U.S. patent number 3,801,309 [Application Number 05/196,448] was granted by the patent office on 1974-04-02 for production of eutectic bodies by unidirectional solidification.
This patent grant is currently assigned to Tyco Laboratories, Inc.. Invention is credited to Abraham I. Mlavsky.
United States Patent |
3,801,309 |
Mlavsky |
April 2, 1974 |
PRODUCTION OF EUTECTIC BODIES BY UNIDIRECTIONAL SOLIDIFICATION
Abstract
Eutectic bodies with controlled morphology are produced by
establishing a thin liquid film of a eutectic composition on a hot
supporting surface, growing a body of said composition from said
film by unidirectional solidification, pulling the body away from
the film at a rate consistent with the rate of solidification, and
replenishing the film so as to sustain continuous growth.
Inventors: |
Mlavsky; Abraham I. (Lincoln,
MA) |
Assignee: |
Tyco Laboratories, Inc.
(Waltham, MA)
|
Family
ID: |
22725462 |
Appl.
No.: |
05/196,448 |
Filed: |
November 8, 1971 |
Current U.S.
Class: |
75/10.11;
148/404; 420/590; 23/301; 420/550; 264/171.1; 117/939 |
Current CPC
Class: |
C30B
21/06 (20130101); C30B 15/34 (20130101) |
Current International
Class: |
C30B
21/00 (20060101); C30B 15/34 (20060101); C30B
21/06 (20060101); C22c 001/02 () |
Field of
Search: |
;75/135 ;23/31SP
;148/1.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bizot; Hyland
Assistant Examiner: Weise; E. L.
Attorney, Agent or Firm: Schiller & Pandiscio
Claims
What is claimed is:
1. Method of producing polyphase eutectic bodies of uniform
morphology comprising:
establishing a thin liquid film of a eutectic composition on a
substantially flat supporting surface and controlling the
temperature of said film so that it has (1) a sharp temperature
gradient along its depth with the film being hottest at said
surface, (2) a substantially flat temperature profile along its
length and breadth, and (3) an average temperature approximately
equal to the eutectic point temperature of said composition;
solidifying and pulling a mass of said composition from the cooler
side of said film at a selected rate; and
simultaneously supplying an additional quantity of said mixture in
liquid form to said surface to replace the liquid consumed in
producing said eutectic mass.
2. Method according to claim 1 wherein said eutectic composition is
a binary composition.
3. Method according to claim 1 wherein said eutectic composition is
an alloy.
4. Method according to claim 3 wherein said alloy essentially
comprises nickel and aluminum.
5. Method according to claim 4 wherein said alloy essentially
comprises nickel, indium and antimony.
6. Method according to claim 1 wherein said mass is turned about
its pulling axis as it is pulled from said film.
7. Method according to claim 1 wherein said flat supporting surface
is porous.
8. Method according to claim 7 wherein said porous surface is part
of a member consisting of a myriad of interconnected open cells,
and further wherein said additional quantity of said mixture is
supplied to said surface via said cells.
9. Method of producing a polyphase eutectic body having a coherent
microstructure comprising:
establishing a thin liquid film of a selected eutecticcomposition
on a substantially horizontal and planar end surface of a heat
conducting member, and controlling the temperature of said film so
that it has (1) a sharp vertical temperature gradient, (2) a
substantially flat horizontal temperature profile, and (3) an
average temperature approximately equal to the eutectic point
temperature of said composition;
growing and vertically withdrawing a coherent polyphase solid body
from said film at a selected rate; and
supplying additional quantities of said composition in liquid form
to said end surface via a passageway in said member as said solid
body is being grown to replace the liquid consumed in producing
said body.
10. Method according to claim 9 wherein the thickness of said film
is held substantially constant during growth and withdrawal of said
body.
11. Method according to claim 9 wherein growth of said body is
initiated by use of a crystalline seed.
12. Method according to claim 9 further including the step of
rotating said body as it is withdrawn from said film.
13. Method according to claim 9 wherein said member is supported in
a heated crucible containing a reservoir supply of said selected
composition in liquid form, and said film is replenished from said
reservoir supply.
Description
This invention relates to production of eutectic materials and more
particularly to production of eutectic compositions by controlled
directional solidification.
It is recognized in the art that unidirectional solidification of
various eutectic compositions may have the effect of providing
products having unique crystallographic and mechanical properties.
In this connection see F. D. Lamkey et al., "The Microstructure,
Crystallography, and Mechanical Behavior of Unidirectionally
Solidified Al-Al.sub.3 Ni Eutectic", Transactions of the
Metallurgical Society of AIME, Vol. 233, pp. 334-341, Feb., 1965;
and R. W. Hertzberg et al., "Mechanical Behavior of Lamella
(Al-CuAl.sub.2) and Whisker Type (Al-Al.sub.3 Ni) Unidirectionally
Solidified Eutectic Alloys", Transactions of the Metallurigcal
Society of AIME, Vol 233, pp. 342-354, Feb., 1965. It has been
demonstrated that if (1) a planar liquid-solid interface is
established in a binary eutectic alloy by proper control of heat
flow during the solidification process and (2) the interface is
moved unidirectionally, it is possible to produce a eutectic
crystal structure consisting of an essentially parallel array of
discrete phases. Thus it has been possible to produce two dominant
phase microstructures: (a) one comprising parallel alternating rods
of one phase embedded in a continuous matrix of the second phase.
The directional solidification technique usually used for this
purpose essentially consists of melting a mixture of the refined
constituents of the desired eutectic, maintaining the melt long
enough to insure complete mixing, and cooling the melt to form
ingots. Then these ingots are remelted in a crucible and
unidirectionally solidified by unidirectionally withdrawing the
crucible from the heat source (or vice versa) at as uniform a rate
as possible with the object of producing a constant rate of growth
and maintaining a constant thermal gradient in the liquid ahead of
the liquid-solid interface.
While this prior art technique has produced eutectic materials
having unique properties, e.g. an alloy of highly anisotropic
mechanical properties comprising single crystal whiskers of
Al.sub.3 Ni in an aluminum metal matrix, it has a number of
limitations. For one thing, the center of the melt tends to cool at
a slower rate (particularly in a large diameter crucible) and hence
the crystallographic structure tends to vary along planes parallel
to the liquid-solid interface. A further limitation is the
inability to grow such eutectic compositions in indefinite lengths.
Further problems are phase discontinuities and difficulty in (a)
maintaining a planar liquid-solid interface, (b) controlling the
temperature gradient at that interface within close limits, and (c)
holding the rate of growth constant.
Accordingly, the primary object of this invention is to provide a
new and improved method of unidirectionally solidifying eutectic
compositions so as to produce bodies that are characterized by
unique crystallographic relationships between the constituent
phases thereof.
Another important object of this invention is to provide a method
of producing binary eutectic compositions as duplex single
crystals.
Still another important object is to provide a method of producing
binary eutectic compositions having microstructures that consist of
substantially parallel alternating lamellae of each phase or long
thin parallel rods of one phase embedded in a continuous matrix of
the other phase.
A further object is to provide eutectic compositions having unique
microstructures.
The foregoing objects are achieved by establishing a relatively
thin molten film of the eutectic composition and growing and
pulling a crystalline body from the thin film while simultaneously
replenishing the film by feeding thereto additional melt under the
influence of surface tension. The process may be conducted on a
continuous basis so as to produce bodies of indefinite length and
the body may be grown to a predetermined arbitrary cross-sectional
configuration.
Other features and advantages of the process and the nature of the
products produced thereby are set forth in or rendered obvious by
the following detailed description of the invention which is to be
considered together with the accompanying drawings wherein:
FIG. 1 is a vertical sectional view of one form of crucible and die
arrangement for practicing the invention;
FIG. 2 is a fragmentary view of the apparatus of FIG. 1 showing a
film of melt and a seed for effecting solidification and growth of
a eutectic body;
FIG. 3 is a vertical sectional view of a second crucible and die
arrangement;
FIG. 4 is a view similar to FIG. 1 showing a die assembly for
growing a hollow eutectic body;
FIG. 5 is a photomicrograph of a transverse section of a eutectic
body comprising LiF and NaCl grown according to this invention;
and
FIG. 6 is a photomicrograph of a transverse section of a eutectic
body comprising LiF and CaF.sub.2 grown according to this
invention.
The present inventions utilizes the so-called EFG process
previously known for growing monocrystalline bodies of materials
such as alumina. The term "EFG" stand for "edge-defined, filmfed
growth" and designates a process for growing crystalline bodies
from a melt. The essential features of the EFG process are
described in U.S. Pat. No. 3,591,348, issued July 6, 1971 to Harold
E. LaBelle, Jr. for METHOD OF GROWING CRYSTALLINE MATERIALS.
In the EFG process the shape of the crystalline body that is
produced is determined by the external or edge configuration of a
horizontal end surface of a forming member which for want of a
better name is called a die, although it does not function in the
same manner as a die. By this process a variety of complex shapes
can be produced commencing with the simplest of seed geometries,
namely, a round small diameter seed crystal. The process involves
growth on a seed from a liquid film or film material sandwiched
between the growing body and the end surface of the die, with the
liquid in the film being continuously replenished from a suitable
reservoir of melt via one or more capillaries in the die member. By
appropriately controlling the pulling speed of the growing body and
the temperature of the liquid film, the film can be made to spread
(under the influence of the surface tension at its periphery)
across the full expanse of the end surface of the die until it
reaches the perimeter or perimeters thereof formed by intersection
of that surface with the side surface or surfaces of the die. The
angle of intersection of the aforesaid surfaces of the die is such
relative to the contact angle of the liquid film that the liquid's
surface tension will prevent it from overrunning the edge or edges
of the die's end surface. Preferably the angle of intersection is a
right angle which is simplest to achieve and thus most practical to
have. The growing body grows to the shape of the film which
conforms to the edge configuration of the die's end surface. Thus
it is possible to grow a substantially monocrystalline body with
any one of a variety of predetermined cross-sectional
configurations, e.g. bodies with circular, square or rectangular
cross-section. Furthermore, since the liquid film has no way of
discriminating between an outside or inside edge of the die's end
surface, it is possible to grow a monocrystalline body with a
continuous hole by providing in that end surface a blind hole, i.e.
a cavity of the same shape as the hole desired in the growing body,
provided, however, that any such blind hole is made large enough so
that surface tension will not cause melt film around the hole to
fill in over the hole. From the foregoing brief description it is
believed clear that the term "edge-defined, film-fed growth"
denotes the essential feature of the EFG process -- the shape of
the growing crystalline body is defined by the edge configuration
of the die and growth takes place from a film of liquid which is
constantly replenished.
It has been determined that essential factors contributing to the
essential monocrystalline character of the bodies that are grown by
the EFG process are the relatively shallow depth of the melt film
supported by the die, the fact that the film-supporting surface of
the die functions as a substantially isothermal heater (i.e. the
film-supporting surface has a substantially flat temperature
profile along its entire expanse), and the fact that melt film is
not affected by perturbations in the melt reservoir and can be
maintained at an average temperature lower than the average
temperature of the melt in the reservoir. The thin melt film has a
sharp vertical temperature gradient and a relatively small
horizontal temperature gradient. It has been found that because of
these factors, coupled with the additional fact that the thickness,
i.e. depth, of the melt film can be maintained substantially
constant by adjusting the rate of heating and the pulling speed, it
is possible to utilize the EFG technique to unidirectionally
solidify eutectic compositions so as to produce coherent eutectic
bodies of indefinite length and controlled cross-sectional
configurations. As used herein, the term "coherent eutectic"
denotes a eutectic composition having a high order of regularity of
dispersal of one phase in another. Eutectic compositions produced
in accordance with this invention are characterized by
crystallographic properties that are substantially more uniform
than eutectic bodies of the same chemical composition produced by
prior art unidirectional solidification techniques. Depending upon
their chemical constituents, eutectic compositions produced as
herein described may be used, for example, as structural materials
for jet engines and to produce components for electrical and
electronic devices and systems.
THe present invention may be used to unidirectionally solidify a
wide variety of eutectic compositions, including, for example,
Al-Al.sub.3 Ni, Al-CuAl.sub.2, Pb-Sn, Zn-Sn, Cd-Zn, Mg-Mg.sub.17
Al.sub.12, NiSb-InSb, and Cu-Cr eutectic alloys, nickel-base super
alloys (such as those commercially designated as PWA Nos. 1011A,
655, 659 and 689), LiF-NaCl, and LiF-CaF.sub.2. Although the
following detailed description of the invention includes specific
examples of producing bodies of only a few of the foregoing
eutectic compositions, persons skilled in the art will appreciate
that the invention is applicable to directionally solidifying all
of the foregoing compositions and also many other compositions,
including, but not limited to, those specified by G. A. Chadwick,
"Eutectic Alloy Solidification Progress In Materials Science", Vol.
12, No. 2, Pergamon Press, Oxford, 1963.
In the following description like reference characters on the
drawings refer to like elements in the several figures.
Turning now to FIG. 1, the illustrated apparatus comprises a
crucible 2 for holding a reservoir supply of a melt 4 of a eutectic
composition which is to be directionally solidified in accordance
with this invention. The crucible is made of a material that will
withstand the operating temperatures and will not react with the
die assembly hereinafter described and will not react with or
dissolve in the melt 4. The crucible is mounted within a susceptor
6 by means of a plurality of short rods 8. The susceptor is made of
a material that will not evolve substances that will react with the
crucible and preferably is spaced from the crucible if it is made
of a material that will react with the crucible or die assembly at
the operating temperatures. The top end of the susceptor is open
but its bottom end is closed off by an end wall 10.
Mounted within the crucible is a die assembly 14 that comprises a
disc 16 that is locked to the crucible by a removable collar 17.
The disc 16 functions as a heat shield to reduce radiative heat
loss from the melt and also supports a die member in the form of a
cylindrical, vertically-extending solid non-porous rod 18 which is
securedly mounted within a centrally located hole in the disc. Rod
18 extends a short distance above the disc and its bottom end
terminates short of the bottom of the crucible. Rod 18 has a flat,
substantially horizontal top end surface 20 and several through
holes in the form of axially-extending bores 22 that are uniformly
spaced about the axis of the rod and are sized to function as
capillaries for the melt 4. Disc 16 and rod 18 are made of a
material that will not react with the crucible and will not react
or dissolve in the melt. Additionally, the rod 18 is made of a
material that is wetted by the melt and the diameters of
capillaries 22 are such as to cause melt to rise up and fully fill
them so long as the bottom end of the rod is trapped by, i.e.
submersed in, the melt.
The apparatus of FIG. 2 is mounted in a suitable induction heating
furnace (not shown) that is adapted to envelope the crucible and
the growing eutectic body in an inert atmosphere and includes a
pulling mechanism that is adapted to position a seed crystal as
hereinafter described and to pull the seed at a controlled rate as
melt solidifies on the seed. One form of furnace that may be used
in the practice of this invention is illustrated and described in
U.S. Pat. No. 3,591,348 and also U.S. Pat. No. 3,471,266, issued
10/7/69 to Harold E. LaBelle, Jr. for GROWTH OF INORGANIC
FILAMENTS. The susceptor 6 is mounted within the furnace by
attaching it to the upper end of a support rod 24 that is mounted
in the furnace. Rod 24 may be mounted to the base 2 of the furnace
shown in U.S. Pat. No. 3,471,266.
Production of a solid eutectic body is initiated by using a seed of
any desired cross-sectional configuration. Thus the seed may be a
round filament, a flat ribbon or a crystalline body of other
suitable shape. The seed crystal serves as a nucleating medium for
the melt and may also be used to establish a film of melt in the
upper surface 20 of the die assembly. The seed may be a single
crystal of one of the components of the eutectic composition or may
be a previously solidified body of substantially the same
composition as the melt. The essential requirement of the seed is
that it be wetted by the melt.
The method of the present invention will now be described with
reference to the apparatus of FIG. 1. Assume for ease of
description that the crucible 2 and the susceptor 6 are mounted in
an induction furance of the type described in U.S. Pat. No.
3,471,266, with the crucible and the capillaries of the die
assembly filled with a melt of a selected binary eutectic
composition and an inert gas atmosphere being continuously
circulated through the furnace. Assume also that a seed 26 in the
form of a monocrystal of one of the constituents of the eutectic or
a single crystal of the eutectic composition is supported by the
crystal pulling mechanism associated with the furnace in coaxial
alignment with rod 18. With the upper surface 20 of rod 18 at a
temperature of about 10.degree.-40.degree.C higher than the
eutectic point temperature of the melt composition in the crucible,
the seed is lowered into contact with surface 20 and held there
long enough for its end to melt and form a liquid film 32 that
connects with the melt in the capillaries 22 (see FIG. 2). It is to
be noted that the capillaries are shown empty in FIGS. 1-2 in order
to render them more distinct to the reader and that before the end
of the seed is melted to form film 32 the melt in each capillary
has a concave meniscus with the edge of the meniscus being
substantially flush with the surface 20. It is to be noted also
that the temperature gradient along the length of the seed is one
factor influencing how much of the seed melts and forms film 32.
The seed 26 functions as a heat sink so that its temperature is
lower at successively higher points therein. However, the thermal
gradients along the seed and vertically across the film 32 are
affected by the power input to the induction heating coil of the
furnace and the height and distance of the heating coil and
susceptor 6 relative to the seed and the die assembly. In practice
these parameters are adjusted so that the film 32 is maintained at
a thickness in the order of 0.2 mm during growth of desired
eutectic solid. After the film 32 has connected with the melt in
the capillaries, the pulling mechanism is actuated so as to pull
the seed vertically away from the surface 20. The initial pulling
speed is set so that surface tension will cause the film 32 to
adhere to the seed long enough for solidification to occur due to a
drop in temperature at the seed-liquid film interface. This drop in
temperature occurs because of movement of the seed away from
surface 20, i.e., because the solid-liquid interface advances
vertically to a relatively cooler region. It is to be noted that
radiative and conductive heat losses from the seed cause it to
exhibit a decrease in temperature with an increase in distance from
the surface 20. If it is desired that the eutectic solid have a
constant cross-section conforming in shape and area to surface 20,
it is necessary to have the film 32 fully cover surface 20.
Accordingly, if the film initially established by melting the seed
does not fully cover surface 20, the pulling speed must be set so
that surface tension will cause the film to spread radially out to
the edge of surface 20 as solidification progresses. Preferably
enough of the seed is melted for the film 32 to fully cover the end
surface of the die assembly, in which case the initial pulling
speed is set at the level at which solidification will occur on the
seed across the full expanse of the film. If the initial film
covers less than all of the surface 20, a pulling speed is used at
the beginning of the solidification process which will cause the
film to spread radially, and once the surface 20 is fully covered,
the pulling speed is increased to a level at which the film is
maintained at a suitable thickness and solidification will occur on
the seed along the full expanse of the film. It is to be noted that
the pulling speed and the temperature of the film control the film
thickness which also controls the rate of film spreading.
Increasing the temperature of surface 20 (and hence the average
temperature of film 32) and increasing the pulling speed (but short
of the speed at which the seed and the growth occurring thereon
will pull clear of the film) each have the effect of increasing the
film thickness.
As the seed is pulled away from surface 20, liquid from film 32
will solidify on the seed at all points along the full horizontal
expanse of the film, with the result that additional accretions of
solid will form a longer and longer solid eutectic body. The liquid
consumed by solidification at the interface of the growing solid
and the film 32 is replaced by additional melt which is supplied to
surface 20 via capillaries 22 under the influence of surface
tension. The rate at which fresh melt is supplied to the surface 20
is determined by the number and size of the capillaries and, within
limits, is always enough to maintain the film 32. The process may
be continued until the solid extension on the seed has grown to a
desired length or until the supply of melt in the crucible has been
depleted to the point where the bottom end of the capillaries are
no longer trapped, whichever event occurs first. Furthermore, the
growth process may be terminated at any time by increasing the
pulling speed enough to cause the growing body to pull free of the
melt film. Once growth has been terminated, the furnace is shut
down and the seed with its newly acquired eutectic extension is
removed for inspection and use.
Because of the sharp temperature gradient that is attainable across
the melt film and because the average temperature of the melt film
can be maintained constant at a level near to but below the
temperature of the melt in the crucible, it is possible to achieve
a constant thermal gradient in the film below the solid-liquid
interface and to maintain a planar solid-liquid interface, with the
result that by preferably adjusting the pulling speed and hence the
solidification rate, it is possible to achieve a predetermined and
uniform micromorphology. This is particularly important for
eutectics that have been shown to exhibit a tendency to undergo a
change in morphology, e.g. a transition from rod-like to lamella
structure, or a change in inter-rod or inter-lamella spacing, with
increasing growth rate. Furthermore, since the film thickness is
relatively small and the film is remote from the crucible, the
solidification process is relatively free of perturbations of the
type that produce localized depletions of one phase in the other.
Such areas of localized phase depletions are known to be sources of
premature failure under stress.
FIG. 3 relates to a preferred modification of apparatus used in
practicing the invention. In this case the die assembly 14A
consists of the disc 16 and a rod 18A which is secured within a
centrally located hole in the disc. Like rod 18, rod 18A extends a
short distance above the disc. The bottom end of rod 18A extends
close to and may even engage the bottom of the crucible. Rod 18A
has a flat substantially horizontal top surface 20 which functions
to support a film of melt 32. However, rod 18A differs from rod 18
in that it is a porous member characterized by a myriad of small
interconnected open cells sized to function as capillaries whereby
melt will rise in the rod by capillary action. Preferably the cells
are sized so that melt will rise to the top surface 20 by capillary
action so long as the level of the melt in the crucible is high
enough to trap the bottom end of the rod. As with rod 18, rod 18A
is made of a material that is wetted by the melt but will not react
with the melt or the crucible at the operating temperatures.
Growth of eutectic bodies with the apparatus of FIG. 3 is
accomplished in the same manner as with the apparatus of FIGS. 1
and 2, except that (1) the melt rises in rod 18A via the open
interconnected cells rather than through discrete bores as shown at
22, and (2) because the capillary action occurs across the full
cross-section of rod 18A, infeeding of melt to the film 32 involves
little or no horizontal flow of melt along surface 20 as may occur
with rod 18. This substantial elimination of flow of fresh melt
laterally along surface 20 minimizes perturbations. Furthermore,
with the film being replenished with fresh melt at a large number
of points instead of at a limited number of points as is the case
when using capillary bores 22, it is easier to maintain an even
melt thickness.
It is recognized that the choice of seed, crucible, susceptor and
die assembly materials and the determination of satisfactory
operating temperatures and pulling speeds will vary in accordance
with the eutectic to be solidified, and also that such choice is
well within the skill of the art. Accordingly, the following
specific examples, which are provided to assure a full and accurate
understanding of the invention, should be considered to merely
illustrate and not to limit the invention.
EXAMPLE I
A crucible having the general appearance of the crucible 2 of FIG.
2 and made of nickel is mounted on rods 8 in a susceptor 6 that is
mounted in a furnace in the manner shown in FIG. 1 of U.S. Pat. No.
3,471,266. The rod 8 is made of alumina and the susceptor 6 is made
of molybdenum. Disposed in the crucible is a die assembly
constructed generally as shown in FIG. 1. The rod 18 is made of
nickel and has four capillaries 22 of about 0.040 inch diameter
spaced uniformly about its center axis. The crucible has an
internal diameter of about 1 inch and an internal depth of about
1.5 inches. The rod 18 has an outside diameter of about 1/8 inch
and a rod length such that its upper end projects about 1/16 inch
above the crucible. The crucible is filled with a solid composition
comprising 23% LiF and 77% NaCl by weight. An elongate seed crystal
26 consisting of LiF is mounted in the seed holder of the crystal
pulling mechanism associated with the furnace so that it is aligned
with rod 18. The seed crystal is supported in axial alignment with
rod 18.
With the crucible mounted in the furnace, the induction heating
coil of the furnace is located so that its upper end is
approximately even with the middle of the susceptor and its lower
end at least even and preferably a little below the susceptor. Then
the furnace enclosure is evacuated and filled with argon gas to a
pressure of about one atmosphere which is maintained during the
growth period, and the induction heating coil is energized and
operated so that the charge in the crucible is brought to a fully
molten condition and the surface 20 is brought to a temperature of
about 700.degree.C. As the charge in the crucible is converted to
the melt 4, columns of melt will rise in and fill the capillaries
22. Each column of melt rises until its meniscus is substantially
flush with the top surface 20 of rod 18. After affording time for
temperature equilibrium to be established, the pulling mechanism is
activated and operated so that the seed 26 is moved down into
contact with the upper surface 20 and allowed to rest in that
position long enough (e.g. about one minute) for the bottom end of
the seed to melt and form a film 32 which fully covers surface 20
and connects with the melt in the capillaries. Then the seed is
withdrawn vertically at a rate of about 0.1 inch per minute. As the
seed is withdrawn, surface tension causes film material to adhere
to the seed and also cause additional melt to flow out of the
capillaries and add to the total volume of film. The liquid film
material adhering to the seed experiences a temperature drop due to
its movement away from the relatively hotter surface 20 and the
fact that the seed functions as a heat sink. As a consequence of
this temperature drop, the liquid that is in contact with the seed
undergoes directional solidification and growth of solid occurs on
the seed. Concurrently with the consumption of film by growth of
solid on the seed, surface tension causes additional melt to flow
up out of the capillaries onto surface 20 to replenish the film.
The pulling speed and temperature are maintained constant and
growth of solid on the seed continues to propagate vertically
throughout the entire horizontal expanse of the film 32, with the
result that successive accretions of solid form an elongate
extension on the seed having substantially the cross-sectional
shape and area of surface 20 (the openings of capillary 20 may be
disregarded in considering what is the configuration of surface 20
since they are filled with melt). Growth is continued until the
growth on the seed has reached a length about 6 inches. Thereafter
the pulling speed is increased rapidly to about 10 inches per
minute, with the result that the grown body pulls free of the film
22. Then the furnace is cooled and the seed retrieved for
sectioning and examination of the grown body.
FIG. 5 is a photomicrograph of a transverse microspecimen,
magnified by a factor of 940, of a eutectic body produced by
practicing the invention according to the procedure of the
foregoing example. The eutectic body was found to be of uniform
morphology throughout its entire volume. As is apparent from FIG.
5, the eutectic body consists of substantially uniformly sized rods
spaced substantially uniformly throughout a matrix phase. The rod
diameters are in the order of 0.0001 inches and the spacing between
rods is in the order of 0.00015 inches. The rods extend parallel to
the direction of solidification. The rods have been found to be of
indefinite length, with the result that the body is characterized
by a high aspect ratio, (i.e., the ratio of rod length to rod
diameter).
EXAMPLE II
A eutectic body consisting of 56% LiF and 44% CaF.sub.2 is produced
by using the same apparatus and following the same procedures as in
Example I, except that the crucible is initially charged with LiF
and CaF.sub.2 in the above proportions, the furnace is operated so
as to hold the top surface 20 of the die assembly at a temperature
of about 775.degree.C, and the pulling speed of the crystal is
maintained at about 0.1 inch per minute.
FIG. 6 is a photomicrograph similar to FIG. 5 of a microspecimen,
magnified by a factor of 940, of a LiF-CaF.sub.2 eutectic body
produced in accordance with the procedure of Example II. As is
evident, the product is a lamellar or plate-type eutectic, the LiF
phase being in the form of plates of indefinite length dispersed
through a CaF.sub.2 matrix. As with the LiF-NaCl eutectic, it has a
coherent microstructure, the two phases having an exceptionally
high degree of regularity with the parallel alternate lamellae
extending parallel to the direction of solidification.
EXAMPLE III
A crucible having the general appearance of the crucible 2 of FIG.
2 and made of alumina is mounted on rods 8 in a susceptor 6 that is
mounted in a furnace of the type shown in FIG. 1 of U.S. Pat. No.
3,471,266, except that the pulling mechanism is constructed in
accordance with the teachings of U.S. Pat. No. 3,552,931, issued
Jan. 5, 1971 to Paul R. Doherty et al., for APPARATUS FOR IMPARTING
TRANSLATIONAL AND ROTATIONAL MOTION, so that the seed crystal (and
the growth that occurs thereon) will undergo rotational motion as
it is being withdrawn. The rods 8 are made of alumina and the
susceptor 6 is made of molybdenum. Disposed in the crucible is a
die assembly constructed as shown in FIG. 2A and made of an alumina
foam or sponge which consists of a myriad of small interconnected
open cells having an average diameter in the order of 0.0002 inch.
The diameter and length of rod 18A and the depth and internal
diameter of the crucible are the same as specified in Example I.
Into the crucible is placed an aluminum-nickel ingot comprising 6.2
weight per cent nickel. The ingot is prepared by inductively
melting substantially pure aluminum and nickel in an argon
atmosphere at 900.degree.C for 1 hour to assure complete mixing,
and then cooling the melt. An elongate aluminum seed crystal is
mounted in the seed holder of the crystal pulling mechanism
associated with the furnace. Then with the induction heating coil
located as described in Example I, the furnace enclosure is
evacuated and filled with argon to a pressure of about one
atmosphere and the heating coil is energized. The furnace
temperature is raised high enough to melt the ingot and then
adjusted so as to hold the temperature of the upper surface of rod
18A at about 675.degree.C. The molten liquid in the crucible
infiltrates the cells of rod 18A and rises up to its top surface by
action of capillary rise. After the cells of rod 18A are filled
with melt, the crystal pulling mechanism is operated to move the
aluminum seed down into contact with the upper surface of rod 18A
and held there long enough for its bottom end to melt and form a
thin film that extends along surface 20. Then the pulling mechanism
is operated to withdraw the seed vertically at a rate of about 2
centimeters per hour while the temperature of surface 20A is held
steady at about 675.degree.C. As the seed is withdrawn, liquid film
material solidifies on the seed and surface tension causes
additional melt to flow up rod 18A to the film on surface 20A to
replace the material lost by solidification. About 10 minutes after
solidification is evident on the seed, the pulling mechanism is
caused to rotate the seed at a rate of about 10 degrees per hour at
the same time that it is being pulled. The pulling and rotational
speeds are held constant and growth of solid continues to propagate
vertically on the seed to a cross-sectional configuration
corresponding to the shape of surface 20. Growth is terminated when
the supply of melt in the crucible is substantially exhausted.
Thereafter the furnace is cooled and the seed retrieved from the
pulling mechanism for sectioning and examination of the grown body.
An Al-Al.sub.3 Ni body grown according to this example has a
eutectic microstructure consisting of a lamellar micromorphology.
The body has substantially parallel alternating lamellae of each
phase with all of the lamellae extending spirally about the body's
longitudinal axis. The lamellae are coextensive and substantially
free of discontinuities. It also is possible to produce a rod-like
eutectic microstructure, i.e., a micromorphology consisting of thin
parallel rods of Al.sub.3 Ni embedded in a continuous matrix of Al,
by increasing the pulling speed (and hence the solidification rate)
to about 8-10 centimeters per hour. If the seed is rotated at an
appropriate speed, the parallel rods of Al.sub.3 Ni will also
extend spirally about the growth axis. If the seed is not rotated,
the lamellae and rods will extend parallel to the growth axis.
Other eutectic alloys also may be grown with a twisted structure
using a pulling procedure like that of Example III. It also is
possible by solidification of the film-supporting surface of the
die, e.g. by providing one or more blind holes or cavities 38 as
shown in FIG. 4 that are too large in diameter to function as a
capillary, to grow eutectic bodies having one or more through holes
extending parallel to the axis of growth. In this case it is
preferred to use a seed crystal in the form of a hollow tube 40 as
shown or a solid body that has a cross section with a substantially
smaller area than the upper surface 20 of the rod of the die
assembly 14B. In the latter case the initial film that is formed
may cover less than all of the surface 20 and must be made to
spread out around the cavity 38 so as to fully cover the surface
20; hence the initial growth of solid will not conform to the
desired shape but will grow to that shape as the film spreads out
over surface 20.
It also is to be understood that the eutectic compositions may
include trace amounts of impurities or minor amounts of selected
elements introduced for reasons obvious to persons skilled in the
art without departing from the present invention. Accordingly, the
term "essentially consisting of" as used herein with respect to the
eutectic composition is intended to allow for such additional
impurities or selected elements.
Eutectic bodies produced according to this invention offer a number
of advantages. The most important advantage is a high degree of
regularity of the phases with the phases being substantially free
of discontinuities. Thus, for example, in a eutectic body having a
rod-like morphology, the individual rods will extend for
substantially the full length of the body. The ability to produce a
body with one or more holes avoids the problem of irregular phase
termination and particle breakout such as occurs when a hole is
drilled in an alloy body. Growing a body so that the phases are
curved about the body's longitudinal axis is advantageous when it
is desired to machine a curved part. By properly controlling the
speed at which the body is rotated as it is being grown, it is
possible to have the phases oriented so that machining transverse
to the direction of the phases can be avoided when the body is
being machined into a finished part of predetermined size and
shape. Furthermore, since the pulling rate is consistent with the
rate of growth along the pulling axis (which in turn depends upon
the temperature gradient across the film from which growth occurs),
it is possible by controlling the rate of heat input and the rate
of heat loss by radiation and conduction to control the rate of
growth within close limits and to maintain the film thickness
substantially constant. Also since the film is supported by the end
surface of the die and its position with respect to the height of
the heater coil is held fixed, the solid-liquid interface is
substantially planar at all times while a eutectic body is being
grown. Another important advantage is that it is possible to grow
eutectic bodies with any one of a variety of arbitrary
cross-sectional configurations, e.g., a body having the general
cross-sectional shape of an air-foil with one or more holes
extending lengthwise of the body. Still other advantages, in
addition to those noted above, will be obvious to persons skilled
in the art.
* * * * *