Note: Descriptions are shown in the official language in which they were submitted.
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ALLOYS FOR HIGH TEMPERATURE
APPLICATIONS, ARTICLES MADE THEREFROM,
AND METHOD FOR REPAIR OF ARTICLES
BACKGROUND OF THE INVENTION
The present invention relates to materials designed to withstand high
temperatures.
More particularly, this invention relates to heat-resistant alloys for high-
temperature
applications, such as, for instance, gas turbine engine components of aircraft
engines
and power generation equipment. The present invention further relates to
methods for
repairing articles for high temperature applications.
There is a continuing demand in many industries, notably in the aircraft
engine and
power generation industries where efficiency directly relates to operating
temperature,
for alloys that exhibit sufficient levels of strength and oxidation resistance
at
increasingly higher temperatures. Gas turbine airfoils on such components as
vanes
and blades are usually made of materials known in the art as
°'superalloys." The term
"superalloy" is usually intended to embrace iron-, cobalt-, or nickel-based
alloys,
which include one or more additional elements to enhance high temperature
performance, including such non-limiting examples as aluminum, tungsten,
molybdenum, titanium, and iron. The term °'based" as used in, for
e:Kample, '°nickel-
based superalloy°' is widely accepted in the art to mean that the
element upon which
the alloy is "based" is the single largest elemental component by atom
fraction in the
alloy composition. Generally recognized to have service capabilities limited
to a
temperature of about 1200°C, conventional superalloys used in gas
turbine airfoils
often operate at the upper limits of their practical service temperature
range. In
typical jet engines, for example, bulk average airfoil temperatures range from
about
900°C to about 1000°C, while airfoil leading and trailing edge
and tip temperatures
can reach about 1150°C or more. At such elevated temperatures, the
oxidation process
consumes conventional superalloy parts, forming a weak, brittle metal oxide
that is
prone to chip or spall away from the part.
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Erosion and oxidation of material at the edges of airfoils lead to degradation
of
turbine efficiency. As airfoils are worn away, gaps between components become
excessively wide, allowing gas to leak through the turbine stages without the
flow of
the gas being converted into mechanical energy. When efficiency drops below
specified levels, the turbine must be removed from service for overhaul and
refurbishment. A significant portion of this refurbishment process is directed
at the
repair of the airfoil leading and trailing edges and tips. For example,
damaged
material is removed and then new material built onto the blade by any of
several
methods, such as, for example, welding with filler material, welding or
brazing new
sections onto the existing blade, or by plasma spraying or laser deposition of
metal
powders onto the blade. The performance of alloys commonly used for repair is
comparable or inferior to that of the material of the original component,
depending
upon the microstructure of the repaired material, its defect density due to
processing,
and its chemistry. Furthermore, in current practice, the original edge
material is made
of the same material as the rest of the original blade, often a superalloy
based on
nickel or cobalt. Because this material was selected to balance the design
requirements of the entire blade, it is generally not optimized to meet the
special local
requirements demanded by conditions at the airfoil leading or trailing edges.
However, maximum temperatures, such as those present at airfoil tips and
edges, are
expected in future applications to be over about 100°C, at which point
many
conventional superalloys begin to melt. Clearly, new materials for repair and
manufacture must be developed to improve the performance of repaired
components
and to exploit efficiency enhancements available to new components designed to
operate at higher turbine operating temperatures.
BRIEF DESCRIPTION
These and other needs are addressed by embodiments of the present invention.
One
embodiment is an alloy comprising, in atom percent, at least about 50% rhodium
(Rh); at least about 5% of a metal selected from the group consisting of
platinum (Pt),
palladium (Pd), and combinations thereof; from about 5% to about 24% ruthenium
(Ru); and from about 1 % to about 40% chromium (Cr); wherein the alloy
comprises
less than about 50% by volume of an A3-structured phase, and wherein the
quantity
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defined by the expression ([Cr) + 2[Ru]) is in the range from about 25% to
about
50%, where [Ru] and [Cr] are the atom percentages of ruthenium and chromium in
the alloy, respectively.
Another embodiment is an article for use in a high temperature, oxidative
environment, comprising the alloy of the present invention.
A third embodiment is a method for repairing an article. The method comprises
providing an article, providing a repair material comprising the alloy of the
present
invention, and joining the repair material to the article.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention
will
become better understood when the following detailed description is read with
reference to the accompanying drawings in which like characters represent like
parts
throughout the drawings, wherein:
Figure I is an isometric view of an airfoil as typically found on a gas
turbine engine
component.
DETAILED DESCRIPTION
The description herein employs examples taken from the gas turbine industry,
particularly the portions of the gas turbine industry concerned with the
design,
manufacture, operation, and repair of aircraft engines and power generation
turbines.
However, the scope of the invention is not limited to only these specific
industries, as
the embodiments of the present invention are applicable to many and various
applications that require materials resistant to high temperature and
aggressive
environments. Unless otherwise noted, the temperature range of interest where
statements and comparisons are made concerning material properties is from
about
1000°C to about 1300°C. The term "high temperature" as used
herein refers to
temperatures above about 1000°C.
The alloy of the present invention balances several competing material
requirements,
including, for example, cost, strength, ductility, and oxidation resistance.
In
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accordance with one embodiment of the present invention, the alloy comprises,
in
atom percent, at least about SO% rhodium (Rh), at least about 5% of a metal
selected
from the group consisting of platinum (Pt), palladium (Pd), and combinations
thereof,
from about 5% to about 24% ruthenium (Ru), and from about 1% to about 40%
chromium (Cr). The alloy comprises less than about 50% by volume of an A3-
structured phase, which is a solid solution containing, among other elements,
Ru and Cr,
and is commonly referred to in the art as "epsilon phase," or s. The presence
of this
phase strengthens the alloy at the cost of some ductility. The remainder of
the alloy
comprises a comparatively ductile AI-structured, or face-centered cubic (FCC)
phase.
In order to achieve a desirable balance of properties, the composition of the
alloy is
maintained such that a quantity defined by the expression ([Cr] + 2[Ru]) is in
the
range from about 25% to about 50%, where [Ru] and [Cr] are the atom
percentages of
ruthenium and chromium in the alloy, respectively. By adding Pd, Cr, and Ru in
the
above proportions to Rh, the present inventors have discovered a material
having
suitable high temperature strength (due to solution strengthening obtained
from the
alloying elements, and in some cases further strengthening due to the presence
of the
A3-structured phase) with sufficient ductility (due to the substantial
proportion of the
AI-structured phase) to be formed into useful shapes. Moreover, the cost of
the alloy
is reduced by adding these alloying elements to the Rh without reducing the
oxidation
resistance of the material below required levels. Those skilled in the art
will
appreciate that Pd is less expensive and less dense than Pt, and so in many
applications where weight and cost are important considerations, optimal
compositions often minimize the use of Pt in favor of Pd. However, Pt may be
used
in place of some or all of the Pd addition where very high environmental
resistance is
desired above all other characteristics.
The mix of the above properties and others such as modulus of elasticity can
be
controlled by varying the relative proportions of constituent elements. For
example,
Cr additions tend to lower the alloy density while increasing the thermal
expansion
coefficient, and Ru additions tend to increase strength and modulus. Moreover,
maintaining the Cr and Ru in accordance with the expression described above
controls
the amount of A3-structured phase in the alloy, allowing further control over
the
strength and ductility of the material. In certain embodiments, the alloy
comprises, in
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atom percent, from about 7% to about 20% ruthenium, and from about 1 % to
about
25% chromium. In particular embodiments, the alloy comprises, in atom percent,
from about 8% to about 20% ruthenium, and from about 1% to about 10% chromium.
Maintaining alloy composition within these ranges tends to minimize the amount
of
A3-structured phase, thereby maximizing the ductility, and therefore 'the
formability,
of the alloy.
Alloys set forth herein as embodiments of the present invention are suitable
for
production using any of the various known methods of metal production and
forming.
Conventional casting, powder metallurgical processing, directional
solidification, and
single-crystal solidification are non-limiting examples of methods suitable
for
forming ingots of these alloys. Thermal and thermo-mechanical processing
techniques common in the art for the formation of other alloys, including, for
instance, forging and heat treating, are suitable for use in maxmfacturing and
strengthening the alloys of the present invention.
Another embodiment is an article for use in a high temperature, oxidative
environment. The article comprises the alloy described above. The article may
be
one that has been repaired, or it may be a newly manufactured article. In some
embodiments, the article comprises a component of a gas turbine engine, such
as, for
example, a turbine blade, vane, or a combustor component. Referring to Figure
l, a
vane or a blade comprises an airfoil 10, which comprises multiple component
sections, including a blade tip 11 (in the case where the component is a
blade), a
leading edge 12, and a trailing edge 13. The alloy of the present invention
may be
suitably disposed anywhere on the component, including, in certain
embodiments, at
one or more of the above component sections. In certain embodiments, the
article
comprises a coating disposed on a substrate, and the coating comprises the
alloy.
Having only particular sections (i.e., those sections known to experience the
most
aggressive stress-temperature combinations) of the airfoil comprise the alloy
of the
present invention minimizes certain drawbacks of alloys comprising significant
amounts of platinum group metals such as, for example, ruthenium, rhodium, and
palladium, including their high cost and high density in comparison to
conventional
airfoil materials. These drawbacks have a reduced effect on the overall
component
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because the comparatively expensive and dense alloy (relative to conventional
superalloys) comprises only a fraction of the overall surface area of the
component.
The properties of the component are thus "tailored" to the expected localized
environments, reducing the need for compromise during the design process and
increasing the expected operating lifetimes for new and repaired components:
A further embodiment of the present invention is a method for repairing an
article. In
this method, an article is provided. The article, in certain embodiments,
comprises a
component of a gas turbine engine, including, for example, a blade, a vane, or
a
combustion component. A repair material is provided, and this repair material
comprises the alloy described above for previous embodiments of the present
invention. This repair material is joined to the article. In some embodiments,
joining
is accomplished, at least in part, by disposing a coating comprising the
repair material
onto the article being repaired. Suitable methods for disposing the coating
include,
for example, thermal spraying, plasma spraying, HVOF spraying, and laser
deposition. In other embodiments, the repair material is joined to the
substrate by one
or more conventional joining processes, including, for example, welding,
brazing, or
diffusion bonding. Regardless of whether the repair material is in the form of
a
coating or a solid section, it may be disposed at any section of the article
deemed to
require the performance characteristics of the repair material. These sections
include,
for example, the leading and trailing edges of airfoils, and blade tips.
While various embodiments are described herein, it will be appreciated from
the
specification that various combinations of elements, variations, equivalents,
or
improvements therein may be made by those skilled in the art, and are still
within the
scope of the invention as defined in the appended claims.
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