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16 1960O CMAR THE SYSTEM -RHENIUM LIBRARN?

by

Peter Schwarzkopf

S.B. Massachusetts Institute of Technology

S.M. Massachusetts Institute of Technology

Submitted in Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF SCIENCE

from the

Massachusetts Institute of Technology

1960

Signature of Author .Of A Department of Metallurgy January, 1960 Signature redacted

Signature of Professor in Charge of Research Signature redacted

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55J1.UNTVJ3 IrnfU6 j6s 2JV )sJUsa $I&IJZX 3 THE SYSTEM TANTALUM-RHENIUM

by

Peter Schwarzkopf Submitted to the Department of Metallurgy January 10, 1960,

in partial fulfillment of the requirements for the

Degree of Doctor of Science

ABSTRACT

A diagram has been proposed for the system tantalum-rhenium.

Melting point, metallographic, and X-ray studies of arc-cast specimens were made.

Rhenium dissolved extensively in tantalum, the maximum solu- bility was 48 weight percent rhenium at,9440*C. The maximum solubility of tantalum in rhenium was 5 weight percent tantalum at 2735*C. Two intermediate phases were found in the system. A sigma phase formed peritectically at 2720*C and existed in a narrow composition range centered at 59.5 weight percent rhenium. The sigma phase decomposed eutectoidally at 2440*C into tantalum solution and the second inter- mediate phase, chi. The chi phase existed fr9m 61 to 80 weight percent rhenium at 2440*C and had a congruent at 2770*C at 79 weight percent rhenium. A eutectic was formed between tantalum solid solution and sigma phase at 2670*C and another eutectic was formed between chi phase and rhenium solid solution at 2735*C.

Thesis Supervisor: John Wulff

Title: Professor of Metallurgy iii.

TABLE OF CONTENTS

Section Page

ABSTRACT ------ii

LIST OF FIGURES ------iii

LIST OF TABLES ------iv

ACKNOWLEDGEMENTS ------v

I INTRODUCTION ------1

II EXPERIMENTAL ------3

III DISCUSSION OF RESULTS ------8

IV SUMMARY AND CONCLUSIONS ------14

V SUGGESTIONS FOR FUTURE WORK ------15

REFERENCES ------17

APPENDIX ------19

BIOGRAPHICAL NOTE ------25 iv.

LIST OF FIGURES Figure Page Number Number 1 Folded Tantalum Sheet Resistance Heating Element ------26

2q. Tantalum-Rhenium Phase Diagram ------27 2b. Detail Tantalum-Rhenium Phase Diagram ------28

3 Dependence of Tantalum Solid Solution Lattice Parameter on Rhenium Content ------29

4 57% Rhenium, 1/2 Hour 2600*C and Quenched (F) X150 -- 30

5 57% Rhenium, 1 Hour 2500C and Quenched (Ta +Q) X350 30 6 51% Rhenium, As-cast (Ta +7) X1000 ------31

7 54% Rhenium, 5 Minutes 2690C and Quenched (Ta + T~) X350 31 8 57% Rhenium, 1 Hour 2450C and Quenched (Ta + U') X350 32

9 57% Rhenium, 2 Hours 2400*C and Quenched (Ta + 1'+ X) X350 32 10 60% Rhenium, As-cast ( 7'+ X) X350 ------33

1 64% Rhenium, 1 Hour 2500*C and Quenched (x) X150 ------33 12 64% Rhenium, 4 Hours 2000*C and Quenched (X + Ta) X350 34

13 Dependence of Chi-Phase Lattice Parameter on Rhenium Content ------35

14 83% Rhenium, As-Cast (X + Re) XIOOO ------34

15 85% Rhenium, As-Cast (X + Re) XOOO36------36 16 88% Rhenium, As-cast (X + Re) XOOO------36

17 95% Rhenium, As-cast (X + Re) X350 ------37 18 81% Rhenium, 1/2 Hour 2730C and Quenched (X + Re) X1000- 37 19 81% Rhenium, 5 Minutes 2740*C and Quenched (X + Re) X1000- 38

20 95% Rhenium, 1/2 Hour 2730C and Quenched (X + Re) X350--- 38

21 95% Rhenium, 4 Hours 2000C and Quenched (X + Re) X350 -- 39

Al Microhardness of Tantalum Solid Solutions ------23a v.

LIST OF TABLES

Table Page Number Number

I Solidus Temperature Determinations ------13

II Tantalum Solid Solution Lattice Parameters ------l3a.

III Chi Phase Lattice Parameters ------I3a. vi.

ACKNOWLEDGMENTS

The author is indebted to Professor John Wulff for his suggestion of the thesis topic and whose advice and criticism were greatly appreciated. Thanks are also extended to Professor

Jere H. Brophy for his supervision and help during the investigation.

The author acknowledges the financial support of the Wright

Air Development Center under prime contract number AF 33(616)-6023 and sub-contract number 3. I. INTRODUCTION

The use of the refractory and their alloys as structural materials requires the knowledge of their equilibrium phase relationships, mechanical and physical properties and oxidation resistance. The fabrication of some of these metals is limited by a ductile-to-brittle transition temperature above room temperature. It is apparent that impurities in these metals raise the transition temperatures observed.

In contrast to other , commercially pure tantalum shows no observable ductile-to-brittle transition. It is strengthened by addi- tions of and ,2 but the transition temperature is raised.

Since rhenium addition to molybdenum and tungsten lowers the ductile-to- brittle transition temperature, it would be worth while to ascertain if rhenium additions to tantalum would increase its strength without an attendant loss in room temperature ductility.

In the initial investigation of this possibility, the limit of solid solubility of rhenium in tantalum and other aspects of the binary phase diagram must first be ascertained. The determination of the phase diagram by melting point, metallographic, and X-ray studies was therefore the pri- mary purpose of the research reported in this thesis.

Several investigators have described the phases present in the tantalum-rhenium system, but no detailed account of the phase diagram has 4 as yet appeared in the literature. Greenfield and Beck reported a solu- bility of 48 to 50 atomic percent rhenium in tantalum, a sigma phase at 41 2.

percent tantalum, and a chi phase isomorphous with alpha from

37 to 25 percent tantalum. Niemic and Trzebiatowski5 observed a lattice parameter a = 9.69 A for the chi phase at a composition of Ta7Re22 but reported no sigma phase. Savitski and Tylkina6 performed a microhardness survey in the system and reported a hardness maximum at 40 atomic percent tantalum and a plateau of low hardness in the tantalum solid solution region.

Knapton, in an X-ray survey of the system, reported results similar to those of Greenfield and Beck.4 3. II. EXPERIMENTAL

Alloys for the determination of the phase diagram were prepared from powders. Commercial 99.7 percent tantalum from Fansteel Metallurgical

Corporation and 99.99 percent rhenium powder from Chase Brass and

Company were used. The powders were weighed to nominal compositions, mixed and then pressed into compacts weighing 5 or 10 grams. The compacts were melted in a non-consumable tungsten electrode arc furnace six times on alternate sides on a water-cooled copper hearth in an atmosphere of titanium- gettered helium at 500 millimeters of mercury pressure. In most cases this procedure yielded buttons of satisfactory homogeneity.

The compositionsof all melted buttons were confirmed by X-ray fluo- rescent analysis. A series of alloys analyzed wet chemically were used as standards. Analysed compositions proved to be within a maximum of 3 percent of the as-weighed nominal compositions.* The accuracy of the technique is estimated to be within 1 percent.

Solidus measurements and high temperature heat treatments were carried out in tantalum or tungsten ribbon resistance heating elements. The ribbon elements are a modification of the Mendenhall wedge blackbody.8 For tem- peratures up to 2800*C, 10 mil tantalum sheet was folded double and formed as shown in Figure 1. The twisted filament shape was necessary to avoid geometric changes caused by . One or more specimens could be fitted in the bottom of the cylindrical cavity in the center of the element. After a specimen was inserted in the cavity, the opening was closed

* All compositions are in weight percent 4. to less than 1/8 inch and the filament was clamped between molybdenum electrodes in a high vacuum chamber. A typical filament two inches long and 3/4 inch deep was heated to 2800*C with a current of 700 amperes at 4 volts. For temperatures between 2800 and 3000C a 4 mil tungsten sheet element was used. The tungsten was warm folded into a wedge and one end clamped in the molybdenum electrode. The other end was held in a segment of 10 mil tantalum sheet twisted to absorb thermal expansion.

This element assembly required 700 amperes at 10 volts to reach 30000C. Specimen temperatures were measured by sighting into the element opening with a Leeds and Northrup optical pyrometer. Two geometric modi- fications of the filament were helpful for solidus temperature determina- tions. A shallow cavity with a depth to opening ratio of less than 3 to 1 permitted direct observation of the specimen with the optical pyrometer.

Since the specimen was clearly visible, blackbody conditions did not prevail, but melting could be directly observed when rounding of sharp features of the specimen occurred. By standardizing against the melting point of.molybdenum, here taken as 2625*C, and keeping the filament geometry constant, the melting temperature was corrected by the following form of Wien's Law:

T To in which T is the true temperature, To is the observed temperature and k is a constant characteristic of the filament geometry and determined ex- perimentally for the molybdenum standard. The resulting corrected melting point served as a first approximation for the second more precise method used for the final determination of the solidus temperature and for alloy heat treatments.

With greater than a 6 to 1 depth to opening ratio the specimen was

indistinguishable from the filament interior which indicated good

blackbody conditions. To determine the solidus temperature, specimens were heat-treated isothermally at temperatures above and below the solidus

temperature obtained by the first method. After several minutes at tempera-

ture, the specimens were quenched by shutting off the furnace power.

Incipient melting was detected by visual or metallographic examination.

The procedure was checked with specimens of pure chromium, molybdenum and tantalum.

Several sources of error were possible in the temperature measurements. These included temperature gradients in the filament or specimen, distortion of the filament geometry by thermal expansion, sight glass absorption, and optical pyrometer errors. No gradients were detected and distortion was minimized by the twisted element shape. A 20*C compensation for the sight glass was added to all temperature readings. This correction was calculated from the observed melting point of molybdenum. The principal source of error was in the operation of the optical pyrometer. The pyrometer used was checked against a new instrument calibrated by the factory and duplicate observations for different operators agreed to within + 10*C. An accuracy estimate of + 20*C was made for all temperatures.

The same deep filament geometry was used for high temperature alloy heat treatments. Because of the irregular shape of the specimens broken from the arc-cast button, no bonding was observed between the specimen and 6. the resistance element and no diffusion of tantalum detected. However, loss of one or the other element by evaporation at the surface was detected metallographically. The specimens were heated in vacuum at pressures of 04 to 10-5 millimeters of mercury as measured by an un- trapped NRC ionization gauge attached to the furnace wall.

Heat treatments at 1000C and 1200C were carried out in evacuated Vycor or silica tubes in a resistance heated muffle furnace. Times of heat treatment ranged from a few minutes at the highest temperatures to

4 weeks at 1000*C. In some cases it was found advantageous to anneal specimens at high temperature before low temperature heat treatment.

Heat-treated and as-cast alloy specimens were examined metallographi- cally and by means of X-ray diffraction. Diffraction specimens were crushed, sieved to less than 200-mesh and mounted on a flat glass slide in a type XRD-5 diffractometer. -filtered copper radiation was used for phase identification and lattice parameter measure- ments. The lattice parameters for tantalum solid solutions and chi phase were determined by step scanning back reflection peaks at 0.01* intervals.

The expected accuracy of the parameter measurements is within 0.001 A.

All single phase boundaries with the exception of the rhenium solid solu- tion limit were determined by the parametric method. In the case of rhenium solid solutions metallographic examination alone was used.

Preparation of specimens for metallographic examination included dry grinding to 4/0 emery paper and final polishing on kerosene lubricated

1 and 1/4 micron diamond laps. All alloys were swab etched with a solution of one part HNO3, one part HF, two parts H2S04, and five parts water. 7.

Phase boundaries determined parametrically were confirmed by metallographic examination in all cases. 8.

III. DISCUSSION OF RESULTS

The proposed tantalum-rhenium phase diagram is shown in Figure 2. Included in this figure are the results of metallographic and X-ray diffraction studies and melting point determinations. The results of melting point determinations are listed in Table I.

An extensive region of tantalum solid solution exists. An alloy of

46 percent rhenium shows evidence by X-ray diffraction of a second phase after quenching from a one-hour heat treatment at 2600*C. The same alloy was single phase after one hour at 2400*C and quenched, but a second phase appeared after annealing for eight hours at 2000*C. X-ray diffraction analysis confirmed that the matrix in this alloy was a solid solution of rhenium in tantalum. The lattice parameter of the body-centered cubic solid solution decreased rapidly with increased rhenium content as shown in Figure 3. The of the change of the lattice parameter made pos- sible the precise determination of the rhenium solubility limit by the parametric method. Metallographic results confirmed that the solubility limit changed slightly with temperature.

At temperatures above 2440*C the second phase in the two-phase region bounding tantalum solid solution was identified by X-ray diffraction as a complex tetragonal structure isomorphous with the sigma phase found in iron-chromium alloys. An alloy of 57 percent rhenium annealed at

2600*C and quenched showed a pure sigma phase metallographically, Figure 4, but X-ray diffraction revealed a small quantity of tantalum solid solution 9. present. The broad black lines in the microstructure are cracks. The same alloy annealed for one hour at 2500*C and quenched, Figure 5, showed a greater quantity of tantalum solid solution present at the grain bounda- ries. A 59 percent rhenium alloy exhibited tantalum solid solution after annealing at 2600*C for one hour and quenched, while in a 60 percent rhenium alloy annealed at 2600*C and quenched, a phase isomorphous with alpha manganese (complex cubic chi phase) was detected. Therefore, the

59 percent rhenium alloy lay in a two-phase region on the low rhenium side of the sigma phase and the 60 percent alloy was in the two-phase field on the rhenium-rich side of the sigma phase. This indicated that the sigma phase occupied a narrow composition region at 59.5 percent rhenium corresponding to the formula Re3Ta2 . The lattice parameter of the sigma phase was a = 9.69 + 0.01 A with c/a = 0.52. The upper boundary of the tantalum solid solution plus sigma phase field was formed by a eutectic horizontal. The as-cast eutectic structure of a 51 percent rhenium alloy is shown in Figure 6. The eutectic tempera- ture found by incipient fusion was 2670*C. Figure 7 shows melting in an alloy heated to 2690*C for 5 minutes and quenched. Areas of tantalum solid solution in the microstructure are part of a divorced eutectic formed by the chilled liquid.

An alloy of 57 percent rhenium annealed at 2450*C for one hour and quench- ed, Figure 8, showed sigma phase plus tantalum solid solution. The same alloy annealed at 2400C for two hours and quenched, Figure 9, showed chil phase in addition to the sigma phase and tantalum solid solution. X-ray diffraction confirmed the decomposition of the sigma phase below 2450*C. 10.

A eutectoid horizontal bounded the two-phase tantalum solid solution plus chi region below 24500C. The extent of the eutectoid horizontal was determined by the extrapolation of the solvus lines determined by the parametric method. The proposed isotherm was placed at 2440*C between 48 and 62 percent rhenium.

The sigma phase formed by a peritectic reaction between liquid and chi phase. No attempt was made rto measure liquidus temperatures and the proposed extension of the peritectic horizontal to the liquidus composition is only arbitrary. The as-cast structure of a 60 percent rhenium alloy is shown in Figure 10. Small areas of chi phase are surrounded by a continuous matrix of sigma. The peritectic reaction isotherm found by incipient fu- sion was 2720*C. The extent of the isotherm found by extrapolation of the chi solvus line was 63 percent rhenium.

The chi phase existed in a broad composition range. A 64 percent rhenium alloy was single phase after a heat treatment of one hour at

2400*C, Figure 11. A precipitate of tantalum solid solution appeared in the same alloy after annealing at 2000C for four hours as shown in

Figure 12. Lattice parameter measurements for a series of single phase chi alloys annealed at 2700*C for one-half hour and quenched were made.

The dependence of the lattice parameter of chi phase with rhenium content is shown in Figure 13 and Table III. The rather sharp decrease of lattice parameter with increased rhenium content enabled the precise determination of chi phase boundaries to be made.

The rhenium-rich side of the chi phase filed was bounded by an almost vertical line at 80 percent rhenium. The solubility of tantalum in chi 11.

phase varied from 61 percent rhenium at the temperature of the eutectoid decomposition 2440C to 66 percent rhenium at 1200*C. Melting point experiments for single phase chi alloys showed a progressive increase of melting temperature from 2720*C at 65 percent rhenium to 2770*C at 79 percent rhenium.

The as-cast microstructure of an 83 percent rhenium alloy is shown in Figure 14. X-ray diffraction analysis identified the matrix as chi phase and the second phase a solid solution of tantalum in rhenium. A eutectic horizontal formed the upper boundary of the chi plus rhenium solid solution two-phase field. The as-cast eutectic structure of an 85 percent rhenium alloy is shown in Figure 15. Figure 16 shows the as-cast microstructure of an 88 percent rhenium alloy. The two-phase alloy con- sisted of primary dendrites of rhenium solid solution surrounded by eutectic.

An as-cast 95 percent rhenium alloy, Figure 17, had a matrix of rhenium solid solution surrounding the chi phase. The microstructure suggested a divorced eutectic. The black spots in the microstructure are pores near the center of the button. A decided inhomogeneity of the button existed as is seen by the greater quantity of the chi phase at the center of the as- cast button. The temperature of the chi-rhenium solid solution eutectic was determined by incipient fusion. Figures 18 and 19 show an 81 percent

rhenium alloy heated to 2730*C and 2740*C respectively. The first shows spheroidized particles of rhenium solid solution after one-half hour at

temperature. The same alloy heated to 2740*C shows evidence of eutectic

liquid which had started to flow in the grain boundaries. The eutectic

temperature of 2735*C was below the maximum melting point of a pure chi 12.

phase alloy of 2770*C at 79 percent rhenium. This observation together with the as-cast microstructures of the two-phase chi plus rhenium solid solution alloys indicated that the chi phase had a congruent melt- ing point at 2770*C. The region of solubility of tantalum in rhenium is relatively narrow. A 97 percent rhenium alloy was single phase after one hour at

2500*C, but the same alloy showed a precipitate of chi phase after a heat treatment of four hours at 2000*C. The decrease with temperature of the solubility limit of tantalum in rhenium is shown in Figures 20 and 21 by the relative amounts and distribution of the chi phase observed after one hour at 2730*C and after 4 hours at 2000*C respectively. The rhenium-rich end of the eutectic horizontal is bracketed between the 95 and the 97 per- cent rhenium alloys.

The melting point of pure rhenium could not be determined by means of the ribbon heating technique. In this case the tungsten sheet element burned out prematurely at 2825*C, the temperature of the rhenium-tungsten eutectic. 9 13.

TABLE I Solidus Temperature Determinations

Composition Temperature *C Temperature *C Wt. Percent Rhenium No Melting Observed Melting Observed

pure Ta 2955 8 2890

32 2800 49 2660 2690 54 2680 2690

57 2670 2690 60 2710 2720 61 2710 2740

65 2700 2710 70 2700 2720

74 2720 2730 77 2700 2740

79 2750 2770 81 2730 2740

89 2740 2760 13a.

TABLE II

Tantalum Solid Solution Lattice Parameters

Wt. Percent Rhenium ao in Angstroms

pure Ta 3.307 2 3.304 8 3.286 15 3.265 22 3.248 23 3.246 25 3.237 26 3.234 32 3.223 40 3.200 41 3.199 42 3.196 43 3.194 44 3.189 45 3.187

TABLE III Chi-Phase Lattice Parameters

Wt. Percent Rhenium a0 in Angstroms

54 9.759 55 9.750 70 9.702 74 9.666 77 9.638 78 9.632 79 9.624 14.

IV. SUMMARY AND CONCLUSIONS

The constitutional diagram for the system tantalum-rhenium in the region above 1000C has been presented. The solubility of rhenium in tantalum ranges from 48 percent rhenium at 2440*C to 40 percent rhenium at 1000C. The solubility of tantalum in rhenium varies from

5 percent tantalum at 2735*C to less than 3 percent tantalum at 2000*C.

Two intermediate phases were found in the system. A sigma phase formed peritectically at 2720*C and occupied a narrow composition range around 59.5 percent rhenium. The sigma phase decomposed eutectoidally at 2440*C into tantalum solid solution and the second intermediate phase, chi. The chi phase had a congruent melting point of 2770*C at 79 per- cent rhenium. The chi phase occuped a composition range of from 61 percent rhenium at 2440*C and 66 percent rhenium at 1200C to 80 percent rhenium at all temperatures. The sigma phase was isomorphous with that found in the iron-chromium system and had a lattice parameter a = 9.69 A with c/a - 0.52. The chi phase was isomorphous with alpha manganese.

A eutectic was formed between tantalum solid solution and sigma phase at 2670*C and 49 percent rhenium. The eutectic isotherm extended from 46 percent to 61 percent rhenium. A second eutectic was formed between chi phase and the rhenium-rich terminal solid solution at 27350C and 16 percent rhenium. The eutectic horizontal extended from 80 to 95 percent rhenium. 15.

V. SUGGESTIONS FOR FUTURE WORK

The determination of the phase diagram forms a part of the develop- ment of tantalum-rhenium alloys for possible practical use. The extensive solubility of rhenium in tantalum permits study of a wide range of tantalum- base alloys. A preliminary investigation was made into the hardness and oxidation resistance of solid solution alloys. These data are discussed in the Appendix. They indicate that small additions of rhenium increase the strength of tantalum but at a loss of ductility. Alloys near the solid solubility limit seemed insensitive to contamination by interstitial dements, and an alloy of 40 percent rhenium showed some oxidation resistance.

This alloy would appear to be of interest in a practical application.

The mechanical properties at high temperature such as hardness on formability and the ductile-to-brittle transition temperatures of tantalum solid solution alloys should be measured. The effect of rhenium could then be determined. However, the strengthening of tantalum by rhenium will probably not prove to be really practical since it can also be strengthened by additions of molybdenum or tungsten. As the ductility of molybdenum or tungsten can be improved by solid solution alloying with rhenium, it would be worth while to determine if tantalum base Ta-Mo or Ta-W can be made ductile by the addition of rhenium. Thus the study of tantalum base ternary systems might be of greater interest for the development of high temperature-high strength alloys. The properties of tantalum-rhenium binary system are an integral part in the investigation of the properties of the ternary system.

The rhenium-rich portion of the tantalum-rhenium system may also prove 16.

interesting for future alloy development. Rhenium, unlike the other refrac- tory metals, does not form a carbide, 1 which suggests that rhenium or rhenium base solid solutions may be useful in carbonaceous atmospheres.

Microhardness results, given in the Appendix, indicate solid solution strengthening of rhenium by tantalum even though the solubility of tantalum does not exceed 5 percent. The high temperature mechanici properties and behavior in reactive atmospheres of rhenium-rich alloys may be useful in certain electronic applications. 17.

REFERENCES

1. Schwartzberg, F. R., Ogden, H. R., and Jaffee, R. I., 'Ductile-

Brittle Transitions in the Refractory Metals," OTS PB 151070; DMIC

Report No. 114, Battelle Memorial Institute, June 25, 1959. 2. Braun, H., Kieffer,. R. and Sedlatschek, K., 1'Hochsmelzende Metalle,"

Third Plansee Seminar, June, 1958.

3. Jaffee, R. I. and Sims, C. T., Technical Report on the Effect of Rhenium on the Fabricability and Ductility of Molybdenum and

Tungsten, to Office of Naval Research from Battelle Memorial Institute,

Contract Nonr-1512(00), April 4, 1958.

4. Greenfield, P. and Beck, P. A., "Intermediate Phases in Binary Systems

of Certain Transition Elements," J. of Metals 8,, #2 (1956) 265.

5. Niemiec, J. and Trzebiatowski, W., Met. Abstr. 24 (1957) 819. 6. Savitski, Y. M. and Tylkina, M. A., !Alloys of Rhenium with High

Melting Metals,1' Zhur. Neorg. Khimii, } (1958) 820.

7. Knapton, 1. G., I'An X-ray Survey of Certain Transition Systems for Sigma Phases,' J. Inst. Met. 26 (1958) 28. 8. Mendenhall, C. E. and Forsythe, W. E., tIThe Relation between Blackbody

and True Temperature for Tungsten, Molybdenum and ,11 Astrophysical

Journal 37 (1913) 380.

9. Dickinson, J. M. and Richardson, L. S., "The Constitution of Rhenium -

Tungsten Alloys,I Trans. ASM 51 (1959) 758.

10. Waite, T. R., Wallace, W. E., and Craig, R. S., IStructures and Phase

Relations in the Tantalum- System from -145 to 70C,"

J. Chem. Phys. 24 (1956) 634. lb.

11. Hughes, J., O'The Alloy System Rhenium-Carbon," Associated Electrical

Industries Report No. A497 (November, 1955). 19.

APPENDIX

In order to evaluate the potential practical applications of alloys of the system tantalum-rhenium, studies of the room temperature microhardness and oxidation resistance at high temperatures were made.

Attention was focused on the effect of rhenium on tantalum in the solid solution region.

Specimens for hardness and oxidation testing were prepared from arc cast alloys and alloys melted in an electron beam. The latter were made from powders weighed to the nominal compositions, pressed into bars, and then sintered in vacuum in a tantalum resistance heating element for one hour at approximately 2200*C. The sintered bars were electron beam zone leveled in a vacuum of 10-5 mm. Hg. Final compositions of the alloys were determined by lattice parameter measurements.

Microhardness measurements were made with a Tukon tester using a

Knoop indenter under a one kilogram load. Measurements for as-cast arc melted alloys are given in Table AI. The microhardness increased rapidly with increased rhenium content after the tantalum solid solubility limit was exceeded and reached a maximum for pure chi phase. A comparison of the as-cast arc melted and electron beam melted solid solution microhard- nesses is plotted in Figure Al. The electron beam melted alloys should be

relatively free of contamination of interstitial impurities such as oxygen, nitrogen, hydrogen and carbon. This is reflected in the lower hardness values of the electron beam melted alloys of low rhenium content as compared

to the arc cast. In an attempt to measure the effect of interstitial 20.

contamination, arc-cast and electron beam melted alloys were annealed in hydrogen. The microhardness results are also plotted in Figure Al.

Hydrogen had a particularly large effect on the low rhenium arc-cast alloys but was negligible on alloys of greater than 30 percent rhenium content. X-ray diffraction analysis of the solid solution series annealed at 1200*C and cooled in hydrogen revealed a tetragonal splitting of dif- fraction peaks at the w mposition 25 percent rhenium, corresponding to the formation of a body-centered tetragonal hydride.10 Orthorhomic splitting was produced by annealing in hydrogen for longer times at 1200*C. This hydride formation was not observed for any other alloy in the solid solu- tion series and was not observed after annealing in hydrogen for 4 days at 1000*C. Cooling the electron beam melted alloys in hydrogen after annealing 2 days at 1000*C resulted in the complete disintegration of alloys of less than 30 percent rhenium. As in the case of the arc-melted alloys the hydrogen treatment had little effect, on alloys near the solid solu- bility limit which showed the relative insensitivity of these alloys to contamination. The electron beam melted series showed clearly the hardening effect rhenium has on tantalum. The 15-percent electron beam melted rhenium alloy shattered when hammered at room temperature. This could be taken as the rough limit of rhenium content before the room temperature ductility of tantalum is destroyed. 21.

The hardness of rhenium solid solutions increased rapidly with small additions of tantalum. Chi phase further hardened these alloys but with accompanying embrittlement.

The oxidation kinetics and scaling characteristics of tantalum- rhenium alloys must be known if these alloys are to be considered for potential high temperature use. A series of tantalum solid solutions were oxidized at 1000*C and 1200*C. Their relative oxidation rates were measured and the scales formed were examined metallographically.

To minimize edge and corner effects arc-cast specimens of spherical shape were oxidized at 1000*C and 1200*C. Metallographic examination of the scales formed revealed in all cases a black adherent layer adja- cent to the metal surface and a light colored flaky scale on the outside surface. The nature of the outer scale varied for different alloys.

Alloys of up to 25 percent rhenium had a very flaky outer layer which spalled readily both in the furnace and on cooling. The outer layer on the 30, 40 and 45 percent rhenium alloys was more adherent. The results for these alloys are listed in Table AII. The weight decreases were the result of spalling and volatilization of the oxide. The rate of the depth of penetration of the oxide was measured on cylindrical specimens of electron beam melted alloys. After 2 hours at 1000C only the 40 and

45 percent rhenium alloys could be determined as alloys of lower rhenium content had deteriorated too rapidly to be measured. The depth of oxide penetration of the 40 percent rhenium alloy was roughly one-half that of the 45 percent alloy. Volatilization was the most severe for the 45 percent rhenium alloy. 22.

TABLE Al Microhardness Results

Knoop Hardness Number Arc-cast Alloys Weight Percent Rhenium Phases As-cast Hydrogen Annealed

pure Ta Ta 459 769 5 i" 650 896 8 t' 706 956

15 737 --

20 II -- 842

25 726 769 30 II 715 758

32 706 --

41 668 --

43 I' 632 650

48 Ta + T 659 911 58 I' 1243 1423

61 IT+X 1452 --

78 x 1512 --

80 1645 --

83 X+ Re 1577 1681

88 te 1113 -- 95 632 632

97 Re 356 --

98 'I' 320 --

99 243 -- ii 100 206 -- 23.

TABLE AI (Continued)

Knoop Hardness Number Electron Beam Melted Weight Percent Rhenium Phases As-cast Hydrogen Annealed

pure Ta Ta 132 --

10 l 352 --

15 ' 492 --

20 529 696

25 'S 650 706

30 II 659 696 40 668 716

43 II 624 650 FIGURE Al

Microhardness of Tantalum Solid Solutions Io0oc

LU 80C z- z cz) (1) 600 z 0 -I

Q- 40C - A ARC CAS T 0 o BEAM 0 ELECTRON z MELTED V ARC CAST HYDROGEN cn ANNEALED 200 O ELECTRON BEAM MELTED HYDROGEN ANNEALED ch

0 10 20. 30 40 50 60 WEIGHT PERCENT RHENIUM 24.

TABLE AII

Oxidation Results

Specimen, Percent Rhenium 30 40 45 30 40 45

Temp. Time, min. weight change, mg/cm2 5 -21 +4 -11 10 -31 -1 -42

10000C 25 -67 -16 -65 60 -107 -044 -144

5 + 17 + 3 -13 10 -1 - 2 -28

1200C 25 spalled -17 -78 60 -48 deep cracks volatilization inches penetration after 2 hrs, spalled 0.01 0.02 1000*C on 0.15 inch dia. rods 25.

BIOGRAPHICAL NOTE

The author was born in Reutte, Tyrol, Austria, on September 25,

1936. He was brought to the United States in 1938 and was naturalized

in 1945. After completing his elementary education in public schools

in New York City, he attended Concordia Collegiate Institute in

Westchester, New York. After graduation he attended the Massachusetts

Institute of Technology where he received an S.B. in Metallurgy in June,

1957. He then entered the Graduate School at M.I.T. and completed work

for an S.M. degree in the Department of Metallurgy.

While at M.I.T. the author has held appointments as a teaching

assistant and research assistant. Figure 1: Folded tantalum resistance element 27.

FIGURE 2a

Tantalum-Rhenium Phase Diagram I I I I I I I I I 3500 o ONE PHASE, A MELTING OBSERVED oc * TWO PHASES + BOUNDARY BY X-RAY 6000 a THREE PHASES NON-EQUILIBRIUM ",1 3000 LIQUID L + X X+L 0' L+o- I 5000 TA +L 0 0 TA + 2500

0- - 14000 0 2000 0 0 0 0 0 c*O@ 00t TANTALUM SOLID TA + X x X+ RE SOLUTION 3000 0 RHENIUM I 1500 - SOLID 0 0 . 4 SOLUTI ON

0 0 0 0000 0 000 +0 ~aa 0+ 0* 2000 1000 - .0o 0 0 0000 0 000 0

I I I I I . I 0 10 20 30 40 50 60 70 80 90 100 TA RE WEIGHT PERCENT RHENIUM 28.

FIGURE 2b

Detail Tantalum-Rhenium Phase Diagram o ONE PHASE I A 'MELTING! OBSERVED -eTWO PHASE + BOUNDARY BY X-RAY OF LIQUID L+X 2800 TA +L L+o - 5000

2600 000

0 4500 TA 0 x

2200 TA +X - 4000

owe0 0 -+ 2000 000 3500 40. 45 50 55 60 65 WEIGHT PERCENT RHENIUM 2.9.

FIGURE 3

Dependence of Tantalum Solid Solution

Lattice Parameter on Rhenium Content. I I I I I I 1 1, II 4 3.300

3.250

3.200

I I I I I' I I I i~9 3.1501 I., I I I I I 0 I 10I I 20I 30 40 50 WEIGHT PERCENT RHENIUM 30.

IG. 4 - 57% Rhenium, 1/2 Hour 26000c and Quenched (gr) X150

6 P3 * 0 z d , 9 &O '~d~

- A . o ' .y -0o - b~~cJ~O j o Li 0 * cjo8 o1 f

~0 0 0 0 'i0i l

0, 0

- * - *

0 FIG. 5 - 57% Rhenium, 1 Hour 2500 C and Quenched (Ta + q-) X350 31.

FIG. 6 - 51% Rhenium, As-Cast (Ta +U') X1000

p J - '0 'e,. (Ci) C) 7' ) 0

L I. '7,-.'' X U

1 (9'

4 K (J~

FIG. 7 - 54% Rhenium, 5 Minutes 2690*C and Quenched (Ta +V-) X350 32. VIC:;i~rP ey b0na 4ex?[

a wo

3 JJ

*~c*":~"''~ 4~cc~Q &,o

LJKu

t(

0 FIG. 8 -57% Rhenium, I Hour 2450 C and Quenched (Ta +) X350

CO9

24000C and FIG. 9 -57% Rhenium, 2 Hours Quenched (Ta + q+ X) X350 33.

~Of

leg

6 V6

F~IG. 10 -60% Rhenium, As-Cast (~+ X) X350

FIG. Hi - 64% Rhenium, 1 Hour 2500*C and Quenched (X) X150 34.

S 011 -- a ~ IM C. - -

(1 ( 1 -

.t.- It

{ - -C t 4 e/ *~e

COO

0' * A

IG. 12 - 64% Rhenium, 4 Hours 2000*C and Quenched (X + Ta) X350

too a

FIG.14 83%RheiumAs-ast X+Re) 100 35.

FIGURE 13 Dependence of Chi-Phase Lattice Parameter on Rhenium Content. I I

9.7501- .0

9700

0' 9.650[-

9;600 I. I 60 ~70I., 80

WEIGHT PERCENT R HENIUM 36.

.1 .11 1 .

*; N ; % * 1* 1 . , IV/ II / 4 //"

All 4 4 1 ~ 111 fill

W1 44 It~

P_ .f4., I 4 It

of's 15 -Q k5 RhnuA-a t(X +1 Re) X

'.* .A' LI f " f- I 1 1 , .-

IG. 16 8% Rhenium, As-Cast (X + Re) X1000 37.

cr- 0r 0 4

I7 ~ 0 Q0

- - *. o

- 4* . Q.,

00.C

'FIG. 17 - 95% Rhenium, As-Cast (X + Re) X350

U 0*~~ '*~~ 0 0. OV 00 v * 0 * 0 - *t 'e. I * S S a * *0 p *a, 0 S S. dop O~e ~ S * U 0 .0o * * C . 5 0 S e -, 0 9 S * 6 S .-. S 0 0 * 0 - 0 O . .- 0~ . 0 0 . a 0 :. 0 0 ** 00 0 46I a . '

.0000-0 0 .* *1'

.. 0 * . , 0 o *o 0 .~ ' IL- 0 :01.0 0

FIG. 18 - 81% Rhenium, 1/2 Hour 2730*C and Quenched (X + Re) X1000 38.

000

C7C

4f 0

0 fFIG. 19 -81% Rhenium, 5 Minutes 2470 C and Quenched (X + Re) X1OOO

A0 AoD* , I 9 0 0 0 O 0 0 o~ 0 0 * 0 0 a IV 9 00 0 0 00 r- 00 0C> 0D *0 / 000o 0 0 o.O 0 00 *~ 0A* 0 0.0 1 0 . 0 00 0 *00 PO o 0 0 0 p 00 0 0 00 0, a ~ 000 00. 00. 00 k0 0 00/1)* . 0 a 0 0., 0 C O 0 00 0 - 0 0 0 v 0 o 0 0~ .%v0 0.0 Q, . .. 0 0 0 .0 00 0 0 -0 0 .000 *a * 0 0 00 0 0 0 .. . 0 00 0o 0 a 0 0 0' fi 0 0 0* , 0 Q i 0 A. A A eCoVS 0 . C, Cp 0 0 0 * 0 O ~ ~ as 1. 0000 0 0 0 0 0 0.00 0 0 0 ? o ~ 0 .0 oO1 .0*

0~0 000 .0.$o0 0 0 0Dof 0~ 0.0 0 00 o 70~ 00 0 1 0 ' 000 000 0 .0 *Z 0 *D 0oo 0 0 ~ * o 0 0 0. 0 0 It 0~ * 0 0 00 *0 00.~ . 0 0o 0 0 0 0 *~ 0 '0 0 0, 0 0 0 -0 0 . 00 .00 ot 06. e 0 00 0 00 /1 * e Soi ~~ 0 SO

~~~'~20 FIG o. 95 Rhnim 0/ or2 and Qenchd (X+ Recoo5 39.

00

c. 0

-C* *'

, 0%,

Db0b &sZ-

0 FIG. 21 - 95% Rhenium, 4 Hours 2000 C and Quenched (X + Re) X350