Apparatus for production of ultrapure amorphous metals utilizing acoustic cooling

Amorphous metals are produced by forming a molten droplet (115) of metal from source (126) and deploying the droplet into a focused acoustical levitating field or by dropping the unit through spheroidizing zone (116) slow quenching zone (118) and fast quenching zone (120) in which the droplet is rapidly cooled by in the standing acoustic wave field produced between half-cylindrical acoustic driver (168) and focal reflector (166) or curved driver (38) and reflector (50). The cooling rate can be further augmented by first cryogenic liquid collar (160) and second cryogenic liquid jacket (170) surrounding the drop tower (112). The sphere (117) is quenched to an amorphous solid which can survive impact in the unit collector (124) or is retrieved by vacuum chuck (20).

TECHNICAL FIELD 
The present invention relates to apparatus for the production of amorphous 
metals and, more particularly, to apparatus for producing amorphous 
metals, alloys or compounds in containerless environments employing 
acoustic cooling. 
BACKGROUND ART 
Recent industrial tests of amorphous alloys under realistic working 
environments have indicated that the wear and corrosive resistances of 
this new category of alloys are at least one order of magnitude higher 
than that of conventional alloys currently in use. Other amorphous metal 
compounds are of interest as superconductors and magnetically soft alloys, 
etc. 
The formation of amorphous metals requires varying degrees of rapid 
cooling. Three techniques currently in use have been most successful in 
fabricating metallic glasses of various geometries and sizes: 1. Liquid 
quenching (LQ), 2. Sputtering, and 3. Electrodeposition (ED). The first 
preparation of an amorphous metal from the corresponding liquid was done 
by a gun technique. In this process, a diaphragm is ruptured by high 
pressure gases, the ensuing shock waves travel down the tube to a crucible 
with a small hole in the bottom. The molten sample is held in the crucible 
by its surface tension before being driven out of the hole in the form of 
small droplets by the shock waves. The droplets then impinge on a metal 
substrate, spreading out and overlapping to form an irregular foil. Other 
variations of this fundamental technique include twin roll technique, melt 
spinning, melt extraction, pendent drop process, laser glazing, chill 
block casting, etc. A variety of atomic deposition techniques have also 
been utilized to form amorphous metals. The latter techniques have higher 
effective cooling rates than liquid quench processes and thus present the 
potential for retention of phases with considerably higher free energy 
excess than the equilibrium phases. 
In all the above-mentioned techniques, a crucible and/or substrate must be 
used at one point in the process. The intimate contact of the melt with a 
foreign surface inevitably introduces impurities into the molten metal, 
which become heterogeneous nucleation sites and detrimentally increase the 
rate of crystalline growth within the melt during its cooling process. In 
fact, recent experiments on PdSi have produced conclusive evidence that 
the extremely high rate of cooling required in the metallic glass 
formation is primarily due to the necessity to suppress this type of 
nucleation process. 
Important progress in the theoretical and experimental areas has been made 
in recent years to provide conclusive evidences that: 
1. Surface heterogeneous nucleations were responsible for activating global 
nucleation process; 
2. Heterogeneous and homogeneous bulk nucleations played insignificant 
roles in an overall crystallization process; and 
3. For the same cooling rate condition, by decreasing the number of surface 
heterogeneous nucleation sites, the size of the amorphous samples was 
increased. 
Logically, if the surface heterogeneous nucleation sites could be reduced 
in number or eliminated altogether, the only crystallization process left 
is that due to the bulk, which could be suppressed with a very modest 
cooling rate. Depending on the size of the sample, the rate could be as 
low as 1.degree. K./sec. With a low cooling rate, the homogeneous 
nucleation rate may be small enough to permit bulk formation of amorphous 
alloys. 
Some earlier attempts to form bulk amorphous alloys have employed 
containerless processing. In this earlier work melts were injected into a 
drop tube. The gaseous atmosphere was selected to minimize surface 
heterogeneous nucleation sites. 
Theoretically, the containerless processing of molten alloys under high 
vacuum will certainly eliminate environmental impurities from making 
contact with the melt during the solidification period, thereby enhancing 
the conditions favorable for bulk homogeneous nucleations. In this case, 
the quench is due to radiative cooling. If the starting alloy is 
idealistically pure, this cooling rate may probably be sufficient for the 
formation of bulk metallic glasses. Realistically, however, this kind of 
condition may never be achievable in laboratory. Or it may not be 
economically feasible. 
In addition, realistic processing time in a drop tube may never exceed 
several seconds. During this time period, the sample must be cooled down 
enough to stand the impact of landing. This may call for a cooling rate 
more rapid than that due to radiation alone. Consequently, some exchange 
gas must be used. This may expose the melt to external impurities such as 
O.sub.2 and H.sub.2 O. 
Preliminary experiments on a PdCuSi system using a drop tube facility to 
produce amorphous solid spheres of several millimeters in diameter have 
been successful. Rapid cooling is provided by a 200 mm Hg helium exchange 
gas in the free-falling path of the droplet. Practical difficulties have 
limited the processing time of this technique to only several seconds. 
Space, on the other hand, provides an ideal containerless and zero-gravity 
environment. Many experiments along this line have been considered and 
proposed. A terrestrial levitation apparatus which is electrostatic, 
electromagnetic or acoustic in nature has also been considered and a 
development of such apparatus is in progress. 
The electrostatic levitation apparatus has been limited to manipulate 
materials of low specific gravity. The electromagnetic system can levitate 
and heat samples of high gravities. However, the rapid quenching of the 
samples is not readily available. Acoustic levitation systems currently in 
use for terrestrial applications in the past could not handle heavy 
materials with acceptable lateral positional stability. In addition, 
depending on the thermal properties of the material, the acoustic 
integrity of the apparatus deteriorates rapidly as the sample is being 
heated to its melting temperature. 
STATEMENT OF THE INVENTION 
An apparatus for contactless cooling of molten metal samples permitting 
extremely high quenching rates has been developed in accordance with this 
invention. A novel acoustical focusing radiator is utilized to increase 
jet streaming. Cooling rates from 10.sup.4 .degree. K./sec and higher are 
achieved by use of acoustic jet streaming. Molten metal samples have been 
cooled without contamination from contact with solid surfaces or exchange 
gases. The cooling rate exceeds the critical quenching rate and converts 
the molten droplet to a viscous amorphous state capable of surviving 
impact in the collection zone. 
Larger spheres can be produced as compared to prior processes. The spheres 
exhibit an ultrasmooth surface characteristic of amorphous glass phases. 
The acoustic levitation eliminates most heterogeneous nucleations at the 
surface and homogeneous nucleations can be suppressed with the cooling 
rates provided by acoustic jet streaming. In the absence of heterogeneous 
nucleation, the quenching rate required for glass formation is much lower 
enabling production of larger amorphous samples, novel amorphous alloys 
and higher volume production. In the focusing radiator approach of the 
invention the molten sample is levitated in a bidirectional acoustic 
standing wave field. In this configuration the rate of cooling is 
approximately 20 times higher than in absence of the bidirectional field 
and a clear pumping activity is observed. Depending on the sound pressure 
level applied, two types of streaming originating from the two 
pressure-antinode surfaces and the solid sample can coexist with different 
relative strength. Their existence is a simple consequence of Newton's 
third law of motion. Their relative strength depends on the sound pressure 
levels and geometries of the resonant cavity and sample. At very high 
sound pressure level, the jet streaming originating from the solid sample 
surface could be dominant, resulting in a vortex pattern. This high 
velocity acoustic jet streaming is regarded as responsible for the high 
cooling rate experimentally observed.

DETAILED DESCRIPTION OF THE INVENTION 
The apparatus of the invention can be utilized to cool and solidify any 
ultrapure molten material while avoiding contaminating from feeding 
mechanisms, cooling gases or the cooling apparatus itself. The molten 
material may be a pure metal, an alloy, a refractory, ceramic or glass 
compound. The system of the invention is partially useful in the 
preparation of amorphous or glassy metal or metallic compounds resulting 
from fast quenching from the molten state to freeze the randomness of the 
atomic distribution resulting in a structureless solid state. 
The apparatus of the invention increases the cooling rate by 20 to 100 
times as compared to radiation and convection cooling experienced by a 
falling body. The melt can contain a pure metal such as nickel or gold 
which requires quench rates of about 10.sup.12 .degree. K./sec and 
10.sup.9 .degree. K./sec, respectively, for metallic glass formation. Most 
alloys require a quench rate in the range of 10.sup.6 .degree. K./sec for 
glass formation while special alloys such as NiNb need a quench rate from 
10.sup.3 to 10.sup.5 .degree. K./sec. Examples of alloys that can be 
processed into metallic glasses in accordance with the invention are 
PdCuSi, AuPbSb, CuZr, etc., which require a quench rate not to exceed 
10.sup.6 .degree. K./sec. 
The invention proceeds by contactless formation of a molten droplet of 
metal glass precursor, deploying the droplet into an acoustic jet stream 
near the focus of a focusing radiator, cooling the droplet to a viscous, 
metal-glass, near-solid state and collecting a solid sphere. The focusing 
radiator can be disposed at an upwardly directed focus capable of 
levitating the molten object or the radiator can be disposed vertically 
with a sideward elongated focus for cooling a molten droplet by jet 
streaming as it falls adjacent the line of focus. 
The molten droplet experiences the following temperature history where Tm 
is the melting temperature of the molten material to be process into an 
amorphous metal glass: 
TABLE 1 
______________________________________ 
Temperature of 
Temperature of 
Location of Droplet 
Ambient Droplet 
______________________________________ 
Melting Zone T &gt; Tm T &gt; Tm 
Spheroidization Zone 
T = T ambient T &gt; Tm 
First (optional) 
T = T Cryogenic 
T &lt; Tm 
Cooling Zone Liquid I 
Focus of Radiator 
T = Cryogenic II 
Liquid II 
Entrance T = 0.6 to 0.9 Tm 
Exit T = 0.1 to 0.3 Tm 
Collection Zone 
T = T ambient T = T ambient 
______________________________________ 
The acoustic source directs acoustic energy generally toward a focus, with 
the source having portions on either side of its axis which vibrate along 
local axes which are not parallel to each other, but which are instead 
directed substantially at the focus. An acoustic reflector positioned near 
the focus, reflects sound to create an intense local field near the 
reflector which stably supports a small object such as a molten droplet. 
The acoustic source can include a curved plate and a plurality of 
transducers in intimate facewise contact with a surface of the plate and 
located on opposite sides of the axis of the curved plate. Each transducer 
vibrates the plate in a direction toward and away from the focus to assure 
the generation of a converging acoustic wave pattern. The reflector is 
positioned much closer to the focus than the acoustic source, and can be 
concavely curved to a much smaller radius of curvature than the source to 
produce an intense localized acoustic field. With the reflector located 
about one-half wavelength beyond the focus, a small object is stably 
supported one-quarter wavelength from the reflector. Suitable acoustic 
sources for practice of the present invention are disclosed in Copending 
Application Ser. No. 272,837, filed June 12, 1981 for ACOUSTIC SUSPENSION 
SYSTEM, the disclosure of which is expressly incorporated herein by 
reference. 
Referring now to FIG. 1 the levitation and jet stream cooling apparatus 10 
includes a droplet deploying means 16, heating means 14, acoustic 
levitation and cooling means 12 and collection means. 
The metal material is not as sensitive to contact when in the solid state. 
Therefore, the metal material and the final glassy material can be handled 
without substantially affecting the required quenching rate for glass 
formation or amorphous characteristics of the final product. The handling 
means can be mechanical fingers, a circular or flat chuck or a vacuum 
chuck 20, as shown. The vacuum chuck 20 is connected to a source of vacuum 
18. The heating means can be resistance or high energy frequency heating 
or a laser 24 having its output optical axis directed at the point of 
levitation 26 occupied by the droplet of metal material 22. 
The apparatus 10 may be contained within a chamber 28 maintained at desired 
pressure by means of a pressure controller 30 connected to the chamber by 
means of a line 34 containing a valve 32. 
The levitation and cooling source is in the form of a hemispherical, 
focusing-radiator acoustical driver 38 having a plurality of transducers 
40 attached to the back-surface of the driver. The transducers are driven 
in synchronism by an oscillator 42, the frequency of which is controlled 
by the output of a voltage source 44. The output of the oscillator is 
amplified in amplifier 46 before delivery to the transducers. The axes of 
vibration of the various transducers 40 converge on a focal point 48. A 
reflector 50 having a concave surface 52 is positioned just outside the 
focal point 48. 
When the transducers 40 are driven by oscillator 42 and amplifier 46, the 
focusing-radiator driver 38 oscillates and generates sound waves 
converging on the focal point 48 adjacent the reflector 50 to form an 
acoustic standing wave field. As an object such as the droplet 22 is 
placed in the standing wave field, a bidirectional acoustic pumping action 
results. The pattern of the acoustic jet stream surrounding the molten 
droplet 22 in a bidirectional acoustic standing wave field is 
schematically shown in FIG. 4. Depending on the sound pressure level 
applied, two types of jet streaming originating from the two 
pressure-antinode surfaces and the solid sample can coexist with different 
relative strengths. Their existence is a simple consequence of Newton's 
third law of motion. Their relative strength depends on sound pressure 
levels and the geometries of the resonant cavity and sample. At a very 
high sound pressure level exceeding about 172 db (reference pressure is 
2.times.10.sup.-4 dyne/cm.sup.2), the jet stream originating from the 
object predominates resulting in the high velocity swirling jet streams. 
The liquid droplet sphere modifies the flow forming new acoustic 
boundaries. Net flow forces create the bidirectional jet streaming which 
provides acoustic levitation, increases the volumetric levitational force 
and stabilizes lateral positioning of the object. The fast rate of flow of 
the acoustic jet stream provides an increase in the rate of cooling from 
20 to 100 times over free fall cooling through a drop tower. 
The system 10 is operated by opening valve 32 and operating pressure 
controller 30 until the pressure in the chamber 28 is adjusted to the 
desired level. The driver 38 is then driven by oscillator 42, voltage 
source 44 and amplifier 46 to create a bidirectional standing wave 
converging on the focal point 48. The metal material is engaged at the end 
of the vacuum chuck 20 and is deployed from the vacuum chuck 20 into the 
point of levitation 26 adjacent the focus 48 by terminating vacuum from 
the vacuum source 18 to the vacuum chuck 20. The laser 24 is actuated to 
melt the metal material to form a droplet 22. The jet stream cools the 
droplet to form a metal glass solid which can be retrieved by terminating 
the acoustic field and dropping the solid into the driver 38 or by 
actuating vacuum pump 18 and applying vacuum to the end of the vacuum 
chuck 20 to collect the metal glass solid. 
Referring now to FIG. 2 the apparatus 110 can be contained within an 
elongated tube or tower 112 having an upper zone 114 for containerless 
production of a droplet 115 of molten material, a spheroidizing zone 116, 
a first slow quenching zone 118, a rapid quenching zone 120 and a 
collection chamber 122 housing a removable collector 124. 
The zone 114 includes a source 126 of pure solid metal or metal compound 
and a means 128 of heating the source to produce a molten unit 115 of 
material positioned to fall into the vertical tube under the force of 
gravity. As shown in FIG. 2 a low rate unit feeder 130 comprises a spool 
132 of foil threaded between clamp electrodes 134, 136 and pulled by 
driven take up spool 138. On application of current from power source 140 
to the electrodes, the foil section 142 between the electrodes 134, 136 
will melt and fall to form droplet 115. 
A higher rate feed mechanism is illustrated in FIG. 3. A rod feeding device 
141 is centrally mounted in the tube 112. The device contains a chuck 143 
holding the metal rod 144 connected to a feeding mechanism 146. A laser 
148 connected to power supply 150 is mounted outside the tube 112 in 
optical alignment with the rod 144 through a window 152. The power supply 
150 and the feeding mechanism 146 are connected to controller 156. When a 
signal is generated by the controller 156 the feeding mechanism advances 
the rod 144 downwardly and synchronously pulses the laser 148 to generate 
a laser beam which melts the rod section 158 to produce a molten falling 
droplet 115. This feeding mechanism can be operated at a very high rate. 
As shown in Table 1 the temperature of droplet is greater than Tm (melting 
temperature of feed material) as the droplet leaves the upper feeding 
zone. Fluid dynamics cause spheroidization of the falling molten droplet 
115 to form a sphere 117 as the droplet 115 falls through a long ambient 
temperature zone 116. In the first quenching zone 118, the sphere 117 is 
subjected to radiative cooling by means of a heat exchange collar 160 
receiving a flow of cryogenic liquid such as liquid nitrogen (77.3.degree. 
K.) from tank 162 through line 164. The sphere 117 exits the zone 118 at a 
temperature of from 0.90 to 0.60 Tm, usually 0.75 Tm. 
The sphere 117 then enters the fast quenching zone 120. In this zone the 
sphere falls down a line just inside a reflector 166 placed adjacent the 
focal point of the half-cylindrical acoustic exiter 168. The transducers 
40 are driven in synchronism by an oscillator 42 controlled by voltage 
source 44 and amplified by amplifier 46. The cooling rate can be further 
augmented by disposing a second cryogenic cooling jacket 170 around the 
zone 120 and feeding a lower temperature cryogenic liquid such as liquid 
Helium (4.2.degree. K.) or liquid hydrogen (20.degree. K.) to the jacket 
170 from the second cryogenic liquid source 172. The sphere is quenched to 
an amorphous solid which falls through the collection chamber 122 into the 
collector 124 and can be recovered by removing end plate 176. 
A chamber 182 is formed by enclosing the tower 112 by a top member 180 and 
an end plate 176. The chamber 182 is maintained under reduced pressure by 
means of vacuum pump 184 connected to the chamber 182 by conduit 186. 
Generally, the chamber is maintained at pressure below half atmosphere, 
usually from 100 to 300 mm Hg. 
It has been reported that preliminary experiments have resulted in 
amorphous bulk spheres about 1.5 mm in diameter for a PdCuSi alloy 
processed containerlessly in a drop tube. Recent experiments in a 45 foot 
stainless steel drop tube have produced amorphous spheres of AuPBSb alloy 
2 mm and larger. The process of the invention utilizing acoustic 
levitation and cooling can produce much larger amorphous bulk spheres of 
the order of 5 mm or larger. 
It is to be realized that only preferred embodiments of the invention have 
been described and that numerous substitutions, modifications and 
alterations are permissible without departing from the spirit and scope of 
the invention as defined in the following claims.