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Signature Redacted Ot ILGVOO 16 1960O CMAR THE SYSTEM TANTALUM-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 Signature of Chairman Department Committee Signature redacted on Graduate Students /I ThW U1 4T I r 4m cJ ' 4 d: Z J 2'e AuZab?8tw. .: t :oTsrnr &11 aI a7 e10: raeiI .A. 'to esC0 aib io flinnmevpsR r f - ' - P Ut) (i .. )N29 i~I44uC N ypiui sieM i mf2lo 1S1k., loau.toi9 to ebLsinp? '' .> N N ~ rbissevfl to Oebd nfi A 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 phase diagram has been proposed for the system tantalum-rhenium. Melting point, metallographic, and X-ray studies of arc-cast alloy 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 solid 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 melting point 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 metals 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 refractory metals, commercially pure tantalum shows no observable ductile-to-brittle transition. It is strengthened by addi- tions of molybdenum and tungsten,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 manganese 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 Copper 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 thermal expansion. 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
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