U.S. patent application number 15/965839 was filed with the patent office on 2018-08-30 for coated articles and method for making.
The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to Leonardo Ajdelsztajn, Thomas Michael Bigelow, Andrew Joseph Detor, Richard Didomizio, Andrew William Emge, James Anthony Ruud, Michael James Weimer.
Application Number | 20180245194 15/965839 |
Document ID | / |
Family ID | 57681233 |
Filed Date | 2018-08-30 |
United States Patent
Application |
20180245194 |
Kind Code |
A1 |
Detor; Andrew Joseph ; et
al. |
August 30, 2018 |
COATED ARTICLES AND METHOD FOR MAKING
Abstract
An article includes a substrate comprising a
precipitate-strengthened alloy and a coating disposed over the
substrate. The alloy comprises a) a population of gamma-prime
precipitates, the population having a multimodal size distribution
with at least one mode corresponding to a size of less than about
100 nanometers; or b) a population of gamma-double-prime
precipitates having a median size less than about 300 nanometers.
The coating comprises at least two elements, and further comprises
a plurality of prior particles. At least a portion of the coating
is substantially free of rapid solidification artifacts. Methods
for fabricating the article and for processing powder useful for
fabricating the article are also provided.
Inventors: |
Detor; Andrew Joseph;
(Albany, NY) ; Ajdelsztajn; Leonardo; (Niskayuna,
NY) ; Bigelow; Thomas Michael; (Glenville, NY)
; Didomizio; Richard; (Amsterdam, NY) ; Emge;
Andrew William; (West Chester, OH) ; Ruud; James
Anthony; (Delmar, NY) ; Weimer; Michael James;
(Loveland, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
SCHENECTADY |
NY |
US |
|
|
Family ID: |
57681233 |
Appl. No.: |
15/965839 |
Filed: |
April 27, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14974755 |
Dec 18, 2015 |
10017844 |
|
|
15965839 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01D 5/08 20130101; B22F
2301/15 20130101; B22F 2301/45 20130101; C23C 4/134 20160101; F05D
2240/24 20130101; Y10T 428/12931 20150115; Y10T 428/12944 20150115;
C22C 19/07 20130101; B22F 1/0085 20130101; B22F 3/115 20130101;
C22C 19/00 20130101; C22F 1/10 20130101; C23C 4/073 20160101; C23C
4/129 20160101; C22C 30/00 20130101; B22F 5/009 20130101; C22C
1/0433 20130101; B22F 9/04 20130101; C23C 4/123 20160101; B22F
2301/052 20130101; F05D 2300/177 20130101; B22F 9/16 20130101; B22F
7/08 20130101; F05D 2230/90 20130101; B22F 2301/20 20130101; F05D
2300/611 20130101; C23C 24/04 20130101; F01D 25/007 20130101; F05D
2300/175 20130101 |
International
Class: |
C23C 4/073 20060101
C23C004/073; F01D 25/00 20060101 F01D025/00; F01D 5/08 20060101
F01D005/08; C23C 4/134 20060101 C23C004/134; C23C 4/123 20060101
C23C004/123; C23C 4/129 20060101 C23C004/129; C22C 30/00 20060101
C22C030/00; C22C 19/07 20060101 C22C019/07; C22F 1/10 20060101
C22F001/10; B22F 9/16 20060101 B22F009/16; B22F 9/04 20060101
B22F009/04 |
Claims
1-18. (canceled)
19. A method comprising: heat-treating a quantity of metallic
powder, the powder having particulates comprising at least two
elements and a plurality of rapid solidification artifacts present
within the particulates, wherein the heat-treating is performed at
a combination of time and temperature effective to remove
substantially all of the rapid solidification artifacts from the
powder, thereby forming a processed powder having a desired
particle size distribution.
20. The method of claim 19, wherein heat-treating further includes
agitating the powder during heat-treatment.
21. The method of claim 19, further comprising mechanically
processing sintered material formed during the heat-treating.
22. The method of claim 21, wherein mechanically processing
comprises milling the sintered material.
23. The method of claim 19, further comprising disposing a coating
material on a substrate, wherein the processed powder is used as a
feedstock for the coating material.
24. The method of claim 23, wherein the disposing step comprises
spraying the feedstock using a technique that does not melt a
substantial portion of the particulates in the feedstock.
25. The method of claim 24, wherein the technique includes
cold-spraying, flame spraying, liquid injection flame spraying, air
plasma spraying, liquid injection air plasma spraying,
high-velocity oxyfuel spraying, liquid injection high velocity
oxyfuel spraying, high-velocity air-fuel spraying, or liquid
injection high-velocity air-fuel spraying.
26. The method of claim 24, wherein the technique includes liquid
injection high velocity air-fuel spraying.
27. The method of claim 23, wherein the substrate comprises a
nickel-based superalloy, a nickel-iron-based superalloy, or a
cobalt-based superalloy.
28. The method of claim 9, wherein the particulates comprise a
NiCrAlY composition.
29. The method of claim 28, wherein the composition comprises
cobalt; from about 28 percent to about 35 percent nickel; from
about 17 percent to about 25 percent chromium; from about 5 percent
to about 15 percent aluminum; and from about 0.01 to about 1
percent yttrium.
30. The method of claim 28, wherein the coating comprises beta
phase, at least about 25 percent gamma phase by volume, and less
than about 1 percent sigma phase by volume.
31. A method comprising: heat-treating a quantity of powder having
particulates comprising a MCrAlX composition at a temperature in a
range from about 925 degrees Celsius to about 1200 degrees Celsius
for at least about 5 minutes to form a processed powder; and
disposing a coating material on a substrate using cold-spraying,
flame spraying, air plasma spraying, high-velocity oxyfuel
spraying, or high-velocity air-fuel spraying, wherein the processed
powder is used as a feedstock for the coating material, and wherein
the substrate comprises a nickel-based superalloy; wherein the
disposing step comprises spraying the feedstock using a technique
that does not melt a substantial portion of the particulates in the
feedstock.
32. A method comprising: disposing a coating onto a substrate by
spraying a feedstock, the feedstock comprising a plurality of
particulates comprising at least two elements and having at least a
portion of the plurality of particulates substantially free of
rapid solidification artifacts; wherein spraying the feedstock
comprises using a deposition technique that does not melt a
substantial portion of the particulates in the feedstock; wherein
the substrate comprises a precipitate-strengthened alloy, the alloy
comprising a) a population of gamma-prime precipitates, the
population having a multimodal size distribution with at least one
mode corresponding to a size of less than about 100 nanometers; or
b) a population of gamma-double-prime precipitates having a median
size less than about 300 nanometers.
33. The method of claim 32, wherein the substrate comprises a
nickel-based superalloy, a nickel-iron-based superalloy, or a
cobalt-based superalloy.
34. The method of claim 32, wherein the feedstock comprises a
MCrAlY composition.
35. The method of claim of claim 34, wherein the feedstock
comprises cobalt; from about 28 percent to about 35 percent nickel;
from about 17 percent to about 25 percent chromium; from about 5
percent to about 15 percent aluminum; and from about 0.01 to about
1 percent yttrium.
36. The method of claim 34, wherein the feedstock comprises a gamma
phase and a beta phase.
37. The method of claim 36, wherein the feedstock includes less
than about 1 percent sigma phase by volume.
38. The method of claim 36, wherein the coating is disposed in
direct contact with the substrate at an interface, and wherein an
interdiffusion zone extending from the interface into the substrate
has a thickness of less than about 5 micrometers.
39. A method comprising: disposing a coating onto a substrate by
spraying a feedstock, the feedstock comprising a plurality of
particulates comprising a MCrAlX composition and having at least a
portion of the plurality of particulates substantially free of
rapid solidification artifacts; wherein spraying the feedstock
comprises using a deposition technique that does not melt a
substantial portion of the particulates in the feedstock; wherein
the substrate comprises a nickel-based superalloy comprising a
population of gamma-prime precipitates, the population having a
multimodal size distribution with at least one mode corresponding
to a size of less than about 100 nanometers.
Description
BACKGROUND
[0001] This disclosure generally relates to articles coated with
protective materials. More particularly, this disclosure relates to
articles coated with oxidation- and corrosion-resistant coatings
for use at high temperature, and methods for fabricating such
articles.
[0002] Materials used for high-temperature applications, such as,
for instance, gas turbine assembly components, are typically
optimized to provide excellent mechanical properties at high
temperatures. This optimization often sacrifices somewhat the
resistance of the materials to high temperature corrosion and
oxidation. To improve the overall performance of components made
with such materials, coatings of various types are often applied to
enhance component surface properties. For example, a substrate made
of a nickel-based superalloy may be coated with an
oxidation-resistant material such as a so-called "MCrAlX" coating,
that is, a coating that includes chromium, aluminum, and (as
represented by the generic "M") one or more of nickel, cobalt, and
iron. The optional "X" component of the coating, if present, is
typically one or more additional elements, such as yttrium, rare
earth elements, or reactive elements added to enhance certain
properties of the material.
[0003] MCrAlX and other coatings are typically applied using
thermal spray techniques. For example, combustion thermal spray
devices are currently used to produce metallic coatings through
panicle melting, or partial melting, and acceleration onto a
substrate. Such devices use a combustion process to produce gas
temperatures above the melting point of the particles and gas
pressures to impart velocity to the particles. One common problem
encountered in the combustion thermal spray process is the
susceptibility of the sprayed metal powder to oxidation. It is
important to reduce the amount of oxygen present in the metal
coating to improve the formability of the coating, and to make the
coating less brittle.
[0004] Combustion cold spray techniques such as those disclosed in
commonly assigned U.S. patent application Ser. No. 12/790,170 have
been developed to enable formation of dense deposits of materials
without substantially heating the materials above their melting
points. While these techniques have provided attractive results,
under certain conditions articles coated using these techniques
have shown sub-optimal mechanical performance. Thus, there remains
a need for coated articles that minimize performance debits
attributable to the presence of the coating, and for methods for
producing such articles.
BRIEF DESCRIPTION
[0005] Embodiments of the present invention are provided to meet
this and other needs. One embodiment is an article. The article
comprises a substrate comprising a precipitate-strengthened alloy
and a coating disposed over the substrate. The alloy comprises a) a
population of gamma-prime precipitates, the population having a
multimodal size distribution with at least one mode corresponding
to a size of less than about 100 nanometers; or b) a population of
gamma-double-prime precipitates having a median size less than
about 300 nanometers. The coating comprises at least two elements,
and further comprises a plurality of prior particles. At least a
portion of the coating is substantially free of rapid
solidification artifacts.
[0006] Another embodiment is a method comprising: heat-treating a
quantity of metallic powder, the powder having particulates
comprising at least two elements and a plurality of rapid
solidification artifacts present within the particulates, wherein
the heat-treating is performed at a combination of time and
temperature effective to remove substantially all of the rapid
solidification artifacts from the powder, thereby forming a
processed powder having a desired particle size distribution. The
processed powder may be used for fabricating a coated article as
described above.
[0007] Another embodiment is a method comprising: disposing a
coating onto a substrate by spraying a feedstock, the feedstock
comprising a plurality of particulates comprising at least two
elements and being having at least a portion of the plurality of
particulates substantially free of rapid solidification artifacts;
wherein spraying the feedstock comprises using a deposition
technique that does not melt a majority substantial portion of the
particulates in the feedstock; wherein the substrate comprises a
precipitate-strengthened alloy, the alloy comprising a) a
population of gamma-prime precipitates, the population having a
multimodal size distribution with at least one mode corresponding
to a size of less than about 100 nanometers; or b) a population of
gamma-double-prime precipitates having a median size less than
about 300 nanometers.
DRAWINGS
[0008] These and other features, aspects, and advantages of the
present invention will become belter understood when the following
detailed description is read with reference to the accompanying
drawing in which like characters represent like parts, wherein:
[0009] FIG. 1 provides a schematic cross-section of an
illustrative, non-limiting embodiment of the invention.
DETAILED DESCRIPTION
[0010] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about", and
"substantially" is not to be limited to the precise value
specified. In some instances, the approximating language may
correspond to the precision of an instrument for measuring the
value. Here and throughout the specification and claims, range
limitations may be combined and/or interchanged; such ranges are
identified and include all the sub-ranges contained therein unless
context or language indicates otherwise.
[0011] In the following specification and the claims, the singular
forms "a", "an" and "the" include plural referents unless the
context clearly dictates otherwise. As used herein, the term "or"
is not meant to be exclusive and refers to at least one of the
referenced components being present and includes instances in which
a combination of the referenced components may be present, unless
the context clearly dictates otherwise.
[0012] As used herein, the terms "may" and "may be" indicate a
possibility of an occurrence within a set of circumstances; a
possession of a specified property, characteristic or function;
and/or qualify another verb by expressing one or more of an
ability, capability, or possibility associated with the qualified
verb. Accordingly, usage of "may" and "may be" indicates that a
modified term is apparently appropriate, capable, or suitable for
an indicated capacity, function, or usage, while taking into
account that in some circumstances, the modified term may sometimes
not be appropriate, capable, or suitable.
[0013] As used herein, the term "coating" refers to a material
disposed on at least a portion of an underlying surface in a
continuous or discontinuous manner. Further, the term "coating"
does not necessarily mean a uniform thickness of the disposed
material, and the disposed material may have a uniform or a
variable thickness. The term "coating" may refer to a single layer
of the coating material or may refer to a plurality of layers of
the coating material. The coating material may be the same or
different in the plurality of layers.
[0014] Coatings of MCrAlX material, such as CoNiCrAlY material,
impart desirable oxidation resistance and corrosion resistance to
superalloy substrates. However, when superalloy substrates were
coated with MCrAlX material via combustion cold-spray high-velocity
air-fuel (HVAF) techniques, the coated specimens showed inferior
low-cycle fatigue life in a specific temperature and stress range
relative to specimens without the coating. Indeed, this problem of
reduction in substrate mechanical properties associated with the
application of overlay coatings such as MCrAlX-type coatings has
been well-documented in the technical literature for many years.
The present inventors discovered that this debit in low-cycle
fatigue life was due at least in part to the presence of brittle
phases in the coating; these phases provided crack initiation sites
during testing. Further analysis demonstrated that these phases
were present either in the as-received powder used to produce the
coating, or were formed during heat-treatment of the coating after
deposition onto the superalloy substrate.
[0015] The source of this problem of deleterious phase content in
these MCrAlX coatings was ultimately traced to the manufacturing
process used to form the powders. These materials are formed via
atomization, in which molten metal of the desired composition is
sprayed through a nozzle to form liny droplets of liquid metal that
rapidly solidify to form solid particles. The solidification of
highly alloyed materials such as MCrAlX material results in several
distinctive features, including but not limited to the formation of
dendrites, the generation of significant chemical segregation
between dendritic and interdendritic regions, and the formation of
deleterious interdendritic phases such as sigma phase. These
features of rapid solidification of highly alloyed materials,
attributable to chemical segregation, are well known in the art of
metal processing and are collectively referred to herein as "rapid
solidification artifacts."
[0016] The HVAF-based process used to produce the MCrAlX coatings
generally did not melt a substantial portion of the powder
particles used as feedstock; as a result, the coating retained the
rapid solidification artifacts present in the as-received powder.
The high degree of chemical segregation in the coating material
provided conditions that favored the retention of artifact phases
during subsequent heat treatment of the coated articles. The time
and temperature combinations for post-coating heat treatment were
limited due to the temperature sensitivity of the superalloy
substrates, but in general the high levels of chemical segregation
could further promote formation of undesirable intermetallic
phases, such as sigma phase and alpha-chromium, if thermal exposure
during heat treatment or service occurs at sufficiently high
temperature and/or for prolonged exposure times. In addition,
coatings produced with typical thermal spray processes which do
melt a substantial portion of the feedstock particles will obtain
rapid solidification artifacts from the solidification of the
feedstock particles upon deposition due to the rapid cooling
occurring during the spray deposition process.
[0017] Superalloys are well known in the industry to have desirable
strength and other mechanical properties at high temperatures, such
as, for instance, temperatures near 800 degrees Celsius. These
properties are typically controlled in large part by certain
features of the alloy microstructure, such as, for instance, the
amount, size, and size distribution of intermetallic precipitates,
the grain size, and grain morphology. These features are known to
be sensitive to temperature; substantial thermal excursions to
temperatures near or above the solvus temperature of a key
strengthening precipitate phase of a superalloy will, for instance,
alter precipitate size and morphology characteristics, which in
turn will alter the properties of the component.
[0018] The temperatures required to remove the rapid solidification
artifacts from the MCrAlX coatings were higher than could be
applied to the coated articles without significantly damaging the
mechanical properties of the superalloy substrates. Thus, the
present inventors have developed techniques as described herein for
producing articles that overcome the noted shortcomings of
conventional processes. As a result, articles in accordance with
embodiments described herein include a heat-sensitive substrate,
such as a superalloy-bearing substrate, that retains its desired
microstructure, yet also bears a coating made of an alloyed
material that is in a slate typically attributed to having
undergone significant high-temperature heat treatment, that is,
having a microstructure that is substantially free of the
deleterious intermetallic phases, dendritic structures, end
attendant chemical segregation that are artifacts of the
conventional powder production process and its associated rapid
solidification from a melt via atomization and/or that are
artifacts of the conventional thermal spray processes and their
rapid solidification from molten particles via deposition.
[0019] Referring now to FIG. 1, an article 100 comprises a
substrate 110 and a coating 120 disposed over substrate 110.
Article 100 is useful for high temperature service, such as for
turbomachinery components. In one embodiment, article 100 is a
component of a gas turbine assembly, such as a turbine disk.
[0020] Substrate 110 includes a precipitation-strengthened alloy,
meaning an alloy that includes one or more populations of
precipitates that function to strengthen the alloy. Superalloys,
such as nickel-based superalloys and nickel-iron-based superalloys,
are examples of precipitation-strengthened alloys. Examples of
nickel-based superalloys include, without limitation, those alloys
known in the art as Rene 88, Rene 88DT, Rene 104, Rene 65, Rene 95,
RR1000, Udimet 500, Udimet 520, Udimet 700, Udimet 720, Udimet
720LI, Waspaloy, Astroloy, Discaloy, AF115, ME16, N18, and IN100.
Other superalloy compositions include those described in U.S.
patent application Ser. Nos. 12/474,580 and 12/474,651. Further
examples of superalloys include, without limitation, those alloys
known in the art as IN718, IN725, and IN706.
[0021] In many superalloy materials, a significant portion of
strengthening is provided by so-called gamma-prime precipitates.
More specifically, the population of gamma-prime precipitates has a
multimodal size distribution with at least one mode of the
population corresponding to a size of less than about 100
nanometers, such as, for instance, from about 10 nanometers to
about 50 nanometers. Such a multimodal distribution is
characteristic of nickel-based superalloys used in, for instance,
turbine disk applications, where discernable modes in the
precipitate size distribution can often be attributed to primary,
secondary, and sometimes tertiary gamma-prime. A superalloy
microstructure in this condition is susceptible to undesirable
coarsening of the fine gamma-prime in the distribution if the alloy
is heated to a temperature above about 800 degrees Celsius,
depending on the particular alloy.
[0022] Moreover, in other superalloys such as IN718, IN706, and IN
725, a significant portion of strengthening is provided by
so-called gamma-double-prime precipitates. More specifically, the
population of gamma-double-prime precipitates has a median size
less than about 300 nanometers, such as, for instance, from about
10 nanometers to about 150 nanometers. Fine gamma-double-prime is
very important to attaining desired levels of high-temperature
properties in these alloys, but a microstructure in this condition
is susceptible to undesirable coarsening of the fine
gamma-double-prime in the distribution if the alloy is heated to a
temperature above about 600 degrees Celsius, depending on the
particular alloy.
[0023] Coating 120 comprises at least two elements. Because it
comprises more than one element, it is potentially susceptible to
chemical segregation during solidification, depending in part on
the nature of the constituent elements and the processing details.
Generally, as the number of constituent elements in a material
increases, the greater the likelihood that solidification of the
material will undergo some chemical segregation.
[0024] Coating 120 further comprises a plurality of prior particle
boundaries, which is indicative of its having been deposited using
a thermal spray method as opposed to other methods, such as
sputtering, electron-beam physical vapor deposition, chemical vapor
deposition, and others that do not involve acceleration of powder
particles onto the substrate. The use of the combustion cold spray
technique noted previously maintains the particles in substantially
solid state, resulting in a coating that includes deformed prior
particles adhered together at their particle boundaries. These
boundaries are generally visible in the finished coating using
microscopy.
[0025] Notably, at least a portion of coating 120 is substantially
free of rapid solidification artifacts, such as dendrites and
dendrite-like structures, significant chemical segregation between
dendritic and interdendritic regions, and deleterious
interdendritic phases. In some embodiments, this portion is at
least about 10 volume percent of the coating, and in certain
embodiments, at least about 50 volume percent of the coating. In
particular embodiments, this portion is at least about 70 volume
percent of the coating. The microstructure of this portion of
coating 120 is more indicative of chemical equilibrium than would
be expected from a coating fabricated from a combustion cold spray
process using conventional, atomized alloy powders as feedstock.
This provides fewer crack initiation sites and increased ductility
within the resulting coating 120 and helps to improve mechanical
performance of article 100.
[0026] In some embodiments, coating 120 includes a composition that
comprises aluminum, chromium, and M, where M is defined to include
one or more of nickel, cobalt, and iron. In particular embodiments,
the coating composition is designed to impart a higher degree of
resistance to oxidation and/or corrosion than is possessed by the
superalloy substrate. The environmental resistance of the coating
composition in this regard is often provided by elevated levels of
aluminum and/or chromium relative to superalloy compositions. For
instance, in some embodiments the coating composition comprises
aluminum at a concentration higher than a concentration of aluminum
in substrate 110. In certain embodiments, coating 120 comprises
aluminum at a concentration of at least about 2 weight percent, and
in particular embodiments the aluminum concentration is at least
about 5 weight percent. In some embodiments, the coating
composition comprises chromium at a concentration of at least about
10 weight percent. In particular embodiments, the coating
composition includes at least about 5 weight percent aluminum and
at least about 10 weight percent chromium. The M component (nickel,
cobalt, iron, or combinations of these) is typically present at
higher levels than the aluminum and chromium, such as at levels of
at least about 50 weight percent.
[0027] The coating composition may further include other elements.
An MCrAlY composition is a typical example, where the composition
described above further includes yttrium, often in an amount less
than about 3 weight percent, such as less than about 1 weight
percent. More generally, in some embodiments the composition is an
"MCrAlX" composition, meaning it comprises M (as defined
previously), chromium, aluminum, and optionally X, where X includes
one or more additional elements such as yttrium, rhenium, tantalum,
molybdenum, rare earth elements, and/or so-called reactive elements
such as hafnium, zirconium, or silicon. In certain embodiments, the
coating includes a CoNiCrAlY composition. Materials of this type
are well known in the art and are readily available commercially.
One example of a CoNiCrAlY composition includes the following (all
percentages are by weight of coating): from about 28 percent to
about 35 percent nickel, from about 17 percent to about 25 percent
chromium, from about 5 percent to about 15 percent aluminum, and
from about 0.01 to about 1 percent yttrium, with cobalt present in
the remainder along with any other alloying elements and incidental
impurities.
[0028] Notably, in certain embodiments the material of coating 120,
such as an MCrAlX material, includes a gamma phase (face-centered
cubic nickel-rich phase) and a beta phase (ordered
body-centered-cubic phase of nominal composition NiAl). Beta phase
is characterized by high resistance to oxidation, but is generally
not present in superalloy compositions. On the other hand,
as-atomized MCrAlX materials often contain very high amounts of
beta, such as 90 volume percent or more. In some embodiments of the
present invention, the coating 120 includes at least about 10
volume percent beta phase, but not more than about 90 volume
percent, and in certain embodiments not more than about 75 percent
by volume. In particular embodiments, coating 120 includes beta
phase in a range from about 10 volume percent to about 60 volume
percent. Typically, obtaining a significant portion of gamma phase
using as-received MCrAlX powder, for instance, as feedstock is
difficult due to the rapid solidification of the powder during its
manufacture. In stark contrast, coating 120 in accordance with some
embodiments of the present invention includes at least about 10
percent by volume of gamma phase, and in certain embodiments
includes at least about 25 percent by volume gamma phase. In
particular embodiments the gamma phase is present at a
concentration of at least about 40 percent by volume. Further, in
some embodiments, the coating comprises beta phase in a range from
about 10 volume percent to about 75 volume percent, and at least
about 25 volume percent gamma phase. Moreover, the microstructure
of coating 120 is remarkably low in deleterious intermetallic
phases; in some embodiments the coating 120 comprising gamma and
beta phases (including any combination of the concentration ranges
of these phases described previously) also has less than 1 percent
of sigma phase by volume. These microstructural attributes may
substantially reduce debits in mechanical properties attributable
to the presence of coating on substrate 110.
[0029] As noted above, with its remarkably low level of rapid
solidification defects, the coating 120 has microstructural
attributes generally associated with material that has been heat
treated to allow, for instance, segregation effects to dissipate
through diffusion over time at temperature. On the other hand, the
substrate material, with its fine precipitate structure, has
microstructural attributes generally associated with material that
has not been heated to temperatures near the precipitate solvus
temperature. In the example where coating 120 comprises a high
temperature material such as MCrAlX, this contrast is remarkable
because the heat treatment required to convert the rapid
solidification artifacts of the MCrAlX material would necessitate
heating the coated article to a temperature that would
substantially alter the microstructure of the substrate 110, if the
article were produced by conventional methods.
[0030] Moreover, in a typical high-temperature heat treatment of a
coated article similar in form to article 100, where a coating and
its substrate meet at an interface, an interdiffusion zone develops
at the interface. This zone develops as a result of diffusion
during heat treatment, as elements diffuse generally toward regions
of lower respective concentration. Depending on the relative
concentrations of various elements within the substrate and the
coating, and the relative rates of diffusion of these elements in
the coating and substrate materials, this interdiffusion zone can
extend into the coating, into the substrate, or both. For the
purposes of this disclosure, regardless of whether it extends into
the substrate, into the coating, or both, the interdiffusion zone
is described to be positioned between the coating and the
substrate.
[0031] Because a substantial heat treatment is not required in
processing article 100 of the present invention to remove rapid
solidification defects from coating 120, for example, there is much
less driving force for interdiffusion zone formation relative to
what would be created in a more conventionally processed article,
which would require substantial heat treatment to achieve similar
microstructural attributes to coating 120 and substrate 110 in
accordance with embodiments of the present invention. In some
embodiments, coating 120 is disposed in direct contact with
substrate 110 at an interface 130, and an interdiffusion zone 140
between coating 120 and substrate 110 has a thickness of less than
about 5 micrometers. It will be appreciated that "less than 5
micrometers" contemplates embodiments in which an interdiffusion
zone is not detectable, i.e., has zero thickness. A reduced
interdiffusion zone 140 enhances the properties of article 100 by
limiting the extent of deleterious phase formation that can occur
in this region of mixed chemical composition.
[0032] Coating 120 thickness is often selected to be as thin as
possible while maintaining a desired level of protection. In some
embodiments, nominal thickness is less than about 250 micrometers;
in certain embodiments, the thickness is less than 100
micromenters, and in particular embodiments, the thickness is less
than about 50 micrometers.
[0033] The following example is provided to further illustrate the
above descriptions. In one embodiment, article 100 comprises a
substrate 110 comprising a nickel-based superalloy. The
nickel-based superalloy comprises a population of gamma-prime
precipitates having a multimodal size distribution with at least
one mode corresponding to a size of less than about 100 nanometers.
A coating 120 is disposed over substrate 110 at an interlace 130.
Coating 120, of which at least about 50 volume percent is
substantially free of rapid solidification defects, includes a) a
MCrAlX composition, b) a plurality of prior particle boundaries,
and c) at least about 30 percent gamma phase by volume of the
coating and at least about 10 percent beta phase by volume. An
interdiffusion zone 140 has a thickness of less than about 5
micrometers.
[0034] The above attributes of article 100 are derived from certain
aspects of methods used in its fabrication. In particular, the
present inventors have found that the composition of the metal
powders used to deposit coating 120 may play an important role in
developing the advantageous features described above. Embodiments
of the present invention thus include methods for preparing
feedstock powders, and the use of such prepared powders in
fabricating article 100.
[0035] In one embodiment, a method includes heat-treating a
quantity of metallic powder. The powder includes particulates
comprising at least two elements and a plurality of rapid
solidification artifacts present within the particulates, as would
be typical for powders formed by atomization techniques or other
techniques involving rapid solidification from a molten state. Heat
treating the powder is performed at a combination of time and
temperature effective to remove substantially all of the rapid
solidification artifacts of the powder, thus rendering the powder
material to a condition that is more indicative of chemical
equilibrium than the material was prior to heat treatment.
[0036] To be effective in eliminating rapid solidification
artifacts, the heat treatment is typically performed at a
temperature at which substantial diffusion of constituent elements
occurs within practical processing times. The selection of time and
temperature thus depends in large part on the type of material
being processed. For example, in one embodiment, the particulates
of the powder comprise a MCrAlX composition as described for
coating 120, above. In such embodiments, the heat treatment
temperature may be in a range from about 925 degrees Celsius (about
1700 degrees Fahrenheit) to about 1200 degrees Celsius (about 2200
degrees Fahrenheit) depending in part on the time allotted for heat
treatment. In some embodiments, the heat treatment temperature is
maintained for a time of at least 5 minutes, and may range up to
several hours.
[0037] Notably, in certain embodiments the MCrAlX material, after
the heat treatment step, includes a gamma phase (face-centered
cubic nickel-rich phase) and a beta phase (ordered
body-centered-cubic phase of nominal composition NiAl). Typically,
obtaining a significant portion of gamma phase using as-received
MCrAlX material, such as CoNiCrAlY powder, for example, as
feedstock is difficult due to the rapid solidification of the
powder during its manufacture. In stark contrast, the powder
composition in accordance with some embodiments of the present
invention includes at least about 25 percent by volume of gamma
phase after the heat treatment step. Moreover, the microstructure
of the powder particulates after heat treatment is remarkably low
in deleterious intermetallic phases; in some embodiments the
composition comprises gamma and beta phases, and also has less than
1 percent of sigma phase by volume. The advantages provided by
these attributes have been described above for coating 120.
[0038] Heat treating the powder may be done in any of several ways.
For example, the powder may be disposed in a thin layer on an inert
surface, such as a ceramic crucible, with the crucible disposed in
a furnace. Generally the atmosphere during heat treatment is
maintained to be substantially inert to the powder material to
avoid detrimental reactions, e.g., oxidation. An argon atmosphere
is one example, and practitioners in the art of metal heat treating
are familiar with this and other alternatives. One prevalent
consideration for the heat treatment of the powders is sintering of
adjacent particulates at the elevated temperature. Where powders
are heated as a static layer, a sheet of loosely sintered
particulate may form during heat treatment. Even in embodiments
employing agitation of the particles during heating, as through the
use of a fluidized bed furnace, a rotary furnace, or ultrasonic
agitation, some degree of sintering may occur. In such cases, the
heat treated product is then mechanically processed, such as by
breaking up sintered sheets and/or milling the sintered material in
a ball miller, a swing mill, attrition mill, or similar apparatus
used in the art of mechanical processing, to achieve a processed
powder having the desired size distribution. The desired size
distribution will depend in large part on the process used to form
the powder into coating 120. In one embodiment, the heat treated
and milled product is passed through a 635 mesh screen to provide a
product having a maximum particle size less than about 20
micrometers.
[0039] One embodiment of the present invention includes the powder
formed from the method described above.
[0040] Having been heat treated and, if needed, mechanically
processed to provide a desired particle size distribution, the
powder is then ready to be deposited onto a substrate, such as, but
not limited to, substrate 110, to form a coating, such as, but not
limited to, coating 120 of article 100. Embodiments of the present
invention thus include disposing a coating material 120 on a
substrate 110, wherein the powder processed as described above is
used as a feedstock for the coating material 120. This disposing
step may be performed as an extension of the powder processing
steps described above, or may be performed as a stand-alone method,
where powder processed as described above is supplied separately as
an input to the method. In either case, the method selected for
depositing the processed powder is a spray method that does not
melt a substantial portion of the particulates in the feedstock.
Here "a substantial portion" means a portion of the particulates
sufficient to form the coating described above. This is done to
preserve the advantageous microstructural attributes of the powder
material achieved by the heat treatment described above; melting
and the rapidly resolidifying the material, as in an air plasma
spray process, may remove all of these advantageous features and
produce coatings with rapid solidification artifacts. Examples of
acceptable methods include cold-spraying, flame spraying, air
plasma spraying (APS) high-velocity oxyfuel spraying (HVOF), and
high-velocity air-fuel spraying (HVAF). The last four techniques
typically include the use of liquid injection to help maintain
feedstock temperatures below the melting point of the material. In
a particular embodiment, the depositing step includes the use of
liquid-injection HVAF, also known as combustion cold spray, as
described in U.S. patent application Ser. No. 12/790,170.
[0041] In embodiments intended to provide a superalloy-based
substrate with enhanced resistance to high-temperature corrosion
and/or oxidation, coating applications that employ liquid
injection, especially those in which the liquid also serves as a
carrier for feedstock particles, such as liquid injection HVAF, are
particularly desirable. This is because in these embodiments, where
the coating serves primarily a chemical function (i.e., corrosion
resistance) rather than a structural function (e.g., mechanical
reinforcement), comparatively thin coatings are desirable to avoid
problems associated with mechanical properties of the substrate,
such as debits in fatigue strength. Fine particles typically
produce thin coatings of higher quality than coarse particles, but
techniques such as conventional cold spray that employ gas-based
powder feed systems are difficult to use with fine powders, as the
particles are difficult to feed well into the gas stream, and are
prone to clogging. Liquid-fed systems, on the other hand, lend
themselves to the use of fine particle feed stocks because the
liquid prevents clogging and provides desired momentum to ensure
the particles are adequately entrained within the gas plume.
[0042] Moreover, the cold spray process, which is capable of very
high particle velocity and momentum, produces coating structures in
which the particles are metallurgically bonded to the substrate and
to themselves. Under some conditions, such a high degree of bonding
can be associated with mechanical property debit of the substrate
material, such as in fatigue strength. Coating processes that
employ liquid injection of particles, in contrast, allow for
sufficient particle velocity for the particles to be mechanically
bonded to the substrate and to themselves. That level of particle
bonding provides for adequate coating adherence to the substrate,
but it reduces the potential for mechanical property debit of the
substrate.
[0043] The substrate 110 upon which coating 120 is disposed in the
step may be any of the materials described above for substrate 120.
In particular embodiments, substrate 120 comprises a nickel-based
superalloy, a nickel-iron-based superalloy, or a cobalt-based
superalloy.
[0044] The resulting article 100 formed by the methods described
herein may haw any of the attributes described for article 100
above. For example, the article 100 may be heat treated after
coating 120 is deposited, but heat treatment is typically
restricted to a time/temperature combination that does not
substantially alter the microstructure (particularly the
precipitate size and/or distribution) of substrate 110. An
interdiffusion zone 140 may form as a result of the coating process
and/or any subsequent heat treatment, but the thickness of
interdiffusion zone is, in some embodiments, maintained below about
5 micrometers.
[0045] In one illustrative embodiment, a method in accordance with
embodiments described herein includes heat-treating a quantity of
powder having particulates comprising a MCrAlX composition at a
temperature in a range from about 925 degrees Celsius to about 1200
degrees Celsius for at least about 5 minutes to form a processed
powder; and disposing a coating material 120 on a substrate 110
using a technique that does not melt a substantial portion of the
particulates in the feedstock, such as cold-spraying, flame
spraying, air plasma spraying, high-velocity oxyfuel spraying, or
high-velocity air-fuel spraying, wherein the processed powder is
used as a feedstock for the coating material. The substrate 110
comprises a nickel-based superalloy having a population of
gamma-prime precipitates, the population having a multimodal size
distribution with at least one mode corresponding to a size of less
than about 100 nanometers. Alternatively, the substrate 110
comprises a nickel-iron-based superalloy having a population of
gamma-double-prime precipitates having a median size less than
about 300 nanometers.
[0046] In another illustrative embodiment, a method comprises
disposing a coating 120 onto a substrate 110 by spraying a
feedstock, the feedstock comprising a plurality of particulates
comprising at least two elements, such as any of the MCrAlX
materials described previously, and having at least a portion of
the plurality of particulates substantially free of rapid
solidification artifacts. Spraying the feedstock comprises using a
deposition technique that does not melt a substantial portion of
the particulates in the feedstock, such as by cold-spraying, flame
spraying, air plasma spraying, high-velocity oxyfuel spraying, or
high-velocity air-fuel spraying, as noted previously. Substrate 110
comprises a precipitate-strengthened alloy, the alloy comprising a)
a population of gamma-prime precipitates, the population having a
multimodal size distribution with at least one mode corresponding
to a size of less than about 100 nanometers; or b) a population of
gamma-double-prime precipitates having a median size less than
about 300 nanometers.
EXAMPLES
[0047] The following examples are presented to further illustrate
non-limiting embodiments of the present invention.
Example 1: Powder Processing
[0048] Approximately 50 grams of CoNiCrAlY powder (.about.10
micrometers average size) was placed into an alumina boat and
shaken lightly to distribute in a thin, uniform layer. The powder
was placed into a lube furnace and heat treated under an argon
atmosphere at 1121 degrees Celsius for a period of 15 minutes,
followed by a natural furnace cool. Following heat treatment, the
metal powders had partially sintered to form a solid sheet. The
sheet was broken into approximately 25 millimeter sized flakes by
hand, and the flakes were then loaded into a swing mill. The swing
mill was operated for 6 minutes, which produced a fine,
free-flowing powder. Powder was finally sieved through a #635 mesh
to form the starting stock for subsequent thermal spray
experiments.
Example 2: Coating Deposition
[0049] Thermal spray experiments were conducted using a
liquid-injection high velocity air-fuel (HVAF) thermal spray
process previously described in detail in U.S. patent application
Ser. No. 12/790,170 to deposit a coating having a nominal thickness
of about 20 micrometers. Powder temperature during spraying was
maintained sufficiently low to prevent melting and excessive
oxidation during deposition. A typical microstructure obtained
using this process with the heat treated CoNiCrAlY powder of
Example 1 included gamma phase and beta phase regions that were
clearly observable via scanning electron microscopy. For
comparison, a coating of the same composition sprayed under the
same conditions but using as-received (as-atomized) powder showed
rapid solidification artifacts from the atomization process. For
example, transmission electron microscopy analysis of the coatings
made using the conventional powder revealed the presence of sigma
phase along with beta phase. In contrast, the coating made with
heat treated powder was composed primarily of the more desirable
gamma phase, and includes beta phase, with no delectable sigma
phase.
Example 3: Mechanical Testing
[0050] In general, the coating made with heat-treated powder is
expected to have improved mechanical properties as the gamma phase
is inherently ductile, while sigma phase is typically brittle. Low
cycle fatigue experiments were conducted to test the benefit of
powder heat treatment. Coatings of approximately 25 micrometer
thickness were applied to nickel-based superalloy lest bars and
cycled to failure at 400 degrees Fahrenheit (about 204 degrees
Celsius) with a peak strain of .about.0.6 percent and an A ratio
equal to 1. Relative to the average life of uncoated material, test
bars coated with the as-received powder showed a debit of
approximately -1.2 standard deviations. In contrast, the use of
heat treated powder resulted in no measurable properly debit and a
fatigue life equal to that of uncoated material.
[0051] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *