U.S. patent application number 11/033099 was filed with the patent office on 2006-07-13 for synthesis of bulk, fully dense nanostructured metals and metal matrix composites.
Invention is credited to Julie M. Schoenung, Jichun Ye.
Application Number | 20060153728 11/033099 |
Document ID | / |
Family ID | 36653427 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060153728 |
Kind Code |
A1 |
Schoenung; Julie M. ; et
al. |
July 13, 2006 |
Synthesis of bulk, fully dense nanostructured metals and metal
matrix composites
Abstract
Bulk nanostructured alloys, such as aluminum 5083 alloys
reinforced with 10 wt. % particulate B.sub.4C, was synthesized by
cryomilling and spark plasma sintering. Material for the alloy are
selected and the selected raw materials are cryomilled, mechanical
milling at cryogenic temperatures, to fabricate nanostructured
alloys at low temperatures. The cryomilled powders are then
degassed, and consolidated using spark plasma sintering into dense
bulk materials. The material thus obtained achieved near full
density bulk materials, while retaining the nanocrystalline nature.
The densities of the compacts were measured using Archimedes
method. XRD, SEM, TEM, and hardness testing were used to
characterize the cryomilled powders and consolidated compacts.
Inventors: |
Schoenung; Julie M.; (Davis,
CA) ; Ye; Jichun; (Davis, CA) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER
801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
36653427 |
Appl. No.: |
11/033099 |
Filed: |
January 10, 2005 |
Current U.S.
Class: |
419/32 ;
419/33 |
Current CPC
Class: |
B22F 2999/00 20130101;
B22F 2998/10 20130101; B22F 3/105 20130101; B22F 1/0044 20130101;
B22F 9/04 20130101; B22F 2202/03 20130101; B22F 9/04 20130101; B22F
3/105 20130101; B22F 2998/10 20130101; B22F 2009/041 20130101; B22F
2003/1051 20130101; B22F 2009/049 20130101; B22F 2999/00 20130101;
C22C 32/00 20130101 |
Class at
Publication: |
419/032 ;
419/033 |
International
Class: |
B22F 3/105 20060101
B22F003/105 |
Goverment Interests
GOVERNMENT INTEREST
[0001] This invention was made with support of government grants
N00014-03-C-0164 from the Office of Naval Research. Therefore, the
United States government may have certain rights in the invention.
Claims
1. A method for producing nanostructured materials, the method
comprising: (a) providing a metal powder and optionally a
reinforcement; (b) mechanically milling (a) at a cryogenic
temperature (cryomilling) to provide a nanostructured powder; (c)
removing gaseous components from the cryomilled powder; and (d)
consolidating the cryomilled powder by spark plasma sintering,
wherein the nanostructured material thus produced has a relative
density of about 99.0% or higher and an average grain size of less
than 100 nm.
2. The method of claim 1, wherein the reinforcement is selected
from the group consisting of oxides, carbides, nitrides, borides,
metals, intermetallics, and alloys.
3. The method of claim 1, wherein the metal powder is selected from
the group consisting of Al, Be, Ca, Sr, Ba, Ra, Sc, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Y, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, and W, and
combinations thereof.
4. The method of claim 1, wherein the metal powder is an aluminum
alloy.
5. The method of claim 1, wherein the reinforcement is boron
carbide.
6. The method of claim 1, wherein the reinforcement is silicon
carbide.
7. The method of claim 1, wherein the reinforcement is aluminum
nitride.
8. The method of claim 1, wherein the reinforcement is aluminum
oxide.
9. The method of claim 1, wherein the reinforcement is in the form
of a particulate.
10. The method of claim 1, wherein the reinforcement is in the form
of a platelet.
11. The method of claim 1, wherein the reinforcement is in the form
of a whisker.
12. The method of claim 1, wherein cryomilling is continued until
an equilibrium grain size of the metal is reached.
13. The method of claim 12, wherein cryomilling is continued
between 6 and 10 hours.
14. The method of claim 1, wherein the removal of gaseous component
occurs at a temperature between about 200.degree. C. and
600.degree. C.
15. The method of claim 14, wherein the removal of gaseous
components occurs at a temperature between about 300.degree. C. and
500.degree. C.
16. The method of claim 1, wherein the spark plasma sintering is
carried out at a temperature between 40% and 100% of the absolute
melting temperature of the metal phase.
17. The method of claim 1, wherein spark plasma sintering is
carried out with different ramping rates and various holding
time.
18. The method of claim 1, wherein removal of gaseous components
and consolidation of the cryomilled-powder occur simultaneously, to
form a fully dense material without a separate degassing step.
19. The method of claim 1, wherein the cryogenic temperature is
provided by liquid nitrogen or liquid argon.
20. The method of claim 1, wherein the mechanical milling is
conducted in a shaker type mill, an attritor mill, a planetary
mill, a ball mill, or a rotary mill.
21. A method for producing nanostructured aluminum alloy, the
method comprising: (a) providing aluminum alloy and a reinforcement
(b) mechanically milling (a) at a temperature of about -150.degree.
C. to about -300.degree. C.; (c) removing gaseous components from
(b); and (d) consolidating (c) by spark plasma sintering, wherein
the nanostructured aluminum alloy thus produced has a density of
2.63 g/cm.sup.3 or higher and an average grain size of less than
100 nm.
22. The method of claim 21, wherein the reinforcement is selected
from the group consisting of oxides, carbides, nitrides, borides,
metals, intermetallics, and alloys.
23. The method of claim 21, wherein the aluminum alloy is aluminum
and another metal powder selected from the group consisting of Be,
Ca, Sr, Ba, Ra, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Nb, Mo, Tc,
Ru, Rh, Pd, Ag, Cd, and W, and combinations thereof.
24. The method of claim 21, wherein the reinforcement is boron
carbide.
25. The method of claim 21, wherein the reinforcement is silicon
carbide.
26. The method of claim 21, wherein the reinforcement is aluminum
nitride.
27. The method of claim 21, wherein the reinforcement is aluminum
oxide.
28. The method of claim 21, wherein cryomilling is continued until
equilibrium grain size of the metal is reached.
29. The method of claim 28, wherein cryomilling is continued
between 6 and 10 hours.
30. The method of claim 21, wherein the removal of gaseous
component occurs at a temperature between about 200.degree. C. and
600.degree. C.
31. The method of claim 30, wherein the removal of gaseous
components occurs at a temperature between about 300.degree. C. and
500.degree. C.
32. The method of claim 21, wherein removal of gaseous components
and consolidation of the cryomilled powder occur simultaneously, to
form a fully dense material without a separate degassing step.
33. The method of claim 21, wherein the temperature is provided by
liquid nitrogen or liquid argon.
34. The method of claim 21, wherein the spark plasma consolidation
is carried out at about 350.degree. C.
Description
FIELD OF INVENTION
[0002] The present invention relates to the production of
nanostructured materials through cryomilling and spark plasma
sintering.
BACKGROUND
[0003] Nanostructured material is material with a microstructure
the characteristic length of which is on the order of a few
(typically 1-500) nanometers. Microstructure refers to the chemical
composition, the arrangement of the atoms (the atomic structure),
and the size of a solid in one, two, or three dimensions.
Nanostructured materials have received increasing attention due to
their superior physical and mechanical properties. They are used in
the electronic industry, telecommunication, electrical, magnetic,
structural, optical, catalytic, biomedical, drug delivery, and in
consumer goods.
[0004] Nanostructured materials have generally been produced by (1)
powder metallurgy, (2) deposition to bulk nanostructured materials,
and (3) structural refinement by severe plastic deformation. In
powder metallurgy processes, nanostructured materials are commonly
made via mechanical milling of powder and subsequent consolidation
of the powder into bulk materials. There are several disadvantages
with this approach. Contamination is unavoidable during mechanical
milling, either from the processing media or atmosphere, and grain
growth during consolidation can occur. Modification of these
methods, however, can lead to the development of processes that are
more practical. For instance, it has been reported that mechanical
milling under liquid nitrogen can prevent the powders from being
severely oxidized from air, and small nitride or oxy-nitride
particles, which are within the size of 2-10 nm, are produced
in-situ during milling. These dispersoids, as they are called, can
both strengthen the metal and enhance the thermal stability (i.e.,
control the grain growth) of the nanostructured materials. As
another example, if the temperature and/or period to consolidate
nanostructured powders into fully dense bulk materials can be
reduced, severe grain growth can be suspended and thus the
nanostructure can again be retained.
[0005] With chemical processes, nanostructured materials are
created from a reaction with organometallics that precipitate
particles of varying sizes and shapes. The process can, however,
introduce excess carbon and/or nitrogen into the final composition.
An alternative approach is the solution-gelation (sol-gel) process
where ceramic production is similar to organometallic processes,
except sol-gel materials may be either organic or inorganic. Both
approaches involve a high cost of raw materials and capital
equipment, limiting their commercial acceptance.
[0006] Physical or thermal processing involves the formation and
collection of nanoparticles through the rapid cooling of a
supersaturated vapor (gas phase condensation, U.S. Pat. No.
5,128,081). Thermal processes create the supersaturated vapor in a
variety of ways, including laser ablation, plasma torch synthesis,
combustion flame, exploding wires, spark erosion, electron beam
evaporation, sputtering (ion collision). In laser ablation, for
example, a high-energy pulsed laser is focused on a target
containing the material to be processed. The high temperature of
the resulting plasma (greater than 10,000.degree. K) vaporizes the
material quickly allowing the process to operate at room
temperature. The process is capable of producing a variety of
nanostructured materials on the laboratory scale, but it has the
disadvantage of being extremely expensive due to the inherent
energy inefficiency of lasers, and, therefore, is not suitable for
industrial scale production.
[0007] Mechanical milling has been widely used to fabricate
nanostructured metal powder and powder for metal matrix composites.
However, it can be difficult to obtain nanostructured aluminum
alloys with conventional mechanical milling, because of the high
recrystallization rate due to the low melting temperature of
aluminum. Cryogenic milling or cryomilling is a modified mechanical
milling technique where the mechanical milling is carried out at
cryogenic temperatures, usually in liquid nitrogen or a similar
chilled atmosphere. Cryomilling has been employed to successfully
fabricate nanostructured aluminum alloy powders and powders for
aluminum metal matrix composites, which exhibit good thermal
stability, because the cryogenic temperature retards the recovery
of the aluminum. Strain is accumulated during cryomilling, leading
to dislocation activity, ultimately causing the formation of
nanoscaled grains within the cryomilled powder. The combined effect
of the ultra-fine dispersion of particles formed during cryomilling
and the reduced grain size is a powder that can be used to make a
bulk material with relatively high strength. This type of material
will also exhibit better creep resistance compared to its
conventional counterpart. It has been reported that cryomilled
aluminum alloys and aluminum metal matrix composite powders have
nanoscaled structures with very good thermal stability. Also,
cryomilling can be easily scaled up to produce tonnage quantities.
Thus, cryomilling is one of the few processing approaches available
for the fabrication of large quantities of nanostructured metal
powders.
[0008] U.S. Pat. No. 4,818,481 to Luton et al. discloses the use of
cryomilling to disperse a second phase within an aluminum alloy
where the repeated fracture and cold-welding of metal powder
involved in ball milling causes strain energy to be stored within
the milled particles. This strain energy is introduced through the
formation of dislocations, which result in decreased grain size
compared to that of the starting powders. The decreased grain size
also corresponds to a dispersed secondary phase within the alloy
which, in turn, results in improved mechanical properties in the
finished product. Different types of oxide dispersions can be
dispersed within aluminum alloys by this method.
[0009] The nanostructured powders described above must be
consolidated into bulk materials. Traditional consolidation
approaches, such as hot pressing (HP), hot isostatic pressing
(HIP), and cold isostatic pressing (CIP) have been employed for
consolidation into bulk materials. U.S. Patent Application
Publication No. 2004/0065173 to Fritzemeier et al. discloses
aluminum alloys produced by blending aluminum with two other metals
by cryomilling. The cryomilled alloy is subsequently consolidated
by HIP. These consolidation methods require degassing to remove the
process control agents that are normally added during cryomilling
and other gases to improve the efficiency of consolidation.
Further, these traditional consolidation approaches need very high
pressures, on the order of GPa, which is provided by high-pressure
argon, and long cycle time that can last for several hours.
Aluminum particles are covered with aluminum oxide films and these
oxides cannot be easily broken during consolidation which will
result in less dense materials thereby negatively affecting the
properties of the nanostructured materials. Therefore, a secondary
processing technique, such as extrusion, is required to improve the
density further and break the oxides. However, the high
temperatures and long consolidation times required by these
processes result in the grain growth of the nanostructured
aluminum.
[0010] Therefore, there is a need for processes for making
nanostructured materials where the processes are scalable for
commercial production of nanostructured materials.
SUMMARY
[0011] The present invention provides methods for the synthesis of
fully dense nanostructured materials, such as nanostructured
aluminum alloys and aluminum metal matrix composites. The
compositions thus synthesized find use in the defense industry,
aerospace industry, electronics industry, and in biotechnology and
drug delivery, among others.
[0012] In one aspect, the invention provides methods for the
synthesis of nanostructured materials where the starting materials
are cryogenically milled and consolidated by spark plasma
sintering.
[0013] In another aspect, the invention provides nanostructured
aluminum alloys and aluminum metal matrix composites with improved
mechanical properties, such as microhardness or strength. The
nanostructured aluminum alloys and aluminum metal matrix composites
thus produced find use in the defense industry, aerospace industry,
electronics industry, and in biotechnology and drug delivery, among
others.
[0014] In another aspect, the invention provides methods for
producing nanostructured materials, where the methods comprise (a)
providing metal powder and optionally a reinforcement, (b)
mechanically milling (a) at cryogenic temperatures to provide
nanostructured powders, (c) removing gaseous components from the
cryomilled powders, and (d) consolidating the cryomilled powder by
spark plasma sintering. The metal powder can be Al, Be, Ca, Sr, Ba,
Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Nb, Mo, Tc, Ru, Rh, Pd, Ag,
Cd, W, or combinations thereof, and preferably is an aluminum
alloy. The reinforcement can be oxides, carbides, nitrides,
borides, metals, intermetallics, or alloys. Thus, the reinforcement
can be boron carbide, silicon carbide, aluminum nitride, or
aluminum oxide.
[0015] In another aspect, the invention provides methods for
producing nanostructured aluminum alloys, the methods comprising
(a) providing aluminum alloy and a reinforcement, (b) mechanically
milling (a) at a temperature of about -150.degree. C. to about
-300.degree. C., (c) removing gaseous components from (b), and (d)
consolidating (c) by spark plasma sintering. The aluminum alloy can
additionally contain a metal powder such as Fe, Co, Ni, Cu, Zn, Y,
Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, or combinations thereof. The
reinforcement can be oxides, carbides, nitrides, borides, metals,
intermetallics, or alloys. Thus, the reinforcement can be boron
carbide, silicon carbide, aluminum nitride, or aluminum oxide.
[0016] These and other aspects of the present invention will become
evident upon reference to the following detailed description. In
addition, various references are set forth herein which describe in
more detail certain procedures or compositions, and are therefore
incorporated by reference in their entirety.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1 illustrates a bright field transmission electron
microscopy (TEM) image for the bulk 5083 Al processes by
cryomilling and SPS.
[0018] FIG. 2 illustrates a dark field TEM image for the bulk 5083
Al processes by cryomilling and SPS.
DETAILED DESCRIPTION
I. Definitions
[0019] Unless otherwise stated, the following terms used in this
application, including the specification and claims, have the
definitions given below. It must be noted that, as used in the
specification and the appended claims, the singular forms "a," "an"
and "the" include plural referents unless the context clearly
dictates otherwise. The practice of the present invention will
employ, unless otherwise indicated, conventional methods of
material science and physical chemistry, within the skill of the
art. Such techniques are explained fully in the literature. See,
e.g., Lu L, Lai M O. Mechanical Alloying, Kluwer Academic
Publishers, 1998, Boston, Mass.; Suryanarayana C. Progr Mater Sci
2001; 46: 1-184; Xie G Q, Ohashi O, Yoshioka T, Song M H, Mitsuishi
K, Yasuda H, Furuya K, Noda T MATERIALS TRANSACTIONS, 42 (9):
1846-1849 SEP 2001; and Cabanas-Moreno J G, Calderon H A, Umemoto
M, ADVANCED STRUCTURAL MATERIALS SCIENCE FORUM, 442: 133-142
2003.
[0020] All publications, patents and patent applications cited
herein, whether supra or infra, are hereby incorporated by
reference in their entirety.
[0021] The term "nanostructured material" generally refers to a
material having average grain sizes on the order of nanometers. For
purposes of the disclosure, nanostructured materials may include
those alloys having an average grain size of 500 nanometers (nm) or
less.
[0022] As used herein, "cryomilling" describes the fine milling of
metallic constituents at extremely low temperatures. Cryomilling
takes place within a ball mill such as an attritor with metallic or
ceramic balls. During milling, the mill temperature is lowered by
using liquid nitrogen, liquid argon, liquid helium, liquid neon,
liquid krypton or liquid xenon. In an attritor, energy is supplied
in the form of motion to the balls within the attritor, which
impinge portions of the metal alloy powder within the attritor,
causing repeated fracturing and welding of the metal.
[0023] As used herein, the term "powder" or "particle" are used
interchangeably and encompass oxides, carbides, nitrides, borides,
chalcogenides, halides, metals, intermetallics, ceramics, polymers,
alloys, and combinations thereof. The term includes single metal,
multi-metal, and complex compositions. Further, the terms include
one-dimensional materials (fibers, tubes), two-dimensional
materials (platelets, films, laminates, planar), and
three-dimensional materials (spheres, cones, ovals, cylindrical,
cubes, monoclinic, parallelepipeds, dumbbells, hexagonal, truncated
dodecahedron, irregular shaped structures, and the like).
[0024] As used herein, the terms "nanopowders" or "nanostructured
powders," are used interchangeably and refer to powders having a
mean grain size less than about 500 nm, preferably less than about
250 nm, or more preferably less than about 100 nm.
[0025] As used herein, the term "alloy" describes a solid
comprising two or more elements, such as aluminum and a second
metal selected from magnesium, lithium, silicon, titanium, and
zirconium. In addition, the alloy may contain metals such as Be,
Ca, Sr, Ba, Ra, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Nb, Mo, Tc,
Ru, Rh, Pd, Ag, Cd, W, or combinations thereof.
II. Modes of Carrying Out the Invention
[0026] The present invention discloses methods for synthesizing
nanostructured materials, and compositions thereof. The synthesis
employs a combined processing route where cryomilling and spark
plasma sintering (SPS) are used to synthesize and consolidate
nanostructured materials. The methods of the invention have the
advantage of having shorter consolidation time and lower
consolidation temperature. The inventive methods do not require the
use of high pressure gases, such as high pressure argon that is
normally required by the existing methods, e.g. HIP. The inventive
methods allow for suspending severe grain growth and maintaining
the nanostructure due to the lower consolidation temperature and
shorter consolidation time. The inventive methods do not require
secondary consolidation steps, e.g. extrusion, because the
potential for formation of an oxide film is lowered as a result of
the process conditions: the sintering occurs in vacuum in the
presence of a strong reducing agent, e.g., the graphite that is
used as the film and die. Further, the consolidation of the
nanostructured material to full density does not require a
degassing step, and the consolidated materials have good thermal
stability, i.e., grains remain in the nanometer scale even after
consolidation and use. The SPS sample contains two distinct regions
with different grain sizes. The small (nano) sized grains
contribute to high strength, while the large (submicron) sized
grains enhance the ductility of the materials. In addition, the
lower processing cost of SPS compensates for the higher processing
cost of cryomilling, thus making the combined processing route
economically feasible and scalable.
[0027] The methods of the present invention can be used with metals
with low melting temperatures, such as Ni, Fe, Cu, and Al, and
mixtures thereof, or with refractory metals, such as Ti, Nb, Mo,
Ta, and W, metal matrix composites, and intermetallics. In one
aspect, the metal powder to be processed is pre-alloyed powder that
can be used directly in the cryomilling process. In another aspect,
the powder to be processed is non-alloyed powder wherein two or
more different metal powders are added to the cryomill, and the
cryomilling process is used to mix together the metal constituents
thereby alloying the metals.
[0028] The methods of the present invention can be used with low
melting metals, such as Ni, Fe, Cu, and Al, and mixtures thereof,
and one or more other metals. Preferably, the starting metals are
manipulated in a substantially oxygen free atmosphere. For example,
if the metal is aluminum, the aluminum is preferably supplied by
atomizing the aluminum from an aluminum source and collecting and
storing the atomized aluminum in a container under an argon or
nitrogen atmosphere. The inert atmosphere prevents the surface of
the aluminum particles from excessive oxidation and prevents
contaminants such as moisture from reacting with the raw metal
powder. Preferably, other metals that can readily oxidize are
treated in the same manner as aluminum prior to and after
milling.
[0029] The metal for use in the invention can be selected from a
Group 2A metal, such as Be or Mg, and mixtures thereof, a Group 3A
metal, such as Al, and mixtures thereof, a Group 4A metal, such Sn
or Pb, and mixtures thereof, a Group V metal, such as V or Nb, and
mixtures thereof, a Group VI metal including Cr, W, or Mo, and
mixtures thereof, VII metal, such as, Mn, or Re, a Group VIII metal
including Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and mixtures thereof, the
lanthanides, such as Ce, Eu, Er, or Yb and mixtures thereof, or
transition metals such as Cu, Ag, Au, Zn, Cd, Sc, Y, or La and
mixtures thereof. Specific examples of mixtures of metals, such as
bimetallics, which may be employed by the present invention include
Fe--Al, Al--Mg, Co--Cr, Co--W, Co--Mo, Ni--Cr, Ni--W, Ni--Mo,
Ru--Cr, Ru--W, Ru--Mo, Rh--Cr, Rh--W, Rh--Mo, Pd--Cr, Pd--W,
Pd--Mo, Ir--Cr, Ir--W, Pt--W, and Pt--Mo. Preferably, the metal is
aluminum, iron, cobalt, nickel, titanium, copper, molybdenum, or a
mixture thereof. The metal or mixture of metals can be processed to
obtain the desired grain size and grain size distribution. Examples
of elemental compositions include, but are not limited to, (a)
precious metals such as platinum, palladium, gold, silver, rhodium,
ruthenium and their alloys; (b) base and rare earth metals such as
iron, nickel, manganese, cobalt, aluminum, copper, zinc, titanium,
samarium, cerium, europium, erbium, and neodymium; (c) semi-metals
such as boron, silicon, tin, indium, selenium, tellurium, and
bismuth; (d) non-metals such as carbon, phosphorus, and halogens;
and (e) alloys such as steel, shape memory alloys, aluminum alloys,
manganese alloys, and superplastic alloys.
[0030] The starting metal powder can additionally be mixed with a
certain amount of reinforcement, also called ceramic composition
(oxide, carbide, nitride, boride, chalcogenide), or an
intermetallic composition (aluminide, silicide) or an elemental
composition. Examples of ceramic composition include, but are not
limited to (a) simple oxides such as aluminum oxide, silicon oxide,
zirconium oxide, cerium oxide, yttrium oxide, bismuth oxide,
titanium oxide, iron oxide, nickel oxide, zinc oxide, molybdenum
oxide, manganese oxide, magnesium oxide, calcium oxide, and tin
oxide; (b) multi-metal oxides such as aluminum silicon oxide,
copper zinc oxide, nickel iron oxide, magnesium aluminum oxide,
calcium aluminum oxide, calcium aluminum silicon oxide, indium tin
oxide, yttrium zirconium oxide, calcium cerium oxide, scandium
yttrium zirconium oxide, barium titanium oxide, barium iron oxide
and silver copper zinc oxide; (c) carbides such as silicon carbide,
boron carbide, iron carbide, titanium carbide, zirconium carbide,
hafnium carbide, molybdenum carbide, and vanadium carbide; (d)
nitrides such as silicon nitride, boron nitride, iron nitride,
titanium nitride, zirconium nitride, hafnium nitride, molybdenum
nitride, and vanadium nitride; (e) borides such as silicon boride,
iron boride, titanium diboride, zirconium boride, hafnium boride,
molybdenum boride, and vanadium boride; and (f) complex ceramics
such as titanium carbonitride, titanium silicon carbide, zirconium
carbonitride, zirconium carboxide, titanium oxynitride, molybdenum
oxynitride, and molybdenum carbonitride.
[0031] In another aspect, the starting metal powders can be mixed
with some compounds other than ceramics. Such compounds may
include, for instance, organometallic compounds such as metal
alkoxides, as well as nitrates, carbonates, sulfates, and
hydroxides. These may be in the form of a powder or a liquid.
[0032] In the case of preparing a mixture containing ceramic and
metals or alloys in obtaining the sintered compact according to the
present invention, there is no particular limitation on the molar
equivalents for the ceramic to be added. However, the molar ratio
of metals to added ceramic reinforcement is preferably 1000:1 to
about 1:1, preferably about 500:1 to about 5:1, and more preferably
about 100:1 to about 10:1
[0033] Once the constituents of the metal or metal mixture and
ceramic reinforcement are selected, powders can be cryomilled,
wherein fracturing and welding of the metal particles is carried
out in a very low temperature environment. The milling can be using
shaker type mills, attritor mills, planetary mills, ball mills, or
rotary mills. Preferably the cryomilling of the metal powder takes
place within an attritor. The attritor is typically a cylindrical
vessel filled with a large number of ceramic or metallic spherical
balls. A single fixed-axis shaft is disposed within the attritor
vessel, and there are several radial arms extending from the shaft.
As the shaft is turned, the arms cause the spherical balls to move
about the attritor. When the attritor contains metal powder and the
attritor is activated, portions of the metal powder are impinged
between the metal balls as they move about the attritor. The force
of the metal balls repeatedly impinges the metal particles and
causes the metal particles to be continually fractured and welded
together.
[0034] The milling of the powders at low temperatures imparts a
high degree of plastic strain within the powder particles. During
cryomilling, the repeated deformation causes a buildup of
dislocation substructure within the particles. After repeated
deformation, the dislocations evolve into cellular networks that
become high-angle grain boundaries separating the very small grains
of the metal. Grain size as small as approximately 10.sup.-8 meter
have been observed via electron microscopy and measured by x-ray
diffraction. Structures having dimensions smaller than 10.sup.-7
meter, such as those found in the material produced at this stage
in the invented process, are commonly referred to as
nanostructured.
[0035] During milling, an organic polymer, such as polyethylene
glycol, polyvinyl alcohol, and the like, or organic acids, such as
stearic acid, ethyl acetate, ethylene bidisteramide and dodecane
may be added as one of the components to be milled with the metal
powder. The addition of organic components promotes the fracturing
of metal particles during milling, and prevents the severe adhesion
of the metal powders onto the milling media and milling tools.
[0036] During milling, the temperature of the metal powder is
preferably about -150.degree. C. or lower, such as about
-300.degree. C. Typically, the temperature of the metal powder is
reduced by using liquefied inert gases, such as liquid nitrogen (bp
-196.degree. C.), liquid argon (bp -186.degree. C.), liquid helium
(bp -269.degree. C.), liquid neon, liquid krypton or liquid xenon.
The use of liquid gases is a convenient way to lower the
temperature of the entire cryomilling system. Further, surrounding
the metal powder in liquid gases limits exposure of the metal
powder to oxygen or moisture. In operation, the liquid gas is
placed inside the attritor, in contact with the metal particles and
the attritor balls.
[0037] The operating parameters of the cryomill will depend upon
the size of the attritor. For example, a 150 liter (40 gal)
attritor is preferably operated at a speed of about 100 to 400 rpm.
The amount of powder added to the attritor is dependent upon the
size and number of balls within the attritor vessel. For a 150
liter attritor filled with 640 kg of 0.25'' diameter steel balls,
up to approximately 20 kg of metal powder may be milled at any one
time. Milling is continued for a time sufficient to reach an
equilibrium nanostructured grain size within the metal.
[0038] After milling, the metal alloy powder is a homogenous solid
solution of aluminum and the secondary metal, optionally having
other added tertiary metal components and optionally having minor
amounts of metallic precipitate interspersed within the alloy and
optionally having ceramic reinforcements interspersed within the
alloy. Grain structure within the alloy is very stable and grain
size is less than 500 nm. Depending on the alloy and extent of
milling the average grain size is less than about 300 nm, and
preferably may be lower than about 100 nm.
[0039] After the metal alloy powder, with the proper composition
and grain structure, is produced, it is consolidated into a form
that may be shaped into a useful object. The consolidation may be
by hot pressing (HP), hot isostatic pressing (HIP), cold isostatic
pressing (CIP), or spark plasma sintering (SPS). The consolidation
is preferably by HIP or SPS, more preferably SPS. If consolidation
is by HIP, the metal powder can be canned, degassed, and then
compacted and welded. After consolidating, the solid mass of the
metal may be worked and shaped. The consolidated metal can be
extruded into a usable metal component, and forged if necessary.
Further, there are no particular limitations concerning the
conditions of the HIP treatment and can be varied. Further, the HIP
treatment above may be carried out under an inert atmosphere such
as of nitrogen, argon, or helium and the retention time at the
treatment temperature and pressure may be in a range of from 0.5 to
3 hours, and particularly, approximately in a range of from 1 to 2
hours.
[0040] The metal alloy is preferably consolidated by SPS. The SPS
system can be commercially obtained, such as Dr. Sinter 1050
apparatus (Sumitomo Coal Mining Co., Japan). Typically in SPS, a
graphite die with an inner diameter of about 20 mm to about 100 mm
is used. The larger inner diameter is selected for the fabrication
of large pieces of bulk materials. The uniaxial pressure for SPS
can be applied by the top and bottom graphite punches thereby
eliminating the need for high-pressure argon. Typically, the alloy
from cryomilling is degassed to remove the gaseous materials,
including stearic acid. The removal of gaseous components is
preferably carried out at a temperature between about 200.degree.
C. and 600.degree. C., more preferably at a temperature between
about 300.degree. C. and 500.degree. C. Then, the alloy in the SPS
system is heated at a rate of about 10-500.degree. C./min and held
at the sintering temperature for about 1 min to about 60 min,
preferably about 2 min to about 15 min. The sintering temperature
is carried out at a temperature between about 40% and 100% of the
absolute melting temperature of the metal phase, preferably between
about 60% and about 95% of the absolute melting temperature of the
metal phase, more preferably about 80% and about 95% of the
absolute melting temperature of the metal phase. Thus, the shorter
sintering time and elimination of the requirement for high-pressure
argon make SPS an economically effective consolidation process.
[0041] The sintered alloy obtained by cryomilling and SPS
consolidation has a relative density with respect to the
theoretical density of about 99.0% or higher, preferably about
99.6% or higher, more preferably about 99.7% or higher, and
particularly preferably, about 99.8% or higher. In this manner, the
residual pores in the nanostructured alloy can be easily expelled
resulting in complete extinction of residual pores in the sintered
materials. A relative density lower than 99.0% is not preferred,
because the resulting alloy exhibits impaired strength and hardness
at room temperature as well as at high temperatures. Thus, the
density of aluminum 5083 according to the present invention is
preferably 2.63 g/cm.sup.3, and more preferably, 2.65 g/cm.sup.3 or
higher (the upper limit is the theoretical density of the resulting
material). Setting the density in the above range is preferred,
because the sintered materials can be sufficiently densified for
improving strength and hardness, while also improving abrasion
resistance.
[0042] At all times from cryomilling through the completion of
consolidation, the alloy powder is handled in an inert atmosphere,
such as a dry nitrogen or an argon atmosphere or in vacuum. The
inert atmosphere prevents oxidation of the surface of the alloy
powder particles. The inert atmosphere further prevents the
introduction of moisture to the alloy and prevents other
contaminants, which might be problematic in the extruded solid,
from entering the powder.
[0043] The size and distribution of grains within the
nanostructured material produced by the present invention may be
verified by any suitable method. One method of verification uses an
X-ray diffraction pattern (XRD). XRD measurements can be performed
using Cu K.sub..alpha. radiation in a Siemens D5000 diffractometer
equipped with a graphite monochromator. The grain size of the
material can be calculated on the basis of the peak broadening. The
methods described above may be used to produce nanostructured
materials with a certain size distribution. When one of the metals
is aluminum, the sintered aluminum according to the present
invention has an average grain size of 500 nm or smaller,
preferably from 1 nm to about 300 nm, more preferably, from 3 nm to
about 200 nm, further preferably, from 5 nm to about 150 nm. In an
embodiment, the nanostructured materials comprise grains between
about 3 nm and about 10 nm in size. In another embodiment, the
nanostructured materials comprise grains between about 5 nm and
about 50 nm in size. In still another embodiment, the
nanostructured materials comprise grains between about 20 nm and
about 40 nm in size. The calculation from XRD peak broadening shows
us the average grain size is 25 nm for as-cryomilled Al powders, 40
nm for degassed powders, and 44-60 nm for SPS-consolidated powders
depending on the sintering parameters.
[0044] Another method of verification is transmission electron
microscopy (TEM). A suitable model is the Phillips CM300 FEG TEM
that is commercially available from FEI Company of Hillsboro, Oreg.
In order to take a TEM micrograph, the metal nanoparticles are
typically thinned to achieve a foil that is thin enough for an
electron beam to pass through. The TEM samples can be prepared
using any of the known art procedures. For example, the powders and
epoxy can be mixed to create a slurry, which can then be mounted
into a stainless steel nut, sliced from a stainless steel pipe with
an outside diameter of 3 mm and an inside diameter of 2 mm, to form
a 3-mm diameter disk. The disk can be ground and dimpled to a
thickness of approximately 30 .mu.m using a dimpler fitted with
alumina grinders. The particle size of the alumina grinders descend
from a 3 .mu.m grade to a 1 .mu.m grade. Further thinning
perforation process can be carried out using a Gatan 600 argon ion
mill at the temperature of near liquid nitrogen temperature (the
extension of sample holder can be soaked in liquid nitrogen) with
an angle range from 22.degree. to 10.degree.. The TEM apparatus is
then used to obtain micrographs of the particles that can be used
to determine the grain size and grain size distribution of the
nanostructure powder created.
[0045] The methods of the present invention synthesize sintered
aluminum 5083 having high strength and hardness. More specifically,
the sintered aluminum 5083 yields a Vicker's hardness of 100 or
higher, preferably 120 or higher, and more preferably, 160 or
higher.
[0046] The nanostructured materials of the present invention have
numerous applications in industries such as, but not limited to,
space shuttle and satellite components, jet aircraft components,
helicopter roof control spiders and swashplates, combustion engine
components, brake rotors, gear box components, missile components,
armor vehicle body and components, diesel pistons, bicycle frames
and components, automotive propeller shaft, corrosion sensitive
applications, biomedical, sensor, electronic, telecommunications,
optics, electrical, photonic, thermal, piezo, magnetic and
electrochemical products.
EXAMPLES
[0047] Below are examples of specific embodiments for carrying out
the present invention. The examples are offered for illustrative
purposes only, and are not intended to limit the scope of the
present invention in any way. Efforts have been made to ensure
accuracy with respect to numbers used (e.g., amounts, temperatures,
etc.), but some experimental error and deviation should, of course,
be allowed for.
Example 1
[0048] Bulk nanostructured aluminum 5083 alloys reinforced with 10
wt % particulate B.sub.4C were synthesized by cryomilling and spark
plasma sintering. Coarse-grained aluminum 5083 alloy and hard
B.sub.4C, having a particle size of a few microns, were cryomilled
at a temperature of -180.degree. C. using a Szegvari attritor with
the ball-to-powder ratio of 32:1 and rotation speed of 180 rpm for
8 h. A small amount of stearic acid (0.2 wt %) was added into the
milling chamber as a process control agent to prevent severe
adhesion of the powders onto the chamber and milling balls. The
cryomilled powder was degassed at 400.degree. C., and loaded into a
graphite die and cold pressed through the punches under a load of
2000 pounds for one minute.
[0049] The powder was consolidated using spark plasma sintering
apparatus under vacuum. The ramping time from room temperature to
350.degree. C. was 3 minutes. The powders were then kept at
350.degree. C. for 3 minutes under the uniaxial sintering pressure
of 80 MPa.
[0050] The densities of the compacts thus obtained were measured
using Archimedes method. The hardness at room temperature for the
sintered composite was obtained by the Vicker's hardness
measurement method under a load of 2.942N.
[0051] The density of the bulk materials is 2.64 g/cm.sup.3, 99.9%
of the theoretical density. The hardness for this bulk composite is
288.7 HV and the average grain size in the aluminum 5083 matrix is
56 nm.
[0052] The x-ray diffraction (XRD) pattern of sintered B.sub.4C
reinforced aluminum composites shows that the compacts are
nanostructured materials.
Example 2
[0053] Bulk nanostructured aluminum 5083 alloys reinforced with 10
wt % particulate B.sub.4C were synthesized by cryomilling and spark
plasma sintering following the procedure of Example 1, except the
cryomilled powder was not degassed at 400.degree. C. before being
subjected to spark plasma sintering and the powders were held at
350.degree. C. for 2 minutes after the temperature reached
350.degree. C. during plasma sintering.
[0054] The densities of the compacts thus obtained were measured
using Archimedes method. The densities of the bulk materials were
2.65 g/cm.sup.3, 100% of the theoretical density. XRD, and hardness
testing were used to characterize the consolidated compacts, and
results showed that the hardness for this bulk composite is 233.3
HV with an average grain size of 44.8 nm in the matrix.
Example 3
[0055] Bulk nanostructured aluminum 5083 alloys were synthesized by
cryomilling and spark plasma sintering following the procedure of
Example 2. The microstructures of the compacts thus obtained were
investigated using transmission electron microscopy (TEM), shown in
FIGS. 1 and 2. The SPS sample contains two distinct regions with
different grain sizes. The region with the large grains is in the
upper left corner in FIG. 1, and the region with the small grains
is in the remainder. In this small-grained region, most of the Al
grains are below 50 nm in size, and some Al grains are as small as
20 nm. As a basis for comparison, the average grain size for
as-cryomilled 5083 Al powder is 25 nm, with a grain size
distribution of 15-60 nm. Thus, the grain size for SPS 5083 Al in
the small-grained region is comparable to the Al grains in the
as-cryomilled 5083 Al powders, indicating that the small grain size
can be retained after consolidation by SPS. Some grains in the
cryomilled powders grew during SPS, forming the region containing
the larger grains. These larger grains have a wide distribution in
size, from 50 to 200 nm. The presence of the small grains in the
SPS 5083 Al contributes to the higher strength of the material,
while the presence of the large grains contributes to the ductility
of the material.
[0056] The densities of the compacts thus obtained were measured
using Archimedes method. The densities of the bulk materials were
2.63 g/cm.sup.3, 99.0% of the theoretical density. XRD, and
hardness testing were used to characterize the consolidated
compacts, and results showed that the hardness for this bulk
material is 165.3 HV with an average grain size of 56.6 nm.
[0057] While the invention has been particularly shown and
described with reference to a preferred embodiment and various
alternate embodiments, it will be understood by persons skilled in
the relevant art that various changes in form and details can be
made therein without departing from the spirit and scope of the
invention. All printed patents and publications referred to in this
application are hereby incorporated herein in their entirety by
this reference.
* * * * *