US3853491A - Tungsten filament having uniform concentration gradient of thoria particles - Google Patents

Abstract

An improved tungsten filament is disclosed containing approximately 1-2 percent by weight thoria particles distributed in the tungsten matrix as a dispersed phase. The thoria particles are distributed in the tungsten matrix to provide a slight but uniform compositional gradient which diminishes continuously with increasing distance from the exterior surface of the filament to the longitudinal central axis. Upon recrystallization of the filament or wire product there is also formed a tungsten grain structure having a more uniform size and shape of the individual tungsten grains than can be obtained by conventional powder metallurgy preparation.

Description

aet 1 Dunham tates TUNGSTEN FILAMENT HAVING UNIFORM CONCENTRATION GRADIENT OF THORIA PARTICLES [75] Inventor: Thomas E, Dunham, Cleveland Heights, Ohio [73] Assignee: General Electric Company, New

York, NY.

[ Notice: The portion of the term of this patent subsequent to Mar. 19, 1991, has been disclaimed.

[22] Filed: Sept. 1, 1972 [21] Appl. No.: 285,939

[52] US. Cl 29/1825, 75/206, 75/207 [51] Int. Cl. C22c l/05, C220 19/00 [58] Field of Search 75/207, 206, 176;

[56] References Cited UNITED STATES PATENTS 12/1913 Coolidge 75/207 X 7/1957 lredell et a1 75/207 X 1*Dec. 10, 1974 Dempsey 75/207 X 3,105,290 10/1963 Sackinger 75/207 X 3,278,281 10/1966 Ehringer 75/207 X 3,475,159 10/1969 Hansen 75/207 X Primary Examiner-Leland A. Sebastian Assistant ExaminerR. E. Schafer Attorney, Agent, or Firm-John F. McDevitt; Henry P. Truesdell; Frank L. Neuhauser [S 7] ABSTRACT 6 Claims, 3 Drawing Figures PATENTEB SE81 0 I974 SHEET 10F 3 STHNDHPD 2.0% 7770 MQTEP/RL PRESENT /.8% W70 MQTER/HL PATENTEU DEC] 0 I974 SHEET 2 OF 3 STRNDHPD 2.0% 7' I70 MATERIAL PRESENT L8? 7770; METER/9L PATENTEL 51E] DIEM SHEET 3 BF 3 5 THNDRRD 1.0% 7/70; F/LflMEN T PRESENT 1.7% 7770 F/LIQMENT TUNGSTEN FILAMENT HAVING UNIFORM CONCENTRATION GRADIENT OF THORIA PARTICLES CROSS-REFERENCE TO RELATED APPLICATIONS Processes for the production of binary alloys of the present invention are described in more detail and are claimed in my copending application entitled METAL PRODUCTS AND PROCESS OF PREPARATION," Ser. No. 186,143, filed Oct. 4, 1971, now US. Pat. No. 3,741,734. Another copending application filed in my name which describes and claims related dispersion alloy products is entitled TUNGSTEN ALLOY PRODUCTS, Ser. No. 248,933 and was filed May 1, 1972. A further copending application entitled Vibration Resistant Lamp, Ser. No. 285,938 and filed Sept. 1, 1972, discloses and claims an incandescent lamp wherein the filament member utilizes the dispersion alloy material of the present invention. All of these copending applications are assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION Thoriated tungsten materials are commonly used as incandescent filaments in electric lamps and as an electron source in such items as power tubes, electric discharge lamps and welding electrodes. It has also been recognized for some time that a higher thoria level in the tungsten material will produce a higher recrystallization temperature and greater mechanical strength. It is also well recognized for thoriated tungsten materials made by conventional powder metallurgy preparation that fabrication of the material into a filament or wire product becomes more difficult with increasing thoria level. As a result of this latter difficulty in processing a dispersion alloy product wherein a powdered compact is prepared from a mixture of the solid starting materials or by Th(NO3)4' additions to the tungsten oxides there are two general types of the alloys being manufactured at this time. More particularly, a 2 percent by weight thoria containing alloy is employed for product applications not requiring a highly wrought or small size filament part whereas a 1 percent by weight thoria containing alloy is being manufactured for smaller diameter lamp and electronic tube filament applications.

Although there has been a long period of production for the two general type tungstenalloy materials above specified, there remains continuing problems. The manufacture of 2 percent thoria material to a finished size is often accompanied by material efficiencies below 50 percent. While the 1 percent thoria alloy is more fabricable, it can also be characterized by a nonuniform tungsten grain structure that occurs upon recrystallization wherein the individual tungsten crystals are unequal in size and develop with a pronounced unevenness in growth along the major wire axis.

A still further problem exists with both types of binary alloys above specified commencing with a dry blend of the starting materials or Th( No h addition to tungsten oxide which is attributable at least in part to non-uniform dispersion of the thoria additive resulting from the deformation and fracture of the larger ThO particles during manufacture. Since the thoria additive can obstruct grain growth of the individual tungsten crystals during the recrystallization which takes place 2 in a wire which has been produced by swaging and wire drawing of an ingot, the above-mentioned non-uniform Th0 dispersion resulting from the deformation and fracture of the larger Th0 particles will tend to cause larger elongated grains to form in some regions of the wire.

It would be desirable, therefore, to provide a tungsten filament containing more than 1 percent by weight thoria particles which can be fabricated readily down to small sizes with a minimum of difficulties.

It would also be useful to provide a tungsten filament containing more than 1 percent by weight thoria particles with a uniform and small grain size of tungsten crystals upon recrystallization.

SUMMARY OF THE INVENTION It has now been discovered that binary alloys of tungsten containing approximately 1-2 percent by weight thoria particles can be prepared in a particular manner hereinafter more fully described to provide a dispersion alloy product which following mechanical working to wire recrystallizes to smaller size and more equiaxed crystals of tungsten than can be obtained by powder metallurgy preparation. More particularly it has been discovered that upon incorporating the above specified thoria levels in a powdered compact of tungsten crystals utilizing a liquid diffusion technique described and claimed in my copending application Serial No. 186,143 there is obtained a more narrow size distribution of the thoria particles than can be obtained by preparing the compact starting with a dry blend of the alloying constituents. When such a dispersion product containing thoria particles less than about 4 microns in diameter is further processed in the conventional manner to obtain a rod, filament, or wire product, the spacial distribution of ThO particles remains more uniform than in the case of conventionally made materials. For example, as will be seen in FIG. 1, most of the large ThO particles in a 93 mil rod W-2 of w/o ThO made by conventional techniques are rod-like segments strung out along the rod axis. The number of this type of particle segment seen in the new material having approximately the same ThO level is markedly less as a result of the improved ThO distribution at the starting ingot stage of processing. The final tungsten wire produced in accordance with the present invention consists of a dispersion strengthened alloy having a relatively small and stable recrystallized tungsten grain size with a more uniform spacial distribution of ThO attributable to the absence of extremely large ThO particles in the precursor ingot. It is also further possible to distribute the thoria particles in a tungsten compact so that a uniform thoria concentration gradient exists with a maximum concentration being located at the exterior surface portion of the compact and with said thoria concentration continuously diminishing with increasing distance to the interior portion of said compact. By varying the thoria concentration in this manner, it can be further expected that a thoria concentration gradient can be produced in the final wire filament such that maximum thoria concentration will be located at the exterior surface portion of the filament and the thoria concentration will continuously diminish with increasing distance to the longitudinal central axis of the filament.

FIG. 1 are photographs taken at 2000 x magnification depicting comparative ThO distribution for 93 mil diameter rods fabricated from a binary alloy of the present invention compared with a conventional W-2 w/o ThO alloy.

FIG. 2 are photographs also taken at 2000 x magnification and depicting the Th distribution in a sintered compact prepared according to the present invention compared with the ThO distribution for a corresponding sintered compact prepared by a conventional powder metallurgy technique.

FIG. 3 are photographs depicting the recrystallized tungsten grain structures for conventional W-l w/o Th0: material compared with W-l .7 w/o ThO alloy of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the liquid diffusion process practiced in the present invention comprises soaking a porous compact of tungsten particles in an inert liquid solvent for a soluble thorium compound until the porous structure of the compact is essentially filled with the solvent and thereafter immersing the solvent-filled compact in a liquid solution of the thorium compound for a sufficient time period to permit liquid diffusion of the dissolved thorium compound into at least a portion of the solvent contained in the pores of the compact. The compact is then removed from the solution after a predetermined amount of liquid diffusion has taken place and all sol- I vent is removed to retain a bulk thoria concentration between approximately 1-2 percent by weight in the pores of the composite formed. By inert liquid solvent" is meant a solvent for the thorium compound which can be removed by drying or heating the liquidfilled compact without leaving a residue in the pore structure or undergoing any significant chemical reaction with the tungsten material. In modifications of the above-described liquid diffusion process, there can be established a concentration gradient of the thorium compound in the pores of the composite such that a slight but uniform decrease in the resultant thoria concentration can be established in the direction of the center.

A specific example will now be given to illustrate preparation of an improved tungsten filament according to the present invention. Accordingly, a pressed compact weighing approximately 3.0 kilograms and having a rectangular cross section of 17 mm X 22 mm with a density of 10.9 gms/cc was prepared from tungsten powder having an average particle size of 2.5 microns by conventional techniques. The compact was slowly soaked in distilled water until the interconnected pore structure of the compact was saturated and the liquid-filled ingot thereafter immersed in an aqueous Th(NO solution having a concentration of 520 grams Th0: per'liter of solution. The liquid diffusion process was interrupted after approximately 30 hours by removing the compact from the thorium nitrate solution. The compact was next dried slowly in a vacuum oven at approximately l30F which removed most of the solvent from the pores of the compact while retaining a slight concentration gradient of the thorium additive in the pores at a bulk ThO concentration of 1.9 weight percent. The thorium additive-containing compact was next subjected to a conventional presintering operation at approximately 1200C in hydrogen for two hours which converted the thorium nitrate in the pores to thorium oxide (thoria).

The presintered compact of the present example was then sintered in conventional fashion by passing an electrical current through the member to raise its temperature nearly to the melting point. The sintering treatment was provided by suspending the presintered compact vertically between electrodes and applying electric current in graduated steps to provide for an escape of any volatile impurities in the compact.

Test specimens were obtained from the sintered compact which had a rectangular cross section in order to measure the thoria concentration along with the distribution and size-range of the thoria particles. The test specimens were examined by known autoradiograph and extraction replica techniques. In one cross-section specimen, the thoria concentration measured by standard X-ray techniques was approximately 2.1 weight thoria at the corner of the compact with the concentration decreasing to approximately 1.9 weight percent thoria at the center longitudinal axis of the compact. The character of the thoria particle size in the present compact as observedv by examining extraction replicas in the electron microscope was also found to be significantly different than that obtained with conventional powder metallurgy preparation as shown in FIG. 2. First of all, the thoria particle size range in the present material was significantly smaller than in the conventional material of the same bulk composition. Secondly, there was not found to be any significant agglomeration or segregation of thoria particles in the present material and the size of the larger thoria particles in the present material was significantly smaller than was found in the conventional material. No thoria particles having a particle size larger than approximately 4 microns diameter was observed in the examined specimen whereas it is not uncommon to see ThO particles in excess of 7 microns in conventional W materials containing 2% ThO Following the sintering step, the compact was rolled, swaged and drawn to approximately 18-mil wire in accordance with further conventional tungsten working practices. The final tungsten wire was further subjected to recrystallization by passing an electrical current through the wire in a hydrogen atmosphere. More particularly, the wire was subjected for a 6-minute time period, to approximately percent of its fusion amperage which produced a smaller and more uniform tungsten grain size than is conventionally obtained for either 1 w/o or 2 w/o ThO alloys. The more uniform recrystallized grain structure is a result of a smaller number of ThO particle segments aligned parallel to the wire axis in the new material.

A further comparison of the recrystallization structure was made between tungsten filaments produced in accordance with the present invention and the same diameter filaments produced in conventional fashion but containing only 1% by weight of thoria. When a 7-mil diameter conventional product was recrystallized by heating for 5 minutes at 2,650C, there was obtained a recrystallization structure characterized by some large elongated grains.As distinct therefrom, the recrystallization structure of a wire product having the same diameter but prepared in accordance with the present invention and containing approximately 1.7 percent thoria by weight after heating for 6 hours at 2,650C lacked the large grain size and non-uniformity in grain structure of the conventional product. The comparative grain structures are depicted in FIG. 3. The same distinctions are even more pronounced at the 2.2-mil significantly higher than for the 1 percent thoria alloy prepared conventionally while the ductility as measured by percent elongation is also equal to that of the 1 percent material. This significant increase in ductility 5 was not expected for the new W-l.7% w/o ThO matewire size which provides confirmation of the more unirial. form size and shape of the recrystallized tungsten It will be apparent from the foregoing description grains that are obtained in accordance with the present that various modifications can be employed to obtain invention. Samples ofa recrystallized tungsten filament the products of the present invention. For example. having such improved grain structure have also been 10 various modifications can be made in the liquid diffuobserved to exhibit higher ductility than is obtained by sion technique with comparable results. Likewise. difconventional preparation. The preferred bulk concenferent processing steps can be employed than above detration of thoria in a tungsten compact to produce scribed in fabricating final tungsten filament from the these desirable results is between 1.3 and 1.9 weight impregnated compact without departing from the true percent spirit and scope of the present invention. It is intended As a still more definite indication of the relationship to limit the present invention, therefore, only to the between tensile strength and ductility for alloy materiscope f th following claims. als of the present invention, certain further physical Wh t I laim a new and de ire to secure by Letters tests are being reported. More particularly, actual P t f h United States is; measurements of the yield strength, ultimate tensile 1, A th lly and mechanically worked dispersion Strength and percent elongation were made p alloy member consisting essentially of a recrystallized mil diameter specimens from two d1fferent1ngots of the tungsten grain Structure h r in the tungsten grain yP of thoriated tungsten all0y Product described are small size equiaxed crystals and which further conthe above speclfic P The Same P y meastains an average l-2 percent by weight thoria particles uremems were made p 18-ml] dlameter Speclmens of smaller size than the tungsten grains distributed in a Obtained from thorlated tungsten alloyproducts. uniformly diminishing concentration gradient having tammg 1 Percent and Pirrierlt y welght thoria but the maximum concentration located at the exterior sur- Prepared by a Convemlonal Powder metallurgy l face of said member with no sharp compositional variaq All test Specrmens were sublected to a mmute tion of thoria particles along said concentration gradianneal in vacuum at 2,400C, prior to the physical tests which were conducted at temperatures from room tem- 2 A swaged member having the alloy composition perature to 2,500C. The strain rate during these physid fi d in claim 1. Cal tests was 005 Inches P mch P mmut? A 3. A rolled member having the alloy composition deparison of tests results for the various specimens apfi d in claim 1 Pears in Table I below- A drawn filament having the alloy composition de- TABLE I YIELD STRENGTH (PSI) ULTIMATE STRENGTH (PSI) ELONGATION Present Present Present Alloy Alloy Alloy Temp 2% 1% 1.7% 2% 1% 1.7% 2% 1% (1.7% (C) T110 Tho ThO Tho Tho ThO Tho Tho ThO It can be noted in Table I that tensile strength for the thoriated tungsten alloy product increases with an increasing level of thoria concentration. The above reported ductility measurements for the conventionally prepared 1 percent and 2 percent by weight thoria alloys also generally indicate that ductility is reduced with increasing thoria levels. In contrast to this latter finding, however, the yield and ultimate strengths for the 1.7 percent thoria alloy of the present invention are fined in claim 1.

5. A dispersion alloy member as in claim 1 having improved strength and ductility compared with a conventionally prepared dispersion alloy member of the same composition.

6. A drawn filament having the physical characteristics defined in claim 5.

Claims (6)

1. A THERMALLY AND MECHANICALLY WORKED DISPERSION ALLOY MEMBER CONSISTING ESSENTIALLY OF A RECRYSTALLIZED TUNGSTEN GRAIN STRUCTURE WHEREIN THE TUNGSTEN GRAINS ARE SMALL SIZE EQUIAXED CRYSTALS AND WHICH FURTHER CONTAINS AN AVERAGE 1-2 PERCENT BY WEIGHT THORIA PARTICLES OF SMALLER SIZE THAN THE TUNGSTEN GRAINS DISTRIBUTED IN A UNIFORMLY DIMINISHING CONCENTRATION GRADIENT HAVING THE MAXIMUM CONCENTRATION LOCATED AT THE EXTERIOR SURFACE OF SAID MEMBER WITH NO SHARP COMPOSITIONAL VARIATION OF THORIA PARTICLES ALONG SAID CONCENTRATION GRADIENT.

2. A swaged member having the alloy composition defined in claim

3. A rolled member having the alloy composition defined in claim

4. A drawn filament having the alloy composition defined in claim 1.

5. A dispersion alloy member as in claim 1 having improved strength and ductility compared with a conventionally prepared dispersion alloy member of the same composition.

6. A drawn filament having the physical characteristics defined in claim 5.

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