NICKEL-BASE SUPERALLOY CAST ARTICLE

ARTICLE COULE EN SUPERALLIAGE A BASE DE NICKEL

English Abstract

ABSTRACT OF THE DISCLOSURE

A nickel-base superalloy cast article is provided
with improved stress rupture and creep properties as a
result of providing its microstructure with aligned cellular
dendrites in combination with the substantial absence of
NiAl, carbon, carbides, and Ti. The superalloy consists, in
atomic percent, essentially of 4-11 Cr. 5-16 Al, at least 0.5
R?, up to about 10V, up to about 15 Co, up to about 5 Ta, up
to about 5W, up to about 1 Mo, up to about 2.5 Mn, up to about
2.5 Rh, with the balance nickel and incidental impurities.
The alloy is suited for use in gas turbines.

Claims

The embodiments of the invention in which an exclu-
sive property or privilege is claimed are defined as follows:
1. A cast Ni-base superalloy article the micro-
structure of which comprises aligned cellular dendrites and
is characterized by the substantial absence of NiAl, carbon,
carbides and Ti, the superalloy consisting, in atomic percent,
essentially of 4-11 Cr, 5-16 Al, about 0.5-5 Re, up to about
10V, up to about 15 Co, up to about 5 Ta, up to about 5 W, up
to about 1 Mo, up to about 2.5 Mn, up to about 2.5 Rh, with
the balance essentially nickel and incidental impurities.
2. The article of claim 1 in which the superalloy
consists, in atomic percent, essentially of 3-8 Co, 4-9 Cr,
8-14 Al, 1-4 Ta, 1-7 V, 0.5-5 Re, up to about 2 W, up to about
1 Mo, up to about 1 Mn, up to about 1 Rh, whith the balance
essentially nickel and incidental impurities.
3. The article of claim 2 in which the V is 4-7 at.
percent and the Re is 0.5-3 at. percent.
4. The article of claim 3 in which the superalloy
consists, in atomic percent, essentially of 3-4 Co, 5-9 Cr, 11-
13 A1, 1-3 Ta, 5-6 V, 0.5-2 Re, up to 1.5 W, up to 1 Mo, up to
0.5 Mn, with the balance nickel and incidental impurities.
5. A cas Ni-base superalloy characterized by the
substantial absence of NiAl, carbon, carbides and Ti and
consisting, in atomic percent, essentially of 4-11 Cr, 5-16 Al,
about 0.5 5 Re, up to about 10 V, up to about 15 Co, up to
about 5 Ta, up to about S W, up to about 1 Mo, up to about
2.5 Mn, up to about 2.5 Rh, with the balance essentially nickel
and incidental impurities.
6. The alloy of claim 5 consisting, in atomic percent,
essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta, 1-7V, 0.5-5 Re, ? to


19


about 2 W, up to about 1 Mo, up to about 1 Mn, up to about 1 Rh, with the
balance essentially nickel and incidental impurities.
7. The alloy of claim 6 in which the V is 4-7 at. % and the
Re is 0. 5-3 at. %.
8. The alloy of claim 7 consisting, in atomic percent,
essentially of 3-4 Co, 5-9 Cr, 11-13 Al, 1-3 Ta, 5-6 V, 0. 5-2 Re, up to
1. 5 W, up to 1 Mo, up to 0. 5 Mn, with the balance nickel and incidental
impurities .

Description

1053482 13DV-6301
Background of the Invention
This invention relates to nickel-base superalloys and to cast
articles having an aligned cellular dendritic structure, for example as a
re~3ult of unidirectional solidification.
More recent efforts in the development of nickel-base
superalloys and their articles for use under strenuous operating conditions
such as are found in gas turbine engines includes emphasis on composite
eutectic alloys. Such alloys include reinforcing carbide members such as
fibers which can be formed in situ during solidification of the alloy. One
form of such solidification which ~as been used and has been widely reported
iæ generally referred to as unidirectional solidification.
Creation of such carbide reinforced alloys obviously requires
the addition of the element carbon. However, during the study of such alloys,
it was recognized that detrimental interaction can occur at the interface
between the carbide and the matrix. In addition, carbides can provide a
source for crack initiation.
It has been recognized that the gamma prime former Ti, when
included in a nickel-base superalloy structure, depresses the alloy's incipient
melting temperature and tends to promote the formation of a eutectic phase
for example, the gamma-gamma prime eutectic. With Ti, the incipient
melting temperature is about 2250F.
Summary of the Invention
It is a principal object of the present invention to provide an
improved cast nickel-base superalloy article, the microstructure of which
is substantially free of carbon, carbides, titanium and phases which are
detrimental to high temperature strength properties.

lOS348Z 1 3DV-630 1
Another object is to provide a castable nickel-base superalloy
sub6tantially free of carbon and titanium and which is particularly useful in
the casting of unidirectionally solidified articles.
These and other objects and advantages will be more clearly
understood from the following detailed description, the drawings and the
examples all of which are intended to be typical of rather than in any way
limiting on the scope of the present invention.
Briefly, the present invention, in one form, provides a cast
nickel-base superalloy article the microstructure of which comprises aligned
cellular dendrites and is further characterized by the substantial absence of
the detrimental NiAl phase, carbon, carbides and Ti. The superalloy
associated with the present invention consists, in atomic percent, essentially
of 4-11 Cr, 5-16 Al, at least 0. 5 Re, up to about 10 V, up to about 15 Co, up
to about 5 Ta, up to about 5 W, up to about 1 Mo, up to about 2. 5 Mn, up to
about 2. 5 Rh, with the balance Ni and incidental impurities. Such incidental
impurities may include Ti at less than 1 and C at less than 0.1 at. % which,
by weight, is less than about 0. 017%. It is also preferred that the elements
Zr and B be omitted from the composition, their presence being limited to
those levels which result from normal pick-up of stray elements during
melting and casting, for example up to about 0. 03% Zr and 0. 01% B by weight.
For comparison purposes, the approximate percent by weight equivalent of
this form of the invention consists essentially of 3. 5-10 Cr; 2. 2-7. 2 Al; at
least 1. 5 Re; up to about 8. 5 V; up to about 15 each of Co, Ta and W; up to
about 1. 5 Mo; up to about 2. 5 Mn; up to about 4. 5 Rh; with the balance Ni
and incidental impurities.
In a preferred form of the present invention, the composition,
and atomic percent, consist essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta,
-- 2 --

1053~8Z 1 3DV - 63 01

1-7 V, 0. 5-5 Re, up to about 2 W, up to about 1 each of Mo, Mn and Rh, with
the balance essentially nickel and incidental impurities. For comparison
purposes, the approximate percent by weight equivalent of this preferred
form consists essentially of 3-8 Co; 3. 5-8 Cr; 3. 6-6. 3 A1; 3-12 Ta; 0. 8-6 V;1. 5-15 Re; up to about 6 W; up to about 1. 5 Mo; up to about 1 Mn; up to about
1. 8 Rh; with the balance Ni and incidental impurities.
Brief Description of the Drawings
FIG. 1 is a photomicrographic view at 100 magnifications of
the structure OI example 105 within the scope of the present invention showing
the aligned cellular dendrites in the transverse directionand the absence of
NiAl;
FIG. 2 is a photomicrographic view at 100 magnifications of
the structure of example 105 within the scope of the present invention showing
the aligned cellular dendrites in the longitudinal direction and the absence of
1 5 NiAl;
FIG. 3 is a photomicrographic view at 100 magnifications in
the transverse direction of the alloy of example 87 outside the scope of the
present invention showing the presence of abundant gamma-gamma prime
eutectic phase;
FIG. 4 is a photomicrographic view at 100 magnifications in
the transverse direction of the alloy of example 145 outside the scope of the
present invention showing the presence of NiAl phase; and
FIG. 5 is a graphical comparison of creep properties.
Description of the Preferred Embodiments
In order to provide an improved nickel-base superalloy article
useful under such strenuous operating conditions as are found in the turbine
section of a modern gas turbine engine, and to remove the carbide
-- 3 --

1053~8Z
1 3DV- 6301
strengthening mechanism from the alloy of such article, it is necessary to
design such alloy with significantly larger amounts of other strengthening
elements. The principal strengthening mechanism remaining after carbide
elimination is the gamma prime phase, which is predominantly Ni3Al, in the
gamma matrix, which is predominantly nickel. However, the gamma prime
and the gamma phases can be strengthened with the addition of alloying
elements. One of the problems which exists in the addition of significantly
higher levels of alloying elements is that the detrimental NiAl phase can be
forced to form, particularly at higher Al values. Further, the combination
of elements can move the alloy into that portion of the alloy's phase diagram
which causes the formation of gamma-gamma prime eutectic. The NiAl
causes a significant and dramatic reduction in high temperature properties
and the gamma-gamma prime eutectic lowers the incipient melting point of
the alloy.
The present invention defines a unique castable nickel-base
superalloy which is not strengthened by any carbide mechanism and which
includes a balance of alloying elements providing strength characteristics
even greater than the carbide strengthened type superalloy structures. At
the same time, its structure is extremely uniform, includes substantially
no NiAl phase and avoids the gamma-gamma prime eutectic. As a result,
the incipient melting temperature of the alloy associated with the present
irnvention is at least about 100F higher than an ordinary superalloy's
incipient melting temperature of about 2250~F. In addition, through the
inclusion of a balance of elements which strengthen the gamma prime
precipitate phase, the gamma prime solution temperature is at least 100
higher than that of the ordinary superalloy. Furthermore, the alloy is
uniquely adapt.od for unidirectional solidification to provide that structure
-- 4 --

l(~S348'~
1 3DV-6301
defined as an aligned cellular dendritic structure. Thus, an article having
such a structure and made from the composition provided by the present
in~rention has a higher temperature operating capability as well as over-
temperature protection in that the chances of causing incipient melting of the
article are reduced.
The present invention will be more fully understood from the
following discussion of representative examples of alloy forms studied during
the evaluation of the present invention. Such examples are grouped for ready
comparison but are not intended to be limitations on the invention's scope.
The alloy associated with the present invention, in atomic
percent, consists essentially of 4-11 Cr, 5-16 Al, at least 0. 5 Re, up to
about lOV, up to about 15 Co, up to about 5 each of Ta and W, up to about
1 Mo, up to about 2. 5 each of Mn and Rh, with the balance essentially nickel
and incidental impurities. However, the preferred form of such alloy, in
atomic percent, consists essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta,
1-7 V, 0. 5-5 and more preferably 0.5 - 3 Re, up to about 2 W, up to about
1 each of Mo, Mn and Rh, with the balance essentially Ni and incidental
impurities. The following Table I lists the compositions of selected forms
of such alloy within the preferred range of the present invention and Table II
lists some of each form's mechanical property data. None of the elements
C, Ti, B or Zr, usually found in nickel-base superalloys, were added and
are to be specifically avoided, except in impurity amounts, according to the
present invention. Unless otherwise specified throughout this specification,
all compositions are in atomic percent.

~05348Z 13DV-6301

TAB LE I -
Preferred form of Invention
Composition (Atomic ~o) Balance Ni
Example Co Cr Al Ta V Re W Mo Mn Rh
105 3.5 5.4 12.7 2.25.5 2.0
106 3.5 5.4 12.8 1.45.5 2.0 0.9
118 3.5 5.4 12.7 2.25.5 1.5 0.5
122 3.5 5.4 12.7 2.25.5 1.0 1.0
123 3.5 5.4 12.7 2.25.5 0.5 1.5
124 3.5 5.4 12.7 2.75.5 1.5
125 3.5 5.4 12.7 3.25.5 1.0
127 3.5 5.4 12.7 2.25.5 1.5 0.5
128 3,5 5.4 12.7 2.25.5 1.0 1.0
133 3.5 5.4 12.7 2.25.5 1.5 0.5
134 3.5 5.4 12.7 2.25.5 1.0 1.0
136 3.5 5.9 12.7 2.25.5 1.5
146 3.5 7.0 12.2 2.15.3 2.0
147 3.5 8.5 11.7 2.15.0 2.0

l~S3~8Z 13~V-6301

TABLE II
Mechanical Properties of Preferred Form
Stress Rupture _ Tensile (1200F)
1650F/60 ksi 1800~F/35 ksiUltimateYield R.A.
Example Life(hrs) R. A. Life(hrs) R. A.(ksi)(ksi) (%)
105 246 21 281 39 171 138 4
106 373 24 172 179 139 12
118 163 19 147 25
122 204 32 184 42 157 136 14
123 98 12 175 40 142 125 8
124 301 24 270 49 139 131 12
125 189 24 130 24
127 253 30 195 39
128 203 37 148 49
133 189 24 159 128 18
134 121 16 162 50
136 247 6 173 26
146 174 3 202 26
147 140 20 247 34

1053482
13DV-6301
As used in tables herein, the terms "ksi" means "thousands
of pounds per square inch" and "RA" means "Reduction in Area". All of
the data were obtained by testing in air under the conditions identified.
In order to provide cast stress rupture, tensile and creep
specimens for each of the alloys evaluated in connection with the present
invention, each alloy form was cast and unidirectionally solidified at the rate
of about 20 inches per hour to create the aligned cellular dendritic structure
which characterizes the article of the present invention. Such structure is
aligned predominantly in the ~001> direction, which is equivalent to the<100>
and ~Ol(~directions. Photomicrographic studies of each of the alloys in
Table I showed no NiAl phase present. Referring to the drawings, FIGS. 1
and 2 are photomicrographs at 100 magnifications of example 105, typical of
the microstructure of the present invention. They show the aligned cellular
dendritic structure which resulted from unidirectional solidification, FIG. 1
being in the transverse direction and FIG. 2 being in the longitudinal direction.
The elongated dendrites are more clearly shown in FIG. 2. The absence of
the dark NiAl phase, shown in FIG. 4,to be discussed later, is particularly
evident in FIGS. 1 and 2.
As was mentioned before, NiAl phase is dramatica~ly
detrimental to stress rupture properties and hence one of the important
characteristics of the present invention is that no NiAl is present in the alloy's
microstructure. The data of Table II clearly shows the significantly improved
stress rupture properties of the present invention at no sacrifice of tensile
proper~ies even though no carbide strengthening is present and the gamma
prime strengthener Ti has not been included as an alloying addition.
The present invention specifically excludes the alloying
addition of the elements C, Ti, B and Zr. As has been discussed, the

lOS3~82 13DV-6301

element C, although it plays a significant part in ordinary nickel-base
superalloys in the carbide strengthening mechanism, can provide a source
for crack initiation. Its elimination, except perhaps as an impurity in very
small amounts, defines the alloy associated with the present invention as a
different kind than the more classical types of nickel-base superalloys.
The elements Zr and B can function in nickel-base superalloys
as grain boundary modifiers but have a tendency to lower melting temperature.
Therefore, Zr and B are not included as alloying additions in the present
invention and are present only as residual elements which can be picked up
during normal melting practices. For example, up to about 0. 03% Zr and
up to about 0. 01% B, by weight, can be tolerated by the present invention
without seriously affecting its characteristics.
Only a trace or very small amounts of Ti, for example up to
about 1 atomic percent, can be tolerated by the present invention because
of the tendency of Ti to form the gamma-gamma prime eutectic phase and
to lower the melting temperature. During the evaluation of the present
invention, a variety of Ni-base superalloys including varying amounts of Ti
were made and tested. A typical one which forms the gamma-gamma prime
eutectic in abundance is eXample 87, the composition of which is, in atomic
percent, 6. 7 Co; 9 Cr; 1 Mo; 2. 4 W; 9. 3 Al; 5. 8 Ti; 1. 6 Ta; 0. 03 Zr; 0.17 B;
0. 25 ~ with the balance essentially Ni and incidental impurities. Particularly
because of the presence of Ti and C, the alloy composition is outside of the
scope o the present invention~ FIG. 3 of the drawings is a photomicrograph
at 100 magnifications in the transverse direction of the example 87 after
unidirectional solidification. FIG. 3 shows the presence of large amounts of
the gamma-gamma prime eutectic which is the lighter constituent in the
photomicrograph. The incipient melting temperature of example 87 is

g _

:105348Z 1 3D~T -6301

about 2250F or at least about lOO~F lower than that of the present invention.
Although Ti generally is an essential element in other nickel-
base superalloys as a strong gamma prime former, it has been eliminated
from the present invention except in trace or residual amounts less than
1 at. %. Accordingly, a significant feature of the present invention is the
substantial elimination of the elements C, Zr, B and Ti normally found in
ordinary nickel-base superalloys.
Because of the virtual elimination of the strong gamma prime
former Ti, a relatively large amount of Al, which in itself is a strong gamma
prime former, is included in the alloy composition associated with the present
invention, In this type of alloy, less than 5 at. % Al does not form sufficient
gamma prime and therefore leads to a weak structure. Greater than about
16 at. % Al, even with a careful balance of other elements, tends to drop out
NiAl or excess eutectic and in some alloys tends to reduce incipient melti~g
temperature. In addition to its being a strong gamma prime former, Al also
improves oxidation resistance. Its preferred range is 8-14 at. %.
Substituting for the eliminated Ti is the element V, a gamma
prime former without titanium's tendency toward the formation of the gamma-
gamma prime eutectic phase which can lower melting temperature. V also
provides some solid solution strengthening. In atomic percent, V is included
in the range of up to 10% although 1-7% is preferred. Greater than about 10%
will have a tendency toward the rejection of NiAl and thus dramatically
reduce stress rupture properties. When higher strength is desired, it is
specifically preferred that V be included in the range of about 4-7 at. ~o.
An important element which is required to be included in the
present invention is Re for solid solution strengthening and precipitation
hardening. It affects both the gamma prime precipitate as well as the gamma

- 10 -

i(~53~ 13DV-6301
matrix. At least 0. S at. % Re, equivalent to at least about 1, 5% Re by weight,is required for its significant effect in strengthening the matrix, particularlyto increase high temperature stress rupture life. In addition, it also affects
the gamma prime in that it has a tendency to force hardeners such as Ta and
V into the gamma prime. In addition to this function, Re can substitute in
amounts up to about 2. S at. % for such elements as W, Mn, Ta, Mo and Cr,
all of which tend to partition between the gamma prime precipitate and the
gamma matrix. Thus, Re is included in the present invention within the range
of 0. 5-5 at. % and preferably in the range of 0. 5-3%. As shown by the
examples of the following Table III, Re in the specific range of 0. 5-2 at. %
is particularly desirable for increasing high temperature stress rupture
properties and while con~idering alloy cost. Comparison of example 123 with
example 110 shows that the absence of Re is not compensated for by an increase
in W to maintain the 1800F stress rupture properties of example 123.

10~348;~ `
13DV-6301
TABLE III
Effect of Re on Properties
Composition (Atomic %) _ Stress Rupture Life (hrs)
Base: 3.5 Co, 12,7 Al, Balance Ni
5 Example Cr Ta V Re W1650F/60 ksi1800F/35 ksi
105 5.4 2,2 5.5 2.0 246 281
124 5.4 2.7 5.5 1.5 301 270
136 5.9 2.2 5.5 1.5 247 173
125 5.4 3.2 5.5 1.0 189 130
131 5.4 2.2 6.5 1.0 150 134
126 5.4 3.7 5.5 0.5 111 96
132 5.4 2.2 7.0 0.5 101 36
123 5.4 2.2 5.5 0.5 1.5 98 175
110 5.7 2.2 5.5 --- 2.1 110 96




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1053~8i~
13DV-63Q1
Ta can be included in the present invention up to 5 at. % and
is preferably included in the range of 1-4 at. %. Ta in the type of alloy to
which the present invention relates partitions between the gamma prime
precipitate and the gamma matrix. Thus, it is both a gamma prime former
as well as a solid solution strengthener. Also, it has a tendency to increase
incipient melting temperature.
Two elements which act similarly to Ta are W and Mo.
Although W can be included up to about 5 at. %, it is preferred that such
element be maintained in the range of up to about 2% for improved properties.
Mo, which can be included up to about 1 at. %, in the absence of Ti will
partition to gamma prime. However, it has a tendency to impair corrosion
and oxidation resistance. Therefore, it is included only up to about 1 at. %.
Required primarily for improvement in oxidation resistance
is Cr which can be included in the range of about 4-11 at. % and preferably
in the range of about 4-9 at. %. Less than 4% is insufficient for oxidation
resistance; greater than 11% tends to introduce alloy instability. At such
higher levels, the alloy is either too weak or is unstable. Therefore, it is
preferred that Cr be included in the range of 4-9 at. % with higher amounts
being tolerable provided other elements, within the range of the present
invention, are balanced to avoid forcing the formation of NiAl or other
mdesirable phases such as sigma, eta and mu. The effect of such unbalance
is shown in the following Table IV.




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~05348Z 13DV - 6301

TABLE IV
Effect of Re and Cr on Properties
Composition (Atomic ~o~Stress Rupture Life (hrs)
Base: 3.5 Co, Balance Ni
5 Example r Al Ta V Re1650F/60 ksi 1800F/35 ksi_
136 5.9 12.7 2.25.5 1.5247 173
146 7.0 12,2 2.15.3 2.0174 202
142 8.0 12.7 2.25.5 1.0121 95
147 8.5 11.7 2.15.0 2.0140 247
143 9.0 12.7 2.25.5 1.0101 102
144 10.0 12.7 2.25.5 1.0 2 3
148 10.0 11.2 2.04.7 2.0 33 48
145 11.0 12.7 2.25.5 1.0 1 3




- l~L -


105348Z 13 DV - 6301

Photomicrographic studies of the examples of Table IV showed
that only examples 144, 145 and 148 exhibited the undesirable NiAl structure.
The dramatic difference in properties can be seen from the stress rupture
data presented in Table IV. Referring to the drawings, FIG. 4 is a
photomicrograph at 100 magnifications in the transverse direction of the
structure of example 145 showing a large amount of the dark NiAl detrimental
phase which produced the dramatic reduction in stress rupture properties in
examples 144, 145 and 148 even though example 148 included 2 at. % Re.
For this reason, the present invention is characterized by the absence of
NiAl in its microstructure which also has the aligned cellular dentrites.
The element Co can be included in the present invention as a
substitute for nickel in an amount up to about 15 at. %. It has a slight
tendency toward the increase of melting temperature and lowers the stacking
fault energy. Preferably, Co is included in the range of about 3-8 at. %.
Mn and Rh can be included as partial substitutes for Re in the
present invention. However, they are not as effective as is Re. Each of Mn
and Rh can be included within the present invention in amounts up to 2.5 at. %
but preferably are included in amounts up to 1 at. % each. The effect of
additions of Mn, Rh and Mo at various levels of Re is shown in Table V.




- 15 -

iQ5348Z 1 3DV- 63 01



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-16-

105348Z 13DV-6301

In addition to a remarkable improvement in stress rupture
properties, the present invention provides significant improvement in creep
properties. This is shown in the data on which FIG. 5 is based, comparing
example 106 with a known cast nickel-base superalloy now in production use
in gas turbine engines and included within the scope of U. S. Patent 3, 615, 376 -
Ross, issued October 26, 1971.
From all of these data, it can be seen that the present invention
provides a different kind of alloy which is particularly useful in the formationof articles having improved high temperature properties as a result of the
combination of the balance of elements and the processing to provide aligned
cellular dendrites in the article's microstructure. Ordinary nickel-base
superalloys include carbon which is then available for the formation of various
types of carbides. The strength mechanism and microstructure of such
ordinary alloy heavily involves carbide formation and accumulation at various
points in the microstructure. The literature in respect to nickel-base
superalloys includes very complete discussions of this type of microstructure
and its problems and benefits based on carbides. Without carbon, there is
defined a completely different kind of alloy, the properties of which depend
on the gamma prime, gamma, eutectic and other phases, some of which can
be detrimental or undesirable. For example, NiAl, which is sometimes
called beta phase, is dramatically destructive toward stress rupture properties;the gamma-gamma prime eutectic tends to lower incipient melting temperature
and hence it is to be maintained at as low a level as is practical. To obtain
high temperature strength which otherwise has been provided by the absent
carbides, the type of alloy involved with the present invention must include
significantly larger or different alloying additions to strengthen both the
gamma prime intermetallic precipitate as well as the gamma matrix while
- 17 -

~Q53~8;~
13DV-fi301
removing the tendency toward NiAl formation and reducing the gamma-gamma
prime eutectic formation. Thus, the present invention adds as much A1 and
C'r as possible while maintaining such stability and balancing the alloy's
stability with other alloying additions to avoid the formation of NiA1.
The cast article of the present invention is characterized not
only by its aligned cellular dendritic structure and the absence of carbides
and NiAl, but also by the fact that it does not include alloying additions of
Ti, Zr and B normally added to nickel-base superalloys. In addition> it
includes relatively large amounts of Re which has been found to provide
improved strength both for the gamma matrix as well as for the gamma prime
precipitate. Because the alloy has a narrower solidus-liquidus range, it is
easier to process by unidirectional solidification and therefore such
processing can be conducted at higher rates. Its improved stress rupture
properties are attained without a sacrifice of tensile properties which are as
lS good or better than ordinary superalloy tensile strength and ductilities.
Although the present invention has been described in connection
with specific examples and embodiments, it will be understood by those
skilled in the art the variations and modifications of which the invention is
capable within its broad scope.




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