Note: Descriptions are shown in the official language in which they were submitted.
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NANOSTRUCTURED SUPERALLOY STRUCTURAL COMPONENTS AND
METHODS OF MAKING
BACKGROUND
The present disclosure relates to superalloys, and more particularly to
structural
components comprising nanostructured superalloys.
Superalloys are metallic alloys that can be used at high temperatures, often
in excess
of 0.7 of the absolute melting temperature. Many structural components, such
as
those used in aircraft engines or power generation devices, are formed from Fe-
, Co-,
or Ni-base superalloys. There is a constant drive towards improving the high
temperature properties of these fatigue-limited structural components in order
to
increase the strength or life of the aircraft engine or power generation
device.
Nanostructured materials often exhibit superior mechanical properties (e.g.,
strength,
hardness, ductility, and the like) relative to their larger-scale
counterparts. Moreover,
the fatigue initiation life of nanostructured materials is significantly
higher than that
of larger-grained materials since dislocation activity may be spread over a
larger
number of grains. Unfortunately, nanostructured alloys, like their larger-
scale
counterparts, undergo the processes of recovery, recrystallization, and/or
grain growth
upon heating. In fact, owing to their non-equilibrium nature, nanoscale grains
are
more susceptible to these processes than are micrometer scale grains.
Consequently,
when thermo-mechanically processing nanostructured alloys into a shaped
article, the
nanostructure and, consequently, the superior properties are often lost.
Furthermore,
during operation of the structural components comprising the nanostructured
alloys,
new opportunities for recovery, recrystallization, and/or grain growth arise
as the
working temperatures increase.
One method of inhibiting recovery, recrystallization, and/or grain growth (and
therefore a method of strengthening alloys) is through Orowan strengthening,
in
which a fine distribution of hard phase particles is incorporated into the
alloy
composition matrix. The strength of such hard phase particle-reinforced alloys
is
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inversely proportional to the spacing between the dispersoid particles, which
can be
controlled by controlling the size of the dispersoid particles. Thus, the use
of
nanoparticles as dispersoids offers the potential of substantially enhancing
alloy
strength.
The introduction of hard phase dispersoid nanoparticles during the processing
of the
alloys presents a major technical challenge. Current processes to disperse
particles
include powder metallurgy routes, such as mechanical alloying of micrometer-
scale
particles, in combination with secondary processes, which include hot-
isostatic
pressing and/or thermo-mechanical processing by hot-forging or extrusion. In
the
mechanical alloying process, nanoparticles are created by repeated fracture of
the
micrometer-scale dispersoid particles during milling. Unfortunately, these
processes
fail to produce a homogeneous distribution of nanoparticles in the alloy
matrix,
especially for large components. In addition, the loading of the hard phase
dispersoid
particles in the alloy composites is frequently limited to less than 2 volume
percent.
Thus, current processes are unable to produce nanostructured alloys having a
sufficiently high enough loading of nanoparticle dispersoids to provide
increased
strength to the alloy or article made therefrom.
There accordingly remains a need in the art for improved methods of producing
nanostructured alloys that have more stable grain structures when exposed to
heat. It
would be particularly advantageous if nanostructured superalloys could be
produced
by such methods. It would be further advantageous if these nanostructured
superalloys
could be used in fatigue-limited structural components, resulting in increased
lifetimes
and/or efficiencies of the devices making use of these structural components.
BRIEF SUMMARY
A superalloy-containing structural component includes a superalloy matrix, and
a
plurality of hard phase nanoparticles dispersed at grain boundaries within the
superalloy matrix, wherein the plurality of hard phase nanoparticles dispersed
at the
grain boundaries comprise about 1 volume percent to about 30 volume percent of
the
structural component, and wherein the superalloy matrix and the plurality of
hard
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phase nanoparticles have been thermo-mechanically processed to form the
structural
component.
In another aspect, a superalloy-containing structural component includes a
superalloy
matrix; a gamma prime phase, wherein the gamma prime phase comprises about 10
weight percent to about 60 weight percent of the nanostructured superalloy
matrix;
and a plurality of hard phase nanoparticles dispersed at grain boundaries
within the
superalloy matrix; wherein the plurality of hard phase nanoparticles dispersed
at the
grain boundaries comprise about 1 volume percent to about 30 volume percent of
the
structural component, and wherein the superalloy matrix, gamma prime phase,
and the
plurality of hard phase nanoparticles dispersed at the grain boundaries within
the
superalloy matrix have been thermo-mechanically processed to form the
structural
component.
A method of manufacturing a nanostructured superalloy-containing structural
component generally includes introducing dislocations into a superalloy
particle
matrix effective to form new grain boundaries within a plurality of superalloy
particles, wherein the grains are nanostructured; introducing hard phase
dispersoid
nanoparticles at a plurality of grain boundaries of the superalloy particles
effective to
pin the grain boundaries; and thermo-mechanically processing the superalloy
particle
matrix and hard phase dispersoid nanoparticles to form the nanostructured
superalloy-
containing structural component.
The above described and other features are exemplified by the following
figures and
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the figures, which are exemplary embodiments and wherein like
elements are numbered alike:
Figure 1 is a graphical representation comparing the tensile strengths of a
prior art
alloy to an alloy made according to one embodiment of the present disclosure;
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Figure 2 is a graphical representation of the high-cycle fatigue properties of
three
different Ni-20Cr alloys;
Figure 3 depicts representative scanning electron micrograph images of a
nanostructured Ni-20Cr alloy, which had dispersoid nanoparticles introduced at
the
grain boundaries both ex-situ and in-situ according to one embodiment of the
present
disclosure; and
Figure 4 is a graphical representation comparing the tensile strengths of a
prior art
alloy to an alloy made according to another embodiment of the present
disclosure.
DETAILED DESCRIPTION
Nanostructured superalloy-containing structural components and their methods
of
manufacture are described herein. In contrast to the prior art, the methods
and
structural components disclosed herein, owing to their nanoscale grain
structure, allow
for increased stability in the superalloy when exposed to heat. Consequently,
fatigue
limited structural components with increased strength can be manufactured,
resulting in
increased lifetimes and/or efficiencies of the devices making use of these
structural
components. As used herein, the term "nanostructured" refers to those
materials having
grains with an average longest dimension of about 1 nanometer (nm) to about
500 mn.
Also, as used herein, the terms "first", "second", and the like do not denote
any order
or importance, but rather are used to distinguish one element from another,
and the
terms "the", "a", and "an" do not denote a limitation of quantity, but rather
denote the
presence of at least one of the referenced item. The modifier "about" used in
connection with a quantity is inclusive of the stated value and has the
meaning
dictated by the context (e.g., includes the degree of error associated with
measurement
of the particular quantity). Furthermore, all ranges disclosed herein are
inclusive of
the endpoints and independently combinable.
The superalloy-containing structural component generally comprises a
superalloy
matrix and a plurality of hard phase nanoparticles dispersed at grain
boundaries within
the superalloy matrix.
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Any Fe-, Co-, or Ni-base superalloy composition may be used to form the
structural
component. The most common solutes in Fe-, Co-, or Ni-base superalloys are
aluminum and/or titanium. Generally, the aluminum and/or titanium
concentrations
are low (e.g., less than or equal to about 15 weight percent (wt %) each).
Other
optional components of Fe-, Co-, or Ni-base superalloys include chromium,
molybdenum, cobalt (in Fe- or Ni-base superalloys), tungsten, nickel (in Fe-
or Co-
base superalloys), rhenium, iron (in Co- or Ni-base superalloys), tantalum,
vanadium,
hafnium, niobium, ruthenium, zirconium, boron, and carbon, each of which may
independently be present in an amount of less than or equal to about 15 wt %.
An exemplary Ni-base superalloy composition, not including the hard phase
nanoparticle dispersoid composition, comprises about 12 to about 20 wt % Cr,
less
than or equal to about 22 wt % Co, less than or equal to about 20 wt % Fe,
about 2 to
about 5 wt % Mo, about 0.5 to about 5 wt % Ti, about 0.5 to about 4 wt % Al,
less
than or equal to about 5 wt % W, less than or equal to about 3 wt % Ta, less
than or
equal to about 3 wt % Re, less than or equal to about 6 wt % Nb, less than or
equal to
about 3 wt % V, less than or equal to about 2 wt % Hf, about 0.02 to 0.2 wt.%
C, less
than or equal to about 0.03 wt.% B, less than or equal to about 0.1 wt.% Zr,
with the
balance being essentially Ni. By "essentially Ni", it is meant that the
composition
may include incidental or trace levels of impurities.
In one embodiment, the superalloy matrix itself is nanostructured. In one
embodiment, the grains within the superalloy matrix have an average longest
dimension of about 10 nm to about 500 nm. In another embodiment, the grains
within
the superalloy matrix have an average longest dimension of about 10 nm to
about
30nm.
The plurality of hard phase nanoparticles may comprise an inorganic oxide, an
inorganic carbide, an inorganic nitride, an inorganic carbonitride, an
inorganic boride,
an inorganic oxycarbide, an inorganic oxynitride, an inorganic silicide, an
inorganic
aluminide, an inorganic sulfide, an inorganic oxysulfide, or a combination
comprising
at least one of the foregoing. Exemplary inorganic oxides include yttria,
alumina,
zirconia, or hafnia. Exemplary inorganic carbides include carbides of hafnium,
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tantalum, molybdenum, zirconium, niobium, chromium, titanium, or tungsten.
Exemplary inorganic sulfides and oxysulfides are cerium sulfide and cerium
oxysulfide, respectively.
In contrast to the prior art, the nanostructured superalloy-containing
structural
components disclosed herein overcome the loading and dispersion limitations
encountered in existing hard phase dispersoid strengthened alloys or
superalloys. In
one embodiment, the superalloy-containing structural component comprises about
1
to about 30 volume percent (vol %) hard phase dispersoid nanoparticles. In
another
embodiment, the superalloy-containing structural component comprises about 10
to
about 30 vol % hard phase dispersoid nanoparticles. This increased loading of
the
hard phase dispersoid nanoparticles results in greater grain boundary pinning
and
therefore greater strength in the structural component.
The plurality of hard phase dispersoid nanoparticles may be spherical, cubic,
rod-like,
needle-like, ellipsoidal, or like shaped. It is not necessary that each of the
plurality of
hard phase dispersoid nanoparticles have the same shape. In one embodiment,
the
plurality of hard phase dispersoid nanoparticles has an average longest
dimension of
about 10 nm to about 500 nm. In another embodiment, each of the plurality of
hard
phase dispersoid nanoparticles has an average longest dimension of about 10 nm
to
about 30 nm.
The structural component may further comprise the so-called "gamma prime"
phase,
which is an intermetallic compound generally based on the formula Ni3(Al/Ti),
and
serves as an additional strengthening mechanism. The gamma prime phase is
particularly resistant to thermal activation, caused by increased
temperatures, which
can lead to recovery and therefore decreased strength. Consequently, a
structural
component comprising an alloy with nanostructured grains, hard phase
dispersoid
nanoparticles, and the gamma prime phase can experience a substantial increase
in its
fatigue life. Depending on the particular conditions to which the structural
component
is exposed, the gamma prime phase may comprise about 10 wt % to about 60 wt %
of
the nanostructured superalloy matrix.
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The structural component may further comprise the so-called "gamma double-
prime"
phase, which is also an intermetallic compound generally based on the formula
Ni3Nb, and like the gamma prime phase also serves as an additional
strengthening
phase. The gamma double-prime, like the gamma prime phase increases in
strength
with temperature up to about 1200 degrees Celsius ( C).
The method of manufacturing a nanostructured superalloy-containing structural
component generally includes introducing dislocations into a superalloy powder
particle matrix effective to form new grain boundaries within a plurality of
superalloy
grains, wherein the grains are nanostructured; introducing hard phase
dispersoid
nanoparticles at the grain boundaries effective to pin the grain boundaries;
and
thermo-mechanically processing the superalloy powder particle matrix and hard
phase
dispersoid nanoparticles to form the nanostructured superalloy-containing
structural
component.
Introducing the dislocations into the superalloy powder particle matrix can be
accomplished by cryomilling, high pressure torsion (HPT), equal channel
angular
pressing (ECAP), cyclic channel die compression (CCDC), accumulative roll
bonding, repetitive corrugation and straightening, twist extrusion, or a
similar severe
plastic deformation technique, or a combination comprising at least one of the
foregoing techniques.
Introducing the hard phase dispersoid nanoparticles at the grain boundaries
can be
done ex-situ and/or in-situ. By ex-situ introduction of the hard phase
dispersoid
nanoparticles, it is meant that the hard phase dispersoid nanoparticles are
intentionally
physically added to the superalloy powder particle matrix during and/or after
the
dislocation formation. By in-situ introduction of the hard phase dispersoid
nanoparticles, it is meant that the hard phase dispersoid nanoparticles are
created (e.g.,
precipitated) within the superalloy powder particle matrix, such as when
cryomilling
in a reactive atmosphere (e.g., in the presence of liquid nitrogen, liquid
hydrocarbons,
oxygen, and the like).
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Thermo-mechanically processing the superalloy powder particles to form the
nanostructured superalloy-containing structural component can be accomplished
by
forging, hot extrusion, hot rolling, and/or like techniques.
Optionally, prior to the thermo-mechanical processing, the superalloy powder
particle
matrix and the hard phase dispersoid nanoparticles may be consolidated into a
compact. Consolidation into a compact may be performed by cold pressing, hot
pressing, hot isostatic pressing, forging, extruding, and/or like
consolidating
techniques.
In one embodiment, a powder particle matrix of a superalloy is cryomilled in
liquid
nitrogen for a time effective to reduce the grain size within the powder
particle matrix
to the desired grain size. During the cryomilling, dispersoid nanoparticles
are formed
(e.g., precipitated) in-situ, for example by oxidizing (if any oxygen is
present) or
nitriding a reactive metal component of the superalloy composition.
Additionally, if
dispersoid nanoparticles are extrinsically added before and/or during the
cryomilling,
then they will be intimately mixed with the powder particle matrix such that
they
serve as pinning agents as well. It should be recognized that there will be a
point after
which no additional cold working (cryomilling) will decrease the grain size of
the
particle powder matrix, but instead will serve to provide an increased
opportunity for
the in-situ formation of dispersoid nanoparticles. This may be desirable
depending on
the specific properties targeted for the final structural component. For
example, in
superalloys comprising aluminum, it may be desirably to have a nitrogen
content of
less than or equal to about 1.0 wt % in order to avoid the increased
brittleness that is
accompanied by a higher nitrogen content. Once the desired grain size
reduction and
nanoparticle dispersoid addition has been achieved, the sample (i.e., the
nanostructured
powder particle matrix and hard phase dispersoid nanoparticles) are
consolidated by
hot isostatic pressing and subsequently forged to form the desired shape.
The nanostructured superalloy-containing structural components disclosed
herein are
suitable for use in at least a portion of a hot gas path assembly, such as a
steam
turbine, gas turbine, aircraft engine, and the like. These hot gas path
assemblies can
have temperatures, to which the structural components are exposed, of about
800 C,
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specifically about 1000 C, and more specifically about 1200 C. Exemplary
structural components include rotating components (e.g., airfoils, discs,
wheels, and
the like), static components (e.g., ducts, frames, casings, buckets, vanes,
and the like),
combustors, and the like.
Advantageously, the nanostructured superalloy-containing structural components
and
methods of manufacture described herein provide for increased stability in the
base
superalloy when exposed to heat. Consequently, fatigue limited structural
components with increased strength can be manufactured, resulting in increased
lifetimes and/or efficiencies of the devices making use of these structural
components.
For example, the finer grains and dispersoids may make possible a doubling, or
more,
of tensile strength and creep resistance. Alloying of the grain boundaries can
inhibit
or eliminate loss in fatigue resistance from environmental exposure.
The present disclosure is illustrated by the following non-limiting examples.
Example 1:
An alloy, comprising nickel and about 20 wt % Cr (Ni-20Cr), was produced by
melting and forging. The average grain diameter after heat treatment of this
prior-art
material is approximately 64 micrometers (gm). The same base alloy composition
was produced as a powder, cryomilled in liquid nitrogen, consolidated, and
heat-
treated. The grain size after heat treatment of this novel material was about
64 nm.
Room temperature tensile tests were conducted on both materials. Figure 1
illustrates
the tensile curves for the two materials. The ultimate tensile strength of the
prior art
micrometer-scale material was about 87 kilopounds per square inch (ksi), or
600
MegaPascals (MPa), while the ultimate tensile strength of the nanostructured
alloy
was about 162 ksi (1117 MPa). This represented an 86% higher tensile strength
in the
alloy produced by the methods disclosed herein.
Example 2:
A nanostructured Ni-20Cr sample was prepared as described in Example 1, except
that, in addition, a plurality of A1z03 dispersoid nanoparticles were
introduced prior to
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cryomilling. Figure 3 presents representative scanning electron microscope
images of
this superalloy composition.
The fatigue properties of 1) this nanostructured Ni-20Cr superalloy, which had
dispersoid nanoparticles introduced at the grain boundaries both ex-situ and
in-situ
(designated "nanostructured Ni-20Cr w/Ab03"), 2) a nanostructured Ni-20Cr
superalloy prepared according to Example 1, which only had dispersoid
nanoparticles
introduced at the grain boundaries in-situ (designated "nanostructured Ni-
20Cr"), and
3) a known Ni-20Cr superalloy, obtained from Special Metals Corporation under
the
trade designation INCONEL MA754 (designated "MA754") were studied. Figure 2
displays the results of the high-cycle fatigue properties of these three
samples. Data is
presented for five samples of the nanostructured Ni-20Cr w/Ab03 superalloy,
five
samples of the nanostructured Ni-20Cr superalloy, and two samples of the MA754
superalloy. As evidenced in Figure 2, each sample of both nanostructured
superalloys
of the present disclosure were able to withstand significantly greater
stresses than the
MA754 superalloy. Furthermore, the nanostructured superalloys of the present
disclosure were also able to experience increased lifetimes before failure
owing to
fatigue.
Example 3:
A Rene 104 alloy is a nickel-base superalloy having a nominal composition (in
weight
percent): 0.05 carbon, 3.4 aluminum, 0.05 zirconium, 3.7 titanium, 0.025
boron, 2.4
tantalum, 3.8 molybdenum, 0.9 niobium, 2.4 tantalum, 13 chromium, 20.6 cobalt,
balance essentially nickel. The alloy was produced by consolidation of
atomized
powder, forging, and heat treatment. One sample of the powder was consolidated
by
hot isostatic pressing, extruded, and heat-treated to yield a micrometer-scale
product.
Another sample of the powder was cryomilled in liquid nitrogen and
subsequently
thermo-mechanically processed by hot isostatic pressing, extrusion, and heat
treatment in a manner identical to the prior-art micrometer-scale product.
The two samples were examined by electron microscopy; and tensile tests were
conducted. In the nanostructured Rene 104 alloy of the present disclosure,
there is a
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distribution of small particles of zirconium and aluminum-rich oxides that
also had
been present on the prior-art powder particle surface; additionally, Ta-rich
carbides
and the gamma-prime phase were present. The grains of the nanostructured Rene
104
alloy are much finer than what was observed for the prior-art micrometer-scale
product. In addition, in the nanostructured Rene 104 alloy of the present
disclosure,
there is a noteworthy distribution of fine titanium-rich particles that are
not present in
the prior-art micrometer-scale product. These titanium-rich particles appear
to form
by a reaction between the milling fluid, (i.e., liquid nitrogen) and titanium
from the
alloy. The titanium particles are associated with regions of much finer grain
size.
Figure 4 illustrates the room temperature tensile curves for the two samples.
The
nanostructured Rene 104 alloy has higher yield (176 vs. 198 ksi) and ultimate
(248 vs.
262 ksi) tensile strengths.
While the disclosure has been described with reference to exemplary
embodiments, it
will be understood by those skilled in the art that various changes may be
made and
equivalents may be substituted for elements thereof without departing from the
scope
of the disclosure. In addition, many modifications may be made to adapt a
particular
situation or material to the teachings of the disclosure without departing
from the
essential scope thereof. Therefore, it is intended that the disclosure not be
limited to
the particular embodiment disclosed as the best mode contemplated for carrying
out
this disclosure, but that the disclosure will include all embodiments falling
within the
scope of the appended claims.
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