Title: Iron base high temperature alloy and method of making
Abstract: The present invention is directed to an iron, aluminum, chromium, carbon alloy and a method of producing the same, wherein the alloy has g good room temperature ductility, excellent high temperature oxidation resistance and ductility. The alloy includes about 10 to 70 at. % iron, about 10 to 45 at. % aluminum, about 1 to 70 at. % chromium and about 0.9 to 15 at. % carbon. The invention is also directed to a material comprising a body-centered-cubic solid solution of this alloy, and a method for strengthening this material by the precipitation of body-centered-cubic particles within the solid solution, wherein the particles have substantially the same lattice parameters as the underlying solid solution. The ease of processing and excellent mechanical properties exhibited by the alloy, especially at high temperatures, allows it to be used in high temperature structural applications, such as a turbocharger component.
Patent Number: 6,841,011 Issued on 01/11/2005 to Lin
| Inventors:
|
Lin; Hui (48 Pennsylvania Ave., Malvern, PA 19355)
|
| Appl. No.:
|
254654 |
| Filed:
|
September 26, 2002 |
| Current U.S. Class: |
148/325; 148/333; 148/419; 148/423; 148/442; 148/548 |
| Intern'l Class: |
C22C 028/00; C22C038/06; C22C038/18; C21D009/00 |
| Field of Search: |
148/333,325,423,419,442,548
420/583,104,79,34
|
References Cited [Referenced By]
U.S. Patent Documents
| 2043631 | Jun., 1936 | Scheil.
| |
| 3785805 | Jan., 1974 | Van Den Boomgaard.
| |
| 3893849 | Jul., 1975 | Brickner.
| |
| 4235623 | Nov., 1980 | Rath.
| |
| 4615732 | Oct., 1986 | Shastry et al.
| |
| 4769214 | Sep., 1988 | Sherby et al.
| |
| 4836981 | Jun., 1989 | Shimada et al.
| |
| 4844865 | Jul., 1989 | Shimada et al.
| |
| 4859649 | Aug., 1989 | Bohnke et al.
| |
| 4961903 | Oct., 1990 | McKamey et al.
| |
| 5084109 | Jan., 1992 | Sikka.
| |
| 5085829 | Feb., 1992 | Ishii et al.
| |
| 5238645 | Aug., 1993 | Sikka et al. | 420/79.
|
| 5286442 | Feb., 1994 | Uematsu et al.
| |
| 5346562 | Sep., 1994 | Batawi et al.
| |
| 6231807 | May., 2001 | Berglund.
| |
| Foreign Patent Documents |
| 19930114921.5 | Jun., 1994 | CN.
| |
| 2339869 | Feb., 1974 | DE.
| |
| 19603515 | Dec., 1996 | DE.
| |
Other References
"Iron Aluminides" by C.G. McKamey, Chap. 9, pp. 351-391.
R.G. Baligidad, et al., "Processing of High Carbon Fe.sub.e AI based
Intermetallic Alloy," Intermetallics 6:765-769 (1998).
R.B. Baligidad, et al., "Effect of Alloying Additions on Structure and
Mechanical Properties of High Carbon Fe-16 wt.% AI Alloy," Materials
Science & Engineering A287:17-24 (2000).
English abstract of Russian patent 384924, Vagin et al., 1971.
Metals Handbook, 1948 edition, published by the American Society for
Metals, p. 13.
ASM Materials Engineering Dictionary, edited by J.R. Davis, p. 231 (1992).
English abstract of Japanese patent 1031922, Yoshizawa, et al., Hitachi
Metals, 1987.
English abstract of German patent 19603515, Luster et al., 1996.
|
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.
Parent Case Text
This is a continuation of application Ser. No. 09/540,403, filed Mar. 31,
2000, now U.S. Pat. No. 6,524,405 and claims the benefit of U.S.
provisional application No. 60/181,936, filed Feb. 11, 2000, all of which
are incorporated herein by reference.
Claims
What is claimed is:
1. A material comprising a body-centered-cubic, solid solution of
Fe--Al--Cr--C, said solid solution having from about 10 to 80 at. % iron,
about 10 to 45 at. % aluminum, about 1 to 70 at. % chromium and about 0.9
to 15 at. % carbon.
2. The material of claim 1, wherein aluminum and chromium are present in a
combined amount of at least 30 at. %.
3. The material of claim 1, said material having a yield strength of
greater than 320 MPa up to about 650.degree. C.
4. The material of claim 1, wherein said material is a polycrystalline
solid solution.
5. The material of claim 1, which is strengthened by
(a) the incorporation of an additional solid solution phase to said solid
solution,
(b) grain size refinement,
(c) the introduction of particles of a strengthening phase, or
(d) the addition of a strengthening element in the solid solution.
6. The material of claim 5, which is strengthened by the addition of
refractory oxide particles to said solid solution.
7. The material of claim 6, wherein said refractory oxide particles
comprise Y.sub.2 O.sub.3.
8. The material of claim 1, said material having a density from about 5.5
g/cm.sup.3 to about 7.5 g/cm.sup.3.
9. The material of claim 1, said material having a yield strength that
stays the same or increases with increasing temperature from room
temperature to about 600.degree. C.
10. The material of claim 1, said material having substantially no weight
change due to oxidation at temperatures up to about 1150.degree. C.
11. The material of claim 1, said material having a tensile ductility
greater than about 95% at temperatures of about 900.degree. C.
12. An article comprising a body-centered-cubic, solid solution of
Fe--Al--Cr--C, said solid solution comprising from about 10 to 80 at. %
iron, about 10 to 45 at. % aluminum, about 1 to 70 at. % chromium and
about 0.9 to 15 at. % carbon.
13. The article of claim 12, wherein aluminum and chromium are present in a
combined amount of at least 30 at. %.
14. The article of claim 12, said article having a density of about 5.5
g/cm.sup.3 to about 7.5 g/cm.sup.3.
15. The article of claim 12, wherein said density is about 6.1 g/cm.sup.3.
16. The article of claim 12 disposed to have a load applied thereto at
temperatures up to about 650.degree. C.
17. The article of claim 12, said article having a yield strength of
greater than 320 MPa up to about 650.degree. C.
18. The article of claim 12, said article having a yield strength that
stays the same or increases with increasing temperature from room
temperature to about 600.degree. C.
19. The article of claim 12, said article having substantially no weight
change due to oxidation up to about 1150.degree. C.
20. The article of claim 12, said article having a tensile ductility
greater than about 95% at temperatures of about 900.degree. C.
21. The article of claim 12, which is a turbocharger part.
22. The article of claim 21, wherein said turbocharger part is a turbine
rotor or a compressor.
23. A method of making an article, said method comprising:
melting a composition comprising about 10 to 80 at. % iron, about 10 to 45
at. % aluminum, about 1 to 70 at. % chromium and about 0.9 to 15 at. %
carbon to form a molten Fe--Al--Cr--C alloy under a controlled atmosphere,
pouring said molten alloy into a mold under a controlled atmosphere, said
mold having a cavity in the shape of said article,
cooling said molten alloy to room temperature to form a solid, as-cast
article, and
removing the solid as-cast article from said mold to form an article
comprising a body-centered-cubic, solid solution of Fe--Al--Cr--C.
24. The method according to claim 23, wherein said controlled atmosphere
consists of an inert gas or a vacuum.
25. A method according to claim 23, further comprising precipitating
body-centered-cubic particles within the solid solution, said particles
having substantially the same lattice parameters as said solid solution.
26. The method according to claim 25, wherein the amount and the
distribution of the body-centered-cubic particles within the solid
solution are adjusted by adjusting the amount of iron, aluminum, chromium
and carbon.
27. The method of claim 23, wherein said article is a turbocharger part.
28. The method of claim 27, wherein said turbocharger part is a turbine
rotor or a compressor.
Description
The present invention is directed to an iron base, heat and corrosion
resistant alloy that has low density, good tensile ductility, and
excellent properties related to oxidation resistance, corrosion
resistance, castability and strength. This new class of alloys is about
20-25% lighter and 20-80% cheaper than most traditional nickel-containing
steels, e.g., stainless steels, heat resistant steels and heat resistant
alloys.
Currently, heat resistant structural applications most often employ heat
resistant steels, heat resistant alloys and superalloys. There is,
however, a need for materials with similar properties having a much lower
density since heat-resistant steels, heat-resistant alloys, and
superalloys have relatively high densities. While alternative materials
such as ceramics and intermetallic ordered alloys are being studied for
their low densities, none of them have achieved the combination of low
density, adequate tensile ductility, high strengths, and good oxidation
resistance that is needed for high temperature engineering applications.
In the case of ceramics, their complete lack of tensile ductility severely
limits the advantage of their low densities. In addition, ceramic
components are usually produced through a powder sintering process which
is a relatively costly process. Because of their lack of ductility and
high cost, ceramics parts can only be used in very limited applications.
Light intermetallic ordered materials have not achieved adequate intrinsic
tensile ductility and exhibit low fracture toughness, especially at room
temperature. As a result of these properties, relatively complex
processing techniques have to be employed to produce these materials and
fabricate them into components. This significantly increases the
production costs and their relatively low toughness at room temperature
can cause handling problems and high component rejection rates.
An example of such an intermetallic ordered material is Fe.sub.3 Al. Unlike
pure iron, which is a body centered cubic (BCC) solid solution and is very
ductile, Fe.sub.3 Al forms an ordered BCC structure (generally defined as
DO.sub.3 at room temperature and B.sub.2 at high temperatures) in which Fe
atoms and Al atoms are arranged in a regular fashion. Fe.sub.3 Al has a
low density and reasonably good oxidation resistance up to about
800.degree. C. because of its high aluminum content. The aluminum in the
material will easily form an oxide scale in an oxidizing environment,
although the oxide scale is not strong and easily spalls at temperatures
above 800.degree. C. Moreover, the raw materials for Fe.sub.3 Al are also
relatively inexpensive. However, Fe.sub.3 Al is very brittle and has a low
room temperature tensile ductility, it easily fractures in both
intergranular and transgranular fashion.
Although chromium containing Fe.sub.3 Al has shown limited improvement in
tensile ductility and is relatively lightweight, as evidenced by a density
of about 6.5 g/cm.sup.3, conventional ordered Fe--Al--Cr compositions
suffer from relatively poor high-temperature strengths, corrosion
resistance and oxidation resistance.
Consequently, the simultaneous achievement of a more affordable heat
resistant structural material that has a low density, good tensile
ductility, excellent oxidation resistance and excellent workability, is a
continuing objective of this field of endeavor. Specifically, there has
been a need for a new iron-base alloy having a low density, high strength,
adequate tensile ductility, defined as .gtoreq.5% tensile elongation, and
excellent oxidation and corrosion resistance. The above-mentioned
objectives can be substantially realized by adding carbon to a
chromium-containing iron aluminum compound such that a body-centered-cubic
iron aluminum chromium carbon alloy is formed.
The immediate application for the present invention includes turbochargers
for high speed diesel engines used in boats, trucks and passenger cars.
Diesel engines are widely used because of better fuel economy than
gasoline engines. To achieve such fuel economy, as well as increase engine
efficiency and reduce pollution, turbo-chargers are routinely used in
high-speed diesel engines. Most industrial trucks as well as about 10% of
passenger cars in the world (up to 20% in Europe and 10% in Japan) are
powered by high-speed diesel engines with turbochargers.
A turbocharger for a diesel engine is made up of a compressor and a
turbine. From a mechanical performance perspective, the turbine is the
most critical part, since it operates at high temperatures, e.g., up to
650.degree. C., and under high centrifugal stress due to high-speed
rotation. The environment in which a turbine operates can also be both
oxidizing and corrosive.
Currently, turbocharger turbines are cast from an iron-nickel base alloy or
a nickel base alloy that is both expensive and heavy. Because of the
weight, it takes time for present turbochargers to overcome inertia before
the turbine can reach the working speed in which it operates most
effectively. As evidenced by the emission of a dark cloud of exhaust on
sudden acceleration, the exhaust gas is not properly burned during the
time it takes for the turbine to reach its operating speed. To solve the
above-mentioned problems associated with Fe--Ni base or Ni base-alloy
turbochargers, turbocharger turbines and compressors from the
body-centered-cubic iron aluminum chromium carbon alloy have been
fabricated of the present invention.
SUMMARY OF THE INVENTION
Accordingly, a subject of the present invention is a material comprising a
body-centered-cubic, single-phase, solid solution of iron aluminum,
specifically Fe--Al--Cr--C. Preferably the material includes about 10 to
80 at. % iron, about 10 to 45 at. % aluminum, about 1 to 70 at. % chromium
and about 0.9 to 15 at. % carbon. The material has excellent properties in
polycrystalline form. In addition, the material can be strengthened by
well-known methods that include solid solution strengthening, grain size
refinement or by the introduction of particles of a strengthening phase.
Preferably, the material can be strengthened by precipitating within the
solid solution, BCC, solid solution particles that have substantially the
same lattice parameters as the underlying solid solution. The inventive
material is oxidation resistant at temperatures up to 1150.degree. C., and
has excellent mechanical properties at temperatures up to about
650.degree. C.
DESCRIPTION OF THE DRAWING
The following drawing, which form a part of the disclosure of the present
invention depict additional aspect of the invention. Of the drawing:
FIG. 1 is a ternary phase diagram showing a BCC phase field.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is embodied in a new Fe--Al--Cr--C
body-centered-cubic solid solution alloy which has a low density (e.g., in
the range of from 5.5 g/cm.sup.3 to 7.5 g/cm.sup.3, and preferably 6.1
g/cm.sup.3), an adequate room temperature tensile ductility, excellent
high temperature strength, oxidation resistance and corrosion resistance.
The inventive alloy preferably comprises about 10 to 80 at. % iron, about
10 to 45 at. % aluminum, about 1 to 70 at. % chromium, and about 0.9 to 15
at. % carbon, wherein the combination of aluminum and chromium is
preferably present in an amount of at least 30 at. %.
Depending on the desired final properties, chromium content may change and
fall into different preferred ranges. For example, cast materials
preferably employ about 5 to 20 at. % chromium, while wrought materials
employ lower amounts of chromium, e.g., about 1 to 10 at. %.
In the present invention, powder x-ray diffraction is used to determine the
existence of a BCC phase from the relative intensities of the diffraction
peaks. In this invention, a BCC phase is either a single BCC phase or a
combination of several BCC phases with substantially the same lattice
parameters. A BCC phase is defined as a phase containing <3% non-BCC
phase. That is, even if a diffraction pattern for a phase shows weak
non-BCC peaks, the phase is still considered to be a BCC phase if the
relative intensity of the non-BCC peaks are <3% of the intensity of the
strongest BCC peak. Such a determination is only necessary to define the
boundaries of the ternary phase diagram shown in FIG. 1, since a
diffraction pattern within those boundaries shows only BCC peaks.
The inventive material has a yield strength of greater than 320 MPa up to
and including a temperature of about 650.degree. C. In addition, that the
inventive material's yield strength increases or stays the same with
increasing temperature from room temperature to about 600.degree. C. In
one embodiment, the yield strength of the material increases sharply with
increasing temperature from room temperature to about 600.degree. C.,
which is contrary to traditional BCC materials. The yield strength for BCC
materials generally decreases with increasing temperature.
This material can be further strengthened by (a) the incorporation of an
additional solid solution phase to said solid solution, (b) grain size
refinement, (c) the introduction of particles of a strengthening phase, or
(d) the addition of a strengthening element in the solid solution.
The incorporation of an additional solid solution phase can be carried out
by the precipitation of body-centered-cubic particles within the solid
solution, wherein the particles have substantially the same lattice
parameters as the solid solution.
Strengthening can also be carried out by the addition of refractory oxide
particles to the solid solution, such as Y.sub.2 O.sub.3.
In has been unexpectedly discovered that the addition of significant
amounts of carbon and chromium transforms light weight iron-aluminum from
an ordered BCC alloy, into a BCC solid solution. In addition, it was found
that the solubility of the carbon in the present invention increases with
increasing amounts of chromium and decreasing amounts of aluminum.
The light-weight alloy possesses an adequate tensile ductility at room
temperature. As illustrated by the properties below, the combination of a
low density, an adequate tensile ductility and high-temperature strengths
is a significant technological breakthrough for light-weight, heat
resistant structural materials.
It has been further discovered that standard processing techniques (e.g.,
casting) can be used to shape the inventive alloy into desired articles.
One object of the present invention, therefore, is to produce, using
standard processing techniques, an article or a composite comprising solid
solution phases of Fe--Al--Cr--C, wherein the solid solution phases are
each body-centered-cubic and single-phase, and their lattice parameters
substantially match each other.
Another object of the present invention is to produce a turbocharger part,
specifically a turbine rotor or a compressor comprising the inventive
alloy.
Properties
A. Oxidation Resistance
The present invention has excellent oxidation resistance, which is defined
as the weight change of the material when exposed to a high temperature,
oxidizing environment. In fact, the inventive materials exhibit oxidation
resistance that is superior to stainless steels, heat-resistant steels,
heat-resistant alloys, and superalloys. In one embodiment, the material
exhibits a weight loss rate of 0.2 g /m.sup.2 day after more than 100
hours at 1000.degree. C. in air. The excellent oxidation resistance is
believed to be due to the large amounts of aluminum and chromium in the
material. If needed, the oxidation resistance can be further improved by
the addition of rare-earth elements to the material.
B. Strength
An article made according to the present invention exhibits
high-temperature strength, e.g., up to 650.degree. C., that is superior to
stainless steels, and most heat resistant steels and alloys. Considering
the low density associated with the material, the specific strength of the
material at temperatures up to 650.degree. C. is even more superior. For
example, the present invention in as-cast form has a yield strength of
greater than 320 MPa up to 650.degree. C. The strength of this alloy can
be further improved with conventional strengthening methods such as grain
refinement (e.g., hot-rolling followed by re-crystallization to change the
microstructure of the article), solid solution strengthening (e.g.,
incorporating into the solid solution a strengthening element), and second
phase particle strengthening.
Second phase particle strengthening can result from the external addition
of refractory oxides, such as Y.sub.2 O.sub.3. Preferably second phase
particle strengthening is done internally, via an in situ technique. By
adjusting the Fe--Al--Cr--C composition, internal particles of
Fe--Al--Cr--C precipitate within the solid solution. For example, the
amount and the distribution of the body-centered-cubic particles within
the solid solution can be tailored by adjusting the amount of iron,
aluminum, chromium and carbon within the composition. These particles are
also BCC, their lattice parameters substantially match the surrounding
solid solution, which eliminates stress related to gradients between
phases, and provides high temperature stability.
The combination of oxidation resistance and high temperature strength
associated with the inventive material allows it to be readily used as
load bearing components exposed to an oxidizing environment at
temperatures of up to 650.degree. C. The present invention can also be
used as non load-bearing parts at temperatures as high as 1200.degree. C.
C. Corrosion Resistance
An article comprising the inventive material also exhibits good corrosion
resistance when tested in a nitric acid solution. The material has a
corrosion resistance rate of less than 0.01 mm/year weight loss in
HNO.sub.3 solution ranging from 20% to 65% at room temperature. The
material also shows no sign of grain boundary corrosion when exposed to
the foregoing conditions.
D. Ductility
The present invention has an adequate tensile ductility at room temperature
and good tensile ductility at over 700.degree. C. providing good hot
workability. For example, the present invention in as-cast form exhibits
tensile ductility of over 5% at room temperature and over 95% at
approximately 900.degree. C. Therefore, the inventive material was readily
hot-rolled at temperatures above 900.degree. C.
E. Castability
Due to the excellent castability properties associated with the present
invention, e.g., a low viscosity when molten, standard metal melting and
casting techniques can be used in producing finished articles. Articles
can be made using conventional induction melting techniques carried out in
a controlled or protective atmosphere, e.g., in an inert gas or under
vacuum. The unique ability of the material to form near net shape articles
is a combination of the fluidity of the molten alloy and the
characteristics of the strengthening phase. Preferably, the material has a
eutectic structure. This microstructure coupled with excellent flow
properties, allows the molten alloy to conform to the shape of the mold,
and results in near net shape articles that do not require additional
finishing steps before use.
The microstructure of an article made in accordance with the present
invention can be-further tailored by adjusting the casting temperature.
For example, it has been discovered that a higher casting temperature can
result in a finer particle size for the secondary, strengthening phase.
For purposes of illustration, a fine microstructure is one where the mean
size of the secondary phase precipitates is less than approximately 50
.mu.m , and preferably about 10-20 .mu.m.
Article
In one embodiment, investment vacuum casting was used to produce a cast
turbocharger turbine rotor with the thinnest blade having a thickness of
approximately 0.5 mm. As shown in Example 1 below, the as-cast
turbocharger turbine rotor exhibited excellent high temperature strengths
up to 650.degree. C. This high temperature strength is similar to cast
iron-nickel base heat-resistant alloys currently used in turbochargers.
However, due to the low density of the inventive material, the specific
strength is approximately 25% higher than current cast iron-nickel base
turbochargers. For example, the turbocharger turbine comprising the
inventive alloy had a density of about 6.1 g/cm.sup.3, compared to cast
iron-nickel base alloys, which have a density of about 8.1 g/cm.sup.3.
Therefore, a turbocharger turbine made in accordance with the present
invention is approximately 25% lighter in weight than standard iron-nickel
base turbocharger turbine rotors.
The light weight turbine rotor of the turbocharger leads to significant
reduction in pollution because it overcomes inertia and reaches operating
speeds faster than the heavier iron-nickel base turbochargers currently
used. Due to this effect, acceleration time can decrease by at least 25%,
leading to a more efficient burn of the exhaust gas during acceleration,
when compared to the heavier iron-nickel turbocharger. In fact, the light
weight alloy of the present invention, when used to make a turbocharger
turbine rotors and compressors would assist diesel engines in meeting
transient (accelerating) emission standards, in addition to steady state
emission standards.
In addition to the above performance benefits, the material costs of the
inventive alloy is substantially cheaper, e.g., at least 50% cheaper, than
conventional nickel-iron turbochargers. This price difference is primarily
associated with the high amounts of nickel present in standard
turbochargers, that are not present in the inventive alloy.
Finally, the present alloy has much better oxidation resistance than
iron-nickel alloy or nickel base alloy turbocharger turbine rotor.
Having disclosed the present invention generally, the following example
further describes the invention.
EXAMPLES
Example 1
An Fe--Al--Cr--C article comprising a composition within the range defined
in FIG. 1 was prepared by a standard melting technique. The composition
was melted under a vacuum to form a molten Fe--Al--Cr--C alloy, which was
then poured into a mold having a cavity in the shape of the article. The
as-poured mold remained under a vacuum until it was sand-cooled in air to
room temperature to form the as-cast article. The-as-cast article was
subsequently removed from the mold, and was found to be a Fe--Al--Cr--C
body-centered cubic, solid solution having a density of about 6.1
g/cm.sup.3.
The mechanical properties of the as-cast article are shown in Table 1. As
can be seen, a material within the present invention exhibits excellent
yield and tensile strength up to 650.degree. C., and good ductility,
particularly at 900.degree. C.
TABLE 1
Mechanical Properties of a bcc Fe--Al--Cr--C alloy
0.2% Offset
Yield Tensile
Temperature Strength .sigma..sub.y Strength .sigma..sub.b
Elongation
(.degree. C.) (MPa) (MPa) (%)
Room Temp. 360 500 5.3
200 375 580 5.8
400 364 617 8.8
500 353 600 8.7
600 361 530 8.7
650 324 403 9.3
700 170 247 33
750 116 168 43
800 90 112 66.7
900 54 68 95.8
1000 26 32 39.2
Table 2 further shows that the inventive material is almost completely
oxidation to 1150.degree. C.
TABLE 2
Oxidation Resistance Properties of a bcc Fe--Al--Cr--C alloy
Weight Change Rate
Temperature after 100 hours in air
(.degree. C.) (g/m.sup.2 d)
600 0.015
700 0.074
800 0.065
900 0.096
1000 -0.2
1100 -2
1150 0.42
Table 3 illustrates the excellent corrosion resistance properties, even in
a 65% nitric acid, of the inventive material.
TABLE 3
Corrosion Resistance Properties of a bcc Fe--Al--Cr--C alloy
HNO.sub.3 solution Corrosion Rate
(%) (mm/yr)
5 0.04
20 0.009
35 0.0084
50 0.0062
65 0.0075
The present invention has been disclosed generally and by reference to
embodiments thereof. The scope of the invention is not limited to the
disclosed embodiments but is defined by the appended claims and their
equivalents.
*
