Fibered Metal Powders

Abstract

Refractory metal powder compacts are sintered and impregnated with a softer metal. The compacts are reduced to rod, wire or sheet. In the process fine fibers of the metal powder are formed.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 807,129, filed Mar. 13, 1969, which is a continuation of application Ser. No. 626,773, filed Mar. 29, 1967, now abandoned.

Other related copending applications are Ser. No. 59,555 filed July 30, 1970, Ser. No. 869,404 filed Mar. 13, 1969, as a division of 626,773, Ser. No. 839024 filed July 3, 1969 as a division and continuation-in-part of Ser. No. 626,773 and 807,129 and 869,404, and Ser. No. 196,812 filed 8 Nov. 1971 as a division of said Ser. No. 839,024.

Description

The present invention relates to composites reinforced by metal fibers particularly the class of hard metals having high strength and high temperature use capability (having at least 50 percent room temperature strength at 500.degree.C and is distinctly advantageous as applied to refractory metals and of extraordinarily small diameter which may be on the order of a micron or less, while having continuous length of several times diameter and as high as ten inches.

BACKGROUND

Metal felts and fine metal wires or fibers or filaments used in such felts 8re known in the art as indicated in U.S. Pat. Nos. 2,903,787 and 3,178,280. These felts are made from standard cold reduced metal wires which are limited to minimum diameters on the order of 0.001 - 0.10 inches or less by the inherent vulnerabilities of standard drawing processes or from shavings from metal blocks which are characterized by many surface defects. Much finer wires can be made by extrusion as indicated in U.S. Pat., No. 3,199,331 to Allen. But production by this process is substantially limited as a practical matter to low melting metals and alloys (e.g., tin). Other prior art of interest is Buehler, U.S. Pat. No. 3,124,455 and the Speidel, Levi and Wulff work cited below.

The present invention involves as a principa object the production of metal fibers of sub-micron size by a new process which is capable of being used with high temperature metals such as tantalum.

It is a further object of the invention to provide an economical method of making metal fibers on the order of 10 microns or less, and preferably sub-micron, in diameter with a single series of processing steps; i.e., free of the expensive supplementary or recycling processing involved, for instance, inSpeidel, U.S. Pat. 3,256,118, Levi, U.S. Pat. No. 3,029,496 and Wulff, January 1966 Journal of Applied Physics, p. 5.

It is a further object of the invention to provide work hardened fibers by a production process free of the need for intermediate anneals as required in the above patents of Allen and Levy, and for use in composites providing a high degree of work hardening in final product form, with or without a final low anneal for stress relief of the matrix only.

Other objects, features and advantages of the present invention will in part be obvious and will in part appear hereinafter.

DESCRIPTION

The invention is now described with respect to typical specific embodiments thereof and with reference to the accompanying drawings wherein:

FIG. 1 is a block diagram of the process of the invention.

FIG. 2 is a copy of a photomicrograph of a composite according to the invention.

FIG. 3 is a copy of a photomicropraph of a metal felt according to the invention.

FIGS. 4-10 are copies of photomicrographs of a composite according to the invention.

The fibers of the invention are made and used by the following process described with reference to FIG. 1 which is a block diagram of the process. First, powders of the metal to be fibered are obtained. The metal may be any of the refractory metals (as elements, alloys or compounds) including tantalum, niobium, molybdenum, tungsten, chromium, beryllium, magnesium oxide, titanium hydride and fabricable aluminides and silicides. The invention would also be of particular utility and distinctly advantageous benefit in fibering other hard metal elements, compounds or alloys which have softening temperatures in excess of about 1,000.degree.C. The starting powder size is variable dependinG upon subsequent processing and reactivity of the powders. The invention has been practiced successfully for instance with tantalum powders as large as minus 100 mesh and as small as a few microns diameter. The powder is consolidated into a compact by pressing and sintering or sintering ina mold. Then a melt of a second metal is provided in vacuum or inert atmosphere and the powder compact of the first metal is impregnated by eipping in the melt. During both the sintering and impregnating steps the compact is degassed and purified to enhance its wettability and ductility.

The second metal may be any of aluminum, copper, nickel, Woods metal, tin, indium, mercury, or any other metal which meets the following criteria with respect to the first metal under the conditions of impregnation:

1. readily wet the skeleton structure of the sintered compact of the first metal.

2. not alloy extensively with the first metal.

3. have similar hardness and fabrication characteristics to the extent necessary for co-working.

4. be easily removable from the compact by chemical or thermal means.

The impregnated compact is then worked down to an this process the adjacent particles of hard metal in the compact begin to form long fibers within the matrix of the second metal.

At this point, the rod or cylinder or plate may be used or fabricated into a useful product in any of tht following ways:

A-1. Removing the matrix metal and

a. using directly as a filter or with further fabrication as a capacitor

b. separating out individual fibers

c. re-impregnating the fibered article

A-2. Using the rod directly as a composite structural element

B- Rolling the rod to sheet prior to (1) or (2) above

C- Drawing the rod to wire prior to (1) or (2) above

D- Heating the rod for diffusion reaction between the hard metal fibers and the matrix prior to (1) or (2) above.

Several permutations of the foregoing can be made. For instance a rod can be drawn for several passes before rolling. A wire or sheet can be heated for diffusion reaction. Similarly a re-impregnated article can be used as a composite, with or without a diffusion reaction, or re-leached. With diffusion reactions, fibers of alloys or compounds can be formed even though such alloys are too brittle to be fibered directly. Another alternative in the scope of the invention is to form a loose fiber bundle or separate fiber (a or b above) and expose it to an oxidizing or nitriding atmosphere. In this way fibers of aluminum oxide or aluminum nitride can be made for use in reinforced composite structures. Also fibers of tantalum or niobium nitride can be made for use as superconductors. In these applications it is of special interest that the fiber diameters are so small as to favor the formation of the above compounds in single crystal form which is especially desirable.

The fibers of the invention are characterized in that each fiber is derived from a single powder particle and its length is dependent on the degree of diameter reduction. For instance, an 8 micron diameter powder particle fibered to 0.1 microns diameter will have a length of about 1 inch, a 30 micron diameter particle fibered to 0.1 microns diameter will have a length of about 70 inches. Further cold working to finer fiber diameters would increase the length. In most applications of the invention, useful fibers will have a length of 10 times the diameter of the fiber or longer (as high as 10.sup.6 times for extreme cases).

The felts of the invention are characterized by substantial cross-linking by metallurgical bonds between tangentially contacting fibers corresponding in part to the bonds between powders in the original powder compact skeleton and corresponding in part to new bonds formed during cold working the impregnated compact down to an elongated article, the new bonds being essentially an extension or stretching out of the old bonds.

FIG. 2 shows longitudinal section photomicrograph of a composite in the form of a wire of 0.039 inch diameter at 133 times magnification. The composite has elongated reinforcing tantalum fibers in a matrix of copper. The starting material for the fibered metal was coarse melting grade powder minus 12 and plus 60 mesh pressed at 18,000 psi and sintered at 2,300.degree.C for one hour to produce a compact of 61 percent density.

FIG. 3 shows a longitudinal section photomicrograph of a tantalum metal felt, encapsulated in a molding resin for microscope examination, at 266 times magnification. The tantalum was made from nominal 8 micron diameter powders (minus 100 mesh and plus 5 microns) which was consolidated to a compact of about 50 percent density and then impregnated with copper and then swaged to rod and rolled to sheet after which the copper was leached out in a nitric acid bath. Upon leaching the metal felt ballooned up to several times its original volume.

Fibers obtained from rod or wire are found to be essentially circular in cross-section and fibers obtained from sheet are found to be rectangular in cross-section. The term "diameter" as used herein refers to diameter of a circle or width of a rectangle.

The practice of the invention is further illustrated by the following non-limiting Examples.

EXAMPLE 1

A mold was filled with tantalum powder of about 8 micron nominal diameter (-100 mesh and plus 5 microns) and the powder was sintered in the mold at 1,500.degree.C for 20 minutes to form a green compact. Then sintering was completed by removing the compact from the mold and heating at 2,300.degree.C for 1 hour to complete consolidation of the powder. The density of the compact was 8.22 gms/cc or 49.5 percent of theoretical density. The compact was vacuum impregnated with copper by dipping in a molten copper bath at 1,170.degree.C for 5 minutes under a vacuum of about 10.sup.-.sup.4 torr. The impregnated compact (0.35 inches diameter by 4 inches long) was enclosed in an iron pipe and then swaged to 0.125 inches diameter. The jacket was removed and the rod was then further swaged to 0.080 inches diameter. After swaging the rod was then leached in nitric acid to remove the copper. The leached compact left a bundle of interwoven tantalum fibers in the form of a felt.

This metal felt was rinsed and removed from the leach bath. The felt was anodized and formed into a capacitor anode and tested for capacitor properties in a wet electrolyte. The formation voltage was 200 volts and the capacitance was 30.6 microfarads and on a specific weight basis 6,120 microfarad -- volts per gram. The felt had a dissipation factor of 32.19 percent making it an over-all operable capacitor anode.

EXAMPLE 2

Tantalum felts were made as in Example 1 but with the difference that the compact was rolled to 0.010 inch thich sheet before leaching. The felt exhibited a vigorous swelling up with a volume incrtase and density decrease of 5-10 increase during leaching and floated on the leaching bath. The capacitor formed from the felt at 150 volts had 7,965 microfarad -- volts per gram specific capacitance.

EXAMPLE 3

Felts were made as in Examples 1 and 2 with the difference that consolidation of the tantalum powder was accomplished by pressing at 18,000 psi and then sintering at 2,250.degree.C for one hour and that some rods were drawn to wire. Densities of 60 - 80 percent of theoretical were obtained in the original compact. Upon leaching the final composite article of this type, the felt did not swell up. However, high values of capacitance were still obtained indicating substantial formation of new surface as in Examples 1 and 2 (surface enhancement of about 2.5 times).

EXAMPLE 4

Several fibers from the felts of Examples 1 and 2 were encapsulated in epoxy resin and measured to yield an individual fiber diameter indication of 0.0002 cm. diameter. The Example 2 fibers were 5 to 10 times as long as the diameter of the fiber; the Example 1 fibers were continuous over much longer length.

EXAMPLE 5

Several compacts made essentially as in Examples 1 and 2 were rolled or drawn to the final sizes indicated below for testing of their composite material properties. These tantalum reinforced copper composites were in the form of 0.020 inch diameter wire and as 0.010 inch thick sheet, both as worked and after being heated (350.degree.C for 1 hour to anneal the copper). The results for these specimens and for comparison, the properties of tantalum and copper, per se, are given in Table 1:

TABLE 1

Example 5 Sample Ultimate Tensile Strength a. 0.01-0.020 inch diameter wire as worked 160,000-195,000 psi b. wire with stress relief 150,000-172,000 c. sheet, as worked 99,000-127,000 d. sheet, stress relieved 93,000 e. Pure tantalum, as worked (0.005 and 0.015 inch thick sheet) 104,000-116,000 f. Pure copper, as worked (0.005 and 0.015 inch thick sheet) 59,000-60,500

EXAMPLE 6

A molybdenum -- copper composite was made and tested in the same manner as the tantalum -- copper composites of Example 5 and formed into 0.06 and 0.08 in wire which displayed ultimate tensile strengths of 81,700 and 108,000 psi, respectively.

EXAMPLE 7

Tantalum felts made as in Examples 4 and 2 were tested for tensile strength after leaching out the copper. The results are in Table 2.

TABLE 2

Example Sample Ultimate Tensile Strength a. 0.01 in sheet 114,700 psi b. 0.04 in wire 90,000 psi EXAMPLE 8 Iron powder of -- 270 mesh was mold sintered at 800.degree.C for 20 minutes and then finally sintered at 1,150.degree.C for 1 hour to a density of 3.45 grams per cc (45 percent theoretical) impregnated as above and worked to 0.025 inch wire and leached to form a fibrous bundle of iron fibers 0.0015 cm diameter, quite continuous and having a surface layer of copper -- iron alloy overlaid by residual copper but with a substantial core of pure iron in the fibers.

EXAMPLE 9

Before leaching, the iron -- copper composite wire of Example 8 was tested for tensile strength and this was found to be 160,000 psi.

EXAMPLE 10

Leaching experiments were conducted and a solution of 5 parts ammonium hydroxide in one part hydrogen peroxide was found to be superior to nitric acid for selectively leaching copper from the iron to free the iron fibers from the composites.

EXAMPLE 11

A sample of -100/+325 mesh tantalum powder was placed inside of a sealed rubber tube and was isostatically pressed at 3,000 psi. The pressed powder bar (1 inch diameter rod) was then vacuum sintered at 2,200.degree.C for 2 hours to give a theoretical density of 45 percent. The sintered porous powder bar was infiltrated with superheated copper to give a composite of 1 inch diameter. The Ta-Cu composite was swaged to 0.2 inch in diameter and was sheared into 1/4 inch slugs. Copper was then leached out from the cut ends of the 1/4 inch slugs leaving an array of tantalum fiber ends. The leached cut ends were then examined microscopically.

FIG. 4 showed a low magnification (29X) photograph of the sheared end of a fiber tantalum slug FIGS. 5, 6 and 7 were magnified (700X) photomicrographs of regions A, B and C marked in FIG. 4 respectively. These were taken end-on from a viewing angle of 15.degree. with respect to the rod (slug) axis. FIGS. 8 and 9 were two different magnifications (670X and 2,650X, respectively; FIG. 9 being a blow-up of the circled area of FIG. 8 - note the corresponding die marks) of the side surface of the slug. FIG. 10 was an inside longitudinal section view at 530X of the slug located half a radius distance from center to periphery of the slug.

The slugs were cut from top to bottom of the FIG. 4 view and ductile failure is apparent at the bottom edge of the slug (region C). Fibers at or near the slug peripheries are of tobacco-leaf form reflecting the characteristic twisting of swaging operations. But fibers below the surface are filamentary in appearance and are believed to be round in cross-section at the center of the slugs (note FIGS. 8 and 9 for surface and sub-surface fibers). All figures (except low magnification FIG. 4) show the essentially straight axial orientation of the fibers. Interconnections can also be seen in the side views.

The best mode of using the invention is believed to be selection of a tantalum - copper pair to produce a tantalum felt suitable for use as a capacitor anode. In addition to the above indicated advantages of ease of processing, surface enhancement and work hardening it is a further useful advantage of the invention that it may be practiced if desired, with relatively coarse melting grade tantalum powder in the original compact rather than the conventional fine grain capacitor grade powder and the desired surface area increase can be obtained in the fibering p7ocess rather than in the original processing of the pooder. A further useful aspect of the invention is the above described feature of swelling when the original compact is made in low density (40 - 60 percent theoretical) and/or when a high degree of working is put into the composite. The swelling of the metal felt, when utilized makes it easier to refill the felt with an anodizing medium and electrolyte.

The extension to other species of the above advantages and variations in processing and still other advantages and variations will be obvious to those skilled in the art from the description herein. For instance, a niobium - tin pair could be utilized to 4btain interconnected niobium fibers in a tin matrix with a better degree of interconnection between fibers than is obtainable in the process of the above described Speidel patent. Then the composite could be heated for diffusion reaction to form a niobium stannide superconductor subsequent to which residual tin would be leached out and replaced with copper by re-impregnation to provide a higher conductivity matrix for electrical stability of the superconductor.

A high degree of control of the final product is obtainable. For instance, use of coarse melt grade powders or low density consolidation of the original compact (40 - 60 percent) tend to limit the number of cross-link bonds formed between fibers thereby enhancing the swelling up of fibers upon leaching the matrix metal and enhancing the ease of separation of fibers.

For superconductor applications it is particularly desirable to use a fine grain powder and form the original compact to a higher density for forming maximum cross-links between fibers. Still other applications within the scope of the present invention will be apparent to those skilled in the art when aided by the foregoing description. The description is therefore intended to be read as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. An elongated composite material comprising a fibrous reinforcing component of a first metal in a matrix of a second metal, said first metal being a refractory metal in elongated fiber form with an average length of at least 0.025 centimeters and said reinforcing component comprising an elongated felt bundle of said fibers axially oriented in the direction of the long dimension of the fibers, the fibers being interconnected to each other be metallurgically bonded cross-links spaced along the fiber lengths, said second metal comprising a metal which is compatible with said first metal.

2. The composite material of claim 1 wherein said fibers were cold-work strengthened to the point that the elongated composite has a tensile strength at least equal to an equivalent volume and cross-section of an elongated member of the first metal alone.

3. The composite material of claim 1 wherein the fibers have an average diameter of less than 0.00025 centimeters.

4. The composite material of claim 1 as produced by forming a powder metallurgy compact of the first metal with sintered bonds between powders and interconnected open space within the compact, inpregnating a compatible matrix metal in molten form into the open space to form a matrix for the first metal cooling the molten phase to solid, mechanically working the impregnated compact to an elongated form and fibering the powders while so doing.

5. The composite material of claim 4 wherein said second metal is the originally impregnated matrix metal.

6. The composite material of claim 4 as produced by removing the originally impregnated matrix metal after elongation and substituting said second metal therefor through impregnation of the fiber bundle with said second metal.

7. The product of claim 1 wherein the fibers comprise niobium stannide and the matrix comprises copper.

8. The product of claim 1 wherein the fibers comprise tantalum and the matrix comprises copper.

9. The product of claim 1 wherein the fibers comprise molybdenum and the matrix comprises copper.

10. The product of claim 1 wherein the fibers comprise tungsten and the matrix comprises copper.

11. The product of claim 1 wherein the felt has a swollen arrangement to exhibit a density of less than half of the theoretical density of the metal.

12. The product of claim 1 wherein the felt has a compact arrangement with a density over half of the theoretical density of the metal.