U.S. patent application number 09/992685 was filed with the patent office on 2002-08-15 for methods and apparatus for using large inertial body forces to identify, process and manufacture multicomponent bulk metallic glass forming alloys, and components fabricated therefrom.
Invention is credited to Johnson, William L..
Application Number | 20020108738 09/992685 |
Document ID | / |
Family ID | 26939666 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020108738 |
Kind Code |
A1 |
Johnson, William L. |
August 15, 2002 |
Methods and apparatus for using large inertial body forces to
identify, process and manufacture multicomponent bulk metallic
glass forming alloys, and components fabricated therefrom
Abstract
To identify and manufacture metallic glass forming alloys, large
inertial forces or "g"-forces are created in a centrifuge assembly
used to sequentially separate crystalline phases (particles) as
they sequentially form and grow in a molten alloy during gradual
cooling of the alloy below its liquidus temperature. These forces
physically remove and isolate the actual crystalline particles from
the remaining liquid as they are formed. Under the influence of a
large g-force, this is accomplished by rapid and efficient
sedimentation and stratification. Further contamination and nascent
solid "debris" in the form of oxides, carbides, or other foreign
particles can be removed from the molten alloy using the same
sedimentation/stratification technique. Finally, a method of
efficiently cooling and solidifying the final low melting
stratified and decontaminated liquid into a solid glass component
is proposed which utilizes convective heat transport by a cooling
gas. The result is a vitrified bulk metallic glass component of
near net shape.
Inventors: |
Johnson, William L.;
(Pasadena, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
26939666 |
Appl. No.: |
09/992685 |
Filed: |
November 14, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60248901 |
Nov 14, 2000 |
|
|
|
60271188 |
Feb 23, 2001 |
|
|
|
Current U.S.
Class: |
164/290 ;
164/118 |
Current CPC
Class: |
B22D 13/04 20130101 |
Class at
Publication: |
164/290 ;
164/118 |
International
Class: |
B22D 013/06 |
Claims
What is claimed is:
1. A high temperature centrifugal processing device for processing
molten metal alloys under very high inertial accelerations,
comprising: a rotor fabricated of a high temperature material
having high strength and fracture resistance at temperatures of
between about 400 and 1200.degree. C. and which is capable of
withstanding inertial accelerations up to at least 50,000 g's; a
plurality of internal cavities within the rotor symmetrically laid
out within the body of the rotor; and a shaft onto which the rotor
is mounted which allows the rotor to be spun at high rotation
frequencies of between about 1000 and 100,000 rpm.
2. The device of claim 1, wherein the rotor comprises two internal
cavities.
3. The device of claim 1, wherein the rotor comprises four internal
cavities.
4. The device of claim 1, wherein the rotor comprises eight
internal cavities.
5. The device of claim 1, wherein the cavities extend to an upper
surface of the rotor and are each sealable with a cover.
6. The device of claim 1, wherein the rotor is enclosed within a
high temperature furnace.
7. The device of claim 1, wherein the rotor is heated using an
element selected from the group consisting of an external induction
heating coil, an external laser, and a resistive heating element
mounted on the rotor itself to couple heat to the rotor.
8. The device of claim 1, further comprising hermetically sealed
sample capsules within said internal cavities.
9. The device of claim 8, wherein the sample capsules are made of a
suitable high temperature material which can withstand high
temperatures of between about 400 and 1200.degree. C. and
concurrent high g accelerations up to 250,000 g's without failure
of the hermetic seal.
10. The device of claim 8, wherein the sample capsules are
fabricated from a material selected from the group consisting of
high temperature steel, Inconel, a Ni-based super-alloy, molybenum
and a refractory metal.
11. The device of claim 8, wherein the sample capsules include
crucibles therein.
12. The device of claim 11, wherein the crucibles are fabricated
from a material selected from the group consisting of refractory
metal, a ceramic, vitreous carbon and fused silica.
13. The device of claim 1, wherein the rotor can withstand inertial
accelerations up to about 100,000 g's.
14. The device of claim 1, wherein the rotor can withstand inertial
accelerations up to about 250,000 g's.
15. The device of claim 1, wherein the rotor is a long rod
including two internal cavities.
16. The device of claim 1, wherein the rotor is made from a
material selected from the group consisting of Inconel, a high
temperature steel, a Ni-based superalloy, vitreous carbon,
ceramics, alumina, zirconia, magnesia and yttria.
17. The device of claim 1, wherein the rotor comprises a plurality
of blade-like protrusions.
18. The device of claim 1, wherein the rotor spans a length of
between about 10 cm to 3 meters.
19. The device of claim 1, further comprising thermocouples to
monitor the temperature of the rotor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/248,901, filed Nov. 14, 2000, and U.S.
Provisional Application No. 60/271,188, filed Feb. 23, 2001, the
entirety of both of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method and apparatus for the
identification and processing of bulk metallic glass forming
alloys, and the manufacture of components fabricated from these
alloys.
[0004] 2. Description of the Related Art
[0005] During the past decade, numerous university and industrial
research groups have invested significant efforts to discover metal
alloy formulations which form glass when cooled from the molten
state at relatively slow rates of cooling. Vitrification or glass
formation in metal alloys was first discovered in about 1960 by P.
Duwez and coworkers at the California Institute of Technology
("Caltech"). They used "rapid solidification" techniques to produce
glassy alloys by cooling the liquid alloy at rates of about 1
million degrees per second. A variety of such rapidly quenched
metallic glasses were produced over the following three
decades.
[0006] Beginning in about 1990, the research groups of Prof. A.
Inoue in Japan and Prof. W. L. Johnson at Caltech developed complex
alloys containing 3, 4, 5 or more components which formed glasses
at far lower cooling rates, typically 1-100 degrees per second when
solidified from the equilibrium liquid state. During the past
several years, these "bulk" glass forming alloys have attracted
substantial commercial interest as engineering materials. The bulk
metallic glasses have high strength, high hardness, high specific
strength, and a variety of other useful engineering properties.
Furthermore, the liquid alloys are highly processable and can be
cast into complex three dimensional very near net shapes. Interest
in utilizing these materials in engineering applications has led to
widespread interest in developing and discovering new bulk glass
forming alloys, in processing these alloys from the melt, and
utilizing these alloys to produce commercially useful materials
such as rods, plates, sheets, tubes, and other more complex
shapes.
[0007] A key parameter which distinguishes alloys with exceptional
glass forming ability is their relatively low melting point. Alloys
which form bulk metallic glass undergo equilibrium melting of the
initial alloy over a range of temperatures which are relatively low
compared with the compositionally averaged melting point of the
pure metals which comprise the alloy. Very often, the optimum glass
forming alloy lies near a minimum in the melting surface (liquidus
surface) of the alloy taken as a function of the alloy composition.
This liquidus surface is conventionally represented in alloy phase
diagrams as a "liquidus projection." For example, in a two
component alloy, the liquidus curve can be represented as a curve
in the composition/temperature plane.
[0008] A simple example is shown in the Au--Si phase diagram of
FIG. 1. This phase diagram of binary Au--Si alloy shows a eutectic
composition 10 with a eutectic temperature 12 of 363.degree. C. The
liquidus line 14 represents the line above which a single liquid
phase is present. The solidus line 16 represents the line below
which the system has completely solidified. Note that the melting
point of Au is about 1064.degree. C. and that of Si is about
1414.degree. C. Also illustrated is the compositional partitioning
of a slowly cooled liquid at an off-eutectic composition 18 during
solidification where x.sub.L is the composition of the remaining
liquid.
[0009] In this simple binary case, the eutectic composition is
precisely the compositional range where Duwez and coworkers
produced the first metallic glass by rapid quenching. More
generally, low lying liquidus temperatures (e.g. near alloy
eutectic compositions) locate the optimum glass forming regions in
higher order ternary, quaternary, quinary, etc. glass forming
alloys. See W. L. Johnson, 24 MATERIALS RESEARCH SOCIETY BULLETIN
42-56 (October 1999). Generally speaking, the ability to form
metallic glass is optimized at or near eutectic compositions, or
more generally near the lowest lying temperatures of the liquidus
surface in ternary, quaternary and higher order alloys. As such,
the search for easy glass forming alloys is very frequently found
to be equivalent to finding those alloy compositions corresponding
to the lowest lying melting temperatures (or lowest lying liquidus
surface). Most often, the best glass forming alloys lie within
about .+-.5 at. % of a minimum in the liquidus surface. The search
for easy glass forming alloys is thus dramatically simplified when
the lowest melting alloys can be identified.
[0010] To locate glass forming compositions, it is critical to be
able to identify or "discover" alloys with chemical compositions
located near the lowest lying regions of the melting curves in the
higher order alloys. Thus, it is of interest to develop an
efficient means of both discovering and isolating the lowest
melting alloys in a complex multicomponent alloys system containing
two, preferably three or more metals. In the case of alloys with
more than three components, the phase diagrams are generally not
available and little or no information exists to guide the
researcher to the optimum low melting compositions.
[0011] It has actually been proposed to develop materials
combinatory methods whereby one searches for low melting alloys by
literally making thousands of alloy compositions (using for example
thin film processing methods) and screening their melting points in
a rapid and parallel manner. See 159 CHEM. WEEK 57 (1997). This
approach requires carrying out literally thousands or tens of
thousands of screening experiments. Thus, what is needed is a
method of identifying the lowest melting alloy in a single or small
number of experiments on a bulk liquid.
[0012] The production of bulk metallic glass, in addition to
developing low melting point alloys, also requires that the low
melting liquid alloy be substantially free of contaminants, oxides,
and debris which induce crystallization. For metals, the most
frequently encountered contamination of the liquid is in the form
of crystalline oxide particles, carbide particles, and a variety of
other types of nascent foreign substances. These contaminants are
ubiquitous and ever present in the processing of nearly all
commercial metals used in casting of metallic components. In most
common cases (e.g., casting of aluminum, iron, and titanium
alloys), they are inevitable inclusions in the liquid. It is well
known that this nascent contamination frequently induces
crystallization of the liquid alloy when it is undercooled below
its melting point. Metallurgists refer to this as heterogeneous
crystal nucleation. Practically speaking, heterogeneous nucleation
is extremely harmful to the glass forming ability of metal alloys.
As such, it would be extremely desirable to develop methods to
reduce or eliminate nascent oxide, carbide, and other debris from
metallic melts when attempting to produce metallic glass. Thus,
what is needed is a direct and efficient means of removing most or
nearly all of this nascent contamination and debris from a molten
alloy.
[0013] Finally, it is of importance to produce bulk metallic glass
in useful shapes. What is needed is a natural means of efficiently
cooling and solidifying or casting a "decontaminated" alloy with an
optimized composition (having the lowest melting point) into
plates, rods, tubes, or other very near net shape castings.
SUMMARY OF THE INVENTION
[0014] The preferred embodiments of the present invention address
these and other needs by providing a rapid and efficient method to
identify and physically isolate alloy compositions of low melting
alloys which readily form bulk metallic glass. Certain preferred
embodiments also process these materials in a manner which removes
unwanted and harmful impurities and debris (such as crystalline
oxide, carbide or nitride particles) in the molten alloy to improve
the glass forming ability of the liquid alloy. Preferred
embodiments also describe the production and manufacture of large
net shape castings, plates, rods, and other useful shapes from the
purified and compositionally optimized liquid alloy.
[0015] In one aspect of the present invention, a method of
identifying the lowest melting eutectic composition of an alloy
having "n" phases is provided, where n .gtoreq.2. An arbitrary
starting alloy is provided. The alloy is heated until it is
substantially molten. Preferably, the molten alloy is subjected to
an inertial force above its melting point for a period of time. The
temperature of the alloy is lowered while subjecting the alloy to a
large inertial force or acceleration. An inertial acceleration is
often also referred to as gravitational acceleration or simply
gravity where the gravity and acceleration have the opposite sense.
The lowering of the temperature causes nucleation and growth of
crystals of a first crystalline solid phase within surrounding
liquid.
[0016] Crystals of the first solid phase are subjected a body or
inertial force (preferably caused by inertial acceleration or
gravity) such that the first solid phase moves either upward or
downward (with respect to the direction of the inertial
acceleration) in the remaining liquid. As used herein, upward and
downward (as well as top and bottom) do not necessarily refer to
the ordinary meanings of these terms, but rather indicate direction
with respect to the inertial acceleration, where bottom is opposite
the direction of acceleration (or along the direction of applied
gravity). The direction of motion (i.e., upward or downward)
depends on the sign of the density difference between the crystals
and the surrounding liquid. Heavier crystals move opposite to the
direction of inertial acceleration; lighter crystals move along the
direction of the inertial acceleration. This process is commonly
referred to as sedimentation.
[0017] Further lowering of the temperature of the alloy while
subjecting the alloy to said inertial acceleration and body forces
causes further nucleation and growth of additional solid phases,
the additional solid phase crystals being subjected to the inertial
forces such that the additional solid phases move upward or
downward, depending on the sign of the difference in density
between these crystals and the remaining liquid. Thus, as
crystalline phases are sequentially formed, the crystals sediment
to the top or bottom of the liquid forming strata at the top and
bottom of the vessel which contains the liquid.
[0018] The temperature is further lowered until the alloy is
substantially completely solidified. The last solid phase to
solidify is desirably located between the previously solidified
strata. However, it will be appreciated that where the solid phases
have substantially the same density, the last solid phase to
solidify does not necessarily have to be between other previously
solidified strata. The fmal liquid will have the lowest melting
point of all the solidified strata. In the case where this final
liquid has a eutectic composition with a minimum melting
temperature, the final liquid remaining will solidify at the
eutectic composition in a well defined layer or strata. An alloy of
eutectic composition will be physically isolated in a well-defined
layer.
[0019] In another aspect of the present invention, a method of
producing a bulk sample of a lowest melting eutectic composition of
an alloy is provided. An arbitrary starting alloy is provided. The
temperature of the alloy is lowered while subjecting the alloy to
an inertial force, the lowering of the temperature causing
nucleation and growth of a first solid phase within surrounding
liquid. The first solid phase is subjected to the inertial force
such that the first solid phase moves upward or downward in the
surrounding liquid. Further lowering of the temperature of the
alloy while subjecting the alloy to said force causes further
nucleation and growth of additional solid phases, the additional
solid phases being subjected to the inertial force such that the
additional solid phases move upward or downward. The temperature is
further lowered until the alloy is substantially completely
solidified. A bulk sample of alloy is then removed from the strata
or layer which was the last to solidify. This lowest melting sample
is then remelted and cast in an effort to produce a bulk glass
casting having the composition of this lowest melting alloy.
[0020] In another aspect of the present invention, a method of
processing a multicomponent alloy, comprising melting the alloy and
subsequently solidifying the alloy, both operations being carried
out in the presence of a centripetal acceleration field. In one
embodiment, the alloy is solidified in the presence of an inertial
acceleration or g field of between about 1 and 10.sup.6 g's
produced by centrifugal motion. Here, g is the acceleration of the
Earth's gravitational field=9.8 m/s.sup.2.
[0021] In another aspect of the present invention, a method of
forming a purified, multicomponent bulk metallic glass forming
alloy is provided. A sample alloy is melted at an elevated
temperature. The molten alloy is subjected to a centripetal
acceleration while holding it above the melting point for a period
of time. The alloy is solidified by lowering the temperature of the
alloy while continuing to subject the alloy to a centripetal
acceleration, the solidified alloy having a portion separated from
the remaining alloy having a lowest melting eutectic composition.
The portion of the alloy having the lowest melting eutectic
composition is isolated. The portion of the alloy having the lowest
melting eutectic composition is re-melted at an elevated
temperature while subjecting this portion to a centripetal
acceleration. The portion of the alloy having the lowest melting
eutectic composition is cooled while subjecting the portion to a
centripetal field, the cooled alloy having a portion with
relatively fewer impurities than the remaining alloy.
[0022] In another aspect of the present invention, a metallic alloy
having a composition optimized for bulk glass forming ability is
provided. The composition is obtained by melting the alloy followed
by a slow and gradual cooling and solidification of an initially
molten alloy in a centrifuge. The centrifuge subjects the alloy to
high inertial forces during solidification which physically
separates crystalline phases from the remaining molten alloy, such
that the optimized composition of the metallic alloy is the last
portion of the alloy to solidify and is physically isolated in a
well-defined layer of the final alloy.
[0023] In another aspect of the present invention, a high
temperature centrifugal processing device for processing molten
metal alloys under very high inertial accelerations is provided. A
rotor is fabricated of a high temperature material having high
strength and fracture resistance at temperatures of between about
400 and 1200.degree. C. and which is capable of withstanding
inertial accelerations up to at least 50,000 g's. A plurality of
internal cavities within the rotor symmetrically laid out within
the body of the rotor. A shaft onto which the rotor is mounted
allows the rotor to be spun at high rotation frequencies of between
about 1000 and 100,000 rpm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a phase diagram of binary Au--Si alloy.
[0025] FIG. 2 is a schematic illustration of a solid spherical
particle located in a liquid and falling in a centripetal
g-field.
[0026] FIG. 3 is a schematic illustration of a centrifuge assembly
for determining the lowest melting eutectic composition of a
multicomponent alloy.
[0027] FIG. 4 is a phase diagram of the Fe--O system.
[0028] FIGS. 5A-5C are schematic illustrations of various rotor
designs that can be used in preferred centrifuge assemblies.
[0029] FIGS. 6A is a perspective view of a hermetically sealed
capsule used for containing a liquid alloy sample.
[0030] FIG. 6B is a cross-sectional view of a sample capsule having
a crucible for containing a liquid alloy sample.
[0031] FIG. 7 is a cross-sectional view of a covered cavity design
for loading sample capsules into a centrifuge assembly.
[0032] FIG. 8 is a cross-sectional view of a split rotor design for
loading sample capsules into a centrifuge assembly.
[0033] FIG. 9 is a perspective view of a centrifuge assembly
according to one preferred embodiment.
[0034] FIG. 10 is a perspective view of a rotor assembly used in
the centrifuge assembly of FIG. 9.
[0035] FIG. 11 is a perspective view of the rotor assembly of FIG.
10, shown open to illustrate the internal cavities.
[0036] FIG. 12 is an end view of the internal surface of one of the
plates forming the rotor assembly of FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] The preferred embodiments illustrated herein describe a
method for identifying, processing and manufacturing a metallic
glass forming alloy, and preferred apparatuses for accomplishing
these methods. More particularly, certain preferred embodiments
describe the use of a centrifuge to create a centripetal force on a
molten alloy as it is cooled in order to separate out the desired
alloy composition and remove impurities. It will be appreciated,
however, that other methods and devices may also be used to produce
the desired separation. It will also be appreciated that the
methods described herein may have applicability to beyond metallic
glass forming alloys.
[0038] Stratification, Sedimentation and Fluid Mechanics
[0039] To accomplish the objectives outlined above, in one
embodiment large inertial forces or "g"-forces are utilized to
sequentially separate crystalline phases (particles) as they
sequentially form and grow in a molten alloy during gradual cooling
below the liquidus temperature. The crystalline particles are
physically removed and isolated from the remaining liquid as they
are formed. Under the influence of a large g-force, this is
accomplished by rapid and efficient sedimentation and
stratification. Further, contamination and nascent solid "debris"
in the form of oxides, carbides, or other foreign particles from
the molten alloy can be removed using the same
sedimentation/stratification technique. The resulting liquid is a
final low melting stratified and decontaminated liquid, which can
be efficiently cooled and solidified into a solid glass component,
such as by utilizing convective heat transport by a cooling gas.
This results in a vitrified bulk metallic glass component of near
net shape.
[0040] The large inertial or g-forces are preferably generated in
one embodiment by the rotational motion of a centrifuge. Using one
of a number of centrifuge designs, inertial acceleration a can be
produced, for example ranging from about 1 to 10.sup.6 g's. Here g
is the acceleration of gravity (9.8 m/s.sup.2). Using more advanced
centrifuges, inertial acceleration of about 10.sup.5 to 10.sup.6
g's or more can be generated. This results in a body force density
.rho. a per unit volume exerted on any object rotating with the
centrifuge where .rho. is the density of the object. More
particularly, a solid particle located in a liquid will be
subjected to a net body force density given by:
f=(.rho..sub.s-.rho..sub.L)a=.DELTA..rho.a
[0041] where .rho..sub.s=density of the solid particle in
kg/m.sup.3 and .rho..sub.L=density of the carrier liquid in
kg/m.sup.3. For a solid particle of volume V, this results in a net
body force of F=f.OMEGA.=.DELTA..rho.a.OMEGA.. This well known body
force causes the solid particle to move or sediment in the g-field
and is responsible for the well known phenomena of
sedimentation.
[0042] To illustrate, consider a solid particle in a liquid column
as shown in FIG. 2. Assuming laminar flow, the drift velocity v of
the solid particle in the liquid can be calculated by knowing the
viscosity of the liquid. For example, in the case of a simple
spherical particle, this is given by equating the frictional drag
on the particle to the body force and is given by the following
equation:
.DELTA..rho.a.OMEGA.=3.pi..mu.vd (1)
[0043] or
v=[.DELTA..rho.a.OMEGA.]/[3.pi..mu.d]=[.DELTA..rho.ad.sup.2]/18.mu.
(2)
[0044] where d is the sphere diameter,
.OMEGA.=(4.pi./3)(d/2).sup.3=.pi.d.- sup.3/6, and .mu.=liquid
viscosity measured in Pa-s. See R. CLIFT ET AL., BUBBLES, DROPS,
AND PARTICLES 380 (Academic Press 1978). As an example, using
a=10.sup.4 g=10.sup.5 m/s.sup.2, d=1 .mu.m, .DELTA..rho.=500
kg/m.sup.3=0.5 g/cc, and .mu.=0.01 pa-s, gives v=drift
velocity=0.28 mm/s. The time required to sediment the particle a
distance L will be .tau.=L/v. For L=10 cm, this requires about 300
seconds for a 1 micron sphere to "sediment" a distance of 10 cm.
For non-spherical particles such as an ellipsoid of non-unit aspect
ratio, the time will be shorter.
[0045] Sedimentation Purification Rates/Time Scales
[0046] For a mixture of "heavy" solid particles in a less dense
melt, geologists have shown that melt segregation along a
centripetal force field over a column length L can be described
by:
d[lnf]/dt=(l/f)df/dt=-v/L (3)
[0047] where f is the volume fraction of solid phase, and v is the
average drift velocity of a solid particle in a liquid in a
g-field. See D. McKenzie, The Generation and Compaction of
Partially Molten Rock, 25 J. PETROL. 713-765.
[0048] In a simple approximation, the fraction of solid decreases
exponentially at the "top" of the column according to:
f=f.sub.0exp (-tv/L)=f.sub.0exp (-t.DELTA..rho.ad.sup.2/18 .mu.L)
(4)
[0049] where the above expression (2) has been used for the drift
velocity v, and f.sub.0 refers to the initial volume fraction of
solid particles.
[0050] The characteristic time scale for exponential decay of the
solid volume fraction at the top of the column is:
.tau.=[.DELTA..rho.ad.sup.2/18 .mu.L].sup.-1 (5)
[0051] If L=10 cm (as above) in a centripetal field of 10.sup.4 g,
then .tau.=300 seconds for a 1 .mu.m particle with .DELTA..rho.=0.5
g/cc, and .mu.=0.01 Pa-s=10 centipoise. In the limit of initial
volume fraction of particles, this means the concentration of
particles decays by a factor of (1/e) in 300 seconds, or a factor
of 10 in about 700 seconds. For a particle of diameter 10 nm, the
time scale for purification becomes 3.times.10.sup.6 seconds, or
about 3 days. If L=1 cm, the same results are obtained for the 1
.mu.m particle in about 70 seconds (about 8 hours for the 10 nm
particles). This is the characteristic time scale for melt
purification with respect to 1 .mu.m (10 nm) particles. This time
scale is proportional to (1/d.sup.2), to .mu.=viscosity, to (1/a)
where a=centripetal acceleration field, and to (1/L) where L
=column length. This gives all of the essential physics required to
estimate temporal rates of melt purification in a two-phase mixture
of liquid containing solid particles (assumed here to be
spherical).
[0052] Ultimate Purification Levels
[0053] Turing now to the ultimate levels of "purification" that can
be achieved in a centrifuge, quite generally, one can use the Lamm
differential equation to obtain transient and steady state
solutions to the problem of particle sedimentation in a centripetal
field. See H. FUJITA, MATHEMATICAL THEORY OF SEDIMENTATION ANALYSIS
(Academic Press 1962). To obtain the ultimate steady state
concentration profiles in a centripetal field, one can use the
Archibald "steady state" solution of the Lamm equation. This is
given by:
C(r)/C.sub.0=exp[s.omega..sup.2r.sup.2/2D].times.[s.omega..sup.2(R.sub.2.s-
up.2-R.sub.1.sup.2)/2D]/[exp(s.omega..sup.2R.sub.2.sup.2/2D)-exp
(s.omega..sup.2R.sub.1.sup.2/2D)] (6)
[0054] where C(r) is the particle concentration (particles/m.sup.3)
at radial location r, C.sub.0 the initial average particle
concentration in the sample, s is the sedimentation coefficient of
the particles, D is the diffusion constant of the particles, r the
radial coordinate of a centrifuge, and R.sub.1 and R.sub.2 the
inner and outer ends, respectively, of a cylindrical sample vile
located along the radial arm of the centrifuge rotating at
frequency of .omega. radians/s.
[0055] This steady state solution can be used to compute a steady
state partitioning (or "purification") coefficient for the
particles. This can be defined as:
K=C(R.sub.1)/C(R.sub.2)=exp[s.omega..sup.2(R.sub.1.sup.2-R.sub.2.sup.2)/2D-
] (7)
[0056] The Svedberg relation can be used to determine the ratio
s/D. See Fujita above. This is given by:
s/D=M(1-.rho..sub.I/.rho..sub.s)/(k.sub.BT) (8)
[0057] where .rho..sub.L and .rho..sub.s are the respective
densities of the carrier liquid and solid particle.
[0058] By way of illustration, the partitioning coefficient is
calculated for a typical case of a spherical solid particle with
density 10% greater than the carrier liquid assumed to have a
typical value of 5000 kg/m.sup.3. For particles with diameters of 1
.mu.m and 10 nm (typical cases of practical interest),
M=2.6.times.10.sup.-15 and 2.6.times.10.sup.-21 kg, respectively.
For k.sub.B=1.38.times.10.sup.-23 J/particle-K and T=1000 K (a
typical temperature of interest), we have s/D=0.1 (M/k.sub.BT)=19
and 1.9.times.10.sup.-2 respectively. Applying equation (7) with
R.sub.1=0.4 m and R.sub.2=0.5 m (a sample column height of 10 cm),
and .omega.=500 Hz, one obtains K=exp (2.14.times.10.sup.7) and exp
(21.4)=2.times.10.sup.9. In the case of 1 .mu.m spheres,
purification is essentially complete while for 10 nm spherical
solid particles, purification is 2 parts per billion. Accordingly,
the method is capable of removing exceedingly small crystalline
particles from the melt to a level of parts per billion measured
with respect to any initial concentration of particles.
[0059] Use of a Centripetal Field during Liquid Alloy
Solidification and Crystal Nucleation--Isolation of Lowest Melting
Liquid in a Multicomponent Phase Diagrams.
[0060] To illustrate the application of the above principles in
discovery, isolation, and processing of bulk metallic glass forming
alloys, several objectives of certain preferred embodiments are
first stated. These are:
[0061] (1) Physical separation of the lowest melting eutectic alloy
from an arbitrary starting alloy with the goal of "discovering" the
lowest melting alloy and physically isolating it.
[0062] (2) Elimination of solid crystalline debris or contamination
(e.g. oxide particles) from the molten eutectic alloy to optimize
glass forming ability.
[0063] (3) Production of a bulk metallic glass component from the
isolated and purified" lowest melting or eutectic liquid.
[0064] To accomplish these goals, a liquid alloy in one embodiment
is sequentially melted and then gradually solidified by slow
cooling in the presence of a centripetal g-field. Preferably, this
can be done with a centrifuge equipped with a high temperature
furnace. FIG. 3 shows one schematic illustration of the device.
Further embodiments of centrifugal devices are described below.
[0065] As shown in FIG. 3, the centrifuge assembly 20 is preferably
confmed in a vacuum chamber 22. A rotor arm 24 rotates about a
central axis 26, and twin furnaces 28 and 30 are provided within
the arm 24 to control the temperature in the chamber. The alloy
samples are loaded in sample holders, described below, which for
the purposes of demonstration can be considered to be cylindrical.
The sample holders are loaded into cavities 32 and 34 of the arm 24
adjacent each furnace. The assembly is preferably instrumented with
rotating high current contacts (not shown) to supply power to the
resistively heated furnaces as well as low current rotating
contacts for use in external measurements of the furnace
temperature using thermocouples.
[0066] As illustrated, .omega. is the angular frequency of the
centrifuge, R is the average radius of the furnace from the
centrifuge point of rotation. The acceleration at the furnace will
be a=.omega..sup.2R. For one preferred device, a rotational
frequency of about 5000 rpm, or about 80 rotations/second, equals
(2.pi.).sup.-1.omega., giving .omega.=500 radians/seconds For this
device, R=0.4 m, giving a=.omega..sup.2R=2.5.tim-
es.10.sup.5.times.0.4 m/s.sup.2=10.sup.5 m/s.sup.2=10.sup.4 g.
[0067] Assuming that the temperature within the cylindrical sample
can be varied and maintained uniform (isothermal conditions) within
the sample, an alloy having an arbitrary starting composition can
be processed as follows. The alloy is first heated above its
liquidus temperature until it is completely molten. The temperature
is then gradually lowered stepwise until the alloy liquidus curve
of the highest melting crystalline phase in the equilibrium diagram
is crossed. The alloy is undercooled below this curve until the
first crystalline phase (call it the .alpha.-phase) nucleates and
grows. For the Au--Si system (FIG. 1), at the initial composition
shown, this first phase is nearly pure Si having the diamond cubic
structure. Being less dense than the remaining liquid, these
growing silicon crystals will be subjected to a g-force. For the
case considered above, this g-force of about 10.sup.4 g's, will
cause rapid sedimentation to the interior of the centrifuge. If the
growing crystals reach only a size scale of about 10 nm, it will
take a time scale of hours (note the large density differential
.DELTA..rho. for Au and Si in this case) for sedimentation over a
length scale of 10 cm.
[0068] The silicon crystals are rapidly and effectively moved to
one end of the sample holder leaving the remaining liquid having
the composition of the liquidus curve at the temperature of
interest. Upon further cooling additional growth of existing
silicon crystals along with further nucleation occurs. These
crystals are in turn removed to the innermost portion of the
centrifuge (as they are less dense than the remaining liquid).
[0069] Finally one reaches the solidus curve at 363.degree. C. (for
the Au--Si system). Further undercooling ultimately produces
nucleation and growth of Au in concert with Si (eutectic
solidification). To the extent that the two phases solidify
simultaneously by growth of the eutectic microstructure, further
sedimentation and segregation is suppressed. The last portion of
the sample to solidify will undergo eutectic crystallization at a
composition very close the eutectic composition in the phase
diagram. Following complete solidification, the sample is cooled to
ambient temperature, the centrifuge brought to rest, and the
"stratified alloy" removed from the centrifuge.
[0070] The sample can then be sectioned into slices perpendicular
to the axis of the original sample cylinder. The cross-sectioned
slices are preferably characterized by x-ray diffraction to
identify phases and by an ion microprobe to determine overall
compositions of the slices. Other methods such as X-ray
spectroscopy, EDS and Auger spectroscopy can also be used. The pure
eutectic liquid will be located in the last "strata" which
undergoes solidification. This "strata" will be characterized by
the presence of all phases (two phases, Au and Si in the example
here) in one stratified section. This is the eutectic alloy.
[0071] The principle of the technique remains the same for the case
of the ternary or quaternary eutectic alloys. If n is the order of
the alloy (e.g. binary, ternary, etc.), then a eutectic point will
be characterized by the existence of n crystalline phases in
equilibrium with the liquid. Generally speaking, the strata which
contains all n-phases will be the eutectic composition
centrifugally segregated and solidified alloy. By this method, one
can physically isolate and identify the eutectic composition in a
higher order alloy by centrifugal processing of an arbitrary
starting alloy provided only that the starting alloy lies somewhere
in the n-phase crystalline coexistence region in the low
temperature phase diagram. In other words, the arbitrary starting
alloy preferably includes n crystalline phases when the alloy is
below the solidus temperature.
[0072] Purification of a Melt Containing Foreign Solid Particles
Such as Oxides
[0073] As case examples of melt purification with respect to
oxides, considered below are representative examples of practical
interest. There has been much effort devoted to synthesizing bulk
metallic glasses base on Fe. One of the most successful approaches
to date has been to use a boron-oxide flux (B.sub.2O.sub.3) to
suppress the effects of oxide particles in inducing crystallization
of glass forming Fe-based.
[0074] The problem can be understood by reference to the Fe--O
binary phase diagram shown in FIG. 4. The phase diagram shows a
very high-temperature liquid miscibility gap. At temperatures below
1500.degree. C., even iron alloys containing small levels of oxygen
undergo liquid phase separation. Extrapolation of the miscibility
gap to much lower temperatures (less than 1000.degree. C.) suggests
phase separation of alloys containing oxygen in the tens of ppm
range. Phase separation produces an oxygen rich melt containing
roughly 50 at. % oxygen. This melt readily crystallizes to the
Wustite phase (a roughly equi-atomic oxide of iron). The Wustite
phase in turn catalyzes nucleation of other crystalline phases
(e.g. crystalline phases of Fe or other intermetallic phases in the
case of higher order alloys). The oxygen rich melt and the Wustite
particles have substantially lower density (about 5.7 g/cc for the
Wustite phase) than liquid iron (about 7.2 g/cc). The difference is
actually over 20% (by comparison with the 10% used in the
illustrative calculations above).
[0075] Both the oxygen rich melt as well as Wustite crystallites
can be removed from the remaining alloy (Fe-rich) by use of
centrifugal separation. The method is based on the sedimentation
behavior described above. The alloy is slowly cooled step-wise from
high temperature until the liquid miscibility gap is traversed.
Phase separation produces an oxygen rich liquid phase (or Wustite
crystal as the case may be) which can be removed by a g-force to a
remote location from the remaining Fe-rich melt. The primary
requirements are that the oxygen rich phases coarsen to a size in
the 0.001 to 1 .mu.m (or larger) range and that the melt be
processed for time scales on the order of hours or days, depending
upon the size of the particles and the exact time scales from
equation (5) above.
[0076] Under these circumstances, the remaining melt will be
"purified" of oxygen rich liquid phase, of Wustite particulate
debris, and of heterogeneous nucleation sites for crystals. Removal
of crystalline Wustite particles down to nm sizes can be
accomplished in time scales of hours/days. The method can be
applied to more complex Fe-based alloys containing a substantial
fraction of Fe (typically greater than 70 at. % Fe). The principal
of the method remains the same to remove oxygen rich phases (both
liquid and crystalline) from the remaining liquid alloy to
decontaminate the melt of heterogeneous nucleation sites. This
method is equally applicable to remove impurities, including but
not limited to oxide and carbide particles, from alloys other than
Fe alloys. These impurities are not nominally intended to be
present in the alloy but arise from the lack of purity of materials
from which the alloy is made.
[0077] Casting of a Bulk Metallic Glass Component
[0078] Once a metallic alloy has been identified and/or purified
using the methods described above, casting of metallic glass
components from purified low melting liquids can be carried out in
at least one of two general ways. These can be referred to as
"ex-situ" and "in-situ" casting, as described below.
[0079] Ex-situ Casting. In one embodiment the alloy can be
solidified entirely in the centrifuge following the initial
processing steps of (1) isolating the low melting eutectic
composition, and (2) purifying this low melting alloy of
crystalline debris (oxides, carbides, etc.) which acts to catalyze
heterogeneous crystal nucleation. The isolated/purified melt is
cooled by shutting down the power to the furnaces on the centrifuge
while maintaining rotation. The centrifuge itself is shut down once
the solidified alloy has been cooled to ambient temperature. The
cooling of the alloy to ambient temperature preferably occurs at a
rate sufficient to suppress crystallization of the alloy. Thus, the
purified eutectic or low melting alloy that is removed from the
centrifuge is preferably an amorphous metal, and can be used as a
feed stock for a traditional casting process.
[0080] In-situ Casting. The purified and isolated low melting point
alloy can be directly cast into a net shape component. In this
case, a casting gate is preferably provided at the appropriate
location in the sample column. This location is situated at a
position on the column where the purified eutectic melt is located.
In practice, the low melting or eutectic alloy is first identified
by a series of initial centripetal experiments aimed at isolating
and identifying the lowest melting alloy as described above. An
alloy of this nominal composition is then used as feedstock for the
centripetal casting. The optimized feedstock material is melted in
the centripetal furnace. It is then purified of any oxide or other
crystalline debris (using the above methods). A gate is provided
which carries the melt to a metal mold or die from a location in
the melt column which is far removed from regions where the
crystallize debris has sedimented. The melt that is removed through
the gate preferably is transferred to the mold or die while still
at or near its eutectic temperature. Once in the mold, the alloy
can be cooled at a rate sufficient to suppress crystallization,
thereby forming an amorphous metallic alloy.
[0081] The above descriptions outline the features of the general
approach to be used. Additional details regarding casting of
amorphous metallic alloys are described in U.S. Pat. No. 5,950,704,
U.S. Pat. No. 5,711,363, and U.S. application Ser. No. 09/879,545,
filed Jun. 11, 2001, the entirety of each of which is hereby
incorporated by reference.
[0082] Components cast using the methods above preferably can be
made in bulk (e.g., having a smallest dimension exceeding about 1
mm). The components preferably have a glassy or amorphous structure
which was produced using the centripetal processing method either
by the "in-situ" or "ex-situ" casting methods described above. A
"bulk" cast component can also be made having a partially amorphous
or glassy structure. More preferably, one embodiment of a bulk cast
component has at least 20 volume % amorphous phase in its
microstructure. Preferred embodiments also describe the production
and manufacture of large net shape castings, plates, rods, and
other useful shapes from the purified and compositionally optimized
liquid alloy.
[0083] Preferred Centrifuge Devices
[0084] As discussed above, a method for identifying and processing
metallic glass forming alloys and fabricating components thereof
using large inertial forces to process liquid metal alloys at high
temperatures (in the molten state) has been provided. This method
is preferably implemented using a centrifugal processing platform.
The implementation of the method preferably uses acceleration or
g-forces. Here, an acceleration of 1 g is the acceleration of
earth's gravity (9.8 m/s.sup.2). Accelerations ranging from about
10.sup.3 g (9.8.times.10.sup.3 m/s.sup.2) up to values as high as
about 1 mega-g or about 10.sup.6 g in the processing of liquid
metal alloys are preferred.
[0085] To implement this method, a device preferably includes: (1)
a centrifugal device/platform capable of generating and sustaining
inertial accelerations up to the mega-g range (10.sup.5-10.sup.6
g's); (2) a centrifugal device capable of holding or containing
both a solid or liquid metal alloy sample on the centrifugal
platform during rotation; (3) the capability to heat the metal
alloy sample to temperatures above its melting temperature while
subjecting the molten alloy to large inertial accelerations (mega
-g) for sustained periods of time (preferably from tens of seconds
up to tens of hours); and (4) the capability to control the
environment in which the molten alloy is processed. Specifically,
to implement the method optimally, one needs to process the molten
metal in either a vacuum on a controlled gas atmosphere. As such,
the liquid alloy should be situated in a controlled vacuum or gas
atmosphere environment during processing at high g
accelerations.
[0086] The actual temperature requirement (3) will depend upon the
melting temperature of the alloy in question. In certain preferred
embodiments, commercially useful structural metals such as
aluminum, titanium, iron, nickel, copper, etc. can be used. The
typical range of melting temperatures will fall between about
400.degree. C. up to about 1200.degree. C.
[0087] For requirement (4), the entire apparatus is preferably
located in a hermetically sealed container, or the alloy samples
are encapsulated in a hermetically sealed container which can be
mounted on the centrifuge and which can remain sealed when
subjected to high temperatures and large inertial accelerations (up
to the mega-g range).
[0088] To the inventor's knowledge, there are no commercially
available centrifuges or centrifugal devices which support all of
these requirements. Requirement (3) is particularly challenging as
the combination of high temperature and large inertial forces will
challenge the mechanical integrity of the rotating centrifugal
platform (hereupon called the rotor).
[0089] To overcome these difficulties, a high temperature
centrifugal device is provided which offers the capability of
processing high melting point liquids under mega-g range (about
10.sup.-5-10.sup.6 g) accelerations for extended times in a
controlled environment.
[0090] High Temperature Rotor
[0091] The proposed device can be realized in several ways. An
exemplifying realization comprises a rotor constructed of a
suitable high temperature material. Here, a high temperature
material is a material which maintains a suitable level of
strength, resistance to deformation, and resistance to fracture and
failure at an elevated temperature when subjected to the high
stress loads associated with mega-g range accelerations. Examples
of suitable rotor materials include, but are not limited to, high
temperature steels such as Inconel alloys; Ni-based super-alloys
(such as used for rotating jet engine components); toughened high
temperature ceramics, such as alumina, zirconia, magnesia or
yttria; and vitreous or pyrolytic carbon.
[0092] All of these materials are known to have significant
strength and toughness at elevated temperatures. For example,
Inconel alloys can have yield strengths of about 500 MPa at
temperatures as high as 900.degree. C. Vitreous carbon, or glassy
carbon rods, crucibles, and components are known to have failure
strength as high as 600 MPa at temperatures ranging up to
1500.degree. C. A monolithic rotor is preferably fabricated from
one of these high temperature materials.
[0093] Preferred rotors will be fabricated of a high temperature
material having high strength and fracture resistance at
temperatures of about 400-1200.degree. C. which is capable of
withstanding inertial accelerations up to at least 100,000 g's.
These rotors can preferably be spun at high rotation frequencies of
about 1000 to 100,000 rpm. In one embodiment, the rotor is capable
of withstanding accelerations up to about 50,000 g's at the rotor
perimeter, more preferably up to about 100,000 g's, and even more
preferably up to about 250,000 g's.
[0094] Simple illustrations of preferred rotor components are shown
in FIGS. 5A-5C. FIG. 5A illustrates a disk rotor 36 mounted on a
shaft 38. FIG. 5B illustrates a rod rotor 42 mounted on a shaft 44.
FIG. 5C illustrates a hub rotor 48 having a plurality of blades 52
mounted on a central hub 50 rotating about a shaft 54. In each of
these embodiments, the rotor is a monolithic fixture of symmetrical
shape which is balanced in order to spin at high frequencies on a
shaft without vibration. This is a common requirement for all
preferred centrifuges.
[0095] The rotors of FIGS. 5A-5C each contain internal cavities for
holding a sample alloy. The cavities are laid out within the rotor
in a symmetric arrangement. For example, four cavities 40 are shown
in FIG. 5A, two cavities 46 are shown in FIG. 5B, and four cavities
56 are shown in FIG. 5C. It will be appreciated that 2, 3, 4, 5, 6,
8, or any other numbers of cavities in a symmetrical arrangement
could be used. The number of cavities to be used will depend on the
overall mechanical load requirements to which the overall rotor
will be subjected.
[0096] The presence of the cavities will reduce the maximum load
that to rotor can withstand and thus reduce the maximum rotation
frequency the rotor can support. As such, the cavities should
generally be relatively "small". For example, if the rotor is as
shown in FIGS. 5A-5C and has a radius R and thickness t (normal to
the page), and the cavities are cylindrical with diameter d and
length L, then D/t is preferably small, more preferably less than
about 0.5. L/R is also preferably small, more preferably less than
about 0.6, in order to maintain sufficient mechanical integrity of
the rotor under the stress load imparted by rotation.
[0097] When the rotor is in the shape of a rod such as in FIG. 5B,
the rotor may have a circular, circular, square, rectangular, or
other shaped cross section. The rod 42 is preferably spun about a
shaft 44 oriented normal to the principal (long) axis of the rod
and located at center of the rod. The rod 42 preferably contains
two internal cavities 46 located between the shaft and the ends of
the rod. The rod/cavity assembly can be spun about the shaft at
elevated temperatures (preferably about 400-1200.degree. C.)
whereby the ends of the rod sustain inertial accelerations of at
least 50,000 g and preferably up to about 250,000 g or more.
[0098] The rotor of FIG. 5C preferably has blade-like protrusions
52 extending from a central hub. Each protrusion contains a sample
processing cavity 56. This rotor is preferably shaped for
optimization of the maximum attainable inertial accelerations.
[0099] The rotors of FIGS. 5A-5C preferably span a length of
between about 10 cm to about 3 meters. In other words, this length
corresponds to the diameter of the disk in FIG. 5A or the length of
the rod in FIG. 5B.
[0100] Sealed Sample Capsules and Crucibles
[0101] The cavities of FIGS. 5A-5C are used to accommodate samples
to be processed. The samples themselves are preferably housed
within hermetically sealed capsules 58, such as shown in FIGS. 6A
and 6B. The liquid alloy sample 60 to be processed is preferably
contained either directly in the capsule 58, or in a crucible 62
housed within the capsule such as shown in FIG. 6B. The liquid
alloy sample 60 is sealed within the capsule 58 either under vacuum
or under an inert gas atmosphere. The capsule 58 may contain the
alloy sample directly (no crucible) provided that the reactivity of
the subject sample in the molten state with the capsule material is
not severe so as to either contaminate the sample or degrade the
mechanical integrity of the hermetically sealed capsule (e.g.
failure of the capsule hermetic seal).
[0102] When reactivity is of concern, a suitable crucible should be
used to contain the alloy. The crucible can be made from a variety
of suitable materials, including but not limited to fused silica,
vitreous carbon, a refractory metal or ceramic material such as
alumina or zirconia. In one preferred embodiment, the crucible 62
can be made of manganese.
[0103] The hermetically sealed sample capsule is preferably made of
a material which can support large inertial stresses (typically
about 10-500 MPa) at elevated temperatures (about 500-1500.degree.
C.) and should be suitable for convenient hermetic sealing of the
crucible and alloy sample under either vacuum or inert gas.
Examples of a suitable capsule include, but are not limited to an
Inconel or high temperature steel capsule which can be sealed by
welding caps onto a cylindrical crucible in a controlled
environment (for example e-beam welding under vacuum or inert gas),
a silica capsule which could be vacuum sealed by glass blowing, a
Ni-based super alloy, and a refractory metal such as
molybdenum.
[0104] A convenient means of loading the sample capsules into the
rotor cavities is preferably provided. For this purpose, various
rotor designs can be employed. An example of a covered cavity
design is shown in FIG. 7. The centrifuge assembly 64 shown in FIG.
7 preferably includes a rotor 66 which can be designed in
accordance with the embodiments of FIGS. 5A-5C, or other
embodiments, and a drive shaft 68. The sample 60 (not shown) is
loaded into a sample cavity 70 lying just beneath the upper surface
of the rotor 66, and a cavity cover 72 is releasably fastened to
the rotor 66 to seal the cavity.
[0105] Another example of a suitable rotor cavity design is shown
in FIG. 8. Here a split rotor design is provided to give access for
loading sample capsules. The assembly 74 includes a rotor split
along the horizontal plane into two plates 76 and 78, and a rotor
drive shaft 80. Sample cavities 82 are disposed in the surfaces of
the components which face each other. Fasteners 84 are preferably
used to seal the components together.
[0106] It will be appreciated that the embodiments of FIGS. 7 and 8
are merely two possible configurations for providing access for
loading samples into a rotor cavity and are not intended as
complete list of possible configurations. In evaluating the merits
of various configurations, the mechanical integrity of the rotor
should be taken into account. Both of the above configuration
provide reasonable designs. Further embodiments are described
below.
[0107] Method of Heating Rotor and Control of Rotor Temperature
[0108] To implement the centrifugal processing method, a means is
preferably provided to heat the entire rotor assembly containing
the sample capsules. Various methods can be employed. In one
embodiment, similar to the testing of rotating aircraft components
at elevated temperatures characteristics of component service,
rotating components are "spin tested" in a furnace. In this method
the entire rotor and shaft assembly are inserted into the furnace
and heated to the ambient temperature of the furnace. Ni-based
super-alloy and Inconel components are routinely tested to
temperatures of up to about 1000.degree. C. using this method.
[0109] In preferred embodiments, the rotor containing cavities and
sample capsules can be inserted into a furnace. In contrast, one
can also raise a cylindrical furnace up over the rotor The furnace
can be preheated or heated following insertion of the rotor. In
either case, the entire rotor assembly will ultimately be heated to
the steady state temperature inside the furnace. Using this method,
the rotor assembly is heated to temperatures sufficient to melt the
sample alloys inside the capsules. As the sample capsules are
essentially enclosed in a blackbody cavity, the sample temperature
will rapidly equilibrate to the rotor temperature. The rotor and
samples can be spun up to maximum rotation frequency of the rotor
and the samples processed under high inertial accelerations.
[0110] Using an Inconel or super-alloy rotor, calculations show
that accelerations of about 1 to 2 times 10.sup.5 g can be achieved
at temperatures up to about 1000.degree. C. The rotor can then be
removed from the furnace or the power to the furnace can be reduced
to provide gradual cooling of the rotor and encapsulated samples.
Thus, solidification can be carried out during the continued
application of the high g acceleration.
[0111] As an alternative to the above method, the rotor can be
heated by providing a heating source for the rotor. For example,
the rotor can be heated while spinning using RF induction heating.
Here, RF coils would be used in a configuration surrounding the
rotor. When driven by an RF power supply, the RF coils couple to
the metal rotor and RF power is coupled directly to the rotor. By
adjusting the intensity of the RF power, one can achieve varying
"steady state" rotor temperatures. This method can be readily
implemented to heat an electrically conducting rotor to
temperatures ranging to 1200.degree. C. or higher where heat loss
from the rotor is primarily by radiation. For a metal rotor of high
thermal conductivity, the rotor will achieve a relatively uniform
temperature in steady state. Likewise, the samples encapsulated
within the rotor will achieve near isothermal conditions as
required for processing liquid alloys at a well defined
temperature.
[0112] Other methods in addition to inductive heating and resistive
heating can be used to heat the rotor assembly. Alternatively, one
could utilize a laser to heat the rotor assembly. Here, relatively
high powered laser would be required. Other heating methods include
direct resistive heating of the rotor. In this case, a high current
rotating electrical feed-through would be required to bring current
from an external power supply into the rotating rotor assembly. All
of the above heating methods can be used to implement the preferred
embodiments of this invention.
[0113] For processing liquid metal alloys, it will also be
desirable to know the temperature of the samples (e.g. the rotor)
as a function of time and acceleration history in order to control
the processing of the liquid metal under high g acceleration. To
measure the temperature of the rotor during processing, several
methods can be employed. These include the use of an infrared or
optical pyrometer to externally monitor the rotor temperature
during processing, and the use of one or more calibrated
thermocouples mounted on the rotor with junctions located within
the sample cavities, preferably in direct contact with the samples.
To utilize thermocouples, a rotating electrical connection is
preferably used to feed the thermocouple signals to an external
voltmeter or monitoring system. Either of the above methods would
be effective for monitoring the sample temperature history during
high g processing.
[0114] Experimental Results
[0115] FIG. 9 illustrates one preferred centrifuge assembly 100
that can be used to process glass forming metallic alloys. The
assembly 100 includes a cylindrical body 102 enclosing a chamber
104, and a lid 106 provided over the body 102 for sealing the
chamber. The cylindrical body 102 preferably acts as a furnace to
control the temperature inside the chamber 104, and is capable of
producing temperatures of up to about 1600.degree. F. (about
875.degree. C.) while the rotor (described below) is spun at
rotation frequencies up to about 35,000 rpm, more preferably up to
1200.degree. C. The body 102 and the lid 104 are preferably made
from a suitable steel or ceramic material which can withstand the
high temperatures without degradation. The sample chamber
preferably has an internal diameter of about 30-60 cm.
[0116] Extending through the lid 106 of the furnace is a drive
shaft 108. The drive shaft is constructed of a high temperature
creep resistant alloy such as Waspalloy or Inconel 100. A rotor
110, described in further detail with respect to FIGS. 10-12, is
provided at the bottom of the shaft. Thus, when the lid 106 is
closed over the cylindrical body 102, the rotor 110 is provided
inside the chamber 104.
[0117] As shown in FIGS. 10-12, the rotor 110 is comprised of two
adjacent plates 112 and 114. As shown in FIGS. 11 and 12, the
internal surfaces of each of these plates contains a plurality of
recesses 120 for housing sample capsules such as described above.
When the two plates are closed together, the recesses together
define internal cavities for holding the sample capsules. As shown
in FIGS. 11 and 12, eight cavities are provided in the rotor,
although any number of cavities could be used. As shown in FIG. 10,
the upper and lower plates of the rotor 110 are held together using
fasteners 116 which extend through openings 118 in the plates.
[0118] The rotor is preferably made of a material which maintains
an elevated strength (preferably greater than about 500 MPa) at
elevated temperatures (preferably up to about 900.degree. C. or
higher) and resists creep under load at such temperatures. One
preferred rotor is constructed from Inconel 100, although any of a
number of materials such as described above can also be used. A
Ni-based superalloy, or a pyrolytic carbon/carbon-fiber reinforced
material would also be suitable for construction of the rotor.
Other rotor components including the fasteners, sample capsules
(contained within the rotor cavities) can also be fabricated from
Inconel 100.
[0119] The overall disk shaped rotor (containing the sample
cavities) in one embodiment has a preferred diameter in the range
of about 10 cm to 50 cm. In one preferred embodiment, the rotor has
a diameter of about 25 cm. The thickness of the rotor disk is
preferably between about 1 cm and 5 cm, more preferably about 2 to
3 cm. The cavities within the rotor preferably each have a length
of about 2 to 10 cm, more preferably about 5 cm, and a diameter of
about 0.5 to 2 cm, more preferably about 1.27 cm. The rotor is
preferably capable of accelerations up to about 60,000 g (600,000
m/s.sup.2), more preferably up to about 200,000 g (2,000,000
m/s.sup.2). One preferred device was tested at accelerations up to
120,000 g (at the outer end of the sample cavities). Cooling times
of about 1 minute to 10 hours may be used, more preferably with
cooling rates of about 0.001.degree. C./second to about 10.degree.
C./second.
[0120] It should be understood that certain variations and
modifications of this invention will suggest themselves to one of
ordinary skill in the art. The scope of the present invention is
not to be limited by the illustrations or the foregoing
descriptions thereof, but rather solely by the appended claims.
* * * * *