US3728168A - Process of making a titanium carbide sheathed titanium filament - Google Patents

Abstract

TITAANIUM FILAMENTS ARE CARBURIZED TO PRODUCE A THICK TITANIUM CARBIDE SHEATH AND A CENTRAL CORE OF TITANIUM METAL WITH A DISPERSED STRENGTTHENINGG PHASE.. THE RRESULTANT PRODUCT IS INCORPORATED INTO A METAL MATRIX TO PROVIDE A COMPOSITE OF IMPROVED STRENGTH ANDD STIFFNESS.

Description

v Filed Oct. 2a, 1958 STRENGTH (s) lO psi April 1 7, 1973 L. R. ALLEN ET AL PROCESS OF MAKING A TITANIUM CARBIDE SHEATHED TITANIUM-'FILAMEN'T' 2 Sheets-Sheet 1 V 5o, I V/ '30 2O loo 500 i000 FIG. 3

EVLA'STICY MODULUS (9103s:

A ril 17, 1973 L. R. ALLEN ET'AL PROCESS OF MAKING A TITANIUM CARBIDE SHEATHED TITANIUM FILAMENT 2 Sheets-Sheet 2 I Filed Oct. 28 1968 FIG. .2

United States Patent US. Cl. 148-205 4 Claims ABSTRACT OF THE DISCLOSURE Titanium filaments are carburized to produce a thick titanium filaments are carburized to produce a thick titanium carbide sheath and 0 central core of titanium metal with a dispersed strengthening phase. The resultant product is incorporated into a metal matrix to provide a composite of improved strength and stiffness.

The present invention relates to filament reinforced metal matrix composites of higher stillness and filament reinforcements for use therein.

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ticity) of a matrix material but will be less reactive with the matrix than the above filamentary reinforcement materials. Titanium carbide, commonly used in powder metallurgy composites for cutting tools, would 'be a good candidate in theory in view of its known chemical inertness and known approximate high strength and stiffness.

The following Tables I and II illustrate on a conservative basis the improvement obtainable through titanium carbide reinforcement of metal matrix materials in comparison with high strength alloy variations of the same metals. The values given for the proposed composite properties--strength in Table I and stilfness (modulus of elasticity) in Table II-are based on a law of mixtures analysis, an assumption of a room temperature stillness of 50x10 psi. for titanium carbide in a form obtain able for use in a composite and known temperature of the reinforcing titanium carbide and matrix metals. The properties of the matrix metals and alloys are taken from handbook data. Densities are also tabulated in similar fashion. The strength and modulus of titanium carbide alone are used in calculating composite properties; the contribution of the matrix is ignored.

TABLE L-STRENGTH (10 P.S.I.) AT VARYING TEMPERATURES Material Density RT. 500 F. 1,000 F. 1,500 F. 2,000 F Aluminum composite (30% TiO) 121 100 85 80 Aluminum alloy (7075) 103 85 3 Titanium composite TiC) 172 140 125 100 85 Titanium alloy (Ti-GAMV) 21 180 94. 1 12. 5

Nickel composite (30% TiC) 280 17 166 150 135 Nickel alloy (Rene 41) 298 250 180 80 TABLE IL-STIFFNESS (10 P.S.I.) .AI VARYING TEMPERATURES Material RT. 500 F. 1,000 F. 1,500 F. 2,000 F.

Aluminum composite (30% T10) 20 17 Aluminum alloy (7075) 10. 7 7. 2

Titanium composite (30% T10) Titanium alloy (TLGAMV) Nickel composite (30% T10) Nickel alloy (Rene 41) NOTE .-R.T.= Room Temperature.

BACKGROUND The art of structural composite materials has developed several plastic-matrix and metal-matrix composite materials utilizing filamentary reinforcing materials of high strength and modulus of elasticity including boron, carbon and silicon carbide and silicon carbide coated carbon. This effort is related to prior developments in regard to glass fiber reinforced composites, dispersion strengthened alloys, filled plastics, ceramics and powder metallurgy. Considerable difficulty remains in effectively utilizing the high modulus reinforcing filamentary materials particularly at high temperatures which may be encountered in forming a composite or in service use (e.g. as turbine blades).

It is desirable to develop a reinforcement material which will increase the strength and stillness (modulus of elas- It does not appear that prior art approaches to utilization of titanium carbide have been successful. This is not surprising since titanium carbide is extremely brittle. Published reports of experiments conducted under sponsorship of the US. Air Force Materials Laboratory cite attempts to produce titanium carbide by decomposition of a carbon vapor source and diffusion of carbon into a titanium wire. These are Defense Documentation Center (DDC) publication AD 611757 (circa 1965) pp. 14-15 (Davies et a1. authors) and AFMLTR-67-39l (February 1968) pp. -91 (General Technologies Corporation, author). Additional background information on titanium carbide properties and preparation and potential use of such materials and other ceramics in composites is given in NASA Technical Translation F-102 (June 1962) pp. 74-93 (Samsonov et al., authors); Schwarzkopt, Refractory Hard Metals (MacMillan, New York, 1953); ASTIA (DDC) Document ADl31071 (1957) (Kieffer et al.); American Society for Testing and Materials, The Technical Potential for Metal Matrix Composites (1967) (Burte et al.); AFML-Tr-66-52 Engineering Properties of Ceramics (June 1966) (Lynch et al.); NASA SP- 5055 (August 1966) Non-Glassy Inorganic Fibers and Composites (Harman).

OBJECTS The general objects of the present invention are to provide an improvement in filament-reinforced composite materials, and methods of making the same, and particularly in reinforced metal matrix composites, which overcomes the prior art problems of high temperature oxidation and/ or reaction between reinforcement and matrix and which affords a high degree of increased composite strength.

Further and specific objects of the invention are to provide filamentary titanium carbide reinforcing materials, and a method of making the same, which provides increased composite stiffness and is reliably and economically manufactured consistent with the foregoing objects.

SUMMARY Generally, the objects of the invention are achieved by production of a carburized titanium filament which is used in a metal matrix at high temperatures. The term filament as used herein includes elongated wires, ribbons and sheets and equivalents and filament radius means wire radius or half-thickness of ribbon or the like. The term titanium includes the element titanium and higher strength alloys of titanium.

The filament product has a characteristic core and sheath in crosssection. The core comprises the characteristic feathery transformed beta structure of titanium normally obtained by heating the metal above the alpha beta transformation temperature (882 C.) and quenching with a dispersed additional phase therein including particles and plates of titanium carbide. Strengthening is obtained from both the titanium carbide dispersed phase and the matrix phase of transformed titanium structure. The sheath comprises the inert refractory material-titanium carbide. The wire product is made by decomposition of a carbon compound vapor, such as methane or acetylene or application of other source of carbon vapor, to the surface of the titanium filament and diffusion of the carbon into the wire. The reaction is controlled to prevent formation of a pyrolytic graphite coating, or hydrogen em'brittlement of the filament. The reaction is conducted with the rate of supply of carbon vapor to the reaction zone being limited so that it is not substantially greater than the rate of diffusion of carbon into the filament and the removal of hydrogen from the reaction zone to achieve this control. Removal of hydrogen and carbon diffusion is enhanced by the removal of titanium oxides during the reaction. Carbon containing gas is held to a pressure less than mm. Hg in the reaction zone and preferably less than 1 mm. The reaction temperature is about 1000 C. below which reaction rate is unduly slow in essentially all significant instances, and below the titanium-titanium carbide eutectic point which is at about 165 C. The range is limited to betwen 1200 C. and 1650 C. for larger filaments (e.g. in wire form with radius greater than .005 inch).

The finished product is incorporated into a metal matrix by conventional compositing techniques to produce a composite which offers substantial improvement in strength and stiffness particularly at high temperature. A matrix of plastic can be impregnated into a bundle of reinforcement in liquid form filaments and cured to solid. A metal matrix can be cast or extruded or hot pressed. One particularly desirable form of composite formation is to stack alternating layers of filaments and matrix metal foil. Within the filament layers, filaments of reinforcing material alternate with filaments of matrix metal (e.g. aluminum wires). The entire stacked assembly is hot pressed to form the composite. Other conventional compositing techniques now utilized with glass, boron, silicon carbide and carbon fibers and whisker reinforcements can also be utilized for making new composites containing titanium carbide. The new composites are characterized by a higher degree of protection from matrix-reinforcement reaction compared to the prior art composites.

Other objects, features and advantages of the invention will be apparent to those skilled in the art from the above general description and the following specific description which includes reference to the accompanying drawings wherein:

FIG. 1 is a schematic cross-section representation of the filament product.

FIG. 2 is a diagram of the carbonizing apparatus used in practicing the method of the invention.

FIG. 3 is a curve of strengthening and stiffening effects as a function of reaction time under a given set of pressure and temperature conditions.

FIG. 1 shows a cross-section of the carburized titanium filament 10. It comprises an outer sheath 12 of completely formed near-stoichiometric titanium carbide and an inner core 14 of titanium with dispersed needle-form precipitates 16 of titanium carbide and additional particle of plate-like precipitates 18 therein. The sheath has an average thickness of from 1 to 50% of wire radius, preferably 10 to 20%.

The impurity content of the filament as a result of the processing or selection of starting materials necessary to produce an effective reinforcing filament, is less than ppm. of hydrogen, less than 3000 ppm. of oxygen and nitrogen combined.

FIG. 2 shows an apparatus which was used for producing carbided titanium wires. The apparatus comprises a vacuum chamber 20 with internal electrical contacts 22, 24 supplied with electric power, via feedthroughs 26, 28 in the chamber wall, from an electric power source 21. A titanium wire 10 to be carbided is connected to the contacts for receiving an internal direct resistance heating current. An optical pyrometer and sight port (not shown) are used to measure wire temperature and power may be adjusted to reach and hold a desired temperature level. A weight indicated at 32 is used to hold the wire from upward distortion as it is heated.

For large scale production, the contacts 22, 24 may be rollers and a reel of titanium wire may be wound and rewound in the chamber using such rollers. A drive would be imparted to the wire and the drive tension would take the place of Weight 32 in holding the wire from buckling.

The vacuum chamber is evacuated by a vacuum pump (rotary mechanical pump) 34 via an intermediate liquid nitrogen cooled trap 36 and control valves 38, 40.

The evacuated chamber is backfilled with gasses obtained from metering inlet valves 50, 52, 54, the gasses being methane, argon and a halogen containing gas (such as carbon tetrachloride, trichloroethylene) respectively. The argon or methane gasses are alternately fed to the vacuum chamber via a valve 56 and when methane is fed, it is preferably combined with the halogen containing gas and the combination is fed to the vacuum system via valve 58 and the cold trap.

The feed of methane or argon is purified by passage through a filter 60 and a Dry Ice cooled trap 66. The filter comprises a quartz tube filled with active uranium chips. A cooler 64 is provided for the reactor ends and a heater 66 is provided for its central zone. The chips are heated to a temperature of between 750 and 850 C. in the central zone for reacting interstitial impurities in the feed gas with the uranium.

The vacuum chamber is evacuated to a pressure of less than 1 mm. Hg and backfilled with purified argon several times. Then the methane-halogen mixture is admitted to the chamber. The wire is then heated to a temperature of between 1200 C. and 1650 C. A 25 mil diameter wire can be brought up to temperatures in this range in 3-4 seconds and cooled below 100 C. in 3-4 seconds. Between heating and cooldown the wire is held at the selected temperature for a time which may be as low as seconds at the highest temperatures up to 1500 seconds at the lowest temperature to produce a carburized titanium wire as described above. The reaction conditions can also be varied by adjusting the partial pressures or proportions of the hydrocarbon and halogen gas and by adjusting total pressure through variation of the reactive gas amounts or by addition of inlet argon. Typically methane pressure is held at .2-.9 mm. Hg during the heating for reaction and is dropped to .005 mm. Hg just prior to completion of the heating-reaction step to remove excess hydrogen. Typical operating temperature for the heating reaction step is 1350 C. In order to monitor temperature properly, the optical pyrometer calibration must be adjusted to account for changing radiation characteristics as the wire surface changes from titanium to titanium carbide.

If the ultimate use of the wire is in a composite where strength is more important than stiffness the shorter times are utilized. In any case the time must be long enough to form the titanium carbide and short enough to retain the titanium core for ductility. If the ultimate use of the composite places the highest premium on stiffness then the longer times are preferred. The given times vary inversely with reaction zone temperature, total pressure in the chamber and partial pressure of the hydrocarbon gas.

Reproducibility of results and to some extent reaction time, are affected by the use of the halogen gas. @If the halogen gas is not used at all, or in insulficient quantity, the titanium wire has a tendency to form compounds on its surface with the oxygen impurity. This oxide and original surface oxide barriers which inhibit out-diffusion of oxygen and hydrogen. Retention of significant amounts of hydrogen during the reaction would produce embrittling hydrides and would interfere with formation of the desired titanium carbide. The oxide would inhibit the diffusion of carbon into the wire, the net result being a formation of a layer of pyrolytic graphite on the wire surface which would further inhibit gas diffusion.

The carbon tetrachloride gas should be present in a volumetric ratio to the methane of at least 121000 and preferably 1:150. A ratio of greater than 1:50 is undesirable. In any case the halogen per se should be between .1% and 1% by volume of total reactive gas. The same proportions apply to alternative halogen and hydrocarbon gasses.

The chloride reacts with surface titanium oxides to form volatile titanium chlorides which move from the reaction zone to cooler walls of the reactor. Other gaseous products may be carbon monoxide and water. A very ef fective oxygen stripping is obtained. The present process would appear to degrade the filament by removing oxygen content Which normally enhances strength. But the net result is an increase of strength.

The result of chlorine addition is that the oxide barrier is dissipated during the reaction and that a net drop in hydrogen content of the wire occurs during the reaction.

EXAMPLES (a) A series of 25 mil diameter wires was carburized utilizing the apparatus and procedures described above.

. The reaction temperature was 135 0 C. and the hydrocarbon gas was methane, with one percent (by volume) of carbon tetrachloride, which was at a pressure of .6 mm. Hg prior to and throughout the heating of the wires for diffusion reaction except just prior (1-2 seconds) to the termination of heating, when pressure was reduced to .005 mm. Hg. The heating times were varied from 50 to 1000 seconds. The resultant samples were tested for strength and elastic modulus. Cross section samples were cut and microscopically examined.

The curve of FIG. 3 shows the variation of strength and stiffness under these conditions. Considerable enhancement of the strength of the titanium filament was achieved in those runs which had shorter heating times with maximum enhancement at about seconds. This strengthening corresponded to the formation of thin sheaths of titanium carbide on the filaments which were within 10 to 20 percent of filament radius. Significant enhancement of modulus of elasticity was also obtained in the same samples as indicated by the FIG. 3 curve.

(b) A series of 4 mil diameter wires was carburized utilizing the apparatus and procedures described above. The reaction temperature was 1350 C. and the hydrocarbon gas was methane, with one percent (by volume) of carbon tetrachloride which was at a pressure of .6 mm. Hg prior to and throughout the heating of the wires for diffusion reaction except just prior (1 to 2 seconds) to the termination of heating, when pressure was reduced to .005 mm. Hg. The heating times were varied from 15 to 60 seconds. Several resultant samples were tested for strength and elastic modulus. Fixture problems prevented readings in most cases, but in some instances valid readings demonstrating enhanced strength and modulus were realized. One sample (which had a reaction time of 30 seconds) exhibited 250,000 p.s.i. strength but could not be tested at higher stresses because of going off-scale on the tensile testers load cell. The modulus of the sample was 46.6 10 p.s.i. Examination of a fracture cross-section of the sample showed that conversion to titanium carbide was almost complete.

Several variations can be made Within the scope of the present invention. Filament strength can be enhanced by selection of high strength titanium alloys, smooth drawn filament wires, or a polishing pre-treatment of the wire by mechanical chemical and/or electrical means prior to carburizing reaction. The polishing can be done just prior to the reaction, for instance, by passing a filament through a pre-treating chamber Where it is exposed to hot chlorine containing vapors prior to entry into the reaction chamber. vPost-treatments to further enhance the inertness of the carburized surface may be performed. These incude nitriding or siliciding the outer surface of the titanium carbide sheath of the filament or depositing a layer of pyrolytic graphite over the titanium carbide to act as a diffusion barrier to metal matrix materials and to provide a reservoir of carbon to compensate for minor inward diffusion of carbon which may occur in the finished product under extreme conditions of high temperature service.

The scale-up of the process of the above examples from batch treatment to continuous or semi-continuous treatment of filament should include a step equivalent to the pump down prior to cooling of the above examples to prevent re-entry of hydrogen. Such equivalent steps would include a continuous passage of the carbon containing gas over a moving titanium filament in a counterflow arrangement, passage of the wire from a reaction chamber to an evacuated or inert gas flushed chamber for cooling or coating the filament prior to cooldown with a protective layer. Once the filament is cooled down to 800 C. the danger of embrittlement is substantially passed.

Still further variations will be apparent to those skilled in the art once given the benefit of the present disclosure. Accordingly it is intended that the above specification and accompanying drawings shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. Method of making high strength, high elastic modulus titanium carbide sheathed, titanium filament comprising the step of reacting at high temperature a titanium filament with carbon-containing vapor to produce a titanium carbide sheath and a beta titanium core while evolving hydrogen from the titanium and clearing the hydrogen from the reaction zone prior to cooling and wherein the reaction is conducted in the presence of a subatmospheric pressure of a carbon-containing vapor and in the presence of halogen catalyst.

7 '2. The method of claim 1 wherein the filament is directly heated to a temperature above 1000 C. and below the titanium-titanium carbide eutectic temperature for a sufiiciently long time to produce a titanium carbide filament sheath which has a thickness of 10-20% of filament 5 radius.

3. The method of claim 2 wherein the pressure of car bun-containing gas is less than 10 mm. Hg.

4. The method of claim 3 wherein the volume fraction of halogen in the carbon-containing gas is between V1000 and /50.

References Cited UNITED STATES PATENTS 2,865,797 12/1958 McCaWley 148-20.3

OTHER REFERENCES Hanzel, R. W.: Surface Hardening Processes for Titanium and Its Alloys, Metal Progress, March 1954, pp. 89-96.

I-IYLAND BIZOT, Primary Examiner 10 G. K. WHITE, Assistant Examiner U.S. Cl. X.R.

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