U.S. patent application number 11/610527 was filed with the patent office on 2008-06-19 for graded metallic structures and method of forming; and related articles.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to SUNDAR AMANCHERLA, KRISHNAMURTHY ANAND, MICHAEL FRANCIS XAVIER GIGLIOTTI, CANAN USLU HARDWICKE, DHEEPA SRINIVASAN, PAZHAYANNUR RAMANATHAN SUBRAMANIAN.
Application Number | 20080142126 11/610527 |
Document ID | / |
Family ID | 39525719 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080142126 |
Kind Code |
A1 |
GIGLIOTTI; MICHAEL FRANCIS XAVIER ;
et al. |
June 19, 2008 |
GRADED METALLIC STRUCTURES AND METHOD OF FORMING; AND RELATED
ARTICLES
Abstract
A metallic structure having a graded microstructure is provided.
The metallic structure comprises a graded region comprising a
plurality of grains having a gradient in grain size varying as a
function of position between a first median grain size at an outer
region and a second median grain size at an inner region and a
plurality of dispersoids dispersed within the microstructure. The
first median grain size is different from the second median grain
size. A method of forming a metallic structure having a graded
microstructure is also provided. The method comprises: providing a
metallic structure comprising at least one reactive species;
diffusing at least one reactant at a controlled rate from an outer
region of the metallic structure towards an inner region of the
metallic structure to form a gradient in reactant activity;
reacting the reactant with the reactive species to form a plurality
of dispersoids; and heat treating the metallic structure to achieve
grain growth so as to form a graded microstructure.
Inventors: |
GIGLIOTTI; MICHAEL FRANCIS
XAVIER; (SCOTIA, NY) ; SUBRAMANIAN; PAZHAYANNUR
RAMANATHAN; (NISKAYUNA, NY) ; AMANCHERLA; SUNDAR;
(BANGALORE, IN) ; ANAND; KRISHNAMURTHY;
(BANGALORE, IN) ; SRINIVASAN; DHEEPA;
(MALLESWARAM, IN) ; HARDWICKE; CANAN USLU;
(SIMPSONVILLE, SC) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
39525719 |
Appl. No.: |
11/610527 |
Filed: |
December 14, 2006 |
Current U.S.
Class: |
148/669 ;
148/320; 148/421; 148/425; 148/426; 148/674; 148/675 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2999/00 20130101; C22C 19/00 20130101; C22C 32/0031 20130101;
C22C 14/00 20130101; C22F 1/10 20130101; B22F 2998/00 20130101;
C22C 19/07 20130101; C22C 32/0026 20130101; B22F 2999/00 20130101;
C22C 19/03 20130101; B22F 2207/13 20130101; B22F 5/04 20130101;
B22F 5/04 20130101 |
Class at
Publication: |
148/669 ;
148/320; 148/421; 148/425; 148/426; 148/674; 148/675 |
International
Class: |
C22F 1/00 20060101
C22F001/00; C22C 14/00 20060101 C22C014/00; C22C 19/03 20060101
C22C019/03; C22C 19/07 20060101 C22C019/07; C22C 38/00 20060101
C22C038/00 |
Claims
1. A metallic structure having a graded microstructure comprising:
a graded region comprising a plurality of grains having a gradient
in grain size varying as a function of position between a first
median grain size at an outer region and a second median grain size
at an inner region, wherein the first median grain size is
different from the second median grain size; and a plurality of
dispersoids dispersed within the microstructure.
2. The metallic structure of claim 1, wherein the metallic
structure comprises a material selected from the group consisting
of cobalt, nickel, iron, and titanium.
3. The metallic structure of claim 2, wherein the metallic
structure comprises a material selected from the group consisting
of a cobalt-based super alloy, a nickel-based super alloy, and a
titanium-based alloy.
4. The metallic structure of claim 3, wherein the metallic
structure comprises a nickel-based super alloy.
5. The metallic structure of claim 1, wherein the metallic
structure comprises an alloy selected from the group selected from
the group consisting of UNS N07718, UNS N13100, UNS N09706, MX4,
RENE104, RENE95, RENE88DT, and UDIMET 720.
6. The metallic structure of claim 1, wherein the first median
grain size has a value in the range from about 100 nanometers to
about 1 micrometer.
7. The metallic structure of claim 6, wherein the first median
grain size has a value in the range from about 100 nanometers to
about 500 nanometers.
8. The metallic structure of claim 1, wherein the second median
grain size has a value in the range from about 10 micrometers to
about 100 micrometers.
9. The metallic structure of claim 8, wherein the second median
grain size has a value in the range from about 10 micrometers to
about 50 micrometers.
10. The metallic structure of claim 1, wherein the dispersoid
comprises a material selected from the group consisting of an
oxide, a nitride, a boride, a carbide, an oxynitride, a
carbo-nitride.
11. The metallic structure of claim 10, wherein the dispersoid
comprises an oxide.
12. The metallic structure of claim 11, wherein the oxide comprises
an oxide selected from the group consisting of alumina, yttria,
hafnia, lanthanum oxide, nickel oxide, thoria, titania, zirconia,
erbium oxide, ceria, and yttrium aluminum oxide.
13. The metallic structure of claim 12, wherein the dispersoids
comprise yttria.
14. The metallic structure of claim 1, wherein the dispersoids have
a median size in the range from about 10 nanometers to about 1
micrometer.
15. The metallic structure of claim 1, wherein the dispersoids have
a median size in the range from about 10 nanometers to about 100
nanometers
16. The metallic structure of claim 1, wherein the metallic
structure is structurally stable in a temperature about 600.degree.
C. to about 1100.degree. C.
17. The metallic structure of claim 1, wherein the metallic
structure is a bulk monolithic structure.
18. A gas turbine component comprising the metallic structure of
claim 1.
19. A turbine airfoil comprising the metallic structure of claim
1.
20. An aircraft engine disc comprising the metallic structure of
claim 1.
21. A method of forming a metallic structure having a graded
microstructure, comprising: providing a metallic structure
comprising at least one reactive species; diffusing at least one
reactant, at a controlled rate, from an outer region of the
metallic structure towards an inner region of the metallic
structure, to form a gradient in reactant activity; reacting the
reactant with the reactive species to form a plurality of
dispersoids; and heat treating the metallic structure to achieve
grain growth, so as to form a graded microstructure, wherein the
graded microstructure comprises a graded region comprising a
plurality of grains having a gradient in grain size varying as a
function of position between a first median grain size at an outer
region and a second median grain size at an inner region, wherein
the first median grain size is different from the second median
grain size; and a plurality of dispersoids dispersed within the
microstructure.
22. The method of claim 21, wherein the metallic structure
comprises a material selected from the group consisting of a
cobalt-based super alloy, a nickel-based super alloy, and a
titanium-based alloy.
23. The method of claim 22, wherein the metallic structure
comprises a titanium-based alloy.
24. The method of claim 21, wherein diffusing a reactant comprises
exposing the metallic structure to an effective activity of the
reactant.
25. The method of claim 24, wherein exposing the metallic structure
to an effective activity of the reactant comprises exposing the
metallic structure to a gaseous phase of the reactant.
26. The method of claim 24, wherein exposing the metallic structure
to an effective activity of the reactant comprises exposing the
metallic structure to a liquid phase of the reactant.
27. The method of claim 21, wherein diffusing the reactant, at a
controlled rate, comprises providing the reactant at a controlled
partial pressure.
28. The method of claim 21, wherein heat treating the metallic
structure to achieve grain growth comprises heating at a
temperature in a range from about 600.degree. C. to about
1200.degree. C.
29. The method of claim 21, wherein the reactive species comprises
a material selected from the group consisting of an oxide former, a
carbide former, a nitride former, and a boride former.
30. The method of claim 29, wherein the reactive species comprises
a plurality of oxide formers.
31. The method of claim 21, wherein the reactive species comprises
at least one selected from the group consisting of aluminum,
yttrium, hafnium, lanthanum, erbium, thorium, titanium, magnesium,
cerium, and erbium.
32. The method of claim 21, wherein the reactant comprises a
material selected from the group consisting of oxygen, boron,
carbon, and nitrogen.
33. The method of claim 21, wherein the dispersoids comprise a
material selected from the group consisting of an oxide, a nitride,
a boride, a carbide, a oxynitride, an intermetallic, and a
carbo-nitride.
34. The method of claim 21, wherein the first median grain size has
a value in the range from about 100 nanometers to about 1
micrometer.
35. The method of claim 21, wherein the second median grain size
has a value in the range from about 10 micrometers to about 50
micrometers.
36. The method of claim 21, wherein reacting the reactant with the
reactive species comprises: decomposing a precursor particles,
dispersed within the metallic structure, into a product comprising
a secondary reactive species and a secondary reactant; and reacting
the secondary reactant with the reactive species.
Description
BACKGROUND OF THE INVENTION
[0001] The invention is related to graded metallic structures. More
particularly, the invention is related to metallic structures
having a gradation in grain size. The invention is also related to
a method of making a metallic structure having a gradation in grain
size.
[0002] Materials having spatial gradients in microstructure or
composition are of considerable interest in disciplines as diverse
as tribology, geology, optoelectronics, biomechanics, fracture
mechanics, and nanotechnology. The potential for improved
mechanical properties in graded metallic structures is attractive
for many high temperature applications. In particular, metallic
structures with graded grain size may achieve a desirable balance
of thermal fatigue resistance and creep resistance in a single
material. Achieving metallic structures with fine grain size and a
systematic gradation in grain size has proven to be a very
challenging task. Many deposition techniques such as thermal
spraying, electrodeposition, and electrophoretic deposition have
been explored as means for preparing ultra-fine grained, graded
metallic coatings. These methods generally have not been successful
in producing bulk metallic structures having high strength and high
temperature stability. There remains a demand for materials having
a graded microstructure, especially for materials with a proper
balance of thermal fatigue resistance and creep resistance. There
is also a demand for methods to produce bulk metallic structures
having an engineered gradation in grain size.
BRIEF DESCRIPTION OF THE INVENTION
[0003] The present invention meets these and other needs by
providing a metallic structure having a gradation in grain size.
The smaller grains at one portion of the material may provide
thermal fatigue resistance for example, and bigger grains at
another portion may provide creep resistance, for example.
[0004] One embodiment of the invention is a metallic structure
having a graded microstructure. The metallic structure comprises a
graded region comprising a plurality of grains having a gradient in
grain size, varying as a function of position, between a first
median grain size at an outer region and a second median grain size
at an inner region and a plurality of dispersoids dispersed within
the microstructure. The first median grain size is different from
the second median grain size.
[0005] Another embodiment is a method for forming a metallic
structure having a graded microstructure. The method comprises:
providing a metallic structure comprising at least one reactive
species; diffusing at least one reactant at a controlled rate from
an outer region of the metallic structure towards an inner region
of the metallic structure to form a gradient in reactant activity;
reacting the reactant with the reactive species to form a plurality
of dispersoids; and heat treating the metallic structure to achieve
grain growth so as to form a graded microstructure. The graded
microstructure comprises a graded region comprising a plurality of
grains having a gradient in grain size. The grain size varies as a
function of position between a first median grain size at an outer
region and a second median grain size at an inner region. The
microstructure further comprises a plurality of dispersoids
dispersed within the microstructure. The first median grain size is
different from the second median grain size.
DESCRIPTION OF THE DRAWINGS
[0006] 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 drawing, wherein:
[0007] FIG. 1 is a schematic of a metallic structure having a
graded microstructure, according to one embodiment of the
invention;
[0008] FIG. 2 is a schematic of a metallic structure having a
graded microstructure, according to one embodiment of the
invention;
[0009] FIG. 3 is a flow chart of a method of making a metallic
structure having a graded microstructure, according to one
embodiment of the invention; and
[0010] FIG. 4 is a schematic representation of an experimental
setup to fabricate a metallic structure having a graded
microstructure, according to one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] In the following description, like reference characters
designate like or corresponding parts throughout the several views
shown in the figures. It is also understood that terms such as
"top," "bottom," "outward," "inward," "first," "second," and the
like are words of convenience and are not to be construed as
limiting terms. Furthermore, whenever a particular aspect of the
invention is said to comprise or consist of at least one of a
number of elements of a group and combinations thereof, it is
understood that the aspect may comprise or consist of any of the
elements of the group, either individually or in combination with
any of the other elements of that group.
[0012] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing one
embodiment of the invention and are not intended to limit the
invention thereto.
[0013] For the purposes of understanding the invention, the term "a
graded microstructure" is meant to describe a microstructure
wherein median grain size varies as a function of position. "Median
grain size", implies a median grain size in a selected region. In
some embodiments, the gradation is substantially continuous, but
this does not always have to be the case. For example, the
rate-of-change in grain size may itself vary from region to region,
increasing slightly in some regions, and decreasing slightly in
others. Any and all of these variations in gradations are meant to
be encompassed by the term "graded". The specific grain size
profile for a given metallic structure may depend on various
factors, e.g., required mechanical and elastic properties, thermal
cycling ranges; material composition, actual grain size, and
thickness of the metallic structure.
[0014] Schematic representation of a metallic structure according
to one embodiment of the present invention is shown in FIG. 1. The
metallic structure 10 of FIG. 1 has a graded microstructure
comprising gradient in grain size as a function of position from an
outer region 12 of the structure towards inner region 14 of the
structure. In particular, the metallic structure comprises a graded
region, for example region 16 comprising a plurality of grains 18
having a gradient in grain size, varying as a function of position,
between a first median grain size at an outer region 12 and a
second median grain size at an inner region 14, and a plurality of
dispersoids 20 dispersed within the microstructure. The first
median grain size is different from the second median grain
size.
[0015] The composition of the metallic structure depends on the
end-use application. The metallic structure comprises any suitable
metal or a metal alloy. Examples of some suitable metals include,
but are not limited to, cobalt, nickel, iron, titanium, various
combinations of these, and alloys thereof. In a particular
embodiment, the metallic structure comprises a material selected
from the group consisting of a cobalt-based super alloy, a
nickel-based super alloy, and a titanium-based alloy. In one
exemplary embodiment, the metallic structure comprises a
nickel-based super alloy. Examples of some suitable alloys include,
but are not limited to, alloys designated by Universal Numbering
System for Metals and Alloys (UNS) UNS N07718, UNS N13100, UNS
N09706; alloys designated by General Electric Company trademarks
MX4, RENE104, RENE95, RENE88DT; and UDIMET 720 (a trademark of
Special Metals Corporation). In a particular embodiment, the alloy
comprises alloy UNS N07718. In another embodiment, the alloy
comprises RENE88DT. In yet another embodiment, the alloy comprises
MX4.
[0016] The resistance to fatigue crack initiation and propagation,
in metals and alloys, is known to be influenced by the grain size.
Fatigue endurance limit typically increases with decrease in grain
size. Studies on grain size variations in metallic materials, in
the nano regime, have shown that nanosized grains exhibit enhanced
resistance to high cycle fatigue. Accordingly, the grain sizes in
the outer and the inner regions are selected based on the elastic
properties required in the two regions.
[0017] In one embodiment, the first median grain size (in the outer
region) has a value in the range from about 100 nanometers to about
1 micrometer. In another embodiment, the first median grain size
(in the outer region) has a value in the range from about 100
nanometers to about 500 nanometers. In another embodiment, the
first median grain size has a value in the range from about 100
nanometers to about 200 nanometers. In one embodiment, the second
median grain size (in the inner region-core region) has a value in
the range from about 10 micrometers to about 100 micrometers. In
another embodiment, the second median grain size has a value in the
range from about 10 micrometers to about 50 micrometers. In one
embodiment, the outer region comprises a region from a top surface
of the metallic structure to about 5% of the depth of the
structure. In another embodiment, the outer region comprises a
region from a top surface of the metallic structure to about 10% of
the depth of the structure. In another embodiment, the outer region
comprises a region from a top surface of the metallic structure to
about 15% of the depth of the structure. In another embodiment, the
outer region comprises a region from a top surface of the metallic
structure to about 40% of the depth of the structure.
[0018] Such microstructural designs involving graded transitions,
from finer surface grain morphology to relatively coarser interior
grain morphology may provide gradual transitions in the properties,
from a surface layer resistant to high-cycle fatigue to a core
region resistant to fatigue damage and crack growth. Further, a
fine-grained surface is expected to yield good thermal fatigue
resistance, and coarse grains at the inner region may provide creep
resistance.
[0019] Typically, the metallic structure comprises a plurality of
dispersoids dispersed within the microstructure. The plurality of
dispersoids comprise at least one material selected from the group
consisting of an oxide, a nitride, a boride, a carbide, a nitride,
an intermetallic, a carbo-nitride, and an oxynitride. In one
embodiment, the dispersoid comprises an oxide. Examples of suitable
oxides include, but are not limited to, alumina, zirconia, yttria,
hafnia, thoria, titania, ceria, lanthanum oxide, nickel oxide, and
erbium oxide. In an exemplary embodiment, the dispersoid comprises
yttria. Dispersoids of suitable size dispersed within the metallic
matrix, are expected to pin the grain boundaries and thus provide
to desired thermal stability and mechanical strength. Typically, at
least about 50% of the plurality of dispersoids is disposed at the
grain boundaries of the plurality of grains. In a particular
embodiment, at least about 90% of the plurality of dispersoids are
disposed at the grain boundaries of the plurality of grains.
[0020] The median size of the dispersoids is selected so as to
obtain desirable mechanical strength and thermal stability. The
dispersoids have a median size in the range from about 10
nanometers to about 1 micrometer. In a particular embodiment, the
dispersoids have a median size in the range from about 10
nanometers to about 50 nanometers. If the dispersoids have too
large a size, they may be less effective in grain boundary
pinning.
[0021] The metallic structure is structurally stable up to a high
temperature, that is, the metallic structure does not undergo a
substantial change in crystal structure, grain growth, or
morphology. The temperature up to where the metallic structure is
stable depends, in part, on the material composition of the
metallic structure. In certain embodiments, the metallic structure
is structurally stable at a temperature up to about 600.degree. C.,
in other embodiments, the metallic structure is structurally stable
at a temperature up to about 800.degree. C., in yet other
embodiments, the metallic structure is structurally stable at a
temperature up to about 1000.degree. C., and in yet other
embodiments, the metallic structure is structurally stable at a
temperature up to about 1100.degree. C.
[0022] The metallic structure is a bulk monolithic structure. As
used herein, a "bulk monolithic structure" means a
three-dimensional bulk structure constituting a single unit without
joint. This is in contrast to a body formed of multiple components,
such as a laminated, or a multi-layered structure, or a thin film,
or a coated layer deposited on a substrate. Accordingly, in some
embodiments, the metallic structure comprises the metal or metal
alloy having the composition and the microstructure as discussed in
the structure embodiments above. The structure may be in the form
of a sheet, a plate, a disc, an annular ring, or a bar, or any
other useful form. Of course, those skilled in the art recognize
that the metallic structures described herein may be coated with
other materials as required for particular applications.
[0023] In an exemplary embodiment, the metallic structure is in the
form of an annular ring 30 as shown in FIG. 2. When the metallic
structure has a plurality of holes, they may be processed such that
they have several graded regions. In such cases, the annular ring
30 has gradation in grain size both from the outer surface 34 and
the interior surface 36. The annular ring may be processed such
that finer grains are present towards both surfaces 34 and 36, and
coarser grains towards the middle region 38.
[0024] The metallic structures of the embodiments with
substantially high mechanical strength, structural stability,
fatigue and creep resistant properties are suitable for various
structural components. They are especially suited for aeroengine
components such as discs that require multifunctional properties.
For example, the central portion of a disc may require high creep
resistance, whereas, the periphery of an aeroengine disc faces high
thermal fatigue damage. It is expected that the grain size
gradation would ensure that the component optimizes itself well to
the differential properties across its thickness. This may result
in improved life as well as enhanced high temperature performance
of the disc. In one embodiment, the metallic structure comprises a
gas turbine component. In an exemplary embodiment, the metallic
structure comprises a turbine airfoil. In an exemplary embodiment,
the metallic structure comprises an aircraft engine disc. It is
expected that a fine-grained microstructure of the metallic
structure at the periphery may provide desired thermal fatigue
resistance. The coarser grains at the core may provide the desired
creep resistance.
[0025] Another aspect of the invention is to provide a method for
preparing a metallic structure having a graded microstructure. A
flow diagram of the method for making a membrane structure is shown
in FIG. 3. The method 40 begins with step 42, wherein a metallic
structure comprising at least one reactive species is provided. In
step 44, at least one reactant is diffused, at a controlled rate,
from an outer region of the metallic structure towards an inner
region of the metallic structure, to form a gradient in reactant
activity. In step 46, the reactant is reacted with the reactive
species to form a plurality of dispersoids, and in step 48, the
metallic structure is heat treated to achieve grain growth, so as
to form a graded microstructure.
[0026] The step of heat treating to achieve grain growth may be
conducted simultaneously with the diffusion step or may be
conducted subsequent to the diffusion step. The nature of the
gradient in reactant activity may be selected by controlling the
partial pressure of the reactant.
[0027] A metallic structure comprising at least one reactive
species is provided in step 42. The selection of reactive species
depends on the thermodynamics of the process. Specifically, the
more negative the value is for the free energy of formation (i.e.
.DELTA.G.sup.o) for a particular dispersoids material, the greater
the tendency (i.e. the thermodynamic driving force) to form the
dispersoid at a given temperature within the matrix metal or metal
alloy. As discussed above, the reactive species comprises a
material selected from the group consisting of an oxide former, a
carbide former, a nitride former, a carbo-nitride, and an
intermetallic. In a particular embodiment, the reactive species
comprises a plurality of oxide formers. In specific embodiments,
the reactive species comprises at least one selected from the group
consisting of aluminum, yttrium, zirconia, hafnium, cerium, erbium,
and lanthanum. In a particular embodiment, the reactive species
comprises yttrium. Yttrium has a strong tendency to form oxides
(substantially low free energy of formation). Further, yttria is
effective in pinning the grain boundaries of the metallic
structure. Therefore, a yttria dispersed metallic structure is
expected to yield a desired graded grain structure, on subjecting
to a suitable oxidizing atmosphere and temperature treatment.
[0028] The gradient in reactant activity formed in step 44 is
expected to create a gradient in precipitation concentration, in
step 46. The dispersoids pin the grain boundaries of the metallic
structure and hence may control the grain sizes of the metallic
material. A gradient in grain size may be obtained by controlling
the precipitation formation and the grain growth rate. As discussed
in detail above, a metallic structure thus processed comprises a
graded microstructure such as a metallic structure 10 as shown in
FIG. 1. The graded microstructure comprises a graded region
comprising a plurality of grains having a gradient in grain size
varying as a function of position between a first median grain size
at an outer region 12 and a second median grain size at an inner
region 14, and a plurality of dispersoids 20 dispersed within the
microstructure. The first median grain size is different from the
second median grain size.
[0029] As discussed in detail in the above embodiments, the
metallic structure may comprise any suitable metal or a metal
alloy. Examples of some suitable metals are cobalt-based super
alloys, nickel-based super alloys, and titanium-based alloys. In an
exemplary embodiment, the metallic structure comprises a
titanium-based alloy.
[0030] Generally, diffusing a reactant comprises exposing the
metallic structure to an effective activity of the reactant. In
some embodiments, the metallic structure may be exposed to a
gaseous phase of the reactant. In other embodiments, the metallic
structure may be exposed to a solid phase of the reactant. The
concentration of the reactants at any region within the metallic
structure, in part, depends on the solubility of the reactant
within the metallic structure and its diffusivity. Hence for a
given metallic structure and reactant system, the nature of the
gradient in reactant activity may be achieved by controlling the
partial pressure of the reactant. Any specific microstructure or
grain size gradation may be achieved by controlling the reactive
species dispersed within the matrix, their volume fraction, partial
pressure of the reactant, temperature, and time duration of heat
treatment, among other parameters.
[0031] In certain embodiments, the metallic structure may be
exposed to a gaseous phase of the reactant. Typically, the parent
metallic matrix comprising one or more reactive species is produced
by casting or any other suitable process. Such a metallic structure
may be subjected to a reactant by immersing the sample in a mixture
of materials capable of releasing the desired reactant. For
example, the sample may be subjected to an oxygen partial pressure
by exposing to oxide powders at the desired temperature in a
controlled atmosphere. The surrounding oxide at least partially
decomposes to yield oxygen that can diffuse into the material to
form oxide particles dispersed in the matrix by the process of
internal oxidation. The extent of the formation of these oxides
with respect to the depth of penetration is a function of the
partial pressure, temperature and the surface area exposed. The
partial pressure of oxygen may be controlled by adjusting the
surrounding oxide mixture, its relative proportions as well as the
temperature. An apparatus 50 for carrying out such a process is
shown in FIG. 4. The article 52 to be processed may be placed
within a chamber 56, with the help of fixtures 53. The article 52
may be surrendered by one or more materials 54 capable of releasing
the desired reactant on heating.
[0032] The various processing parameters may be evaluated for any
specific material system. It is explained, by way of example, where
article 52 is a metallic matrix comprising yttrium surrounded by a
nickel oxide powder 54:
[0033] If the alloy with yttrium is surrounded by NiO, without any
additional partial pressure of O.sub.2, the existing partial
pressure at 1000.degree. C. is computed analyzing the
reactions.
NiO.fwdarw.Ni+1/2O.sub.2 [1]
1/2O.sub.2.fwdarw.[O].sub.alloy [2]
2Y+3[O].sub.alloy.fwdarw.Y.sub.2O.sub.3 [3]
[0034] Gibbs energy for the above mentioned reactions are shown
below.
.DELTA.G.sub.1=234300+85.2T J/mol
.DELTA.G.sub.2=85353+18.5T J/mol
.DELTA.G.sub.3=-1640382.24+245.31T J/mol
[0035] From reactions [1] and [2], using the rate constant
calculations, it can be computed that the oxygen concentration at
1000.degree. C. would be 0.0024%. This would also mean a partial
pressure of 3.56.times.10.sup.-8 torr. Typically, about 10.sup.-5
torr oxygen partial pressure would be needed to form a surface
layer. (About 10.sup.-5 torr oxygen partial pressure would be
needed to form a surface layer of aluminum oxide). The passive
layer may hinder the flow of reactants through the surface of the
structure. Since the partial pressure is three orders of magnitude
smaller, partial pressure may be increased using gaseous oxygen to
increase the penetration depth.
[0036] From a similar analysis, it can be shown that very small
quantity (10.sup.-39 torr) of oxygen is needed to react with yttria
to form Y.sub.2O.sub.3. This would mean that Y.sub.2O.sub.3 would
form instantaneously.
[0037] To estimate the penetration distance X, the following
equation may be used:
X=(2C.sub.o/vC.sub.s*D.sub.t)1/2 [4]
[0038] where, C.sub.o is the concentration of the reactant, v is
the stoichiometric value of the reactive species to the reactant,
C.sub.s is the concentration of the reactive species, and D.sub.t
is the diffusivity of the reactive species. Using the value of
oxygen concentration at the surface and the bulk diffusivity of O
in Ni as 1.54.times.10.sup.-8 cm.sup.2/sec, the penetration
distance would be about 40 micrometers. By varying the temperature
and the mixture of the oxides to change the partial pressure of
oxygen, the penetration distance may be optimized.
[0039] Diffusing the reactant at a controlled rate comprises
providing the reactant at a controlled partial pressure. The
partial pressure required for any reactant-metallic system may be
computed, as discussed in detail below. Typically, heat treating
the metallic structure to achieve grain growth comprises heating at
a temperature greater than about 2/3.sup.rd of the melting
temperature, as measured on an absolute scale. In some embodiments,
the temperature is in a range from about 600.degree. C. to about
1200.degree. C. The exact temperature profile chosen depends on the
composition of the metallic structure.
[0040] As discussed above, the reactant comprises a material
selected from the group consisting of oxygen, boron, carbon, and
nitrogen. The dispersoid comprises a material selected from the
group consisting of an oxide, a nitride, a boride, and an
oxynitride. The first median grain size has a value in the range
from about 100 nanometers to about 1 micrometer. The second median
grain size has a value in the range from about 10 micrometers to
about 50 micrometers.
[0041] In an alternative embodiment, the reactant for reacting with
the reactive species may be obtained by decomposing a plurality of
precursor particles dispersed within the metallic structure. The
precursor particles are chemically less stable than the
dispersoids. On decomposition, the precursor particles decompose
into a product comprising a plurality of secondary reactive species
and secondary reactants. The secondary reactants may be further
reacted with the reactive species to form dispersoids. Dispersion
of precursor particles within the metallic structure gives an
additional degree of freedom in altering the reactant activity
profile within the metallic structure. This change in the reactant
activity profile may be utilized for altering the microstructure of
the processed metallic structure.
[0042] The metallic structures and methods disclosed herein provide
many advantages over conventionally used methods. The method is
capable of providing a material having a gradient microstructure.
These graded metallic structures may provide multifunctional
capabilities in a single component and may also enable high
temperature performance of the metallic structure. Metallic
structures with a hole (rotating parts) or multiple holes (internal
cooling holes) may be processed using the disclosed method to
achieve several graded regions.
[0043] The embodiments of the present invention are fundamentally
different from those conventionally known in the art. There have
been reports of graded metallic structured layers. In such cases,
the layers are extremely thin (less than a few micrometers). The
metallic structures disclosed herein provide bulk structures with
the right balance of creep and fatigue properties within the same
monolithic structure. The embodiments of the invention provide
simpler and versatile methods to obtain bulk structures of graded
metallic structures.
[0044] The following example serves to illustrate the features and
advantages offered by the present invention, and not intended to
limit the invention thereto.
EXAMPLES
[0045] The following examples describe the preparation method a
graded metallic structures.
[0046] Example: Method for fabricating a graded metallic structure
by internal oxidation of yttria using nickel oxide.
[0047] The part made of Ni based superalloy with nano-structured
grains is placed in a bath of nickel oxide powder. The superalloy
has the reactant solutes such as yttria that would readily oxidize.
This whole set up is placed in an inert gas furnace (if the
component is big) or encapsulated in a glass fixture (for small
components). Depending on the partial pressure of oxygen required
to form the surface oxide layer such as AlO (about 10.sup.-5 torr),
the excess partial pressure could be supplied by passing O.sup.2
gas through the furnace or the fixture. The internal oxidation
results in the reactant solute getting oxidized to form the oxide
particles in the alloy matrix preferrably at grain boundaries due
to enhanced rates of diffusion. The extent of the depth of
oxidation is proportional to the temperature and the partial
pressure of oxygen. The combination is optimized to get the correct
size distribution of the oxide particles up to required depths.
[0048] While the invention 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 invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof. 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. For example, it should be understood that
though the above embodiments are discussed with respect to a
airfoil disc, the embodiments of the invention may be utilized in
any other metallic component, in which the excellent creep and
fatigue resistant of these graded metallic structures are
essentially beneficial. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed as
the best mode contemplated for carrying out this invention, but
that the invention will include all embodiments falling within the
scope of the appended claims.
* * * * *