Note: Descriptions are shown in the official language in which they were submitted.
WO 00/08219 PCT/CA99/00722
PREPARATION OF METAL-MATRIX COMPOSITE MATERIALS
USING CERAMIC PARTICLES WITH MODIFIED SURFACES
TECHNICAL FIELD
s This invention relates to the preparation of metal-matrix composite
materials
and, more specifically, to a method for rapid preparation of such materials
and to the
modification of the surfaces of ceramic particles used in such composite
materials, prior
to formation of the composite materials.
BACKGROUND ART
1o In one form of a metal-matrix composite material, a reinforcement phase is
embedded in a metal matrix. The reinforcement is typically equiaxed or
elongated
particles of a ceramic phase such as aluminum oxide, and the matrix is a pure
metal or
alloy such as aluminum. The particle phase and the matrix metal phase each
retains its
physical and chemical identity in the composite material, and each phase
contributes to
~ s the properties of the final composite material.
In order to achieve good mechanical properties, the metal matrix should wet
the
surfaces of the ceramic particles. The wetted interface ensures good
mechanical load
transfer between the phases, and also minimizes the possibility of internal
failure modes
such as cavitation failure at the matrix/particle interfaces during
deformation.
2o The achievement of a wetted interface may be the result of a stirring
process or
an infiltration process. These processes are limited, in the first case by the
maximum
solid loading (i.e., volume fraction of particles in the composite material}
that may be
achieved, and in the second case, by the time taken to infiltrate a particle
bed
completely. It is therefore desirable to find a rapid means of obtaining a
wetted
2s interface that is effective even for high solids loading.
However, the close interfacial contact achieved as a result of good wetting
may
in some cases lead to degradation of the properties of the composite material
through
chemical interdiffusion and the formation of brittle and/or unstable phases at
the
interface between the particles and the matrix. For example, the alloying
elements
3o typically found in aluminum casting alloys used as the matrix of the
composite material
may chemically react with aluminum oxide or silicon carbide particles, forming
brittle
phases that reduce the fracture resistance of the composite material.
A number of techniques have been developed to achieve particle/matrix wetting
while alleviating the problems associated with chemical interdiffusion and
interaction
CA 02339803 2001-02-06
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between the matrix and the particles. The particles may be coated with a
surface Iayer
that resists chemical interdiffusion. Coating of the particles is usually
expensive, and
some coating techniques may not achieve total coating over the entire surfaces
of the
particles. 1n another approach, uncoiled particles tray be used to reduce the
cost, and a
wetting layer such as aluminum nitride formed during the composite
fabrication. The
chemical reactions associated with this approach are usually slow, resulting
in long
processing times. In another alternative, the composition of the metallic
matrix may be
carefully selected so as to minimize chemical interactions. While operable,
this technique
limits the types of matrix alloys that may be used.
1D There is a need for an approach that achieves a wetted rnatrixlparticlc
interface at
a rapid rate, and which also minimizes the degradation of the composite
material due to
chemical interaction. The present invention fulfills this need, and fulthcr
provides related
advantages.
DItS~'T.OSL7RE Of THF ITTVENTION
The present invention provides an approach for modifyins the surfaces of the
particles to be used in a metal-matrix composite material, and prepatirtg the
composite
material using the modified particles. Tho composite material has a wetted
matrixlparticle interface, and cnay include a passivation barrier that reduces
the incidence
of deleterious diffusional reactions during fabrication and service. The
approach is
applicable to a wide variety of combinations of particles and matrix alloys,
but is most
preferably applied to the commercially important composite material containing
aluminum oxide particles in an aluminum-aDoy matrix. The technique of the
invention is
readily utilized, and may be accomplished much more rapidly than available
alternative
procedures. lncxpcnaive uncoiled particles may be used as the starting
material, and the
ZS preferred passivation treatment of the particles is accomplished as part of
the composite
fabrication process. It is therefore more economical than the alternatives.
According to one aspect of the invention, there is provide a method of
preparing a
composite material, in which a mass of etramic particles is furnished aid a
source of
molten matrix metal is contacted with the mass to form a metal-containing
composite
material; characterized in that, prior to the contact of the matxix metal with
the mass, a
source of molten magnesium is contacted with the mass in the absence of added
oxygen
or nitrogen.
According to aaother aspect of the invention, there is provided a method of
preparing a composite material, in which a mass of ceramic particles is
famished and a
source of molten matrix metal is contacted with the mass to form a metal-
conlainutg
composite material; characterized in that, prior to the contact of the molten
matrix metal
Ab~ENDED SHEET
CA 02339803 2001-02-06
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with the mass, the ceramic particles are contacted, in the absence of added
oxygen or
nitrogen, with a source of molten metal that is reactive with the ceramic
particles to an
extent that a continuous, non-porous layer of chemical reaction product
compound is
fonnod on the ceramic particles upon the contact of the source of molten met
al with the
ceramic particles.
Expressed in other terms, the invention, in at least preferred forms, provides
a
method for preparing a oomposite material, in which a mass of ceramic
particles is
furnished and a source of molten matrix metal is contacted with the mass to
form a tnetal-
containing composite material; wherein, prior to the contact of said matrix
metal with the
mass, a source of a passivating compound is contacted with the mass such that
a
passivating layer is formed on the ceramic particles.
In accordance with a preferred aspect of the invention, a method for preparing
a
composite material comprises the steps of furnishing a mass of ceramic
particles,
contacting a source of magnesium to the mass o~ ceramic particles, such that
magnesium
is deposited onto at least a portion of the surfaces of the particles in the
absence of added
oxygen and nitcogczt, and contacting a source of aluminum to the surfaces of
the
magnesium-treated particles to farm an aluminum-containing composite material.
In one preferred cnnbodiment, a method for preparing a composite material
comprises the steps of furnishing a bed of ceramic particles in a container,
the bed of
ceramic particles being at a temperature greater than the melting point of
aluminum, and
placing a layer of molten magnesium on an upper surface of the bed of ceramic
particles,
whereupon the layer ofmolten magnesium flows into and through the bed of
ceramic
particles, and forms a magnesium layer on the particles. The method further
includes
thereafter placing a layer of molten aluminum on an upper surface of the bed
of ceramic
particles through which the magnesium layer is flowing, and creating a
pressure
differential between the layer of molten aluminum and an interior of the bed
of ceramic
particles, whereupon molten eluminunt flows into the bed of ceramic particles
to form an
aluminum-containing composite material.
1n accordance with another preferred embodiment of the invention, a method of
preparing a composite materiel comprises the Steps Of furnlshtng a mass of
ceramic
particles, and pessivating the surfaces of the ceramic particles by forming a
substantially
continuous, non-porous lays of a reaction product compound thereon. The
passivating
approach includes contacting a reactive metal to the ceramic particles
~j~~E(~GED S~iEE'~
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WO 00/08219 PCT/CA99/00722
4
to form the reaction product compound as a chemical reaction product of the
reactive
metal and the ceramic particles. A first composite material is formed of the
passivated
ceramic particles and a first matrix metal. A second composite material having
a lower
volume fraction of particles may be formed by furnishing a source of a second
matrix
metal, wherein the second matrix metal is molten, and dispersing the first
composite
material in the second matrix metal.
The approach of the invention may be practiced with a wide variety of
particles,
first matrix metals, and second matrix metals. In a preferred application, the
particles
are an oxide-based ceramic such as aluminum oxide or spinet, and/or silicon
carbide
1o ceramic. The reactive metal is preferably magnesium (i.e., either pure
magnesium or
magnesium alloys}, and the first matrix metal and second matrix metal are
aluminum
(i.e., either pure aluminum or aluminum alloys).
Thus, a preferred method for preparing a composite material comprises the
steps
of furnishing a mass of aluminum oxide particles, and contacting a source of
~ 5 magnesium to the mass of ceramic particles, such that magnesium is
deposited onto the
surfaces of the particles in the absence of added oxygen and nitrogen. The
magnesium,
which may be in the form of pure magnesium or an operable source of a
magnesium
alloy, reacts with the ceramic particles to produce a continuous, non-porous
layer of
reaction product thereon. The reaction layer is a dense spinet composition on
the
2o surfaces of the aluminum oxide particles. A source of aluminum (first
matrix metal) is
contacted to the surfaces of the magnesium-treated particles to form an
aluminum-
containing composite material. The resulting composite material may be
dispersed into
a second source of aluminum (second matrix metal) to dilute the composite to a
lower
volume fraction of particles.
25 The present approach may be distinguished from prior approaches. The
particles are not furnished in a coated form, but instead are protected by a
reaction layer
or coating layer formed during processing. The reaction layer is distinct from
a
deposited layer. In some prior techniques, a magnesium nitride layer is
deposited upon
uncoated particles. This wetting-enhancement layer is formed by the reaction
of
3o magnesium with atmospheric nitrogen, a slow process. The present invention
produces
its passivation layer by the reaction of a reactive metal with the particles
themselves,
which occurs much more rapidly than the nitrogen gas reaction of the prior
approach.
The technique of the invention provides an approach for producing a metal-
matrix/ceramic particle composite material wherein the ceramic reinforcement
particles
CA 02339803 2001-02-06
- WO 00/08219 PCT/CA99/00722
have passivated surfaces to inhibit subsequent deleterious reactions during
subsequent
processing, or coated surfaces to enhance subsequent wetting of the matrix to
the
particles. The composite material is readily manufactured in a rapid, economic
fashion.
It is to be noted that the term particles is intended to mean generally
equiaxed or
5 slightly elongated regular or irregular particles, or substantially
elongated particles such
as fibers or whiskers. In the present invention, the use of regular, equiaxed
particles is
preferred.
Other features and advantages of the present invention will be apparent from
the
following more detailed description of the preferred embodiment, taken in
conjunction
1o with the accompanying drawings, which illustrate, by way of example, the
principles of
the invention. The scope of the invention is not, however, limited to this
preferred
embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA-1B are idealized depictions of the microstructures of composite
materials made according to the invention, wherein Figure 1 A illustrates a
composite
material having a passivation layer and Figure 1B illustrates a composite
material
having a magnesium surface layer that enhances wettability;
Figures 2A-2E are schematic views illustrating stages during the production of
a
composite material by one preferred approach of the invention, wherein Figure
2A
2o depicts an initial stage with a layer of molten magnesium overlying a bed
of ceramic
particles, Figure 2B depicts the magnesium infiltrated into the bed of ceramic
particles,
Figure 2C depicts a layer of molten aluminum overlying the bed of magnesium-
treated
ceramic particles, Figure 2D depicts the bed of magnesium-treated ceramic
particles
infiltrated with aluminum, and Figure 2E depicts dispersing the resulting
composite
material into additional matrix alloy;
Figure 3 is a block flow diagram of a preferred approach for practicing the
invention;
Figure 4 is a schematic view of a second embodiment of an apparatus for
practicing the invention; and
3o Figure S is a schematic view of a third embodiment of an apparatus for
practicing the invention.
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6
BEST MODES FOR CARRYING OUT THE INVENTION
Figures lA and 1B illustrate composite materials 20 made according to the
present invention. In each case, the composite material 20 has ceramic
particles 22
embedded in a metal matrix 24.
In the embodiment of Figure lA, a passivating/wetting layer 26 is present at
the
surfaces of the particles 22. The passivating/wetting layer 26 is typically
thin, in the
order of less than about 1 micrometer in thickness. Its thickness is
exaggerated in
Figure 1 A so that it may be depicted.
In the embodiment of Figure 1B, a wetting layer 28 of magnesium is present at
1o the surfaces of the particles 22. The wetting layer 28 is also typically
thin, in the order
of less than about 1 micrometer in thickness. Its thickness is exaggerated in
Figure 1 B
so that it may be depicted. According to the fabrication approaches discussed
subsequently, the magnesium layer 28 is deposited on the surface of the
particles, and
the matrix alloy is provided thereafter to wet the surfaces of the magnesium-
coated
15 particles. Because magnesium readily dissolves in common matrix metals and
alloys,
such as aluminum, all or some of the wetting layer 28 may be dissolved away as
the
aluminum is wetted to the particles, and for this reason the wetting layer 28
is depicted
by dashed lines. However, by the time that dissolution occurs, the wetting
function of
the magnesium layer 28 has been performed, and its dissolution is acceptable.
Thus,
2o Figure IB is intended to depict a time after the wetting layer 28 has been
deposited and
the matrix alloy has been introduced, but prior to partial or total
dissolution of the
wetting layer. It does not necessarily, but may, depict the final product.
The particles 22 may be substantially equiaxed or elongated, and may be of any
operable material. Preferred particles in the Figure I A embodiment include
aluminum
25 oxide particles. Preferred particles in the Figure IB embodiment include
silicon
carbide and/or spinet (magnesium aluminum oxide) particles.
The matrix 24 for both embodiments may be any operable material. The
preferred matrix material is aluminum, which term herein includes both
substantially
pure aluminum and aluminum alloys. The volume fraction of particles in the
matrix
30 may be varied over a wide range, according to the fabrication processes to
be described
subsequently. In the preferred infiltration approach, the particles may be
present in
relatively large fractions of from about 35 to about 70 volume percent of the
total of
particles and metal in the as-infiltrated material, which volume fraction may
be reduced
to a value of less than about 30 volume percent by subsequent dilution.
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WO 00/08219 PCT/CA99/00722
In the Figure lA embodiment, the passivating/wetting layer 26 is selected
according to the composition of the particles and the composition of the
matrix
material. The passivatinglwetting layer 26 is the reaction product of a
reactive metal
having a composition different from that of the matrix material 24 with at
least one
chemical constituent of the ceramic of the particles 22. The reactive metal
must be
amenable to the selected fabrication technique. In the case of an aluminum
oxide
particle and an aluminum matrix, the reactive metal is preferably magnesium.
The term
"magnesium" as used herein includes both substantially pure magnesium and
alloys of
magnesium having su~cient magnesium to permit the surface reaction with the
I o ceramic particles. For the cases of most interest, operable alloys of the
magnesium
reactive metal must have at least about 10 weight percent magnesium. Alloys of
magnesium are preferred to pure magnesium for several practical reasons.
Molten pure
magnesium has a tendency to rapid chemical reaction with oxygen, leading to
the
possibility of a magnesium fire during commercial operations. Alloys have a
lower
melting point and allow lower working temperatures to ensure complete
infiltration of
the bed of particles by the magnesium. The density of pure magnesium is fixed,
but the
density of a magnesium alloy may be selected so as to be closer to that of the
aluminum
that follows the magnesium into the bed, thereby reducing the likelihood of
gravity-
induced mixing of the magnesium infiltrating layer and the aluminum that
follows it.
2o Figures 2A-2E illustrate, in diagrammatic form, one preferred process for
practicing the invention to produce eithex the structure of Figure lA or the
structure of
Figure 1B, depending upon the nature of the particles used, and Figure 3
illustrates the
process in block diagram form. A bed 30 of ceramic particles is furnished in a
container 32 (numeral 50 in Figure 3). The ceramic particles are, as discussed
previously, typically aluminum oxide particles for the Figure 1 A embodiment
and
silicon carbide and/or spinel particles for the Figure 1B embodiment. The bed
30 of
ceramic particles is heated to a temperature greater than the melting point of
the
magnesium-source material used to infiltrate the bed in the next step, and
also greater
than the melting point of the aluminum-source material used in a subsequent
step. A
layer 34 of molten magnesium is placed on the top or upper surface of the bed
30 of
ceramic particles in Figure 2A, numeral 52 in Figure 3. The molten magnesium
may be
pure magnesium or, more preferably, a magnesium alloy such as Mg-9 weight
percent
Al-lweight percent zinc (AZ 91 alloy). If a magnesium alloy is used for the
Figure lA
embodiment, the magnesium alloy must have at least about 10 percent by weight
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WO 00/08219 PCT/CA99/00722
8
magnesium so that the magnesium chemically reacts with the particles such as
aluminum oxide in the subsequent steps. The amount of magnesium is sufficient
to
produce a molten pool on top of the bed 30, and to produce, in the final
composite after
infiltration with both the magnesium and subsequent aluminum alloy, a desired
magnesium composition in the matrix alloy of the composite. This depth is
about 25
millimeters in a preferred embodiment.
As shown in Figure 2B, the material of the magnesium layer 34 spontaneously
wicks into the bed 30 of ceramic particles having an upper surface 90 as a
discrete band
of material 92 in the upper portion of the bed, infiltrating the bed 30 to an
initial
to infiltration front 94, numeral 54 of Figure 3. As shown in Figure 2C, after
a sufficient
time for this process to commence (typically a few minutes or less) but not
necessarily
be completed, a layer 36 of molten aluminum (which may be substantially pure
aluminum or an aluminum alloy) is placed on top of the bed 30, numeral 56 of
Figure
3. Aluminum will not usually spontaneously wick into the bed 30 but, as shown
in
Figure 2D, the presence of the magnesium containing band 92 in the top of the
bed 30
assists in such infiltration, and in addition a pressure differential is
created between the
aluminum layer 36 and the bed 30 to force the aluminum layer 36 to flow into
the bed
30 behind the magnesium band 92, numeral 58 of Figure 3. The pressure
differential,
which need be no more than a few tenths of an atmosphere, is created by any
operable
approach. It may be created by ensuring that the aluminum layer 36 completely
covers
the exposed surface of the bed 30 of ceramic particles and is created
immediately after
the magnesium band 92 is infiltrated into the bed 30. The reaction between the
magnesium and any oxygen and nitrogen trapped inside the bed 30 essentially
getters
the oxygen and nitrogen, and the resulting partial vacuum, which the aluminum
layer
36 seals, is sufficient to draw the aluminum into the bed. In an alternative
embodiment,
illustrated in Figure 4, a vacuum may be applied to the interior of the bed 30
to draw
the aluminum layer 36 into the bed 30.
The aluminum layer 36 forms a substantially continuous infiltration layer with
the magnesium band 92 initially infiltrated into the top of the bed 30, and
through
3o interdiffusion of the aluminum from the aluminum layer 36 into the
magnesium-rich
band 92 and vice versa, the magnesium concentration at the infiltration front
is
gradually reduced towards the bulk concentration, which is the concentration
resulting
after full infiltration and diffusional equilibration. However, as the
diffusion process is
slower than the magnesium- and vacuum-enhanced infiltration rate, the
infiltration front
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WO 00/08219 PCT/CA99/00722
9
remains rich in magnesium and therefore the infiltrating magnesium metal
continues to
perform the functions of the initially infiltrated magnesium band 92.
The initially infiltrated magnesium band 92, and the magnesium-rich
infiltration
front 94 thereafter provide enhanced wetting through the high magnesium
content
and/or provides, in the Figure lA embodiment, a reaction with the aluminum
oxide to
form the layer 26 of spinel (magnesium aluminum oxide) on the surface of the
particles
22. This layer 26 forms as a continuous layer around the entire periphery and
surface
of the particles and renders the particles non-reactive to further attack by
the aluminum.
The layer 26 is typically less than about 1 micrometer thick, but may be
thicker. The
layer 26 is dense (i.e., not porous) and therefore, once formed, serves as a
protective
passivating barrier against chemical reaction with the aluminum infiltration
layer which
does not contact the particles until after this passivating reaction has
occurred. By
contrast, if a spinel layer is formed at the surfaces of the particles by
reaction of the
particle material with the magnesium in a relatively dilute magnesium-
containing
aluminum alloy in either the initially infiltrated layer or at the
infiltration front, the
layer is less dense and more fuzzy in appearance when viewed microscopically,
and
offers little resistance to subsequent, continuing degradation by the
following
aluminum infiltration layer.
For particles which are non-reactive to form a passivating layer (such as SiC
or
spinel particles), the above process is operative to provide enhanced wetting
and fast
infiltration, even though no chemical reaction occurs with the particle
surface to form a
passivating layer.
In an alternative approach, as in the Figure 1 B embodiment, the magnesium
may be delivered to the bed 30 by other operable means. For example, the bed
30
could be fluidized by the introduction of an inert fluidizing gas at the
bottom of the bed.
The magnesium in the foam of a magnesium compound may be introduced into the
bottom of the bed 30 in the fluidizing gas stream so as to deposit a magnesium
layer on
the surface of the particles throughout the bed. Similarly, magnesium vapor in
an inert
carrier gas may be used to permeate the bed and coat the bed particles with a
layer of
3o magnesium. A layer of aluminum alloy is provided as in the previous
approach and a
vacuum applied using the approaches previously described. The deposited
magnesium
may react to form a passivating layer (for example, if the particles are
alumina, a dense
spinel layer may be foamed) about 1 micrometer in thickness, or in other cases
may
form a layer 28 of magnesium which is continuous over the surface of the
particles and
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WO 00/08219 PCT/CA99/00~22
preferably less than about 1 micrometer in thickness. The aluminum layer
readily wets
the passivating/wetting layer foamed on alumina particles, or the wetting
layer 28
(Figure 1 B embodiment) previously formed on the particles, resulting in a
well-wetted
matrix/particle interface.
5 In either approach, there may be a small amount of oxygen, nitrogen, or
other
reactive gas present initially, which is quickly gettered by the magnesium. No
further
oxygen or nitrogen is permitted to contact and react with the magnesium inside
the bed
30. Thus, there is an "absence of added oxygen and nitrogen", as that term is
used
herein. There is therefore very little, if any, aluminum oxide, magnesium
oxide,
10 aluminum nitride, or magnesium nitride formed. This approach is to be
contrasted with
a prior approach wherein excess nitrogen gas is intentionally provided in the
bed to
react with infiltrated magnesium to produce magnesium nitride, which deposits
upon
the particles. In the present approach, by contrast, the magnesium reacts with
the
alumina particles, not the gas, at the particle surface. The present technique
produces a
more adherent and denser layer to passivate the surfaces of the particles, as
compared
with this prior approach.
The infiltrated material in Figure 2D typically has a relatively high volume
fraction of particles in the matrix, on the order of 40 volume percent or
higher as
determined by the relative amount of particles and matrix alloy used in the
infiltration
procedure. For some applications, a lower volume fraction of particles is
desired. To
produce such a lower volume fraction, the composite material mixture of Figure
2D is
diluted, numeral 60 of Figure 3, such as by mixing the composite material with
a
second matrix material 38. In one approach, as illustrated in Figure 2E, the
composite
material of Figure 2D may be poured into a mass of molten second matrix
material 38
in a second container 40 and mixed together. Equivalently for the present
purposes, the
second matrix material may be poured into the container 32 and mixed with the
infiltrated composite material. In either case, the relative amounts of the
infiltrated
composite material prepared in steps 50-58 and the second matrix material 38
are
selected to yield a final composite material having a preselected volume
fraction of
3o particles. The second matrix material 38 is typically a metallic alloy
which may be of
the same composition as the first matrix alloy material of the layer 36, or it
may be of a
different composition selected to yield a preselected net composition of the
final matrix.
Another implementation of the invention is illustrated in Figure S. A mixture
of
operable particles (e.g., silicon carbide or spinet particles) and about 1
percent by
CA 02339803 2001-02-06
WO 00/08219 PCT/CA99/00722
weight of a source of magnesium is introduced into a mixing container 70
through an
input line 72. The preferred source of magnesium is an alloy having the
composition of
the magnesium-aluminum eutectic at about 69 weight percent magnesium, balance
aluminum. The eutectic composition is preferred in order to minimize the
required
operating temperature of the mixing container 72. The mixing container 70 is
heated to
a temperature above the melting point of the source of magnesium, and is
provided with
paddles 74 that stir the mixture of particles and the source of magnesium to
ensure
complete coating of the particles. The magnesium-coated particles flow from
the
mixing container 70 through an outlet line 76.
to The magnesium-coated particles flow into a composite mixing container 78.
The matrix alloy is added to the composite mixing container 78 through an
input line
80. The matrix alloy and the magnesium-coated particles are mixed in the
composite
mixing container 78 to produce a final composite composition. The composite
material, with the matrix in molten form, is removed through an outlet line 82
to be cast
or otherwise processed. The composite mixing container 78 may be operated in
either a
batch or a continuous-flow mode of operation.
EXAMPLES
The present invention has been practiced using the approach discussed in
relation to Figures 2A-2E and Figure 3. In one test, 2.8 kilograms of 320 mesh
silicon
carbide powder was added to a 3 inch diameter by 18 inch long tube container
32 to
form the 12-inch deep bed 30. The container and bed were heated to
700°C. About 150
grams of molten magnesium was poured onto the upper surface of the bed 30. The
magnesium had wicked into the bed after about 3 minutes, and no magnesium
layer was
visible on the surface. Approximately 1.6 kilograms of aluminum alloy 359 was
then
poured onto the upper surface of the magnesium-infiltrated bed to form a layer
6 inches
deep. The upper surface of the aluminum alloy layer fell 3.75 inches in 10
minutes, and
thereafter remained at a stable height. This amount of metal was sufficient to
completely infiltrate the bed at 60 percent packing density. The resulting
composite
material was resuspended with additional aluminum alloy 359.
3o In a second test, 5.0 kilograms of Placor 20 aluminum oxide particles
having an
average size of 20 micrometers was heated in a five inch diameter steel
crucible to
700°C. About 866 grams of alloy AZ91 (Mg-9 weight percent aluminum-1
weight
percent zinc), preheated to 750°C, was added to the surface of the
particle bed. After
about 15 seconds the magnesium alloy had wicked into the bed, and an
additional 6.0
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12
kilograms of commercial purity aluminum, preheated to 750°C, was added
to the
surface of the bed. The aluminum completely infiltrated the bed in about 1
minute.
The composite was re-suspended by stirring to produce a 32 volume percent
aluminum
oxide composite, which was thereafter diluted by the addition of fmrther
commercial
purity aluminum to produce a 20 volume percent aluminum oxide composite
material.
The composite materials produced in the first and second tests were studied
metallographically. In each case, the interfaces between the particles and the
matrix
alloy were clean. The absence of a visible reaction product at the aluminum
oxide/metal matrix interface in the second test suggests that a thin
passivating layer of
l0 spinal had formed on the aluminum oxide particles.
Although a particular embodiment of the invention has been described in detail
for purposes of illustration, various modifications and enhancements may be
made
without departing from the spirit and scope of the invention. Accordingly, the
invention is not to be limited except as by the appended claims.
CA 02339803 2001-02-06
