Method of preparing copper-dendritic composite alloys for mechanical reduction

Copper-dendritic composite alloys are prepared for mechanical reduction to increase tensile strength by dispersing molten droplets of the composite alloy into an inert gas; solidifying the droplets in the form of minute spheres or platelets; and compacting a mass of the spheres or platelets into an integrated body. The spheres preferably have diameters of from 50 to 2000 .mu.m, and the platelets thicknesses of 100 to 2000 .mu.m. The resulting spheres or platelets will contain ultra-fine dendrites which produce higher strengths on mechanical reduction of the bodies formed therefrom, or comparable strengths at lower reduction values. The method is applicable to alloys of copper with vanadium, niobium, tantalum, chromium, molybdenum, tungsten, iron and cobalt.

FIELD OF INVENTION 
The field of this invention is alloys of copper with metals forming a 
dendritic phase, and the mechanical reduction of such composite alloys to 
obtain increased tensile strength. 
BACKGROUND OF INVENTION 
Because of its electrical and heat conducting properties, copper has many 
important uses in the form of wire, sheet, etc. However, pure copper has 
relatively weak tensile strength. One promising approach to improving the 
strength of copper is alloying it with a metal which forms a dendritic 
phase in the copper matrix. Such multi-phase copper alloy mixtures have 
been referred to as "in-situ" composites. The alloying metal is present as 
an array of dendrites. 
It has been demonstrated that quite high strength copper-dendritic alloys 
can be produced by alloying copper with elements such as niobium, 
vanadium, or iron. See Bevk et al. (1978); and Bevk, et al. (1982). High 
strength sheets or wires may be fabricated by a casting and mechanical 
reduction process. The casting is first produced as a microstructure of X 
dendrites in a Cu matrix, and the alloy is then mechanically reduced by 
either rolling or drawing operations. This kind of mechanically worked 
copper composite alloy is described by Downing, et al. (1987), and 
Verhoeven, et al. U.S. Pat. No. 4,378,330. 
Cu-X dendrite type alloys are quite ductile and may be mechanically reduced 
to very large drawing strains without breakage. Mechanical reduction, such 
as by drawing, extrusion, or rolling, converts the X dendrites into 
elongated filaments, which serve to reinforce and greatly increase the 
strength of the formed wire, sheet, or other configuration. 
In the development of this copper-dendrite technology for practical use, a 
problem has arisen which remains to be resolved. As the reduction in area 
ratio, A.sub.o /A (where A.sub.o =original area and A =final area) is 
increased the strength of the alloy is observed to increase. However, wire 
diameters for the highest strengths are extremely small, such as 25 mm 
(0.001 inch). 
Reduced strengths with larger size ingots for a given A.sub.o /A value 
result because the dendrite size in the larger ingot is increased. For 
example, the dendrite size in the 15 gm ingots of Bevk (1982) was about 2 
.mu.m compared to 7 .mu.m in the larger ingot. Ultimate tensile strengths 
correlate approximately with S.sup.-0.5, where S is the spacing of the X 
filaments produced from the X dendrites in the casting. Consequently, for 
a given composition of the X component, the dendrite spacing will 
inherently increase as the casting size increases because of the reduced 
solidification rates required with the lower surface to volume ratio of 
larger sized ingots. For scale up to larger sized ingots, therefore, the 
ingots need to have larger A.sub.o /A values to achieve comparable 
strengths to the smaller ingots. Heretofore, however, no method has been 
known for overcoming this limitation. 
SUMMARY OF INVENTION 
The present invention comprises a new method of preparing copper-dendritic 
composite metal alloys for mechanical reduction, and thereby to increase 
tensile strength. The method has particular application to preparing high 
tensile strength copper wire, but can also be used for preparing copper 
sheet or other copper forms. 
The method is carried out by dispersing molten droplets of the composite 
alloy into an inert gas such as argon. The dispersed droplets are 
solidified to particles such as spheres or platelets. The solidified 
particles have sizes corresponding to the droplet sizes. If the size of 
the droplets or platelets as produced is not sufficiently uniform, the 
particles may be sorted by size. The spheres or platelets as produced, or 
as size-selected, are compacted to form integrated bodies ready for 
mechanical reduction by drawing or rolling. 
Because the droplets produced by the gas dispersion step can be frozen at a 
rapid rate, (viz. by solidification while gas-borne or by impingement on a 
cool surface), the resulting spheres or platelets will contain ultra-fine 
dendrites. By consolidating the spheres or platelets, larger diameter 
bodies can be formed with finer dendrite structures. High tensile 
strengths are therefore obtainable by mechanical reduction at lower 
A.sub.o /A values. The result is that larger size wire or sheet can be 
produced while obtaining maximized strengths.

DETAILED DESCRIPTION 
The method of this invention may be practiced with any copper-dendritic 
composite alloy. Such alloys when initially formed and cast are composed 
of a copper matrix in which there is a dispersed, solid solution phase of 
dendrites of the alloying metal. Metals that are particularly suitable for 
forming such composite alloys include vanadium, niobium, tantalum, 
chromium, molybdenum, tungsten, iron, and cobalt. Such alloys may be 
formed by conventional melting, fusing and casting procedures. Verhoeven 
et al. U.S. Pat. No. 4,378,330 describes a Cu-Nb alloy which is 
representative of this class of in-situ or composite alloys. One or more 
of the above-listed dendrite forming metals can be substituted for the 
niobium as described in the cited patent. 
As an alternative to conventional melting or casting, the alloy may be 
formed by a consumable arc melting method, as described in Verhoeven et 
al. U.S. Pat. No. 4,481,030. In that process, a consumable electrode is 
prepared which has a copper matrix with a plurality of the 
dendrite-forming "X" metal strips embedded therein. The electrode is 
subjected to direct current arc melting in an enclosed chamber containing 
an inert gas (e.g., argon). Reduced gas pressures, such as about 2/3 
atmosphere, can be employed for most of the dendritic metals. However, 
more refractory high melting point metals, superatmospheric pressure may 
be used as described in Verhoeven, et al. (1986). The elevated pressure 
process is advantageous for forming alloys of copper with molybdenum 
and/or tungsten. The inert gas pressure around the electrode should be 
sufficient to suppress boiling of liquid copper at the liquidus 
temperature of the alloy being produced. 
In practicing the present invention, as indicated by the flow sheet of FIG. 
1, after the Cu-X alloy has been prepared it is melted and formed into 
fine droplets. This operation is carried out within an enclosed chamber 
containing an inert gas atmosphere (viz. argon or helium). The molten 
droplets are dispersed into the inert gas. Droplets are rapidly solidified 
either while gas-borne or by impingement on a cooled surface. When the 
droplets are solidified while gas-borne they will have a generally 
spherical shape. If solidified by surface impingement, the solidified 
particles will have a flattened, wafer-like shape, referred to herein as 
"platelets". Particle sizes may be controlled by selecting the method and 
conditions of dispersement. For example, gas jet atomization or electrode 
sputtering may be used to produce the dispersed droplets. It is preferred 
to form droplets having average sizes in the range from about 50 .mu.m to 
2000 .mu.m. Alternatively or additionally, the solidified particles 
(spheres or platelets) can be subjected to size sorting and oversize or 
undersize particles can be eliminated. 
In the next step of the process, a mass, comprising a loose body, of the 
spheres or platelets is compacted into an integrated body. For example, 
size-selected particles may be formed into a cylindrical shape. In this 
step, bonding is obtained between the copper surfaces of the particles. 
Integration at a low temperature is preferred to avoid possible coarsening 
of the dendrite phase. Generally suitable compaction processes include 
packing the particles into a cylinder form by pressing in a die and/or 
cold isostatic pressing. A suitable procedure is described by Foner 
(1982), the particles being introduced into a cylindrical copper container 
and subjected to compaction by extrusion of the container. 
The compacted bodies may be in the form of billets ready for mechanical 
reduction processing. Such billets can be processed by any mechanical size 
reduction process, including rotary forging, rod rolling, swaging, or 
drawing. Such processing is carried out as previously described. (See, for 
example, Verhoeven, et al. U.S. Pat. No. 4,378,330.) 
FIG. 2 illustrates an atomization apparatus that may be used in practicing 
this invention in one preferred embodiment. In this method the Cu-X alloy 
is melted in a crucible, and the melt is dispersed by inert gas jet 
atomization. For a description of similar metal atomization processes, 
reference may be had to the Metals Handbook, Vol. 7, "Powder Metallurgy", 
9th ed. (1984), pages 25-39. 
As indicated in FIG. 2, the Cu-X alloy in the crucible is melted by an 
induction heating coil, and is discharged through a pour sprout by lifting 
a flow-out plug. Surrounding the outlet passage are gas jet inlets 
provided in a nozzle plate. The inlets are connected to a source of inert 
gas under pressure. The gas is preferably argon. The atomized liquid 
droplets thus formed are cooled and solidified as they fall through the 
inert gas atmosphere and form spherical particles, which are collected in 
a suitable chamber. 
The apparatus of FIG. 2 is particularly suitable for use with relatively 
non-reactive metals, viz. chromium, iron, cobalt, etc. More reactive 
metals may possibly become contaminated by reaction with the crucible 
material. However, the crucible can be formed of materials which are 
non-reactive or non-contaminating. 
An alternative apparatus is shown in FIG. 3. In this procedure, melting in 
a crucible is not required, thereby avoiding crucible contamination of 
reactive metals. The composite alloy is formed into an electrode, which is 
melted by an electric arc. Droplets are formed by sputtering from the 
melting electrode tip. Instead of solidifying the droplets in the 
surrounding atmosphere of inert gas, such as argon or helium, the droplets 
while still molten may be impinged on a cooled surface. For example, as 
indicated in FIG. 3, a vibrating water-cooled copper plate can be employed 
for this purpose. The solidified platelets thus formed fall into a 
collection chamber. 
The apparatus of FIG. 3 may also be employed for depositing thin coatings 
of the Cu-X alloy on copper plates or copper cylinders, which can then be 
subjected to mechanical reduction. Procedures for carrying out this 
alternative process are described below in Examples IX and X. 
It will be understood that prior to the operation of the apparatus of FIG. 
2 or FIG. 3, the enclosing chambers are evacuated, and then filled with 
the inert gas. 
While the diameters of the spheres produced by the apparatus of FIG. 2 or 
the diameters of the platelets produced by the apparatus of FIG. 3 can 
vary, it is preferred to produce the spheres in size ranges of diameters 
from 50 .mu.m to 2000 .mu.m, and the platelet thicknesses in the range of 
100 to 2000 .mu.m. 
The method of this invention is further illustrated by the following 
specific examples. 
EXAMPLE I 
Cu-Nb spheres are prepared by an atomization process similar to the 
illustration of FIG. 2. Mixtures of Cu and Nb metals having % Nb in a 
preferred range of 10 to 20 wt % are placed in the crucible. The crucible 
material can be ThO.sub.2 or ZrO.sub.2 stabilized with Y.sub.2 O.sub.3 or 
Y.sub.2 O.sub.3 or Mo or W, where the preferred crucible materials are 
ZrO.sub.2 or Y.sub.2 O.sub.3. The large enclosure chamber is vacuum purged 
and filling to a pressure of 0.7 atm of inert gas is preferred. The 
induction coil is turned on and the metals melted and mixed by convection 
currents from the induction current plus natural convection. The flow out 
plug is lifted a controlled amount and the molten Cu-Nb alloy flows out 
the pour spout. At the same time Argon gas is introduced into the nozzle 
plate, causing the molten Cu-Nb stream to fly outward in a spray of fine 
molten droplets. The droplets solidify "in flight" and are deposited as 
solid spheres in the collection chamber, comprising a fine powder. The 
powders can be sized by passing through sieves. Sized-fractions giving the 
smallest Nb dendrites are compacted to cylindrical shapes by pressing in 
dies and/or by cold isostatic pressing. The resulting cylinders can be 
either extruded and drawn, swaged, or rolled to final form, or hot 
isostatically pressed followed by reduction to final size by any of 
extrusion, rolling, forging, swaging, or drawing. The droplet size of the 
atomized liquid is controlled by: gas velocity out of the nozzle, the 
angle .alpha., (indicated in FIG. 2), the diameter d (also indicated), and 
temperature of the molten bath. The preferred droplet size and 
corresponding particle sizes are in the range 50 to 2000 .mu.m. The 
smallest obtainable dendrite size is related to selection of droplet size. 
Too small droplets (e.g., below 50 .mu.m) may not produce Nb dendrites. 
Too large droplets (e.g., above 2000 .mu.m) tend to produce too large 
dendrites. 
EXAMPLE II 
Following the procedure of Example I, Nb is replaced by Ta or V. The 
temperature of the molten alloy should be about 100.degree. C. hotter 
(viz. 1800.degree. C. instead of 1700.degree. C.) for Ta, and about 
100.degree. cooler for V. The crucible may be formed of Y.sub.2 O.sub.3. 
EXAMPLE III 
Following the procedure of Example I, Nb is replaced with Fe, Cr, or Co. 
The temperature of the molten alloys is lower, around 1600.degree. C. The 
crucible material may be Al.sub.2 O.sub.3. 
EXAMPLE IV 
Following the procedure of Example I, Nb is replaced with a mixture of 
Fe+Cr, or Fe+Co, or Co+Cr. The preferred range is 10 to 30 weight percent 
of Cr or Co in Fe. The molten alloy temperature is lower, around 
1600.degree. C. The crucible may be formed of Al.sub.2 O.sub.3. 
EXAMPLE V 
Following the procedure of Example I, a prealloyed Cu-Nb alloy is charged 
into the crucible rather than the individual Cu and Nb metals. Alloying 
occurs before discharge and droplet dispersion. 
EXAMPLE VI 
A prealloyed Cu-Nb rod is placed in the chamber of FIG. 3 and the chamber 
is vacuum purged and backfilled with 0.7 atm of inert gas (Ar preferred). 
An arc is struck across the tungsten electrode and the ingot bottom, 
thereby slowly melting the Cu-Nb rod. Drops of molten Cu-Nb sputter from 
the rod tip and fall downwardly. The rod is lowered as it is consumed. The 
drops fall onto a vibrating, water-cooled copper plate, which causes rapid 
solidification, forming platelets. The vibration of the Cu plate moves the 
platelets down its slope and they are collected in a chamber. The 
platelets can be compacted into a cylindrical form by pressing in a die 
and/or cold isostatic pressing. These cylinders are then mechanically 
reduced as described in Example 1. Drop size can be controlled by the size 
of the W electrode and the Cu-Nb electrode, the voltage, and the current 
in the arc. These parameters are adjusted to produce drops giving a 
minimum mean dendrite size, preferred drop sizes are in the range of 0.1 
to 1.5 mm. Dendrite sizes of the order of 0.2 .mu.m are achievable. 
EXAMPLE VII 
Following the procedure of Example VI, the tungsten electrode is replaced 
by a Cu-Nb composite electrode. In this case, both electrodes can be 
identified as Cu-Nb alloy cylinders. The electrodes may be arranged 
perpendicularly, or placed horizontally. This modification eliminates the 
possibility of tungsten contamination, but usually some tungsten 
contamination is not harmful to the process. 
EXAMPLE VIII 
Following the procedures of Examples VI and VII, Nb is replaced by any one 
or a combination of V, Ta, Cr, Mo, W, Fe, or Co. 
EXAMPLE IX 
The procedure is identical to Example I except the collection chamber is 
modified as follows. Above the collection chamber shown in FIG. 2, a thin 
plate of water cooled Cu-X is placed below the diverging spray of liquid 
droplets, that is, close to the point of divergence of the spray. The 
molten dropets are thereby caused to solidify either directly on the thin 
plate, or to solidify partially in flight and finish solidification on the 
plate. Platelets can be formed rather than spherical particles. 
Alternatively, the cooled plate can be moved in a rectangular pattern so 
as to become uniformly coated with the solidified droplets. Following 
deposition of the Cu-Nb alloy the plate is removed, and reduced to sheet 
form by either hot or cold extrusion. 
EXAMPLE X 
The procedure is the same as Example IX, except the plate is replaced with 
a small diameter water cooled rod of Cu-X alloy. As deposition occurs the 
rod is rotated and translated thereby building up its diameter with the 
solidified droplets of Cu-X alloy. This cylinder is removed and reduced to 
wire by any of the mechanical reduction processes, extrusion, forging, rod 
rolling, swaging, or drawing. 
While the foregoing examples have illustrated certain proportions of the 
dendritic metal to copper, such proportions may vary over a wide range, 
including from as little as one part by volume of dendritic metal to 99 
parts by volume of copper up to as much as 50 parts dendritic metal to 50 
parts by volume copper. In general, the range of dendritic metal in the 
alloy will be from about 5 to 39 volume percent. 
REFERENCES 
Bevk, et al. (1982), "In Situ Composites IV", Ed. Lempket et al., Elsevier 
Sci. Publ. Co., pages 121-133. 
Downing, et al. (1987), J. Appl. Phys. 61:2621-2625. 
Foner (1982), Prog. Powder Met. 38:107-114. 
Bevk, et al. (1978), J. Appl. Phys., 49:6031-6038. 
Verhoeven et al. (1986), J. Metals, Sept. Issue, pp. 20-24. 
Verhoeven, et al., U.S. Pat. No. 4,378,330. 
Verhoeven, et al., U.S. Pat. No. 4,481,030.