010] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 depicts a turbine bucket that can be formed from the
nickel-base superalloy of the present invention.
[0012] FIG. 2 is a represents a cross-sectional view of a mold for
casting a large single-crystal component in accordance with an
embodiment of this invention.
[0013] FIGS. 3 and 4 are scanned images showing external freckle
chains on the surfaces of buckets formed from alloys whose
compositions are, respectively, in accordance with the production
specification for Rene N5 and modified in accordance with an
embodiment of the present invention.
[0014] FIG. 5 is a graph plotting freckle length measurement data
obtained from buckets cast from an alloy whose compositions is in
accordance with the production specification for Rene N5 and from
two alloys modified in accordance with embodiments of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention was the result of an effort to develop
a nickel-base alloy having properties similar to the nickel-base
alloy commercially known as Rene N5, but with a chemistry that
reduces the tendency for freckling while maintaining the mechanical
properties of the alloy, such as high temperature strength
(including creep resistance), oxidation resistance, fatigue
resistance, castability, and microstructural stability for use in
such applications as the hot gas flow path of a gas turbine. As an
example, FIG. 1 depicts a first stage bucket 10 for a land-based
gas turbine, such as the H and FB class gas turbines used in the
power-generating industry and manufactured by the assignee of the
invention. The bucket 10 has an airfoil 12 and shank 14, with a
dovetail 16 formed on the shank 14 for anchoring the bucket 10 to a
turbine disk (not shown). As is common with turbine buckets, the
bucket 10 is preferably unidirectionally cast to have a columnar
single crystal (SX) or columnar polycrystalline (DS)
microstructure. While the advantages of this invention will be
described with reference to components of a gas turbine, such as
the bucket 10 shown in FIG. 1, the teachings of this invention are
generally applicable to other components that benefit from being
cast from a high-temperature material such as N5.
[0016] As known in the art, freckles form in part as a result of
molten metal convection in the casting mold which disrupts
unidirectional solidification processes, producing irregularities
seen on SX and DS casting surfaces as little chains of equiaxed
crystals. Freckles can act as grain initiators during the
solidification process that cause unacceptable off-axial grains,
and may reduce fatigue life of the casting. According to a
preferred and unexpected aspect of the invention, external and
internal freckling can be inhibited and even eliminated in an alloy
whose constituent levels fall within the ranges disclosed for N5
levels in U.S. Pat. No. 6,074,602, the contents of which relating
to composition of the N5 alloy are incorporated herein by
reference. Furthermore, the alloy can be used to produce large
castings that are typically more prone to freckling, including the
large turbine buckets of land-based gas turbines such as that of
FIG. 1, whose overall lengths are often in the range of about
fifteen to about forty inches (about forty to about one hundred
centimeters).
[0017] The compositional modifications to the nickel-base N5
superalloy that achieve the reduction in freckling were initially
based on data generated from a modeling technique, through which
correlations were identified and analyzed regarding the overall
effect of each element of the N5 alloy on freckle formation. Broad
ranges disclosed in U.S. Pat. No. 6,074,602 for N5 are summarized
in Table I below.
TABLE-US-00001 TABLE I Constituent Weight Percent Chromium 5-10%
Cobalt 5-10% Molybdenum 0-2% Tungsten 3-10% Tantalum 3-8% Titanium
0-2% Aluminum 5-7% Rhenium 0-6% Carbon 0-0.07% Hafnium 0-0.5% Boron
0-0.015% Yttrium 0-0.075% Nickel Balance
[0018] Through this modeling, certain alloying constituents and
certain combinations of constituents were predicted to have a
strong affect on freckle formation, while others were predicted to
have a lesser or essentially no affect. In particular, the modeling
technique was used to quantify the effect of each alloying
constituent on freckle formation, using what will be termed here
the Freckle Susceptibility Index, R.sub.e, defined by the following
equation:
R.sub.e=A(VG).sup.-1.SIGMA.(P.sub.iC.sub.0,i)
where A is a constant, V and G are casting process factors
(respectively, withdraw rate and thermal gradient), and P.sub.i and
C.sub.0,i are chemistry factors (respectively, a convection potency
factor for each element i and the initial concentration of each
element i). The convection potency factor is a factor developed for
use in the modeling technique, and takes into account several
variables, including the slope of the liquidus line in the phase
diagram, the segregation coefficient, solutal volume expansion
coefficient, and thermal volume expansion coefficient. From the
equation, it can be seen that elements with a negative convection
potency factor (P.sub.i) reduce the value of the Freckle
Susceptibility Index, R.sub.e, and therefore are expected to reduce
the risk of freckling, whereas elements with a positive P.sub.i
value increase R.sub.e and are therefore expected to increase the
risk of freckling. P.sub.i values for various elements of N5 are
listed in Table II below:
TABLE-US-00002 TABLE II Constituent P.sub.i Value Chromium 16
Cobalt 0 Molybdenum 0 Tungsten 24 Tantalum -16 Columbian -22
Titanium 29 Aluminum 12 Rhenium 31 Carbon -467 Hafnium -12768
Zirconium 4522
[0019] Based on the equation for calculating R.sub.e values using
the P.sub.i values of Table II, it can be seen that hafnium should
have the strongest influence on freckle formation, and also the
greatest beneficial effect on freckle prevention if present at
sufficiently high levels. Tantalum and carbon also have beneficial
effects, though not as pronounced as hafnium. On the other hand,
aluminum, tungsten, and rhenium are expected to have a moderate
detrimental effect due to their moderate P.sub.i values in
combination with their significant alloying levels in the N5 alloy.
Other elements, including cobalt, chromium, titanium, niobium, and
molybdenum should have negligible effects, either because of their
low P.sub.i values (cobalt, chromium, and molybdenum) or their very
low amounts in N5 (titanium and columbium). Though an impurity in
N5, the very high P.sub.i value for zirconium suggests that it may
also have a detrimental effect on freckling within the typically
allowed range of up to 0.01 weight percent in the N5 alloy.
[0020] Independent of the process parameters, the influence that
any one element might have on the Freckle Susceptibility Index,
R.sub.e, can be predicted based on the product of its convection
potency factor, P.sub.i, and its concentration tolerance
(.DELTA.C.sub.i) in the production specification for the N5 alloy.
For example, this product for hafnium is -12768.times.0.06=-766,
evidencing a very strong positive affect. The same calculation
indicates that tantalum (-3) and carbon (-5) should have moderate
positive affects, rhenium (8), tungsten (6), and aluminum (2)
should have moderate negative affects, and cobalt, chromium, and
molybdenum should have negligible affects (approximately 0), as
will titanium, columbium, and zirconium if present as impurities.
On the other hand, zirconium may significantly promote freckling
within the typically allowed range of up to 0.01 weight percent
(4522.times.0.005=23).
[0021] The extent to which a given constituent promoted or
inhibited freckling influenced the targeted level for that
constituent in the alloy, and consequently the nickel-base alloy of
the present invention nominally contains more hafnium, tantalum and
carbon and less rhenium, tungsten, aluminum, and zirconium than the
nominal N5 composition. Because tantalum and aluminum are both
gamma prime (Ni.sub.3X) formers, a lower level of aluminum can be
offset by the higher level of tantalum to maintain the gamma prime
content desired for N5. However, rhenium and tungsten are both
solid solution strengtheners, and as such attempts to offset
reductions in both of these elements must be by increasing another
solid solution strengthener in order to maintain the desirable
properties of N5. As will be discussed below, experiments conducted
to verify the predictions of the modeling technique suggested that
the influence of hafnium may be sufficiently strong to eliminate
freckling if hafnium levels exceed the hafnium upper limit for N5
(0.50 weight percent).
[0022] According to the invention, the nickel-base alloy has the
following broad, preferred, and nominal compositions, by
weight.
TABLE-US-00003 TABLE III Constituent Broad Range Preferred Range
Nominal Chromium 6 to 8% 6.75 to 7.25% 7% Cobalt 6 to 9% 7 to 8%
7.5% Molybdenum 0 to 2% 1.3 to 1.7% 1.5% Tungsten 4 to 6% 4.5 to
5.0% 4.75% Tantalum 6.4 to 6.9% 6.5 to 6.8% 6.7% Titanium 0 to 2% 0
to 0.05% 0 to 0.02% Aluminum 5 to 7% 6.0 to 6.2% 6.1% Rhenium 2.7
to 3.0% 2.75 to 2.9% 2.8% Carbon 0.04 to 0.08% 0.05 to 0.07% 0.06%
Hafnium 0.3 to 0.7% 0.4 to 0.6% 0.5% Boron 0.002 to 0.006% 0.003 to
0.005% 0.004% Yttrium 0 to 0.075% 0 to 0.075% 0 to 0.03% Zirconium
0.002 to 0.004% 0.002 to 0.003% 0.0025%
The balance of the alloy is nickel and incidental impurities. Aside
from freckling reduction, the alloy of this invention preferably
has essentially the same properties as, and potentially superior
properties to, the N5 alloy, and therefore provides an excellent
alternative to N5.
[0023] As noted above, based on the above modeling technique and
subsequent casting experiments, upper and lower limits of certain
constituents in Table III are higher or lower than the production
specification ranges for the N5 alloy of Table I. Though the
alloying ranges of these alloying constituents are within or
overlap the broadest ranges disclosed for N5, these ranges are very
narrowly tailored to avoid the incidence and degree of freckling
that has previously not been controlled. In view of the complexity
of the N5 alloy and the interactions of its constituents, the
reduction in freckling was viewed as unpredictable prior to the
teachings of the present invention. Furthermore, it was unexpected
that an alloy with specific constituent levels within the broad
ranges taught for the N5 alloy could exhibit substantially reduced
freckling as compared to other alloy chemistries within the N5
composition ranges and processed under identical conditions.
Notably, certain elements noted in the past as inhibiting
freckling, for example, carbon, must be limited to a low and narrow
range in N5 to ensure the attainment of certain mechanical
properties.
[0024] Components of the alloy having SX or DS microstructures can
be produced by generally conventional casting unidirectional
casting techniques. For example, single-crystal castings are
produced from a melt of the alloy, for example, prepared by known
vacuum induction melting techniques. As known in the art, heat
transfer conditions during the solidification of the casting are
controlled so that one section of the casting article does not cool
faster than the rest of the casting to avoid the nucleation and
formation of secondary grains from the melt in competition with the
primary columnar single crystal. Secondary and multi-grains are
further controlled by adjusting the heat transfer conditions during
the withdrawal of the mold into a cooling bath or radiation cooling
zone to ensure that all parts of the casting cool at approximately
the same rate.
[0025] FIG. 2 represents a shell mold 20 of a type suitable for
producing a single-crystal casting of the alloy of this invention.
As known in the art, the mold 20 is preferably formed of a material
such as alumina or silica, and has an internal cavity 22
corresponding to the desired shape of a casting 32, represented as
a turbine bucket similar to that of FIG. 1. As such, the cavity 22
is configured to produce the casting 32 with an airfoil portion 34,
shank 36, and dovetail 38. The mold 20 is shown secured to a chill
plate 24 and placed in a heating zone 26 to heat the mold 20 to a
temperature equal to or above the melting temperature of the alloy,
and more particularly above the liquidus temperature of the alloy
(about 1397.degree. C.). The casting process is preferably carried
out in a vacuum or an inert atmosphere. The molten alloy is poured
into the preheated mold 20, after which unidirectional
solidification is initiated by withdrawing the base of the mold 20
and chill plate 24 downwardly at a fixed rate to a cooling zone
(not shown), which may contain a liquid metal cooling bath or a
vacuum or ambient or cooled air for radiation cooling. The casting
32 grows epitaxially (for example, with the <100>
orientation) based on the crystalline structure and orientation of
a small block of single-crystal seed material 28 at the base of the
mold 20, from which a single crystal forms from a crystal selector
30. The columnar single crystal becomes larger in the enlarged
section of the cavity 22. A bridge 40 connects protruding sections
of the casting 32 with lower sections of the casting 32 so that a
unidirectional columnar single crystal forms substantially
throughout the casting 32. The casting 32 is deemed to be a
substantially columnar single crystal if more than 50% of the
casting is single crystal. Those skilled in the art will appreciate
that a DS casting can be produced in a similar manner, though with
modifications to the mold 20, such a growth zone at the base of the
mold 20 that is open to the chill plate 24, and omission of the
crystal selector 30.
[0026] In the experiments used to verify the predictions of the
modeling technique, nine stage 1 buckets were prepared for the 9FB
class gas turbines manufactured by the assignee of the invention.
The compositions of the buckets were formulated on the basis of
data generated from the aforementioned modeling technique, by which
the overall effect of each element of N5 on freckle formation was
predicted. Targeted chemistries were then proposed, taking into
consideration those elements believed to have a strong effect on
freckle formation, as well as those believed to have a less effect.
Two experimental compositions, Alloy A and Alloy B, were evaluated,
along with an alloy formulated to have a baseline N5 chemistry
("Baseline Alloy"). As evident from Table IV below, Alloys A and B
were substantially identical to the Baseline Alloy except for their
levels of tungsten (which differed by only 0.15% from the
Baseline), tantalum (which differed by only 0.24% or 0.25% from the
Baseline), aluminum (which differed by only 0.15% from the
Baseline), rhenium (which differed by only 0.11% and 0.13% from the
Baseline), hafnium (which differed by only 0.06% or 0.36% from the
Baseline), and zirconium (which differed by only 0.004% from the
Baseline). Nonetheless, it will be seen that these small changes in
chemistry had a significant impact on the incidence of freckling in
their respective alloys.
TABLE-US-00004 TABLE IV Constituent Baseline Alloy Alloy A Alloy B
Chromium 7.12% 7.10% 7.10% Cobalt 7.42% 7.42% 7.41% Molybdenum
1.44% 1.42% 1.42% Tungsten 4.90% 4.75% 4.75% Tantalum 6.44% 6.68%
6.69% Titanium 0.02% 0.02% 0.02% Aluminum 6.20% 6.05% 6.05% Rhenium
2.91% 2.80% 2.78% Carbon 0.058% 0.058% 0.058% Hafnium 0.14% 0.20%
0.50% Boron 0.004% 0.004% 0.004% Zirconium 0.007% 0.003% 0.003%
Nickel Balance Balance Balance
[0027] Three single-crystal castings of each alloy composition were
prepared in accordance with commercial practices for N5, generally
in accordance with the casting process described above, as well as
a heat treatment process disclosed in accordance with
commonly-assigned U.S. Pat. No. 5,100,484, the contents of which
relating to processing of the N5 alloy are incorporated herein by
reference. The casting molds were about 400 millimeters in length
and about 150 mm in width, and filled to contain about 15 kg of
molten alloy. The variables in the experiment were based solely on
the chemistry factors and not the process factors described for the
Freckle Susceptibility Index, R.sub.e. Casting parameters including
furnace temperature, withdrawal rate, and thermal gradient in the
castings during cooling were kept as constants throughout the
experiment. Therefore, results of the experiment reflected only
chemistry affects, and process parameters were not of significance
when comparing the results obtained with Alloys A and B to the
results obtained with the Baseline Alloy.
[0028] Following casting, the single-crystal buckets were examined
by visual and fluorescent penetrant inspection for internal and
external freckles. FIGS. 3 and 4 are scanned images showing
external freckle chains in buckets formed of the Baseline Alloy and
Alloy B, respectively. In FIG. 3, more than twenty freckle chains
can be seen on the bucket root of the Baseline Alloy casting. In
contrast, only two short freckle chains can be seen at the upper
left corner of the bucket root of the Alloy B casting in FIG. 4.
FIG. 5 is a graph plotting freckle length measurement data obtained
from the buckets cast from all three alloys. The data evidence a
reduction in total freckle chain length of about 30% with Alloy A
as compared to the Baseline Alloy, and by a factor of about ten for
the Alloy B castings. Because Alloys A and B were substantially
identical to each other except for their levels of hafnium (0.20%
versus 0.50%), it was concluded that haffiium was the single
greatest chemistry factor in reducing freckling, and that superior
results could be obtained with hafnium levels in excess of about
0.20%, more preferably at least 0.30%.
[0029] The significant reduction in freckling observed in the Alloy
A castings evidenced the influence that small changes in aluminum,
tantalum, tungsten, rhenium, and zirconium levels can also have on
freckling. In particular, the experimental results indicated that
N5 castings with aluminum levels of less than 6.20%, tantalum
levels of more than 6.44%, tungsten levels of less than 4.90%,
rhenium levels of less than 2.91%, and zirconium levels of less
than 0.007% can exhibit significantly improved resistance to
freckling over an N5 castings identical in every other aspect.
Furthermore, it was evident that the resistance to freckling is
extremely sensitive to the levels of these elements.
[0030] Finally, the results from the casting experiments were
believed to confirm the predictions made with equation for the
Freckle Susceptibility Index, R.sub.e.
[0031] While the invention has been described in terms of specific
embodiments, it is apparent that other forms could be adopted by
one skilled in the art. For example, the physical configuration of
the castings could differ from that shown, and processes other than
those noted could be used to produce the castings. Therefore, the
scope of the invention is to be limited only by the following
claims.
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