Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention - This invention relates to the
fields of directional solidification and directionally
solidified cast eutectic articles. Articles made according
to the present invention are particularly suited for use in
high temperature~ high stress applications such as blades and
vancs in gas turbine engines.
Description of the Prior Art - The metallurgical art has
long appreciated the fact that differences in microstructure
of a given alloy can result in wide differences in mechanical
properties. An example of the importance of microstructure
in determining mechanical properties is presented in the
field of directionally solidified eutectic alloys. U.S.
Patent 3,124,452 issued to Kraft and assigned to the present
assignee discloses the production of metallic eutectic
alloys having a microstructure consisting of parallel second
phase plates, fibers, or rods. Directionally solidified
eutectic articles possess extremely high tensile strengths
in directions parallel to the microstructural alignment,
however, such directionally solidified alloys have aniso-
tropic properties and the tensile strengths of such alloys
drop off rapidly when tensile forces are applied in directions
which do not coincide with the orientation of the microstruc-
ture. Directionally solidified eutectics generally have low
elongation values and this lack of ductility can be a
serious impediment to the successful application of such
materials U.S. Patent 3,790,303 presents one method by
which directionally solidified eutectic articles may be
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produced having a combi~ation of different properties in
different parts of the article. This patent discloses an
article in which one portion is directionally solidified
producing an aligned eutectic structure with a high degree
of alignment and a second portion is a nondirectionally
solidified portion having a random microstructural orientation.
The specific article described is a gas turbine blade in
which the root portion which must be attached to the turbine
disk has a non-orientated microstructure and this non-
orientated microstructure is claimed to have a relatively
high ductility so that the blade may be satisfactorily
attached ~o the disk. The nondirectionally solidified root
portion of the article has a microstructure which is commonly
referred to as dendritic. The airfoil portion of the article
has an aligned microstructure with a high degree of alignment
and a resultant claimed high yield strength. Consequently
the airfoil portion is able to withstand relatively high
stresses which are encountered due to centrifugal forces
and gas pressures.
Other U.S. patents in the field of directionally
solidified eutectics include 3,434,827; 3,528,808; 3,554,817;
3,564,940; 3,671,223 and 3,793,010, all assigned to the present
assignee.
SUMMARY OF THE INVENTION
The present invention includes a eutectic article which
is completely directionally solidified but which has portions
with different microstructures and different mechanical
properties. One portion of the article is ~irectionally
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solidified under conditions which produce a very high degreeof microstructural alignment with the average degree of mis-
alignment being less than 5 and preferably less than 3. This
portion of the article is characterized by high tensile
strengths at elevated temperatures and relatively low ductilities.
A second portion of the article is directionally solidified
under conditions which produce a cellular microstructure in
which the individual lamellar plates or fibers are less well
aligned with the average mi~alignment of the individual plates
or fibers from the common axis of solidification falling between
about ~ and 15. The cellular microstructure is characterized
by having significantly greater ductilities while at the same
time having higher mechanical properties than a dendritic or
nondirectionally solidified equiaxed type of structure.
The article is produced by directional solidification
and the solidification conditions are changed during the process
so as to produce the,different types of microstructure. The
process of the present invention is particularly suited for
the production of gas turbine blades in which the root portion
which operates at lower temperatures and requires more ductility
would be cellular and the airfoil portion which operates at high
temperatures and has less need for ductility then would be a
completely oriented lamellar or fibrous structure.
In accordance with a specific embodiment, a method of
producing a directionally solidified eutectic article, having
a cellular microstructure in one portion of the article and a
completely oriented fibrous microstructure in another portion
of the article, includes the steps of: preparing a ceramic in-
vestment mold having a mold cavity of the desired shape, pre-
heating said mold, filling said mold with a mass of molten
material of approximately eutectic composition, directionally
solidifying the mass of material at a rate which will produce
a fully aligned fibrous microstructure in one portion of the ,
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~ 068454article, increasing the directional solidification rate so as
to produce a cellular microstruction in at least one other
portion of the article.
In accordance with a further embodiment a method of
producing a directionally solidified eutectic article, having
a cellular microstructure in one portion of the article and a
completely oriented fibrous microstructure in another portion
of the article, includes thQ steps of: preparing a ceramic in-
vestluent mold havin~ a mold cavity of the desired shape, pre-
1~ heating said mold, filling said mold with a mass of moltenmaterial of approximately eutectic composition, directionally
solidifying the mass of material at a rate which will produce
a cellular microstructure in one portion of the article, de-
creasing the directional solidification rate so as to produce
a fully aligned fibrous microstructure in at least one other
portion of the article.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a comparison of the rupture life of a
directionally solidified eutectic alloy solidified at
different rates, tested at 1400F/110 ksi.
Fig. 2 shows the rupture life of the alloy/conditions
shown in Fig. 1, tested at 1800F/40 ksi.
Fig. 3 shows the rupture life of the alloy/conditions
shown in Fig. 1, tested at 2000F/20 ksi.
Fig. 4 shows the stress required to produce rupture
in 100 hours in a eutectic alloy, directionally solidified
so as to produce two different microstructures.
Fig. 5a and 5b shows longitudinal and transverse micro-
structures in the root and airfoil sections of a eutectic, gas
turbine blade produced according to the invention.
Fig. ~ shows longitudinal microstructures taken in the
root and airfoil section of a eutectic~ gas turbine blade
produced according to the present invention.
Fig. 7 shows a representative microstructure of a
nondirectionally solidified equiaxed eutectic delta phase
reinforced alloy.
DESCRIPTION OF THE PREFERRED EMBODDMENTS
The present invention concerns the field of directionally
solidified eutectics. As used in this application the term
eutectic is intended to encompass both monovariant eutectics,
bivariant eutectics, and multivariant eutectics and is meant
to encompass eutectics involving elements as well as eutectics
involving compounds. The present invention is broadly
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applicable to all types of eutectics, accordingly, the term
lamellar, rod like,plate like and fibrous will be generally
used interchangeably in the application. In the description
which follows~ all compositions are given in weight percent
unless otherwise specified.
The orientation perfection of a directionally solidified
eutectic may be defined as follows:
Consider projections of the normal axes to the second
phase regions in a small volume of solidified eutectic. If
the eutectic is perfectly aligned these normals will all fall
along the equator of the sphere surrounding the volume element,
if one considers the growth axis of the eutectic to be the
same as the north-south axis of the sphere. At the other
extreme, consider a completely non-oriented eutectic structure
in which case the projected normals to all the second phase
regions in a small volume of solidified eutectic would strike
a surrounding sphere with a completely random and uniform
distribution.
As a practical matter, a perfectly oriented structure is
considered to be one in which the projected normals from the
lamellar plates would strike within about 5 of the equator of
the surrounding sphere and preferably less; that is to say
preferably within about 3 of the equator. A cellular
structure is considered to be one in which the projected
normals from the lamellar plates in the small volume of solidi-
fied eutectic fall within from 6 to 15 of the equator of the
surrounding sphere; that is to say about 10~ from the equator.
In a completely non-oriented eutectic structure the projected
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normals to second phase planes in a volume of solidified
eutectic material would intersect a surrounding sphere at an
average angle of 45 from the equator.
In systems studied to date it has been determined that
completely oriented microstructures possess optimum yield,
tensile and creep rupture strengths at elevated temperatures
while cellular microstructures possess lower strengths but
much greater ductilities at intermediate and elevated
temperatures. Dendritic structures appear to have lower
strengths than cellular structures and usually have lower
ductilities than the perfectly oriented or cellular structures.
Directionally solidified eutectic structures are generally
formed by solidifying molten material of eutectic composition
under conditions such that the plane separating solid
solidified material from liquid material is substantially
uniform and flat and moves along an axis from one portion
of the article to another portion of the article. As the rate
of solidification increases past a certain critical speed,
the interface loses its microscopic flatness and assumes a
cusped configuration. This interface configuration leads to
the production of a cellular colony microstructure. At still
higher speeds, a dendritic growth form may result. Whether
a particular solidification condition produces a perfectly
oriented or cellular microstructure depends upon the ratio of
G to R where G is the thermal gradient in the liquid at the
interface and R is the rate of motion of the interface. The
preceding discussion applies to directional solidification,
when nondirectional solidification occurs and an equiaxed
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structure results. The particular ratio of G to R at which
the microstructure changes from perfectly oriented to
cellular is substantially constant for a particular alloy
system. The critical ratio of G to R may be readily determined
by simple experimental techniques. Nondirectional solidifica-
tion produces a randomly oriented cellular or dendritic
structure.
A turbine blade is made according to the present
invention as follows: a ceramic shell investment mold is
produced of the desired blade configuration. The lost wax
or investment technique may be used to produce the mold. In
the following discussion it will be assumed that the bottom
portion of the mold, which contacts a chill plate, corresponds
to the root of the blade and the upper portion of the mold
having the opening through which molten metal may be intro-
duced corresponds to the airfoil portion of the article. Natur-
ally the mold could be produced in an inverted form. It will
be further assumed that solidification takes place first in
the bottom portion of the mold and moves toward the top
portion of the mold. Of course the solidification process could
be reversed if means were provided to supply molten metal
to the bottom portion of the mold. The mold is preferably
preheated to above the melting point of the alloy in question
and is then filled with molten metal and maintained at a
temperature above the melting point of the eutectic alloy.
In order to induce directional solidification the mold is
moved through a thermal gradient, from a temperature above the
melting point of the alloy to a temperature below the melting
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point of the alloy, at a particular rate. In the process
of the present invention, the rate at which the mold is moved
is selected so that the ratio of the thermal gradient G to the
rate R will produce a cellular microstructure in the root or
bottom portion of the casting. The rate at which the mold is
moved through the thermal gradient closely approximates the
ra~e at which the casting solidifies, however, those skilled
in the art will appreciate that when the cross sectional area
of the cast article changes drastically the ra~te of movement
of the solidification interface will change in a fashion which
is not proportional to the motion of the mold. As the solidi-
fication interface approaches the junction between the root
portion of the article and the blade portion of the article
the rate of motion of the mold is decreased to a rate which will
produce a completely oriented microstructure in the airfoil
portion of the article. Care should be taken to ensure that this
transition between cellular and completely oriented microstruc-
ture occurs in the root section of the blade which is subject
to lower unit stress rather than in the airfoil portion of the
article.
As noted above, the parameter which controls the
microstructure is the ratio of G (thermal gradient) to R
(solidification ra~e~. The article of the present invention has
a cellular microstructure in one portion and a fully oriented
microstructure in at least one other section. One way in
which this result may be achieved is by changing the solidifica-
tion rate R, so as to change the ratio of G to R.
A similar result may be achieved in another, less
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obvious fashion. Those skilled in the art will appreciate
that although the thermal gradient G, at the solidification
interface is largely controlled by the external applied thermal
gradient, other factors may have some affect. The chief of
these other factors is the shape of the cross section of the
part being solidified. Under a constant applied external thermal
gradient, a steeper internal thermal gradient will result in
a part having a thin cross section and a large ratio of surface
~o volume, than a thick section with a low rate of surface to
~0 volume. This fact may be used to produce parts with variable
microstructures at a constant solidified rate R, where a fully
oriented microstructure is desired in a thin section with a
high surface to volume ratio and a cellular microstructure in a
thick section with a low rate of surface to volume. To produce
this desired result, the solidifcation rate R must be carefully
selected, so that when G is high (thin section) a fully oriented
microstructure results and when G is low (thick section) a
cellular microstructure results.
Using this approach, R is critical and the process ,is
more difficult to control than one in which R is varied. This
approach is also limited to the case where a cellular structure
is desirable in a thick section. If R is varied, a fully
oriented structure may be obtained in a thick section and a
cellular structure may be obtained in a thin section. Accordingly
the method in which R is varied is preferred.
It must be emphasized that the process of the present
invention involves total directional solidification throughout
the complete article as compared with the prior art which
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suggests uncontrolled dendritic solidification in one portion
of the article followed by directional solidification in one
portion of the article. Cellular directional solidification
has been found to provide mechanical properties superior to
those obtained by uncontrolled dendritic solidification.
The invention will be explained further by the following
examples, which though illustrative, are not intended to be
limitative upon the claims.
Example_I
In this example a nickel base eutectic alloy containing
20% columbium, 6% chromium, and 2 1/2% aluminum was utilized.
The structure of this alloy in its solidified state is a gamma
nickel solid solution matrix containing a precipitate of gamma
prime formed in the solid state and reinforced with a delta
(Ni3Cb) second phase. Ceramic shell molds were produced and
preheated and were then filled with the molten eutectic which
was directionally solidified by withdrawing the molds from a
furnace having a hot zone temperature of approximately 3,000F.
The molds were withdrawn through a radiation shield so that a
relatively steep temperature gradient could be produced.
Three withdrawal ra~es were used; three centimeters per hour,
7.5 centimeters per hour, and 50 centimeters per hour. The
3 centimeter per hour rate yielded a fully oriented fibrous
microstructure while the 7 1/2 centimeter per hour and 50
centimeter per hour withdrawal rates both yielded cellular
microstructures. The 50 centimeter per hour withdrawal rage
produced a much finer cellular microstructure than the 7 1/2
centimeter per hour withdrawal rate. Samples of these materials
were tested to determine creep rupture properties at 1400F
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under a stress of 110,000 psi, at 1800F with an applied
stress of 40,000 psi and at 2,000F with an applied stress
of 20,000 psi. The results of these tests are shown in
Figs. 1, 2 and 3. Consideration of these figures shows that
the completely oriented fibrous microstructure had significant-
ly less ductility than the cellular microstructure under the
three test conditions. The difference is most noticeable at
1400~F where the completely oriented microstructure had about
1% ductility while the cellular microstructure produced at
the 50 cent~meter per hour withdrawal rate had a ductility
in excess of eight percent. It should be noted that the
scale of rupture life in the figures varies from figure to
figure. Thus, at 1400~F the rupture life of the fine
cellular structure was about 200 hours as compared with the
rupture life of the cellular structure at higher temperatures
which was in the range of 60 to 90 hours. In actual operating
practice the root portion of the blade operates at significantly
lower temperatures than does the blade portion and the root
portion is subjected to shear and bending stresses which require
a certain amount of ductility. Thus it can be seen that if a
blade were made of the alloy by the process of the present
example it would possess an extremely desirable combination of
mechanical properties in that it would have significantly higher
ductilities in the root section which operates in the temperature
range of 1400F coupled with relatively high rupture strength
in the blade portions which operate at higher temperatures and
higher stresses.
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Example II
In this example a nickel base alloy containing 23.1%
columbium and 4.4% aluminum was utilized. This alloy in
its solidified form consists of a gamma prime matrix reinforced
by a second phase delta (Ni3Cb) structure in plate like form.
Samples of this alloy were directionally solidified in a
manner similar to that described for example 1, by withdrawing
them from a heated furnace into a cool zone. It was found
that a withdrawal rate of 2 centimeters per hour produced a
fully oriented lamellar structure while a withdrawal rate of
15 centimeters per hour produced a fully cellular structure.
Several samples of each type structure were tested over a
range of temperatures and the stress to produce rupture in
100 hours was determined as a function of temperature for the
two types of structures. The stress levels required to
produce rupture in 100 hours are shown in Figure 4. It is
evident from consideration of Figure 4 that the cellular
microstructure withstands a higher stress to rupture at
temperatures below 1600F while the lamellar microstructure
produces a higher stress to rupture at temperatures above
1600F. Since the operating conditions of the blade are such
that the root portion operates at a temperature of about 1400F
while the airfoil portion operates at a temperature of approximate-
ly 1800~F it can be seen that for this particular alloy a
superior combination of properties could be produced in the
single blade if the root portion had a cellular microstructure
and the airfoil portion a lamellar microstructure.
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Example III
A ceramic shell mold was produced using standard commercial
practices. The internal mold cavity conformed to a standard
blade configuration except that an elongated root section was
utilized. The cavity was oriented with the root section down.
A gamma prime (Ni3A1) plus delta (Ni3~b) eutectic alloy (23.1%
Cb, 4.4% Al, bal Ni) was melted and poured into the mold which
had been preheated to a temperature of 3,000~F. The filled
mold was solidiied by immersion in a liquid tin bath at a
controlled rate. The liquid tin bath was maintained at a
temperature of 650F. The root portion of the mold was immersed
at a rate of two inches per hour and the immersion rate was
reduced to one inch per hour for solidification of the airfoil
section. The solidified blade was etched and examined.
Figure 5 shows longitudinal and transverse photomicrographs
in the root and airfoil portions of the casti~g, and shows the
cellular andcompletely oriented lamellar microstructure which
resulted. Figure 6 shows longitudinal photomicrographs and
relates them to the position in the blade from which they were
taken. The purpose of the elongated root section (labeled
starter in Figure 6) was to ensure a stable and uniform micro-
structure in the root section. Such a starter portion might
not be necessary in a fully developed commercial process; if it
were necessary, it would be discarded after solidification.
Figure 7 shows an equiaxed eutectic structure in a delta
reinforced alloy which was nondirectionally solidified. The
wide differences in orientation of the second phase regions
are readily visible.
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Example IV
Rupture samples were machined from the root and air-
foil portions of the article produced in Example III.
Samples from the airfoil and root had rupture elongations
of 1.06 and 2.19% respectively, when tested at 1400F/120
KSI. The increased ductility found in the root sample
is desirable since it permits the root section to conform
to the mounting means without rupturing.
Although the invention has been shown and described
with respect to a preferred embodiment thereof, it should
be understood by those skilled in the art that various
changes and omissions in the form and detail thereof may
be made therein without departing from the spirit and the
scope of the invention.
