Tungsten Metallurgy Has Received Considerable Attention in the Aerospace Industry Because of Its High Strength at Very High Temperatures

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TECHNOLOGY UTILIZATION REPORT

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0 TUNGSTEN POWDER METALLURGY

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA SP-5035 .e

I TECHNOLOGY Technology Utilization I UTILIZATION REPORT Division

TUNGSTEN POWDER METALLURGY

Prepared by V. D. BARTHand H. 0. MCINTIRE Battelle Memorial Institute Columbus, Ohio

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington, D. C. November 1965 *. .

This document was prepared under the sponsorship of the National Aeronautics and Space Administ,ration. Neither the United States Government nor any person acting on behalf of the United States Government assumes any liability resulting from the use of the information coritnined in this document, or warrants that such use will be free from privately owned rights.

~ __ __ For sale by the Superintendent of Documents U.S. Government Printing Office Washington, D.C. 204021 Price 35 cents FOREWORD The Administrator of the National Aeronautics and Space Administration has established a technology utilization program for rapid dissemination of information on technological developments which appears to be useful for general industrial application. Tungsten metallurgy has received considerable attention in the aerospace industry because of its high strength at very high temperatures. This study on tungsten powder metallurgy was undertaken to explore the work done by NASA and others and to make this advanced state of the art more generally available. This report was prepared by V. D. Barth and H. 0. McIntire of Battelle Memorial Institute, under contract to the Technology Utilization Division of the National Aeronautics and Space Administration.

The Director, Technology Utilization Division National Aeronautics and Space Administration

iii V LIST OF FIGURES

1 Combustion temperatures of rocket propellants plotted against the melting and vaporizing temperatures of various component materials------2 2 High-temperature materials requirements------2 3 Structural continuity in bcc metals and the corresponding Young's modulus, illustrating the relative position of tungsten in the series______.._--_-_ 3 4 Correlation of the activation energies for creep, stress-rupture, and self-diff usion with the melting temperature______------__ ------4 5 Correlation of the thermal component of the yield stress T* with the parameter (T- T,),/T, for the bcc metals ______-- 4 6 Metal-metal oxide equilibria in hydrogen atmospheres------6 7 As-sintered tungsten, ultrafine powder (0.015-0.050 micron) pressed at 5000 psi and sintered at 2280O F (1250' C) in hydrogen for 15 hours (Battelle) ------9 8 Compressibility of tungsten powder compared to various other powders------13 9 Comparison of the compressibilities of several metal powders compacted by both conventional and isostatic pressing techniques------14 10 Density-pressure relationship for hot pressing tungsten powder held 30 minutes at 1500" C and 1800O C______~~~------~----~------15 11 Density-temperature relationship for hot pressing tungsten powder at 5000 psi- 15 12 Tungsten carbide case depth versus pressure for hot pressing tungsten powder at 1500Oand 1800O C-____.______-~~-~~------~------15 13 This photomicrograph, taken at 500X from an unetched specimen, shows the silver evenly dispersed throughout the tungsten matrix- - .------23 14 Specific impulse and exhaust velocity as functions of the chamber temperature and the molecular weight for various values of k and p,/p,------26 15 Schematic representation of heat absorption by silver in an infiltrated tungsten nozzle liner wall- - - __ -.__ - ______- __.- __ - - - - - ______-- - - 27 16 Sintering rates of porous-tungsten ionizers. Compacting pressure, 34 000 pounds per square inch; atmosphere, dry hydrogen. -.- - -.-. ------.- 30

vi LIST OF TABLES Pape I Young's Modulus and Melting Point for Group I11 through Group VI11 Transition Metals------3 I1 Approximate Temperature of Recrystallization Onset for Increasing Deformation of Niobium, Tantalum, Molybdenum and Tungsten------4 I11 Equilibrium Data for the System W02-H2-W-H20 ______- - - - __ ------5 IV Chemical Analyses of Forging Billets ______7 V Typical Purity Specifications for Tungsten Powder as of 1960------8 VI Materials Advisory Board Recommended Sheet Composition: Maximum ' Allowable Chemicals------.------9 VI1 Impurity Limits, SAE Specification AMD 78 BU, for Tungsten Forgings- Pressed, Sintered, and Forged, 1-15-63- ___------__ ------9 VI11 Characteristics of Powder Lots and Blends Evaluated------9 IX Chemical Analysis of Tungsten Powder Selected for Full-Scale Pilot Phase- - 10 X Suggested Tungsten Powder Requirements for Tungsten Sheet Production- 10 XI Submicron Tungsten Metal- - - __ __ ------.. ------.------10 XI1 Compression of Tungsten Powder..- __ - - - _.------__ __ ------.- - 12 XI11 Mean Tension-Test Results on Pressed-and-Sintered Tungsten of 85y0 Theoretical Density______---~~..------~--~~~------~-~17 XI v Mean Tension-Test Results on Pressed-and-Sintered Tungsten of 90% Theoretical Density____-__---_------18 xv Mean Tension-Test Results on Pressed-and-Sintered Tungsten of 95% Theoretical Density______~------..-_~~-~~-~~------~~~18 XVI Pressed-and-Sintered Tungsten Creep-Rupture Properties------19 XVII Typical Purity of Vapor-Deposited Tungsten------20 XVIII Approximate Comparison of Heat Sources------21 XIX Working Rules for Production of Infiltrated Structures------24 xx Effect of Powder Size, Compaction Pressure, and Sintering Temperature on Sintered Porosity of Tungsten- - - __ - -.._ ------25 XXI Low-Pressure Gas-Flow Pore-Size Determinations for Sintered Tungsten Matrices______.______-__-_---.~~-~------..-~~~~------26 XXII Heat Absorption Above 77" F for Several Metals -______26 XXIII Tension-Test Results on Pressed-and-Sintered Tungsten Infiltrated with 20 wt % Silver ______------28 XXIV. Tensile Strengths of Copper Composites Reinforced with 5-mil Tungsten Fiber______-----~--~---~--.~---~------~------~29 xxv Modulus of Elasticity of Copper Composites Reinforced with Tungsten Fiber______--_-______~~.~-~~~~~----~-~--..------~-30

vi i INTRODUCTION It is the purpose of this report toEFmarize and several neighboring elements are illustrated recent developments in tungsten powder metal- in figure 3.(18) From time to time, efforts have lurgy technology as related to space vehicles been made to arrive at a semi-theoretical cor- _and the less traditional applications. The relation between melting point or some other customary use of tungsten as a carbide or as a physical property, such as heat of fusion and minor alloying element is not considered here. mechanical strength.(l8* a* The excep- According to a 1910 paper by Fink, "An- tionally high melting temperature of tungsten nouncement has been recently made of the makes such a correlation of more than ordinary production of tungsten in a form in which it is interest. Table I, from a recent critical survey ductile. This ductile tungsten would seem to by Gschneider(B), indicates that the elastic be a new substance from the viewpoint of the modulus increases continuously in a horizontal physical chemist, . . .".(l)* A contemporary direction from Group I11 to the first member of account by Coolidge noted that "when work Group VIII, after which it decreases. The was first started on the problem of producing a minimum melting points, however, tend to lie ductile form of tungsten, the metal looked very in Group VI (actually at Group V in the first uncompromising."(2) The succeeding techno- series), and, therefore, the maximum melting logical developments over the following 40- to temperature of tungsten does not coincide with 50-year period have been detailed in a number the maximum ambient temperature modulus of papers and book^.(^-'^) Reference (15) in in the third transition series. At higher tem- 1959 marks the approximate beginning of peratures, the elastic strength of tungsten production for space technology uses. compares more favorably. For example, the The importance of tungsten as an elevated modulus of tungsten at 1650' F is approximately temperature resistant material is suggested by the same as that of rhenium. The 20 000 000 the service temperatures of figures 1"") and 2."') psi advantage if iridium over tungsten at room temperature is reduced to 10 000000 psi at Early advances in tungsten metallurgy were 1650' F.(24)The same effect is shown in bulk made in industrial laboratories with private modulus comparisons. e5) funds. The greatest contributions to space- The melting point-strength relationship is oriented tungsten technology have been made further illustrated for tungsten and other metals in Government funded laboratories or other by an activation energy-melting temperature facilities, of which the National Aeronautics plot of the type advanced by J. E. Dorn. Fig- and Space Administration is a notable example. ure 4(*") illustrates the generally good correlation Tungsten as a Tmnsition Metal obtained in such a plot, in which AH is an observed activation energy for creep, stress Solubility and melting temperature relation- rupture, or self-diffusion. It has been shown ships with some corresponding Young's modulus that creep in metals is an activated process, and values for bcc Group V to Group VI11 metals that, at high temperatures (above about 0.5

*Referencesindicated throughout as superior numbers in parentheses are listed at the end of the report.

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I I -TaC, HfC, melt I 7000 I1 I I 1 I/Ol I -C, sublimes 0 1000 2000 m ow0 5000 m 7m TEMPERATURE I°F I ZrC, melts propel Ionts Figure 9.-High-temperature materials requirements. (I7) W, melts

NHqC104 - 6000 -Hf 62 melts table 11. (27) Such recrystallization is ordinarily + AI + Bii To €52 melts associated with rapid loss in mechanical strength.

Fc, boils Finally, Conrad and Hayes (28) have arrived rider I To, melts at a relation between the thermal component, RFNA + polyethylene T*, of the yield stress and the incremental fractions of the absolute melting temperatures 5000 Ni, boils N2O4 + N2H4 for vanadium (V), chromium (Cr), iron (Fe), Mo, melts H202 + C~HSOH Cu, boils niobium (Nb), tantalum (Ta), and tungsten Nb, melts (W) (fig. 5). The fact that the thermal com-

Block powder ponent can be correlated, as illustrated, suggests Ag, boils that the same dislocation mechanism may be . 4000 rate-controlling in these metals. Futher, since the thermal component is independent of structure, it is concluded that the yeilding mechanism operates in overcoming the Peierls-Nabarro Pb, boils stress; i.e., the inherent resistance to strain of 3000 the bcc lattice. Fe, melts The forging considerations illustrate the inherent strength advantage of tungsten at high temperatures. The fundamental reason for this characteristic strengt,h in terms of electron configuration interaction has not been fully explained. There is little doubt however, that an optimum number of electrons in the FAHRENHEIT unfilled 5d shell is involved.(m) For example, it has been suggested that the exceptionally Figure 1 .-Combustion temperatures of rocket propellants plotted against the melting and vaporizing temperatures of high cohesive energy and elastic isotropy of various component materials. (I") tungsten have their origin in the powerful exchange spin interactions between the inconi-

Tm),the activation energy for creep is usually pleted d-~hells.'~~1 31) the stme as that for self-diffusion. The resist- imce of four refractory rnetal lattices to thermal Some Considerations in Hardware Design disturbance is also reflected in the approximate The foregoing summary has noted the inherent threshold-of-recrystallization temperatures in strength of tungsten metal. This is only one 2 6330

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Figure 3.-Structural continuity in bcc metals (upper) and the corresponding Young's modulus (lower), illustrating the relative position of tungsten in the series.(l*)

Table I.-Young's Modulus and Melting Point for Group 111 through Group Vlll Transition Metals Cz3)

Group: I11 IV V VI VI1 VI11

~~~~~

Element: ~~~~-__-~Sc Ti V Cr Mn Fe co Ni

Meltingpoint, OF______2802 3034 3461 3407 2271 2798 2717 2647 Young's modulus, 108 psi ______11 15 19 35 29 30 30 28 Pd Element: ~~~~~___~~Y Zr Nb Mo Tc Ru Rh

Meltingpoint, OF______._ 2735 3362 4474 4739 3938 4136 3560 2826 Young's modulus, 108 psi ------9.4 13 15 48 54 60 54 18

Hf Ta W Re OS Ir Pt Element: ___La. -. ~~~~~___ Meltingpoint,OF ______1688 4032 5428 6126 5720 5481 4429 3216 Young's modulus, 106 psi ______5.5 20 26 58 67 78 77 25 of several factors which may be important in a 3. Creep strength high temperature material. Following is a list 4. Stress-to-rupture strength slightly modified from Promisel: (1') 5. Fatigue 1. Operation to 4000' C (or higher 6. Thermal shock resistance temperature) 7. Ductile-to-brittle temperature transi- 2. Strength-to-weight ratio tion 3 Table 11.-Approximate Temperature of RecrystallizationOnset for Increasing Deformation of Niobium, Tantalum, Molybdenum, and Tungsten

Begining of recrystallization, after % deformation at OC Material Purity Mode of deformation ~.

10% 30% 60% 90 %

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Nb 99% Nb______Cold rolled ______1200 1100 1025 1020 0.11% 0 0.0027% H 0.06% C Ta 99.8% Ta ______Cold rolled, cold pressed (hydrostatic pressure)- - 1280 1250 1250 1200 Mo 0.1-o.2Y0 C 1200 1070 980 -__-- Some 1/1070 Fe Traces of Si N 0.00270 0 W 99.670 W ______-.Hammered, with process annealing (below re- 1560 1500 1430 1420 Commercially crystallization temperature).

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Melting Ternperoture,T,,,, K Figure 5.-Conelation of thermal component of the yield (mess T* with the parameter (T-T,)/T, for the bcc metals.‘28) Figure 4.4orreIation of the activation energies for creep, stress-rupture, and self-diff usion with the melting temperature. (2e) 12. Impact resistance 13. Erosion resistance 8. Fabricability and joinability 14. Availability-sheet, bar, forgings, tub- 9. Oxidation resistance ing, extrusions. 10. Thermal, electronic, and nuclear prop- In design work, more or less weight is given erties to one item or another in the foregoing list, 11. Resistance to molten metals depending on the details of application. Com- 4 monly this takes the form of a "trade off" in of the differences in details of applications which a less critical requirement is sacrificed in requirements. favor of a fully necessary property. Tungsten is outstanding in general with Mineral Reserves respect to items 1, 3, 4, 11, and 13. Item 9, Estimates of US. reserves of tungsten have oxidation resistance, is perhaps the greatest been reported by McEelvey and by Nel- problem associated with this metal and its These amount to approximately 70 000 alloys. Strength-to-weight considerations com- to 80 000 tons as metal. This tonnage appears monly restrict tungsten to operating environ- adequate for the present, but the long range ments above 3000' F. Tungsten is notably outlook is questionable. Tungsten is currently less favorable in comparison to many lower imported for the major portion of U.S. require- melting metals with respect to items 7 and 8. ments because of the lower price of foreign It is difficult to generalize much further in view concentrates.

POWDER PRODUCTION

Reduction to Metal The favorable circumstances for reduction Because of the high melting point of tungsten, are illustrated by the comparatively large its production is dominated by powder metal- equilibrium PH,o/PH,values. Figure 6 from lurgy processes rather than by fusion associated Chang'35' is based on thermodynamic calcula- techniques. The common raw materials, wolf- tions and compares the equilibria for the general ramite and scheelite, may be treated as follows: reaction : MmOn(,)+~Hz(,) =mWs) +nH& For W02 and other oxides, the areas below each CaWO, +2HCl= H2W04+ CaClz of the curves are those in which metal production is favored. For either the production of The subsequent purification steps are de- metal powder from the oxide or sintering of signed to yield as pure a furnace feed as is tungsten in hydrogen, it is apparent that sub- consitent with economic considerations. The stantial amounts of water vapor can be tolerated purified product is ordinarily the ammonium at even fairly low temperatures. However, in paratungstate : 5 (WH,)zO~12W03~xH20or tung- the production of powder, water vapor is stic acid : ordinarily maintained at a low level to keep the reduced product as free of oxide as possible. 5 (NHJzO.1 2W0,.5H20 + 1OHCl+2H,O The production of tungsten metal powder by = 1ONH,Cl+ 12H,WO,

The paratungstate is ordinarily fired in air Table Ill.-Equilibrium Data for the System to form yellow WO,. It is also possible to WOa-Hz-W-Hz0 hydrogen-reduce from the paratungstate. The oxide is reduced to WOz, by degrees, in a Temperature, OC Kp= PH~O/PH~P' flowing hydrogen atmosphere. The equilib- I rium, under furnace conditions, of the reaction 0. 12 0. 21 0. 35 0. 48 is very favorable to the production of tungsten 0. 68 metal. Griffis'") obtained experimental equi- 0. 94 librium data for the above reaction. These data are averaged in table 111. (4 Averaged from Oriffis' data. 5 80 furnaces is reported to yield a more uniform LL 60 ‘.w prodiict, than thnt produccd in B four-tube 40 a muffle furnace which is also described. Fur- 20 ther, the rotary furnaces are more amenable to $0 automation. Three-stage reduction of WO, in the manufacture of thoriated tungsten pro- ,-- 40 duced high quality wire in Czech research.(39) &- 60 A number of studies dealing with WO, reduction have been reported by Philips Research 2 -100 Laboratories workers. The retarding action 0 -120 of water vapor during the hydrogen reduction

-I 40 of WO, is less pronounced with doped products. 00 0 -2 Potassium silicate and K,SiW120, increase the TEMPERATURE. *F reduction rate of WO, in deep layers.(40) The influence of doping agents on the growth of Figure 6.--Metal-metal oxide equilibria in hydrogen tungsten crystals during hydrogen reduction atmospheres. (35) of the oxide has been studied. The action of potassium silicate in the growth of large crystals the passage of WO, in boats through a hydrogen is related to the formation of a-tungsten rela- tube furnace is affected by several factors: tively early in the reduction process.(41) Alu- Reduction temperature mina, on the other hand, opposes growth of Kate at which temperature is increased primary tungsten crystals. Grain sizes and Hate of hydrogen flow shapes of pure, thoriated, and silicate doped Total load and depth of load in each boat tungsten powders have been studied by means Rate of boat passage through the tube of an electron microscope.(42) furnace. Most tungsten powder is produced in the Powder, Size and Purity size range from about 1 to 10 microns. Very In the last 10 years, there has been a distinct fine powders require equally fine oxide starting trend toward the production of higher purity material, a rapid flow of hydrogen through the tungsten powder intended for the fabrication of furnace, a low reduction temperature, and a shapes to be severely worked. At the same minimum time in the furnace consistent with time, it should be noted that powder purity complete reduction. Coarse powders are pro- is only one factor in the successful production duced by higher reduction temperatures and of mechanically worked parts, that is, accept- longer furnace dwell times at temperature. able powder chemistry will not guarantee The foregoing outline is intended only to freedom from failure during deformation of a furnish a perspective view of tungsten powder shape made from the powder. The metallurgi- production. For further details, references cal structure of a wrought preform of powder cited in the precedingsection should be reviewed. metallurgy origin may determine the success Recently, hexamine has been used as an or failure of a forming operation. Because tiuxiliary reducing agent for W03.(3e,37)The some impurity levels in starting powders may type of reduction involved is illustrated by the be altered during subsequent consolide tion following expression : and shaping, and because factors such as grain (CH,),N4+6WO3=6 W +6C02+6H20 +N2 size are important, it is difficult to specify critical impurity limits. “Substantial” im- The use of hexamine is said to permit the rapid purity contents ordinarily are damaging. Table production of high purity tungsten with par- IV(*,) illustrates the starting powder composi- ticle size distributions which may be prede- tion differences between two materials ex- termined arid reproduced over a wide range. hibiting satisfactory forging response (Vendors Dettiils of two Soviet rotary furnaces for A and B) and two materials with which severe producing tungsten powder have appeared. (,8) cracking was experienced during efforts to The hydrogen reduction of wo, in these forge them (Vendors C and D). Cracking 6 Table IV.4hemical Analyses of Tungsten Forging An extensive powder evaluation program was carried out as part of a Navy contract to develop large (48 in. X 18 in.) tungsten sheet Unalloyed sintered W production techniques by powder metallurgy (powder metallurgy) as contrasted with an arc-melting approach. I Element, I Table VIII(48’49) illustrates the variety of PPm Vendor Vendor Vendor powders investigated. The undoped powder A C D produced by single-stage hydrogen reduction of ammonium paratungstate, ranging in particle size from 1 to 20 microns, and having the analysis shown in table IX, was selected for the full- scale pilot phase of this work. The powder particle size distribution appeared to be the major factor involved in the pre- diction of powder behavior through consolidation and subsequent working. Fine powders generally sintered the highest densities, but were also the most difficult to press and to work in the sintered condition. The coarse, heavy density powders pressed well and provided high green strength, but were difficult to sinter up to the desired levels. The use of blended com- bination of fine and coarse powders provided types with excellent consolidation and working properties, but the size distribution tended to be carried through into the sintered and wrought product, resulting in a nonheterogeneous structure difficult to work. The undoped powder produced by single stage reduction of paratungstate appeared to provide a material which would best suit the

(*) Analyses were supplied by sources. production facilities and proposed material application at the start of the program (table was expected in the latter, considering the high X). At the end of the program, it was sug- levels of some impurities. Table V(44)illus- gested that oxygen content be held to 300 ppm trates the variety of impurity level specifica- maximum, and that the combined carbon and tions as of about 1960. These specifications nitrogen should not amount to more than 20 may be compared to Materials Advisory ppm. Further, if possible, residual impurities Board recommendation for maximum allow- other than interstitials should be held to below able impurities in tungsten sheet (table VI(45)) 20 ppm. and those contained in an AMD Specification The problem of developing a rigid specifica- (table VII(46)). Carbon, oxygen, hydrogen, tion for tungsten powder to produce a satis- and nitrogen, if not present in too great factory product in different consolidation fa- amounts, may be reduced to acceptable levels cilities should be noted. For example, the during sintering; nonvolatile impurities will 3.74.0 optimum particle size range of table remain in the sintered product. Producers X might possibly be increased to 4.04.3 and of powder intended for the production of still produce suitable end product, with the tungsten to be mechanically worked to hard- availability of higher sintering temperatures. (48) ware shapes generally like to keep Fe, Si, All Mo, and Ni at low levels. The effects of Ultrafine Powders powder quality on subsequent properties of The interest in ultrafine metal powders is tungsten have been summarized by Allen. (47) illustrated by extensive NASA work.(50) In 7 Table V.-Typical Purity Specifications for Tungsten Powder as of 1960 ('I)

I Impurity content, wt%, a OINIH/C/Al /Ca/ Fe 1 Mo 1 Ni 1 Si /W

FOTuse in powder metallurgy product Producers: Cleveland Tungsten ____ 0. 20 ______0. 01 ______0. 01 0. 005 ______0. 01 0. 030 Fansteel. .______0. 10 0. 0050. 0050. 02 0. 005 _____ 0. 02 0. 02 0. 02 ------Firth Sterling_____--___ 0. 30 ______0. 01 0. 01 0. 01 0. 02 0. 01 0. 02 0. 01 General Electric (Lamp h.Ietals and Compo- nents Department)--- 0. 01 __ _--__ ___ 0. 003

FOTuse in melted product

General Electric Research Lab______0. 01 0. 001 _____ 0. 005

the case tungsten, there have been several tungsten (surface area: 3.7 m2/g) to conven- incentives for fine powder production. Among tional powder can increase as-sintered densi- these are the development of dispersoid strength- ties up to about 9 percent greater than densi- ened alloys, the consequent decrease in diffusion ties obtainable using conventional powder. distance where solid solution alloys are being Lamprey and Ripley have made ultrafine made from elemental powders, and reduction tungsten powder by hydrogen reduction of sintering temperature. Figure 7 illustrates WC16.(b2*53-64)The powder so produced had the high as-sintered density and small, uniform a diameter of about 0.02 to 0.03 micron. Green grain size achieved by the low temperature compacts of this powder were sintered to 94 sintering of ultrafine tungsten powder. It percent theoretical density at the comparatively has been demonstrated in Los Alamos work(51) short time and low temperature of 30 minutes that additions of up to 25 percent of ultrafine at 1550' C, that is, at about 1000° C lower 8 Table Vlll.4haracteristics of Powder Lots and Blends - Evaluated (46.49) I Average Code particle lensity, No. size, g1in.a microns -

1 1. 18 34. 5 2 7. 10 70. 4 3 6. 00 47. 8 4 5. 80 52. 9 5 5. 60 54. 2 6 4. 80 51. 0 7 i Figure 7.-Ar-rintered tungsten, ultrafine powder (0.01 5- 4. 30 52. 2 0.050 micron) pressed at 5000 psi and sintered at 2280" F 8 4. 30 49. 8 (1 250" C) in hydrogen for 15 hours (Battelle). 9 6. 10 90. 8 10 3. 90 59. 5 Table VI.-Materials Advisory Board Recommended Sheet 11 3. 60 56. 4 Composition Maximum Allowable Chemicals 12 4. 80 60. 4 13 2. 60 61. 2 Element ppm, max 14 2. 00 43. 8 15 2. 50 54. 6 18 5. 00 79. 3 19 1. 15 45. 2 20 3. 65 67. 7 - than is required for ordinary tungsten powder in this length of time. Ultrafine tungsten powder currently offered for sale by one producer is advertised to have the following properties : Surface area--- 10 to 20 meters2-g I Particle size-- - 0.015 to 0.030 micron Particle shape- Spherical Particle size Practically no size varia- distribution. tion. Bulk density.. - Less than 1 g/cm3 Stability in air- Reactive (possibly pyrophoric) I m Stability in No apparent reaction water. Fluidity- - - - __ Very fluid. Hz ______0.001% N2 _--_---0.001% 02------1.0% Max. W ------Nominal balance As described in unpublished work at Bat- telle Memorial Institute, powder ranging in average particle size between 0.1 and 0.4 micron has been made by hydrogen reduction of sus- pended submicron particles of WO,. Tung-

788-307 0 - 65 - 3 9 Table IX.-Chemical Analysis of Tungsten (4a~4Q) Powder Table XI.-Submicron Tungsten Metal Selected for Full-Scale Pilot Phase Typical analysis: Impurity Content, % % element element wt% C ______0.07 Mn ______0. 001 Al----____-_0.002 Na ______0.001 cr _---_-_--_0. 001 Mg______0. 001 0 ______.____0. 026 0.001- Be ___._____0. 002 K ______0. 001 N ______0. 0005 0.001- Fe______0. 01 cu______0.001 c_____.__-_. 0.001 0.001- Mo ______0. 005 WSO______99.88 Ag ____._____0. 001 0. 010 Si ______0. 02 0 _____-_-__ 1. AI .______0. 001- 0.001- Ca ______0. 001- 0. 005- Typical physical properties: Color______.._.____ Black cu___...____ 0.001 Crystal form ______W Particle shape- - -.. . - - Crystalline

Particle size ___..~ _.__<0.01-0.07, avg 0.011 Table X.-Suggested Tungsten Powder Requirements for micron. Tungsten Sheet Production Surface area ____._____29 sq m/g Bulk density ______35 lb/cu ft Stability ____ - - ___ - __ - Pyrophoric in air

lower surface area (5 to 10 meters2/gram). At the beginning of the work described above, a state-of-the-art survey for submicron powders was made.@') An Air Force program to develop defect free, high formability tungsten flat rolled sheets using ultrafine tungsten powder (0.08 to 0.2 micron), inert working atmosphere, and high fabrication temperatures is in progress. (5*) sten oxide, aerosolized in a hydrogen stream, A summarizing, final report for this program was carried into a horizontal reaction chamber has not been received. at 1450'F. There was little settling out of Discussions of subsieve particle size analyses the reduced product in the suspending hydro - of tungsten have recently appeared.(60-Bo) gen stream at linear travel rates of 3 to 11 feet per minute. Spherical Powders Submicron tungsten powder has also been Spherical tungsten metal powders have be- made by arc vaporization of WO, followed by come important with the development of reduction by hydrogen of the very fine oxide evaporation-cooled rocket nozzle throat sections BET surface measurements gave and porous tungsten ionizers for rocket engines. IL value of 29 meters2/gram or 0.011 micron NASA has sponsored a substantial amount of on a spherical diameter basis. In this work, the research and development in these and bor- it was observed that tungsten particles having dering areas, as well as having carried out much surface areas greater than 20 meters*/gram were research in its own laboratories. pyrophoric when exposed to air. Table Several procedures have been followed in characterizes the tungsten produced by this making small tungsten spheres. One process method in pilot scale runs. Low temperature uses a fluidized bed reactor in which seed oside reductions, for example, 600' C for a particles in the 1- to 10-micron size range 24-hour period, yielded powders with high constitute nuclei for the growth of spherical surface iireas (about 30 meters*/gram), but powder by the accretion of tungsten from dso with high oxygen content. Correspond- hydrogen reduced WF,. C6l) Tungsten granules ingly reductions at 800' to S50' C produced in the 200- to 400-micron size range, containing powders with lower residutd oxygen, but also a few percent by volume of seed, are now 10 ~~

available commercially. This material has Electrolytic Powder been consolidated by gas pressure bonding into Pure tungsten can be deposited from an special shapes or into sheet bar material. aqueous solution only with difficulty if at all. Spherical tungsten can also be made by the Ordinarily, only an alloy of one kind or another use of a plasma torch.(s2-ea) Tungsten powder can be formed. Greater success has been can be spheroidized, in principle at least, by possible by the deposition of tungsten from dropping it through a resistance-heated graphite molten salt baths. Using either a WO, or a tube. (W One producer has made spheres in the scheelite feed, and an Na4PZO7-NaC1-Na2B40, range 75 to 150 microns diameter, starting electrolyte, coarse dendritic powders of tung- with X6-inch diameter tungsten wire. (65) Pre- sten have been deposited.(68) The product sumably, an arc process of one kind or another had higher flow rates and better compressibility is used. than hydrogen-reduced tungs ten powder. (W The use of the plasma arc for producing However, the large particle size peculiar to the tungsten spheres has been described in detail in electrowinning process effectively prevented a British publication. (W conventional sintering to a high-density ma- The sizing and application of spherical terial. Densities 92 percent of theoretical were tungsten powders has been reviewed in detail obtained by hot swaging the presintered in an AIME publication.(67) tungsten in stainless steel sheaths.

CONSOLIDATION

Since tungsten metallurgy begins with the sequent demand on the mechanical press- metal in powder form, powder consolidation is ing stage of consolidation has encouraged of interest whether or not the final product the continued study of tungsten powder com- passes through a fusion stage. Although the pactions. There is no fully acceptable expres- most important metal powder consolidation sion to describe accurately the extent of densi- procedure consists of compaction to shape in a fication versus applied pressure for tungsten, as die followed by solid state sintering, there is a well as other powders, although many have been variety of modifications of this general pro- suggested. Heckel ('O) found the relation be- cedure. Only those of either current or tween as-pressed density and applied pressure potential interest in the consolidation of tung- to be expressed by the equation: sten are noted in the following sections. In (&)=KP+A Cold Mechanical Compaction-Some General Considerations where : D is the relative density (that is, the The only important tungsten powder com- ratio of the density of the compact to pacting method for the production of green full density) ingots for manufacture of rod, wire, and narrow P is the applied pressure strip is solid die pressing. The compacts K is a proportionality constant related customarily range in size up to 2 feet in length, to the yield strength of the metal 1 inch in thickness, and 2 inches in width, and A is a constant. are made by simple lateral pressure in a rectangular steel die. These are presintered at For tungsten, Heckel found K=7.6X llOOo to 1300' C, followed by self-resistance psi-' and A=0.62. Heckel's and other densi- sintering to about 90 percent of theoretical fication expressions have been critically re- density, and then worked to shape by various viewed by Donachie and Burr (71), who conclude mechanical means. In the newer applications that the previously noted equation allows at for consolidated tungs ten, large pressings and least an educated first guess at pressure-density more complicated shapes have been required. values. Values of percent porosity as a function Some of these must be able to withstand of pressure have been reported for several sizes machining in the green condition. The con- of samples, by Shapir~.(~*)The mean particle 11 Table XIL-Compression of Tungsten Powder(’*) pacts made from these irregular particles I cmtinscs to increiise for pressures beyond yo Porosity that which produces maximum density.

Pressure, ~ Density and strength values approach kg/cm2 Mixed “A91 “B” “C” maximum values at about the same size pressure for compacts made from the ~___ more regular-shaped powder particles.

Zero ___.___.___60. 3 58. 1 56. 6 56. 3 (3) It has also been shown that irregular- 616____.______- 47. 9 48. 5 48. 6 48. 1 shaped electrolytic powder particles pro- 921 ______._____45. 7 46. 2 46. 4 46. 2 duce stronger compacts than hydrogen- 1246_____.___._ 43. 8 44. 3 44. 5 44. 4 reduced powder of the same particle size. 1578______.__._ 42. 0 42. 5 42. 6 42. 8 (4) Finer tungsten powders produce less 1904______._._ 40. 6 41. 1 41. 1 41. 2 2208___.__.____ 39. 0 39. 7 39. 7 39. 9 dense green compacts than do coarser 2852 _____..____36. 4 37. 1 37. 1 37. 3 powders. The reason for this is the 3475 ______._-_-34.0 34. 4 34. 6 35. 1 increased friction during pressure appli- 4125 _____._.___31. 7 32. 3 32. 5 32. 9 cation for the fine powders. 4843 _____._._.-29. 7 30. 1 30. 3 30. 9 (5) Particle-size distribution plays a major 5480______-_- 27. 7 28. 3 28. 2 29. 0 6137 ______._-.-26. 1 26. 5 26. 5 27. 2 role in the formation of dense tungsten 6761 ______24. 4 24. 9 25. 0 25. 9 compacts. The maximum packing density obtained with two powders which measure 25 and 1.5 microns is at a ratio diameters of samples “A,” “B,” and “C” of table of approximately 80 percent of the XI1 were 39, 45, and 58 microns, respectively. coarser and 20 percent of the finer powder. Data for sample “C” are plotted in figure 8, along with data for some other powders to Isostatic Compaction illustrate the comparative resistance of tungsten Isostatic compaction of tungsten powders to compaction forces. The curve for tungsten has been a very successful way of circumventing is in good agreement with data for isostatic the problems of obtaining uniform densities in compression of medium size tungsten particles large compacts by solid die pressing.(73*75) reported in reference (73), but the density is Isostatic compaction dates back at least as far greater than that also reported in reference (73) as 1913, when tungsten slugs were compacted for fine particles, as is to be expected from by Madden (H. D. Madden, ITS. Pat. 1 081 618 powder metallurgy experience with the finer Dec. 16, 1913, to Westinghouse Lamp Corpora- tungsten powder sizes. tion). A 1954 paper describes the hydraulic Factors affecting the conipaction of tungsten pressing of tungsten tubing by a radial com- powders have been investigated and summa- pacting process.(i6) The isostatic process is in rized by Poster, (’*) who reached the following general use for the production of large tungsten general conclusions: slabs and billets where sintered densities must (1) The formation of green compacts from ordinarily reach minimums of about 90 to 92 tungsten powders depends to a large percent if mechanical working is to be effected extent upon mechanical interlocking of without failure by fracturing. The term “iso- the particles, since there is little defor- static pressing” refers to any compacting opera- nirttion under pressure. Interlocking de- tgel, (2) Irregular-shaped tun g s t e n p arti c 1e s, rubber, suitable powders, gases, and such. which can be produced by the standard Hydrostatic pressing refers to the use of a mcthod of hydrogen reduction of tungstic fluid for tratisniitting the pressure, and pas oxides, produce stronger but less dense pressure bonding refers to particular situations green cornpacts than do regular-shaped where gases are employed at elevated tempern- particles made by the same process. It tures. In some instances, the process may be has been found that the strength of coni- only partially isostatic, for example, when 12 60

-Q) 30 0 V u) .-V E .-f L 0 0 -0 20 .-c u) 2 0 a c c Q,z aQ,

7 00 Pressure, kg/c m2 I I I I I I I I 0 14,230 28,460 42,700 56,900 71,100 85,400 99,600 Pressure, psi

Figure 8.--Comprenibility of tungsten powder compared to various other po~dm.''~'

13 only equal radial pressures are applied. The use of an isostatic pressing procedure avoids the disadvantages which arise from the influenco of solid die walls during die pressing and which result in noniiniform density in either a large powder mass, unfavorable shape, or unfavorable 0 dimensions. Commonly, metal powder is \0 *- 10.0 loaded into a rubber or plastic container. The x &---- Conventional container is partially evacuated, after which E I+* ;9.0 hydrdic pressures are applied to the external f I surfaces of the container. The as-cold com- Lc \Copper 21 pacted product ordinarly will withstand rea- ._fu) 8.0 sonable handling, without crumbling. This - e-- JL general procedure is especially useful in con- E 7.0 x Iron solidating tungsten powders where density 8 inhomogeneities have a more than ordinary 6.0 effect on the subsequent workability of the as-sintered shape. An interesting recent modificaton of the fluid isostatic technique involves the sub- stitution of a preformed gel, elastomer, or 40 50 60 other flexible mold material as the pressure- Compacting Pressure,1000 psi A-48786 transmitting medium, This allows the isostatic pressing of tungsten shapes.'") The improve- Figure 9.-Compariron of the comprerribilities of reveral ment in density with increasing increments of metal irortatic prerring techniquer. pressure for pliable mold compaction of tungsten as compared to conventional solid die compac- Hot Pressing tion has been demonstrated in Frankford Temperature adequate for the hot pressing Arsenal work.('*) At a compacting pressure of of tungsten powders are too great for the use 60 000 psi, there was a 13-percent improvement of steel dies. Graphite dies are fairly satis- in density of a simple cylindrical tungsten shape factory, but they suffer from the following when tho pliable mold was used (fig. 9). A disadvantages : NASA technical brief(7e)suggests the use of (1) Graphite die life is relatively short at the barrel shaped chambers in rubber molds for the pressures and tenperatures required for isostatic pressing of tungsten cylinders. For hot pressing tungsten powders. molding tungsten microspheres, pressures up (2) Usually some contamination of the tungs- to 56 000 psi were applied. Decrease in ram ten from the die occurs, and must be pressure was accompanied by deflection of the taken into consideration. The com- mold wall away from rather than toward the pact may require subsequent machining compact. This avoided fracture of the weakly to remove carburized surface. bonded compact during unloading. In the work on the compaction of tungsten (3) Uniform heating of large dies by induc- powder for sheet production, previously re- tion is difficult. ferred to (I8), it was concluded that isostatic (4) For large production runs, hot pressing in pressing in the range 30000 to 35 000 psi graphite dies becomes too expensive. should be satisfactory for preparation of green Where graphite-die pressing is applicable, compacts. Pressing at 20000 psi tended to combined pressure-molding and sintering is result in low sintered densities. There did not feasible in one comparatively short time oper- appear to be any adverse effect, but also there ation ; closer dimensional control and higher was no advantage in the subsequently sin- final densities are possible. There also may tered sheet bar, from compacting pressures as be special cases in which graphite-die pressing high as 50 000 psi. is advantageous, for example, where the die 14 100

80 i ;

60 1400 1500 1600 1700 I8W

Hot Prsrnng Ternperofwe. C 223s. Figure 11 .-Density-temperature relationship for hot pressing tungsten powder at 5000 psi. t 2 in. dio ingot iI 2 in dio ingol (2% Tho21 8000 # 4 in dio ingot (2% Tho,) 50 0 2000 4000 6000 8000 6000 0 28 Pure lungsten Pressure. psi 0 48 Tvlqrlen +I% ThoZ . A 2op Pure tungrten Figure 1O.-Density-pressure relationship for hot pressing a 5 4000 tungsten powder held 30 minutes at 1500" c and 1800" C. a

2000 remains as part of the finished shape, as in a composite nozzle structure.'" 0 Graphite-die hot pressing has been described 0 4 8 12 16 20 24 28 Core Depth in.) by Hoyt, St. Germain and Slosarik, (82) and by Die and press designs for hot Figure 1 %-Tungsten carbide case depth versus pressure for pressing refractory materials have been con- hot pressing tungsten powder of 1 500" and 1800" C. (8) sidered in detail by Jackson.(s5) Figure 10 from White and Jurkowitz(s3) illustrates the for transpiration cooled structures, hot press- densities obtained in graphite dies in 30 minutes ing of coarse powders may be advantageous. at two temperatures for various tungsten pow- In the consolidation of refractory metal alloys ders. Figure 11 illustrates the temperature- (SOW-1OMo-1OCb and 68W-20Ta-12Mo) from density dependence for a fixed compacting elemental powders, hot pressing at temperatures pressure. The carbide case depth formed on above 2100' C has been encouraging where all tungsten appears to be strongly dependent on constituent powders are finer than about 5 particle size as shown in figure 12. microns.(%) The use of substantially coarser It was concluded in this work that densities powders leads to alloy inhomogeneity, requiring above 95 percent can be obtained by hot press- a high temperature postpressing anneal. ing in graphite dies at temperatures from A variation of hot pressing has been developed 1500' to 1800° C and at pressures from 5000 in the last few years, referred to as "gas pressure to 8000 psi for fine particle sizes (<5 microns) bonding." It has been used in consolidating a of either pure tungsten or thoriated powder. variety of metallic and ceramic powders as well Results also indicated that there was little bene- as in making diffusion bonds in parts. Gas fit to be obtained by going to higher tempera- pressure bonding is essentially a technique for tures or pressures. Hot pressing did not appear elevated temperature isostatic pressing using a practical for powders coarser than about 20 gas as the pressure medium. Canned materials microns in view of the low densities obtained are heated and subjected to pressure in a high (less than 85 percent). Where controlled temperature autoclave. The equipment used interconnected porosity is desired, for example, has been described in reference (86). 15 For the consolidation of tungsten powder, powder types and sintering schedules, that pre- temperatures of 2100’ to 3200’ F, times of dictions of rates cannot gct bcyord approxi- 0.5 to 4 h----..UUL~depending on the mass of con- mations. However, data from some of the tained material, and pressures of 2000 to 15 000 references cited should be of interest in this psi are commonly used. The details for press- regard. ing tungsten powder into billets, diffusion According to Matt,(187)for tungsten powders bonding of tungsten foils, or preparation of nose of any particular particle size and sintering cones from tungsten powder, depend on the end temperature, the maximum density will be objective. Contamination inherent in graphite- nearly reached within the first hour of time. die hot pressing is avoided in the gas pressure Additional sintering time will only tend to round bonding technique. On the other hand, use of pore passages, with consequent removal of notch gas pressure bonding methods ordinnrily makes effects and an increase in strength. necessary the removal of the container either by As a rule, temperatures at which the onset solution or by machining. References (87), of densification of metal powders becomes (88), and (89) detail the bonding of tungsten significant are about 0.3 Tm-0.45T,, where T, powder, and reference (90) illustrates tt bonded is the melting temperature in degrees absolute. spherical tungsten product. For tungsten, this temperature is 0.40 T, according to Hiittig, as noted in reference (117). Sintering Normally, sintering is carried out at temper- The general behavior of tungsten powders atures in excess of at least 0.6 T,. In a high during sintering has been studied for the past 50 melting temperalure metal such as tungsten, years. The state of the art as of about 10 years there is an unusual incentive to resort to some ago has been discussed in detail by Agte and sort of (‘activated” sintering, in which case the Vacek”) and briefly summarized by Jones. desired densities can ordinarily be achieved at With the development of tungsten as a space lower sintering temperatures. For example, hardware material, there has been a renewed Brett and Sieg1e‘”l) have found that clean interest in sintering in view of the fact that it is tungsten wire surfaces in contact will not bond still and probably will continue to be the prin- below lG00’ C, but, in the presence of even very cipal means of tungsten consolidation. This small amounts of nickel (0.00001 in. of electro- resurgence is illustrated, for example, by plate on 0.001 in. tungsten wire), bonding and references (92) to (99) for the nonactivated, and neck growth occurred at a temperature as low by references (100) to (115) for the activated as 1050’ C. In work described in reference sintering of tungsten. There has been no (I 15), bromine atmosphere activation was found comprehensive study of data from various to occur between 1200’ and 1700O C, with an investigators in order to arrive at a generally activity maximum at about 1400’ C, at which applicable metal powder sintering theory. As temperature densification was increased by 40 a matter of fact, there is some disagreement percent . among powder metallurgists concerning the The overriding factor determining strength in exact meaning of the term “sintering” itself. a tungsten powder shape that has not reached Kothnri (e5) examined data from a number of full density (and ordinarily 100 percent full sources and concluded that “in general, all the density cannot be reached by sintering alone) results, experimental and theoretical, support is the amount of porosity. The effect of porosity the idea that grain boundary diffusion is the on mechanicd strength has been critically predominant mechanism of material transport reviewed by Knudsen (11*) who found that the during the sintering of tungsten in the tempera- relation between the strength S, the porosity ture range of 1100-1500 C.” For coarse P, and the grain size G of a porous sintered particles in the range 14 to 16 microns surface metal powder or ceramic body can be approxi- diffusion appears to be the dominant operative mated by equations of the form mechanism during sintering.(ee) At the present S= k&ag-bP time, it appears that densification of tungsten powder compacts during sintering is sufficiently where XI, a, and b are empirical constants. In dependent on the properties of individual earlier work on the strength of slip-cast tung- 16 sten, the empirical relationship S=861000e-3.e7p (4) The scatter in data for modulus at various was observed.(110) In general, the drop in strain rates below 1000° F precludes any strength for any particular difference in porosity evaluation; however, within the rate between two as-sintered specimens is greater range investigated, tbe modulus should in the high density than in the lower density be insensitive to strain rate variations. ranges. (5) The creep rate (up to 0.001 in strain) Tables XIII, XIV, XV, and XVI summarize becomes progressively sensitive to tem- mechanical properties data from a series of perature and is somewhat insensitive to measurements on porous tungsten.(lZ0) The stress in the temperature range of 2000O general conclusions reached as the result of this to 5000O F. work were as follows: (6) The creep behavior of pressed-and-sin- (1) A large scatter in mechanical property tered tungsten does not appear to be data of pressed-and-sintered tungsten of influenced by variations in percent theo- 85, 90, and 95 percent of theoretical retical density in the range investigated. density can be attributed primarily to The data presented in tables XI11 to XVI the Variations in processing schedules and are not valid for use in detailed design work, powder lots used by the material sup- but are of value in illustrating gross trends. pliers. The critical study by Jenkins(l1') should be (2) Ductility, as measured by percent elonga- checked for A general summary of the effect of tion and percent reduction of area, was porosity on sintered metal powder bodies. determined to be inversely related to strain rate and directly related to percent Explosive Compaction theoretical den sity . Explosive compaction of tungsten powder to (3) The tensile properties of pressed-and- yield high densities is feasible, but, at the sintered tungsten are strain rate depend- present time, has not found wide application. ent. Relative brittleness at the lower Explosive compaction of met,al powders will temperatures prevented the establish- most likely find application in those areas which ment of consistent quantitative trends. require either large or complex shapes, that

Table XIII.-Mean Tension-Test Results on Pressed-and-Sintered Tungsten of 85% Theoretical Density -

0.2% offset Ultimate Elongation Modulus of Temperature, Strain rate, yield tensile in 1.0 in., Reduction of elasticity, No. of data OF in./in./min strength, strength, % area, % 106 psi points 103 psi 103 psi

Room 0.017 (a) 58.4 0. 0 0. 0 20. 8 4 500 0.017 (a) 55. 3 0. 0 0. 0 18. 3 4 500 0. 1 (a) 43. 0 0. 0 0. 0 30. 0 4 1000 0. 005 (a) 51. 0 0. 0 0. 0 47. 0 2 1000 0.017 (4 47. 6 0. 0 0. 0 17. 0 4 1000 . 0. 1 (a) 55. 0 0. 0 0. 0 32. 6 3 1000 10. 0 (a) 42. 9 0. 0 0. 0 53. 5 2 1500 0.017 37. 9 38. 0 1. 2 2. 6 15. 5 2 2000 0.017 29. 7 30. 0 2. 0 2. 7 14. 0 3 2000 0. 1 39. 5 42. 0 4. 8 6. 5 39. 0 2 3000 0.017 16. 9 17. 4 4. 4 12. 9 9. 5 3 3000 0. 1 21. 6 22. 2 7. 8 24. 6 32. 0 3 3000 10. 0 16. 9 34. 2 13. 2 46. 2 36. 0 1 4000 0.017 6. 8 7. 5 11. 5 15. 3 3. 2 3 4000 0. 1 10. 2 11. 0 5. 4 23. 6 10. 0 2 5000 0.017 2. 8 3. 1 6. 5 11. 5 1. 3 2 5000 0. 1 4.0 4. 1 5. 2 12. 0 1. 8 1 6000 0.017 0. 6 0. 8 12. 4 25. 0 0. 2 1

17 I I I I 1 I 0.273 offset Ultimate Elongation Modulus of Temperature, Strain rate, yield tensile in 1.0 in., Reduction of elasticity, No. of data OF in./in./min strength, strength, % area, % 106 psi points 103 psi 103 psi

Room 0.017 (a) 53. 3 0. 0 0. 0 40. 8 3 500 0.005 (a) 34. 8 0. 0 0. 0 54. 5 2 500 0.017 (a) 39. 4 0. 0 0. 0 40. 4 5 500 0. 05 (a) 40. 1 0. 0 0. 0 51. 0 2 1000 0.005 47. 9 50. 0 3. 2 3. 9 45. 5 2 1000 0. 017 35. 8 39. 6 2. 4 2. 0 34. 2 10 1000 0. 05 (a) 59. 3 0. 0 0. 0 52. 0 2 2000 0.017 28. 7 36. 4 6. 9 16. 0 21. 6 9 2000 0. 05 23. 9 30. 9 9. 7 18. 8 38. 5 2 2000 0. 1 22. 9 29. 2 4. 8 19. 1 37. 5 2 3000 0.017 18. 0 20. 5 8. 8 31. 0 19. 6 6 3000 0. 05 18. 8 19. 8 7. 2 24. 8 26. 0 2 3000 0. 1 22. 3 22. 3 6. 3 25. 6 35. 0 1 3000 10. 0 31. 7 43. 4 12. 0 23. 1 47. 0 1 4000 0. 017 9. 2 9. 7 9. 8 43. 0 (b) 1 5000 0. 017 2. 3 2. 8 8. 8 13. 5 1. 7 2 6000 0. 017 0. 7 0. 7 10. 6 33. 0 0. 6 1 I

- __ ~____~___

Room 0.017 (4 61. 0 0. 0 0. 0 44. 5 1 500 0.005 (a) 36. 4 0. 0 0. 0 62. 0 2 500 0.017 (a) 52. 5 0. 0 0. 0 37. 6 4 500 0. 1 (4 44. 7 0. 0 0. 0 42. 0 2 500 10. 0 (8) 46. 1 0. 0 0. 0 54. 0 1 750 0.005 (a) 44. 8 0. 0 0. 0 52. 0 1 750 0. 1 (a) 44. 8 0. 0 0. 0 48. 0 2 750 10. 0 (a) 33. 2 0. 0 0. 0 45. 5 2 1000 0.005 37. 7 51. 1 10. 7 17. 2 29. 5 2 1000 0.017 41. 0 48. 0 9. 9 28. 9 34. 8 2 1000 0. 1 38. 4 54. 8 9. 9 28. 9 25. 5 2 1000 10. 0 (a) 52. 1 0. 0 0. 0 50. 0 2 2000 0. 017 17. 8 33. 6 31. 5 51. 5 22. 7 3 2000 0. 1 24. 4 41. 2 32. 1 59. 5 21. 5 2 3000 0.017 13. 0 20. 5 16. 8 39. 5 13. 0 3 3000 0. 1 22. 7 24. 2 24. 3 58. 5 15. 0 2 4000 0.017 8. 6 8. 9 8. 5 13. 6 4. 2 3 4000 0. 1 10. 0 11. 1 7. 3 15. 8 17. 0 2 5000 0.017 4. 9 5. 0 8. 0 17. 0 2. 0 1 6000 0.017 1. 1 1. 2 14. 0 15. 5 0. 4 2

______~-~______-~ Table XVI.-Pressed-and-Sintered Tungrien Creep-Rupture Properties I Average Time, in minutes, to elongate Creep elongation strain Density, Test Stress, rate to % of temp, psi 0.1% ;heoretical OF longetion 0.1% 0.5% 1.0% %/see

2000 26800 1. 3 ______------___- 0.0014 92. 5 2000 26800 3. 5 .______------___- 0.0005 93. 2 2000 16800 4.0 ______------_- 0.0004 95. 3 3000 19000 0. 02 0.04 ______0.0833 84. 0 3000 19000 0. 02 0. 38 0. 72 0.0833 85. 0 3000 11900 0. 12 ______- 0.0139 84. 4 3000 11 900 0. 29 2. 32 ______0.0051 85. 5 3000 9500 0.320 2. 144 ______0.0052 90. 0 4000 7780 0. 003 0.012 0.024 0.5555 90. 0 4000 4970 0.019 0. 99 2. 03 0.0087 95. 3 4000 3100 0. 01 0. 05 0. 08 0. 1666 84. 9 4000 3100 0. 15 0. 85 1. 98 0.0111 93. 8 5000 3010 0.006 0.030 0.070 0.2777 90. 0 5000 2330 0.020 0. 086 0. 169 0.0833 87. 6 5000 2040 0. 05 0. 24 0. 45 0.0333 92. 6 5000 1880 0. 140 0.610 0.946 0.0119 90. 0 5000 1270 0. 06 0. 43 1. 45 0.0277 95. 8 would be impractical to compact by more con- and less average diameter did not roll well. ventional powder metallurgy techniqnes. In Coarser powders tended to cohere well and experimental work wit.h t,ungst,en powders, yielded higher as-rolled densities. The best “green” densities as high as 95 to 97 percent combinations of thickness and density appeared have been achieved.(121-122)According bo calcu- to result from powders of high bulk density lations in a Soviet paper, during the shock compacted at 1 fpm, with roll openings of 10 compression of porous tungsten, exceptionally mils and a powder head as high as possible (that high temperatures (up in the 20 000 to 30 000 is, up to 4 inches). Maximum thicknesses K range) may be reached mornentarily.(lz3) attained in 2%-inch-widestrip were in the rauge 60 to 6s mils with green and sintered (1700’ C, Powder Rolling 3090‘ F) densities about 80 to 85 and 90 to 95 Tungsten powder can be consolidated by percent, respectively. Lamination tendencies rolling in an appropriate sheath or can.(124) were not encountered until thicknesses near 70 From a pracbical standpoint, however, this is mils were reached. Strips several feet in length of less interest than t’he cold compaction and were rolled. Mixtures of W with UO, could be sintering by direct rolling. The first effort rolled successfully. In later, related work(128) toward the direct rolling of tungst,en strip from on the flow of tungsten powder into roll gaps, powder is represented by a German patent in it was shown that atmospheric humidity greatly 1902 (DRP 154 998, Nov. 14, 1902).(125)In a affectts the flow characteristics of trhe powder. more recent work, it was possible to roll tung- The humidity effect may depend strongly on sten powder directly into a weak but manage- the type of powder and on the powder particle able strip. (126) Further success was encountered size. in IAOSAlamos work.(127)A horizontal mill was Other efforts to roll tungsten powder in- used to permit vertical feeding into the com- to sheet was reported by Ang, but no date pacting rolls. Tungsten powders varying from were given.(lZ9) A cyclic process for the 1 to 10 microns average particle size were compaction of tungsten into strip has also tested for rollability. Powders of 3 microns been described. (130) 19 Extrusion of Powder Extrusion of a tmngsten powder-bbder ~k- ture is not a new concept, having been used in filament manufacture earlier. There has been a recent suggestion that extrusion of tungsten powder-binder mixtures may be a useful way of making larger shapes. Tungsten powder alone is not well adapted to direct extrusion at ambient temperatures. However, there are indications that tungsten powder can be hot Element Concen- Element Concen- extruded to yield rod and tubing.(132,133) tration, ppm tration, ppm

Vapor Deposition 02 ______engineering procedures. The deposits deposition as a process is applicable under spe- are readily machined. Joining by diffusion cial conditions where less expensive processes bonding is ordinarily preferable to welding, will not yield the required product. which results in a cast structure. A major advantage of vapor-deposited tungsten is its Plasma-Arc Deposition resistance to grain growth. This resistance Investigation of tungsten metal spraying as a appeitrh to be related to the characteristic low- method of building up rocket nozzle throat angle grain boundaries and the absence of strain- shapes was begun about 1958 at a time when related stored energy, both of which would combustion temperatures of newly developed 20 aluminized solid propellants exceeded 5000' F. include thin, high-quality impervious coatings, Because tungsten was difficult to fabricate thick high-quality and relatively void-free de- by conventional methods, flame spraying seemed posits, deposits of varying thickness which may to offer an ideal method of forming nozzle contain intentional or unintentional voids, and hardware or of building up other solid tungsten thick deposits such as forging blanks which may bodies. It was concluded in the final report be subsequently mechanically worked to pro- on this work 05~)that: duce a wrought structure. It is also possible, (1) The plasma (powder) spray process pro- for example, to make graded deposits. Arc- vides a stronger tungsten deposit than plasma spraying of tungsten is often a very good does the arc (wire) spray process. method for making irregular shapes that would (2) Plasma spraying of tungsten in a con- be difficult to produce by some other procedure. trolled environment offers strength and The properties of as-sprayed materials are purity advantages over spraying in air. adequate for many applications, but there are (3) Activated sintering by alloy additions occasions when maximum attainable strength is feasible for improving strength and or some other particular property is required. density of sprayed tungsten. Strength properties in as-sprayed tungsten are (4) Sprayed tungsten rocket nozzles func- commonly improved by a postsintering oper- tioned satisfactorily for firings at mod- ation as described by Brophy et a1'(168)and erate chamber pressures (-600 psi); Singleton. (170) at higher pressures, erosion was en- A substantial contribution to the under- countered. standing of sprayed deposit-substrate bondings Arc-plasma devices are the only practical is represented by NASA work, principally that means of producing and maintaining extremely of Grisaff e and Spitzig.(158-160* 162, In their high temperatures for the spraying of high work, during spraying of tungsten powder melting metal powders. Table XVIII illus- on a molybdenum surface, coating-to-substrate trates the comparative position of the arc- bonding occurred when the substrate was pre- plasma with respect to other heat sources heated to 2200' F or higher. The metallugrical from the standpoint of temperature, stream bond resulted from the growth of substrate velocity, and rate of heat transfer. The grains into the tungsten coating. The molyb- power requirements of any arc-plasma device denum substrates were completely recrystallized depend on its particular use, but 10 to 100 kw even though the total time for preheating, is sufficient for spheroidizing, spraying to con- spraying, and cooldown to 400' F was only solidate, and the manufacturing of ultrafine about 30 seconds. In spraying tungsten on a metal powders. tungsten substrate, it was found that metal- The arc-plasma jet spraying of tungsten lurgical bonds could be effected below the has been covered in detail in a number substrate recrystallization temperature. of places (154--171) and, therefore, will not be described in detail here. Pressureless Consolidation A variety of deposits can be made by the Loose powder sintering has been used t.0 plasma spraying of tungsten powder. These consolidate beryllium powder to densities as

Table XVII1.-Approximate Comparison of Heat Sources (168)

Heat transfer Type of heat source Temp., "C Velocity, rate to object ft/sec in flame, Btu/in.2 sec

20 0-5 400 5 1,800 45 20 70 15,000 250

21 high as 95 to 98 percent of theoretical.(174)Vibra- slip while blending, at the rate of one tory compacting has been used to cornprt’ drcp fm every 10G grams of powder various nietal p~wders.(l~~-”~)A third process to be used in the slip. of pressureless forming, slip casting, is the only (d) The remainder of the powder is slowly one of current technological interest as far as added to the slip while blending. tungsten is concerned. (e) The mixture is allowed to blend for The principal advantage of tungsten powder several minutes after the powder has slip casting as compared to many other fabrica- been added to assure complete dis- tion procedures lies in its relative ease of pro- persion of the powder. duction and low cost. It is, in general, appli- (f) The mixture is poured into a beaker cable for small production runs of complex and the viscosity and pH checked. shape having thin walls. Thick sections, or The average viscosity should be ap- sections too large to be self-supporting on proximately 8000 cps and the pH removal from the mold, or shapes to be exten- should be 6.0 to 7.0. sively deformed mechanically, are usually not (g) The slip ratio and viscosity may be adaptable to slip casting production. adjusted at this point. If the viscosity The slip making process utilizes three basic is low, powder is added to the slip. materials: a powder, a liquid suspending agent, If the viscosity is high, additional and, normally, an additive which helps to blending may drop it to the proper maintain a stable suspension. The most useful level. Otherwise, binder will have to deflocculants appear to be the ammonium and be added to the slip to lower the vis- sodium salts of alginic acid. cosity. The preparation of a tungsten powder slip for (3) The slip is poured into a filter flask and casting a box shape is illustrated by a General vacuum deaerated for 30 seconds. Dynamics as follows. Vibration was applied to the plaster mold (1) The 4-percent ammonium alginate solu- during pouring and for 4 hours after, or until tion is mixed at lenst one day before it is shrinking had virtually ceased. Additional slip used. Distilled or deionized water is was fed into the mold to make up shrinkage recommended for use in preparing the during consolidation. The casting was allowed Mares solution. to solidify over night before removal. This (2) The ammonium alginate solution and particular work was part of our effort to densi- tungsten powder are mixed in a Waring fy slip-formed or plasma-sprayed shapes by blender. HERE” (high energy rate forming). This effort A slip ratio of 1O:l is obtained with to work the slip-formed and plasma-sprayed 2500 grams of tungsten powder and shapes proved impractical in both cases. 250 cc of alginate solution. The ratio Further details of the slip-casting process may be increased according to the applicable to tungsten powders are available in viscosity of the slip after it is mixed. references (172), (173), (179), (180), (1811,

I The entire amount of ammonium (182), and (183). alginate solution is poured into a beaker and the tungsten powder slowly Infiltration added while hand stirring until the The infiltration of porous tungsten skeletons mixture becomes reasonably flowable. to yield a composite structure has been in use Less than one-half of the powder will for some time in the production of electrical have been consumed at this point. Kieffer and associates, in a gen- The contents of the beaker are then eral discussion on infiltration, note a 1916 poured into the blender. This method patent covering porous tungsten-silver or -cop- is preferred over complete mixing in per infiltrated composites.(185) The problem of the blender because it avoids excessive achieving complete infiltration is magnified in splashing of the binder which may the transition from contact materials to porous retard accurate blending. tungsten sweat-cooled nozzle hardware. The Ammonium hydroxide is added to the empirical rules from Kieffer(185)for the production of infiltrated structures (table XIX) control is helpful in reducing the incidence and illustrate the factors which should be considered severity of these defects. Figure 13 illustrates in the infiltration of any large object. In the a properly infiltrated structure.(lSs) The proc- case of a porous tungsten nozzle preform, the essing schedule required to produce any specified principal problem is in getting a form com- struct,ure will vary with the characteristics of pletely receptive to the infiltrant. Commonly, the starting tungsten powder. Table XX(lg9) for a preform approximately 80 percent dense, illustrates the density and interconnected po- the efficiency of silver infiltration is about 90 rosity obtained in a particular set of experiments. percent.(lsR) Major defects in the pressed-and- Calculated pore sizes are listed in table XXI.(18g) sinterd body are gross porosity, flash-sintered Processing details are discussed in references fines, and chemical contamination.(187) Flash (190) to (192). A theoretically based index of sintering may occur in areas within the pressed infiltration of general applicability has been shape where there is a disproportionately large developed by Shaler(lg3)on the basis of thermo- concentration of tungsten fines (particles of dynamic considerations. average diameter less than about 2 microns). Infiltration of tungsten is of particular im- This results in localized high density areas with portance in the production of sweat-cooled associated gross porosity. Closer processing nozzles.

STRUCTURES Tungsten as a structural material for vehicle strength requirements exceed those that can be hardware is characterized by specialized appli- provided by lower melting metals, for example, cations. In general, it is used in, but is not as a backing plate to withstand reentry stagna- restricted to, cases where high temperature tion temperatures. The following sections illus-

Figure 13.-This photomicrograph, taken at 500 X from an unetched specimen, shows the silver evenly dispersed throughout the tungsten matrix. Ultimate tensile strength: Density------16.80-17.76 g/cc Minimum ______50 000 psi Electrical conductivity ______Approx. 33% IACS Typical value- - ______75 000 psi Grain size, ASTM- - ___---____10-12 23 Table XIX.-Working Ruler for Production of Infiltrated Structures

1. There should be a marked difference between the solidus temperature of the skeleton and the infiltration temperature of the liquid. 2. The mutual solubilities of the components at room temperature should be as low as possible. Any significant solubility should occur only at higher temperatures. 3. The development of any eutectic, solid solution, or intermetallic phase which would impede the capillary ad- vance of the infiltrant by means of a volume increase, or by an increase in the fluid viscosity of the infiltrant, should be avoided. 4. The infiltration temperature should be just above the liquidus of the infiltrant in order to minimize mutual solution. 5. Where interference by metallurgical reactions may occur, the infiltration time should be held to a minimum. 6. The presence of non-wettable oxide films should be avoided either by preventing their formation or by their removal prior to infiltration. 7. Infiltration in vacuum should be employed for systems exhibiting poor wetting characteristics. Even under unfavorable conditions, this may practically eliminate residual porosity. 8. Skeleton and infiltrant metals forming oxides which are not reducible by hydrogen should be introduced in pre-alloyed form. 9. In cases where the infiltrant is an alloy, it is advantageous to use an alloy composition in equilibrium with the skeleton at the infiltration temperature. Infiltrants exhibiting significant solubility for the skeleton can be used in the form of presaturated alloys. 10. Where any phase has a low boiling polnt, it may be necessary to infiltrate in a closed container under pressure. 11. If diffusion or annealing treatments are carried out subsequent to infiltration, they should be performed at temperatures below the melting point of any liquid phase, and possibly under pressure. 12. In the case of systems which alloy readily, complete infiltration can be achieved if skeletons with very large pores are used and the infiltration time is kept to a minimum. 13. The use of fluxes of one kind or another may facilitate wetting and infiltration when oxide films are present. 14. The infiltration process can be facilitated by adding to the skeleton interior auxiliary metals which alloy with both components. trate the application of tungsten powder metal- early as 1942, a patent was filed for the injection lurgy to hardware. In the example of ion cooling of a nozzle.(l") Sweat-cooling of tur- rocket engines, the superior contact ionization bine engines was discussed in detail in a 1952 efficiency of cesium on tungsten as compared to paper.(lQ7) The adaptation of porous tungsten that on molybdenum, tantalum, or rhenium, is for this purpose is the result of considerable developmental work. Table XXII lists some possible infiltrants to Infiltrated Nozzle Materials function as coolants during firing. Although The premium for high combustion tempera- some other metals appear preferable on the tures in rocket engine operation is illustrated basis of heat absorption per unit volume (last by the relation between specific impulse and column), silver is currently the preferred coolant combustion chamber (fig. 14) .(lQ4) The most for a number of reasons, including ease of severe consequent temperature and abrasion infiltration, compatibility with tungsten, and problem commonly occurs at the throat section. adequate vapor pressure at exhaust tempera- Where throat dimensions must be closely main- tures. Figure 15 is a schematic representation tained, tungsten is commonly used as the liner of heat absorption in the case of silver infiltrated material ut the constriction zone. With rising tungsten. The preparation of an adequate engine temperatures, the advantage of cooling tungsten preform is necessary both for the is obvious. Various cooling procedures have reception of infiltrant during fabrication and been discussed in detail by Steurer, (Ig5) and will for adequate boil-off rates during firing. For not be considered here. From a gross design these reasons, optimum permeability of the weight per unit area of surface, at very high tungsten preform must be built into the cooled heat fluxes, trsnspirntion (or sweat cooling) is structure. Numerous permeability studies have preferable to practically all other types. As been made for this purpose. In NOTs work 24 ~ ~~~~

Table XX.4ffect of Powder Size, Compaction Pressure, and Sintering Temperature (a) on Sintered Porosity of Tungsten (*w

Powder Porosity, yo Compaction Sintering pressure, ksi iemperature, Size, Shape OF Bulk Intercon- microns nected

7. 6 10 4000 30 30 20 4000 26 26 25 4000 23 23 25 4500 16 13 40 4000 19 17 50 4000 17 15 50 4500 10 4 4. 1 10 4000 16 12 25 4000 12 5 40 4000 10 1 50 4000 10 1 10 10 4000 27 26 10 4500 16 15 25 4000 24 22 25 4500 14 12 40 4000 20 18 40 4500 12 8 50 4000 19 17 50 4500 11 6 20 10 4000 30 30 25 4000 27 27 25 4500 27 27 50 4000 22 22 50 4500 22 22 30 10 4000 33 33 25 4000 30 30 25 4500 29 29 40 4000 25 25 50 4000 23 23 50 4500 23 23

(8) Pressed compacts presintered in dry hydrogen for 6 hours; final sintering in vacuum for 4 hours at the temperature noted.

(ref. 198), Robinson found the permeability, K, Pertinent thermal shock studies, noted in of tungsten powder compacts to be related to references (201) and (202), have been made on porosity 4 by the expression both infiltrated and fully dense tungsten nozzle liner structures. K= a4" A current program has as its objective the development of the powder metallurgy and where n is a porosity coefficient equal to 4.38 infiltration technology necessary for producing and CY is a variable whose value is dependent controlled porosity skeletons for fabrica tion as on the characteristics of the powder in the rocket nozzle inserts to operate in temperature particular compact. Strengths of infiltrated environments as high as 7000' F.(2m) A general tungsten throats are ordinarily increased by analysis of the cooling process in infiltrated increased densification of the tungsten matrix preform. At the same time, this reduces the nozzle inserts during engine firing has been total pore volume available for coolant storage, given by Gessner et a1.(*04) and also the permeability. An optimum density Table XXIII(206)illustrates the type of data appears to lie in the vicinity of 80 percent. obtained in studies on the properties of silver 25 Initial powder Intercon- nected Calculated porosity, pore size, Size, Shape % microns microns

7. 6 23 7. 4 7. 6 17 2. 9 10 12 2. 9 10 6 2. 1 -_Tx - Chamber temperature.*R 20 27 5. 4 m Molecular might of gases. Iblmole 20 22 3. 3 30 27 13 Figure 14.-Specific impulse and exhaust velocity as func- 30 23 6 tions of the chamber temperature and the molecular weight 80 30 34 for various values of k and p1/p2.('~') 80 24 24 percent variation and reflects the state of the art of infiltrated tungsten in 1961. The infiltrated and copper infiltrated nozzle ma- advances in the state of the art since that time terial. The following tentative conclusions have produced a markedly improved, re- were reached following an examination of the producible product. The strength properties data. obtained in the program are, therefore, not 1. The variation in properties measured for representative of currently used material and several vendors' products ranged up to 50 should not be used directly for design purposes.

Table XX1I.-Heat Absorption Above 77" F for

Heat absorbed in To include Btu/cu in. Metal solid state, To melting To include To include vapor 5000" from 77" F % of that point, btu/lb AH,,,, btu/lb AH y, btu/lb F, btu/lb to vapor at absorbed up 5000' F to 5000" F -

7.7 105 153 1,320 4. 7 286 459 5,970 8.6 1,580 2,140 18,050 12.0 296 385 1,638 4.8 31 54 487 7.4 198 286 2,660 12.1 453 570 3,470 1.9 260 446 11,700 8.7 316 474 3,010 14. 1 373 508 2,530 9.6 348 477 3,640 3.2 18 28 482 10.1 240 322 2,370 5.8 71 120 98 1 8.1 282 378 3,480

~~ ~~~__ -- ~~~ ~ ~ ~~ .. ~~ ~ ~ (a) References: Hultgren, R., Orr, R. L., Anderson, P. D., and Kelley, E;. K., Selecfed Thermodynamic Properlies of Metals and Alloua, Wiley (lea). Stull, D. R., and Sinke, 0. C., Thermodynamic Properties ofthe Elements, Am. Chem. Soc. (1956). (b) Vaporizes above 5woO F at 1 atmosphere, % to and including AH,. (c) Solid tungsten to 5000' F, 180 Btu/lb. 26 --

I500 I 1 the two silver tungsten composites manifests 1400 itself in two temperature ranges. I300 P a. An apparent reduction in rotch sen- 1200 sitivity of the tungsten matrix and a resultant Q I IO0 increase in the ultimate tensile strength at- ._s 1000 c tained is effectedover the range 70" F to 700" F. g 900 b. A decrease in ultimate tensile strength Is 800 a is observed over the range 3000" to 4000" F, c 700 which may be attributed to the presence of 600 infiltrant vapors and their disruptive effects ._p 500 c within the tungsten matrices. E 400 4. An analysis of yield strength for the 300 u5 uninfiltrated as well as for all four composite 200 materials as a function of test temperature 100 reveals the following: 0 a. The activation energy governing the (4 LL J n yielding process within the tungsten matrix of all four composites over the range 500' to 2500" F-11 400 cal/mole as compared to -24 600 cal/mole for the uninfiltrated tungsten. b. The activation energy governing the yielding process in all four tungsten composites, Q as well as in the uninfiltrated tungsten, changes significantly over the range 2500" to 3500" F to a common one, -300 000 cal/mole for all five materials, and exhibits extremely high temperature sensitivity over the range 3500" to 4000' F. 5. The stress-dependence of the average creep rate to 0.001 in./in. strain decreases with increasing test temperature, approaching no dependence on applied stress for test temperatures greater than 3000" F. 6. The average creep rate to 0.001 in./in. strain of the uninfiltrated tungsten (85 percent Solid-Liquid theoretical density) is considerably more stress- +Vapor dependent than that of the two copper infiltrated tungsten composites (64 and 78 percent Figure 15.-Schematic representation of heat absorption by theoretical densities). silver in an infiltrated tungsten nozzle liner wall. 7. The creep rate at a specified test tempcra- ture establishes a lower limit of tensile strain 2. The differences in strength levels observed rate below which significant creep-unloading for the two copper and the two silver infiltrated of the specimen will occur concurrently with tungsten composites at a specified temperature tensile loading and modify the true tensile in the range 700" to 3000" F are primarily behavior. attributed to the differences in percent-of- 8. The four infiltrated tungsten composites theoretical-density of the uninfiltrated pressed- tested during this program do not manifest a and-sintered tungsten matrices, rather than to sensitivity-to-strain rate over the range tested, the presence of either infiltrant within the 0.005 to 0.1 in./in./min. tungsten matrix. The dynamic Young's modulus of tungsten- 3. The influence of the infiltrants on the 20 volume percent silver infiltrated material mechanical behavior of the two copper and was evaluated up to 1020' C. Above 800' C, 27 Table XXIII.-Tension-Test Results on Pressed-and-Sintered Tungsten Infiltrated with PO wt % Silver

0.2% Offset Ultimate Modulus of Density of Temp, OF yield tensile Elongation Reduction elasticity, tungsten stress, strength, in 1 inch,% in area,% 108 psi matrix, ksi ksi g/cc - Strain rate=0.006 in./in./min

52. 7 0. 5 0. 0 33. 3 12. 5 69. 0 0. 0 0. 0 27. 9 12. 8 72. 3 2. 5 3. 5 35. 1 12. 9 56. 1 1. 5 0. 0 26. 7 12. 9 49. 1 1. 0 1. 0 (b) 13. 0 32. 1 0. 5 0. 2 (b) 13. 0 19. 0 0. 2 0. 2 20. 7 12. 2

Strain rate= 0.06 in./in./min

88. 5 0. 0 0. 0 31. 8 13. 1 61. 5 0. 0 0. 0 32. 0 13. 1 102.0 1. 7 0. 0 34. 0 13. 1 70. 1 1. 2 0. 0 35. 2 12. 8 68. 0 0. 0 0. 0 33. 5 12. s 38. 3 1. 2 0. 2 49. 0 12. 5 66. 8 0. 6 0. 0 27. 3 13. 1 55. 5 0. 6 0. 0 36. 0 12. 7 56. 6 0. 2 0. 0 34. 5 12. 8 40. 3 0. 2 0. 0 30. 1 12. 8 25. 4 0. 2 0. 2 21. 4 12. 3 21. 6 3. 2 1. 8 16. 1 13. 0 20. 5 2. 5 2. 0 23. 0 12. 7 13. 8 0. 0 0. 0 10. 7 13. 0 14. 5 0. 0 0. 0 16. 1 13. 0 12. 8 3. 2 6. 1 6. 2 13. 0 6. 7 0. 0 0. 0 4. 4 12. 4 4. 8 2. 4 14. 4 (b) _____._____ 4. 1 2. 4 23. 2 5. 5 12. 4 15. 3 0. 0 0. 0 10. 2 12. 7 6. 9 0. 8 2. 7 (b) 12. 8 4. 1 1. 6 4. 8 6. 2 12. 8

Strain rate=0.1 in./in./min

I I I 51. 8 0. 5 0. 0 29. 9 12. 8 73. 4 1. 0 0. 0 24. 4 12. 8 40. 0 0. 0 0. 0 (b) 12. 9 49. 4 1. 5 0. 6 27. 1 12. 9 47. 4 1. 0 0. 2 23. 2 12. 8 25. 4 0. 2 0. 2 21. 4 12. 3 I __

(8) Specimen failed prior to 0.2% offset yield. (b) Modulus not obtainable from stress-strain curve.

28 the modulus decreased more rapidly than ex- Table XXIV.-Tensile Strengths of Copper Composites pected. Above the melting point of silver, the Reinforced With 5-mil Tungsten Fiber 018) modulus of the composite structure was approximately equal to the modulus of a tungsten sample containing an equivalent percentage of Fiber, vol. Diameter, Number of Tensile % in. wires strength, pores. (208) 1Oa pi Reference (207) notes the infiltration of porous tungsten with copper and a copper-2 percent beryllium alloy. The age-hardened copper-beryllium alloy substantially increased the composite strength over that containing 12. 6 0.097 48 47. 7 pure copper. 15. 8 .089 50 52. 0 16. 1 . 125 100 63. 5 High Strength Composites 27. 2 .096 100 98. 3 27. 2 .096 100 100.1 High strength composites in which embedded 28. 0 .191 ______97. 5 tungsten wire functions as a strengthening 33. 6 . 122 177 102.2 agent are of considerable interest as a way of 33. 9 . 113 172% 122.4 reinforcing materials of lower elastic modulus 40. 2 .093 139 137. 0 47. 7 . 117 261 154.5 and lower yield strength. A substantial fraction 52. 7 . 113 267 189. 6 of the work to the present time, in which tung- 53. 6 . 123 325 182. 2 sten fibers have been used, has been carried out 54. 4 .095 197 188. 4 in National Aeronautics and Space Administra- 57. 2 . 118 317 178. 8 tion facilities.(208.208. 212. 214. 218. 21% 220) Much of 57. 3 .121 338 189. 7 62. 4 . 124 372 210.9 the work has been carried out on tungsten- 62. 7 . 077 150 215. 1 reinforced copper in view of the practically 63. 5 .121 369 213.9 insolubility of tungsten in copper. Some other 65. 0 . 112 325>/4 225. 7 fiber matrix combinations that have been 67. 1 .122 400 231. 0 studied are tungsten fibers plus aluminum 67. 4 .119 385 228. 3 68. 1 . 117 371 227. 7 oxide,(210)ceramic mixtures,(211)aluminum ,@la) 69. 2 . 108 323 235. 6 and, in work covered by reference (217), cobalt, 70. 2 . 108 326 238. 0 cobalt alloy, and Nichrome. Reinforcement 70. 2 .085 204 238. 4 theory has been discussed in several papers, for 71. 6 . 118 399 242. 9 example, references (219), (220), and (221). 72. 4 . 083 200 238. 9 72. 9 . 110 356 239. 9 Strengths achieved by the incorporation of 75. 4 . 127 483 249. 8 tungsten fibers in copper are illustrated by 76. 8 . 087 231 254.9 tables XXIV and XXV.(218) The composites were reinforced with either continuous or discontinuous fibers. The tensile properties and (b) Discontinuous reinforcement stress-strain behavior of composites reinforced with either type of fiber were similar, and, in 12. 4 0.143 101 41. 1 both cases, the full strength of the fiber was 14. 1 . 143 117 57. 3 utilized. The ultimate strength of the com- 14.4 .135 105 56. 6 115 62. 6 posites was found to be a linear function of the 18. 2 .126 21. 3 . 125 133 49. 0 fiber content. 23. 3 . 129 154 57. 7 25. 2 . 111 124 91. 1 Tungsten Ionizers 27. 9 .120 161 115.8 32. 3 .lo1 132 110.0 The electrostatic acceleration or ion rocket 32. 4 .117 178 100.3 engine is a propulsion candidate for distant 35. 7 . 101 146 120.0 space exploration. In principle, the thrust 36. 0 . 128 234 90. 3 chamber incorporates an alkali metal vaporizing 37. 7 . 118 210 93. 5 chamber, an ionization section, an accelerator 29 Table XXV.-Modulus of Elasticity of Copper Composite 2. Pores available on approximately 5 micron Reinforced with Tunssten Fiber center-to-center distances eyer the full emitter area Fiber, Modulus of 3. Long term dimensional stability of vol yo elasticity, f0.001 inch (at operating temperatures in excess of 2000’ F for times in the order of 1 to 2 years) (a) Continuous reinforcement 4. Possibility of making reliable joints (braz- ing, welding, and so on) which are vapor 18.7 26.4 tight, and which will not contaminate the 24.3 26. 7 emitter surface. 26.7 28.3 29.0 30.0 Woven wire cloth, finely spaced holes drilled 32.3 29.8 in metal sheets, closely packed wire bundles, 35.9 30.2 and sintered powder compacts made from 55.0 41.6 both angular and spherical tungsten powders 57.6 41.7 have all been investigated as ionizing struc- 64.9 44.4 66.6 44.3 ture~.‘~~~)It has been concluded that porous 67.3 45.6 tungsten materials processed by powder metal- 75.5 48.3 lurgy methods from spherical powder appear Copper 17.7 most satisfactory at this time. However, tung- Tungsten 58.8 sten ionizers still remain a problem from a life standpoint. In a recent assessment, Mickelsen (b) Discontinuous reinforcement and Kaufman note:(243)“The performance of ______the contact-ionization thrustor is severely limited by porous-tungsten technology. The 23.0 26. 1 requirements for submicron, close-spaced pores 25.0 28. 9 without in-flight sintering appear formidable.” 25.7 28.2 26.9 27. 9 Figure 16 from Mickelsen,(za) and earlier from grid, and an electron emitter. It has been described in simple terms in a number of places, for example, in references (222-227). The greatest research and development work in the IO 00 L direction of producing an operable ion rocket r engine has been carried on at National Aero- a) .-E nau tics and Space Administration facilities or F at other facilities under NASA contracts as 0, 100 .-C illustrated by references (228-246). Some ad- L a) c ditional efforts are represented by references .-C (247-249). 0 Tungsten performs a critical function in the ionization section of the engine, and, for that IO reason, much work has been done to develop a workable structure. The basic concepts of the contact-ionization thruster were first presented by E. Stuhlinger in 1954 and 1955.(260*251) I 50 60 70 80 100 Studies of porous ionizers started with the application of porous tungsten in ion engines in Percent of Theoretical Density late 1958. The general metallurgical require- Figure 16.-Sintering rates of porous-tungsten ionizers. ments may be summarized as follows:(247) Compacting pressure, 34 000 pounds per square inch, 1. Pore sizes in the submicron range atmosphere, dry hydrogen. (*43) 30 Turk,(252)illustrates the tendency of porous starting powders, accelerated sintering behavior, shapes made from fine tungsten powders to lack of uniformity of pore parameters, and continue sintering in service tests. insufficient product reproducibility. A second Recent efforts to produce an improved ionizer problem associated with porous tungsten ionizer involve compacting and sintering studies on development has been the general inability in closely graded spherical tungsten powder to the past of ion engine technology to accurately yield average pore sizes of 0.5, 1, 2, and 3 define and specify pore parameter requirements. microns.(27) The use of angular powder for Any future success in ion engine development ionizer preparation has several disadvantages, depends very greatly on successful powder including the difficulty of size classification of metallurgy development of the ionizer itself.

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