Isothermal Transformations in Eutectoid Zirconium-Niobium Alloys
ISOTHERMAL TRANSFORMATIONS IN EUTECTOID
ZIRCONIUM-NIOBIUM ALLOYS
by
MALCOLM JOHN FINLAYSON
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in th© Department
of
MINING AND METALLURGY
We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE.
Members of the Department of
Mining and Metallurgy
THE UNIVERSITY OF BRITISH COLUMBIA
November, 1957- ABSTRACT
Isothermal transformations in eutectoid zirconium-niobium alloys
have been studied by resistometric techniques at high temperature, and by room-temperature hardness measurements, metallography, and X-ray methods,
Room-temperature measurements were performed on specimens which had been heat-treated in evacuated vycor capsules. The resistometric method gave data which were not in agreement with data obtained by room-temperature measurements. A T-T-T curve established by room-temperature hardness and
metallography was found to be similar to one obtained by a previous investi•
gator. The lack of agreement between measurements made at high temperature
and those made at room temperature suggests that a structural change is occurring in these alloys during the quench from the transformation tempera•
ture. For this reason, room temperature metallography is unsatisfactory for following transformations in these alloys. The analysis of micro-structure
is complicated by the presence of a needle-like •phase' which was not
identifiable by the X-ray techniques employed.
It is shown that the resistometric technique is a sensitive method
for observing transformations in zirconium-niobium alloys. In presenting this thesis In partial fulfilment of the requirements for an advanced degree at the University
of British Columbia, I agree that the Library shall make
it freely available for reference and study. I further
agree that permission for extensive copying of this thesis
for scholarly purposes may be granted by the Head of my
Department or by his representative. It is understood
that copying or publication of this thesis for financial
gain shall not be allowed without my written permission.
Department of ML^^.^ <^~ The University of British Columbia, Vancouver 5\ Canada. Date 7 M 7 ACKNOWLEDGEMENT The" author is grateful for financial aid in the form of a research assistantship provided by the Defence Research Board of Canada. Funds for the present work were nrovided by the Defence Research Board of Canada under Research Grant DRB 7510-18. The author gratefully acknowledges the assistance of Dr. V. Griffiths, under whose direction this investigation was performed, and the technical advice and assistance given by Mr. R, Butters and Mr. R. Richter. TABLE OF CONTENTS Page I. INTRODUCTION ...... o..oo.o«oo. 9. O0.o««*o I A. Object of the Investigation . . . 1 B. The Zirconium-Niobium Alloy System 3 II. PROCEDURE AND RESULTS . 14 A * AllOy Melt 6 1*3.3.1 S o«o90«4>ftoooooo«««e » © # 1A- ,'B.. Preliminary Work 16 C. Isothermal. Transformation Study ...... 23 1. AllOyS ..o.eao.o...... 23 2. Transformations in Vycor Capsules . 25 3. Resistance Measurements ... 30 III. DISCUSSION OF RESULTS AND CONCLUSIONS 49 IV. APPENDICES 1. Resistance Data «.«...... 53 2. Related Phase Diagrams .o...o...... oo 55 3. ASTM d-Spacings for Related Zirconium Compounds ..... 59 V. REFERENCES »»••»«»»•••••»•*•»••••••••••• 60 LIST OF ILLUSTRATIONS Figure Page 1. A comparison of the neutron cross section of zirconium with that . „ „ of other elements ...... 2 2. The zirconium-niobium phase diagram (after Rogers and Atkins) .... 7 3. Room temperature resistivities of zirconium-niobium alloys in two conditions ..oo.. ..»...•. .o ... o.o.oo 7 4. Change in resistance of a 17.5/5 Nb alloy on slow heating ...... 8 5. Variation in lattice parameter with composition for alloys quenched from 1100°C ' 8 6. Microstructure of a zirconium - 15% Nb alloy quenched from 800°C. . . 10 7. Microstructure of a zirconium - 12% Nb alloy quenched from 1250°C . . 10 8. The zirconium-niobium alloy system (after Bychkov et al). 12 9. Hardness vs composition of zirconium-niobium alloys for different thermal treatments 12 10. T-T-T curve for a zirconium - 14.6% Nb alloy (after Domagala). .... 13 11. Diagram of the levitation melting apparatus of Polonis et al .... 17 12. Vickers hardness vs weight percent Nb for as-cast Zr-Nb alloys ... 19 13. Pure zirconium, as-cast 20 14. Zirconium - 4.16% niobium, as-cast 20 15. Zirconium - 4.62% niobium, as-cast 20 16. Zirconium - 14.2% niobium, as-cast 20 17. Zirconium - 13% niobium, as-cast ,.. • ...... 21 18. Zirconium - 19.6% niobium, as-cast ...... 21 19. Zirconium - 4.6% niobium - heat treated . 21 20. Zirconium - 19.6% niobium - heat treated 21 21. Typical ingot produced by the levitation method...... 24 ILLUSTRATIONS (continued) Page 22. Microstructures of a Zr - 17.4% Nb alloy isothermally transformed at 630°C 26 23. Microstructures of a Zr - 17.455 Nb alloy isothermally transformed at 354°C 27 24. Microstructures of a Zr - 17.4% Nb alloy isothermally transformed at 630°C 28 25. Change in Vickers hardness for the 17.4% Nb alloy isothermally transformed at the temperatures shown • 29 26. T-T-T curve for a zirconium - 17.4% niobium alloy based on hardness changes ...... *..».•••.•*•. 32 27. Diagram of furnace and vacuum chamber assembly ...... 33 28. General view of apparatus 34 29. View of main vacuum furnace elements . . 34 30. Diagram of resistance measuring circuit ...... 35 31. Vacuum furnace assembly showing position of furnace and connections. 36 32. Close-up of lid showing the method of attaching the specimen .... 38 33. The change of the ratio of resistance to initial resistance .... 39 34. Change of resistance on heating the near-equilibrium structure of a Zr - 16.4% Nb alloy 40 35. Some typical resistance - time curves obtained on a Zr - 16.4% alloy 42 36. Tentative T-T-T curve for a Zr - 16.4% Nb alloy, based on resistance data .....*•••• 43 37. Hardness change in a Zr - 16.4% Nb alloy, isothermally transformed at 515°C 43 38. Microstructures of the 16,4% Nb alloy isothermally transformed at 515°C 45 39. Needles in a Zr - 16.4% Nb alloy water quenched after 48 hrs.at 800°C 46 40. Back reflection Laue pictures of the Zr - 16.4% Nb alloy 47 LIST OF TABLES Page 1. Some Physical and Mechanical Properties of Zirconium and Niobium. . . 3 2. The Mechanical Properties of Zirconium-Niobium Alloys at Room- Temperature 4 3. Yield Strengths of Zirconium-Niobium Alloys at 649°C ...... 5 4. Analysis of the Alloy Materials of Domagala et al 14 5. Analysis of Foote Crystal Bar Zirconium 15 6. Spectrographic Analysis of Niobium 15 7. Decrease in Gas Content of Niobium on Vacuum Sintering 15 8. Data Pertinent to the Coil Design 17 9. Composition and As-cast Vickers Hardness of Zr-Nb Alloys Prepared by Levitation Melting r 18 10. D-spacings (Angstroms) from X-ray Measurements for Alloys Made with Sponge Zr . . 11. Dimensions of Coil Used to Prepare Alloys of Crystal Bar Zirconium 12. Weight Data for Alloys of Crystal Bar Zirconium Base 23 17„ D-spacings (Angstroms) for the 17.4% Niobium Alloy Isothermally Transformed at 514°C 31 14. D-spacings (Angstroms) for a 16.4% Niobium alloy Isothermally Transformed at 515°C 44 ISOTHERMAL TRANSFORMATIONS IN EUTECTOLD .ZIRCONIUM-NIOBIUM ALLOYS INTRODUCTION A. Object of the Investigation For use within the fission zone of nuclear reactors which have to work at high temperatures-, the usual engineering materials are unsuitable, either because they absorb too many neutrons or react with the fuel. Thus attention has been directed to metals formerly regarded as rare, and great advances have been made in the development of such materials for engineering use. Of the metals with relatively high melting points, zirconium has the lowest absorption cross-section for thermal neutrons (see Fig. l). It was therefore considered to be a potentially important structural or canning material for reactors operating at medium and high temperatures. However, since the creep strength of zirconium at elevated temperatures was found to be poor, extensive studies of the alloying behaviour of zirconium have been undertaken. These studies, which are described in a recent volume,^ constituted a search for alloys having high-temperature strength coupled with the retention of the attractive nuclear and chemical pronerties of pure zirconium. It was therefore the object of the present work to add to the data on zirconium alloys by studying isothermal transformations in zirconium-niobium alloys of eutectoid composition and thereby determining the related Time- Temperature-Transformation (T-T-T) diagram. - 2 - Table I Thermal-Neutron Cross Section (Barns) of Commonly Available Elements With Melting Points of Metallic Elements In Low and Intermediate Cross Section Group* Low Cross Section Intermediate Cross Section High Cross Section ( Table II Thermal-Neutron Cross Sections and Melting Points of Metallic Elements With .Melting Points Above S0O°C Lou I . t Sectiun • Intermediate Crosi Sixtion- Metal Mfltluii Point (~C) Metal Melting Point (JC Beryllium o.oin I2K0 Columbium 1.1 241S Magnesium O.OS't 6S1 Iron 2.4 1539 Zirconium 0. 18 1815 Molybdenum 2.4 2625 Aluminum 0.22 660 Chromium 2.9 1890 Copper 3.6 1083 Nickel 4.5 1455 Vanadium 4.7 1710 Titanium 5.6 1725 Table III Selected Refractory Metals and Alloys Low Cross Section Intermediate Cross Sections ( < 1.0 Barn) r- (<10 Bams. >1.0 Barn)- Metal ff« Metal Zirconium 0.18 Iron 2.4 Molybdenum 2.4 Stainless steels ~1 Nickel-base alloys (Hastelloys, Monels. Inconels, Nichrome) Titanium Figure 1. A comparison of the neutron cross section of zirconium with that of other elements (after Miller2). B, The Zirconium-Niobium Alloy System Zirconium and niobium are both members of the second transition group of elements having atomic numbers of 40 and 41 respectively, Table 1 lists some of their physical and mechanical Drooerties. Zirconium exists in two allotropic modificationsj alpha, which is close-packed-hexagonal and is stable below 862°C, and beta, which is body-centred-cubic and is stable between 862°C and the melting point of 1852°C. Niobium is body-centred-cubic at all temperatures below its melting point of 2415°C. The addition of alloying elements to zirconium affects its allo• tropic transformation temperature. Some elements raise this temperature while others lower itj that is, certain elements stabilize the alpha phase while others stabilize the beta phase. Pfeil3 and Smoluchowski"'" have discussed extensively the theoretical aspects of the alloying behaviour of zirconium. Mcintosh has reviewed the alloy systems of niobium. Of the elements of interest to this work, oxygen, nitrogen, and hafnium stabilize the alpha phasej niobium and tantalum stabilize the beta. It is also felt that hydrogen tends to be a beta stabilizer. The related r>hase diagrams for these elements in zirconium may be found in the Appendix. Table 1 Some Physical and Mechanical Properties of Zirconium and Niobium. Zirconium Niobium Atomic Number 40 41 Atomic Weight 91.92 92.91 Crystal Structure a cph a = 3.232A bcc a = 3.300A c - 5.147A c/a = 1.593 0 bcc a = 3.61A (~900°C) Allotropic Transformation 826 °C Melting Point 1852°C 2415°C Resistivity R.T0/*<-ohm-cm 40 15.22 Hardness R.T. ~70 VPN ~ 40 VPN The earliest work on the zirconium-niobium system appears to have been done by Anderson and co-workers at the U.S. Bureau of Mines.-> Three alloys were prepared containing 0,6, 5.1, and 12.9%* niobium respectively. The 0.6% alloy, which was composed mainly of the Widmanstatten structure, as seen in pure zirconium under similar conditions, contained also a little of a second phase. Pfeil^1 believed this second phase to be retained beta. In the 5.1% alloy, the needles are described as 'fine needles', and small spheroids of a second phase randomly dispersed are mentioned in the description of the 'as- swaged microstructure*. The 12.9% alloy was composed of very large grains filled with finely foliated fern-like patterns. The microstructures of the zirconium-niobium alloys prepared by 7 Litton are not reported, but the mechanical properties at room temperature were appreciably different from those obtained on the alloys of Anderson et al,(see Table 2). TMs may be due to the difference in heat treatment. Litton's alloys were annealed at 725°C. Neither the time of annealing, nor the rate of cooling to room temperature is stated. The Bureau of Mines alloys were annealed 20-30 minutes at 850°C after swaging and then air-cooled. Table 2. The Mechanical Properties of Zirconium-Niobium Alloys at Room Temperature. Author Wt.% Yield Strength (psi) Ultimate Strength Niobium psi 0.05% offset 0.2% offset Anderson et al 0.6 54,600 83,700 Litton 2.5 63,300 87,500 Anderson 5.1 79.800 . 104,700 Litton 7.5 93,700 107,000 Litton 12.5 81,500 102,000 Anderson 12.9 150,300 Litton 17.5 78,700 - 91,200 Litton 22.5 67,500 72,500 Litton 27.5 57,800 74,300 ... ';.„, ... _ 4 Weight percentages are used throughout unless otherwise seated. The decrease in the yield strength of Littons alloys containing above 7.5% niobium, may mean that under the condition of the experiments, the alloys contained an increasing amount of the beta phase. The yield strengths at 649°C of the Bureau of Mines alloys are shown in Table 3. Table 3. Yield Strengths of Zirconium-Niobium Alloys at 649°C. Composition Yield Strength (psi) Wt. % Nb. 0.2% offset 0.6 9,320 5.1 7,910 12.9 10,020 These values are readily explicable on the hypothesis that niobium depresses the a-p transformation of zirconium. The alloy containing 5.1% niobium would appear to contain an appreciable amount of beta phase at 649°C. 8 Simcoe and Mudge observed an increased strength in both 0.5 and 1.0% niobium alloys made with hafnium-containing zirconium and also stated that less than 0.5% niobium is soluble in zirconium at 800°C. o According to Keeler the strength of zirconium is increased by additions of niobium to a content of at least 3%. Keeler"^ also observed a maximum in hardness at about 10 atomic percent niobium and commented on the brittleness of alloys of this composition. In 1952, Hodge-*"'- investigated the zirconium-niobium system UP to about 25% niobium. His data on melting points were not sufficiently numerous to distinguish with certainty between the alternatives of a narrow eutectic horizontal and a wide flat minimum in the solidus curve. Hodge tentatively suggested that transformations in the solid state appeared to indicate that the - 6 - eutectoid in the zirconium-rich alloys lay at about 625°C and 10% niobium and that the solubility of niobium in zirconium at 625°C was near 6%. 12 In 1955, Rogers and Atkins published the Phase diagram shown in Figure 2. Complete mutual solid solubility exists for an interval below the solidus line, a continuous curve with a flat minimum near 22% niobium and 1740°C. The liquidus was not determined, but the narrow range of melting at about 20% niobium was noted. On cooling, the solid solution breaks UP, except at the niobium-rich side, from two causes: zirconium-rich alloys transform under the influence of the B —• a transformation in pure zirconium; . alloys of intermediate composition decompose into two solid solutions below 1000°C. The combined effect is the formation of a eutectoid at a temperature of 6l0°C and a composition of 17.5% niobium. The eutectoid horizontal extends from 6.5 to 87.0% niobium. Some age-hardening effects were observed in the zirconium-rich alloys but the position of the solvus lines remained uncertain. The analysis given for the material used showed that the impurity content of the zirconium was low. This material was low-hafnium (less than 0.05%) grade zirconium which had been produced by the iodide process, melted under a protective atmosphere and fabricated to plates. The niobium analysis was known only approximately, the principal impurities being tantalum (0.5%) and carbon (0.25%) plus a few hundredths of one percent each of iron, silicon, and titanium. Certain of their results are reproduced as Figures 3, 4> and 5 since these data affected the course of the present investigation. Figure 3 shows the comparison of room temperature resistivities for alloys quenched to room temperature after 20 hours at 900°C (curve A) and for alloys annealed at temperatures below the eutectoid in order to obtain the equilibrium structure (curve B). For compositions between 4 and 13% niobium this annealing consisted of 120 hours at 590°C plus 160 hours at 575°C whereas compositions between - 7 i ' . 1 C, 1 , 1 i !• ! ' 1 A o B ! ft P 3 UJ Q- F >-— lit hr hr Cb '•'Cb 3 i i 400 /i a + J 200 I 0 10 20 30 40 50 60 70 80 90 IOC; COLUMBIUM, PER CENT Figure 2. The zirconium-niobium phase diagram (after Rogers and Atkins1 ) 0 20 40 60 80 .100 WT.% NIOBIUM Figure 3: Room Temperature Resistivities of Zirconium-Niobium Alloys in Two Conditions; A, Quenched from 900°C, B, Equilibrium Structure, (after Rogers and Atkins ) - 8 - Figure 4. Change in resistance of 17.5% niobium alloy on slow- heating. Break indicates the transformation of the low-temperature equilibrium structure at the eutectoid temperature. (After Rogers and Atkins-*-2). 3.20! I 1 J 1——I 0 20 40 60 80 100% Nb Figure 5» Variation in lattice parameter with composition for alloys quenched from 1100°C. All are body-centered- cubic structures. No points occur for compositions below 15% since some transformation occurred at the rates of cooling employed. (After Rogers and Atkins^-2). - 9 - 15 and 90% niobium were held for 240 hours at 590°C plus 160 hours at 575°C. Figure 4 shows the plot of simultaneous measurements of resistance and tempera• ture on a wire specimen of an alloy of eutectoid composition as it was subjected to slow heating. The sharp decrease in the resistance at 6l0°C represents the crossing of the eutectoid horizontal. Figure 5 gives the variation of lattice parameter with composition for alloys between 15 and 100% niobium which had been quenched from 1100°C„ All such alloys were in a single phase condition and possessed a body-centered-cubic structure. No points were plotted for compositions less than 15% niobium since the zirconium-rich alloys were seen to undergo some transformation at the rates of cooling attained. The rate of cooling was sufficiently effective to retain the high temperature condition in all but the low-niobium alloys because the transformation in the solid state became increas• ingly sluggish as the niobium content was increased beyond 5%. Alloys rapidly cooled from 900°C were all single phase except those between 44 and 77% niobium. It was also suspected that some precipitation of a from the B-zirconium occurred during rapid cooling. No microstructures were presented in the above report. In 1956, Domagala and Mcpherson"^3 described their investigation of the zirconium-niobium system. From experiments on alloys prepared from iodide zirconium and 'high purity' niobium powder, they obtained data which did not agree with certain aspects of the work of Rogers and Atkins. They described a 15% niobium alloy quenched after 136 hours at 8009C as a two-phase a + 3 structure (see Figure 6). From this and other considerations, they believed that the eutectoid horizontal in this system was at a temperature of 800°G, the eutectoid composition was at 17% niobium, and a continuous series of solid solutions existed only above 1180°C. Rogers and Atkins' reply to this criticism was that their alloys cooled from 700°C and 800°C showed strong lines of two body-centered-cubic phases. Figure 6. Microstructure of a zirconium - 15% Nb alloy quenched from 800°C. Considered to be two phase a + 3 structure by Domagala and McPherson13. Figure 7. Microstructure of a zirconium - 12% Nb alloy quenched from 1250°C. (After Rogers and Atkins13). - 11 - A few weak lines were also present in the X-ray patterns and these were believed to be representative of a which had formed as the alloy passed through the transformation temperature. They presented a photomicrograph of a 12% niobium alloy which was quenched from 1250°C (see Figure 7). This needle-like structure was stated to be characteristic of alloys containing from 8 to 15% niobium quenched from an extensive range of temperatures. It was believed that the network came into existence when the alloy went through the transformation range and that it was probably related to the high electrical resistance and consider• able brittleness of rapidly-cooled alloys in this range of composition. Rogers and Atkins did not believe that this was a two-phase structure. In 1957, Bychkov et al"^ published the zirconium-niobium phase diagram shown in Figure 8. This diagram is similar in,form to that of Rogers and Atkins but the eutectoid horizontal here is at 550°C and the eutectoid composition is at 12% niobium. Also the minimum in the solidus curve occurs at 1600°C rather than at 1740°C as found by Rogers and Atkins. Bychkov et al used iodide nrocess zirconium containing 1.5% hafnium and niobium containing 1.0% tantalum. The methods used by the two groups to fabricate their alloys by hot-rolling and forging were essentially the same. The hardness measurements on the alloys of Bychkov are included as Figure 9. After the experimental part of the present investigation was completed, a report was received of the work performed at Armour by a group headed by 15 Domagala . Figure 10 shows the T-T-T curve which this group obtained for a zirconium-14o6% niobium alloy. Data were obtained by measuring resistivity and hardness of quenched specimens at room temperature. Their alloys were prepared from sponge zirconium and niobium sheet by arc-melting in a water-cooled copper hearth under an argon atmosphere. The ingots obtained were hot-forged to 3/8'' diameter rods and centreless ground to 3/16" diameter. Three to four inch lengths - 12 2500 HOC ^ "00 ° .1800 1/700 >-• § 1500 f A20 ,| ItOO BOO — 700 _5l Zrl0Z03O*0SOS0P0809O!W Bee. %Hb Figure.8. The zirconium^niobium alloy system (after Bychkov et al1^). 20 40 60 80 100 Wt.% Nb Figure 9« Hardness-vs comppsition of Zr-Nb alloys for different thermal treatments'; 1. As-cast. 2. Furnace cooled from 650°C. < 3. Quenched from 750°C. lz (after Bychkov et al 0. - " - 13 - o 800 CO CD CO U HO CO § "TO IDTT TOOTT 1U,U00 Time-Minutes Figure 10. T-T-T curve for a Zr --14.6% Nb alloy (after Domagala1^), of this material were used for resistivity measurements. The heat treatments consisted of first heating the specimen to 1000°C in a globar furnace, in which a dynamic helium atmosphere was maintained. After 8 to 10 minutes at 1000°C, the specimen was quenched into another resistance furnace and isothermally transformed, in a helium atmosphere, for a specific length of time, after which it was quenched to room temperature by plunging it into water. The ends of the rods were then ground prior to the measurement of room-temperature resistivity. A slice was taken off the specimen for hardness measurements and metaliographic observation! the specimen was then heated again to 1000°C and the cycle repeated, The analysis of their materials is given in Table 4. The zirconium used probably contained about 2.0% hafnium. - 14 - Table 4. • Analysis of the Alloy Materials of Domagala et al^ Impurity Sponge Zirconium Niobium As received Arc-melted and Imnurity Wt. % forged bar-stock Oxygen 0.131% 0.124% Ta 0.5 Nitrogen .02% 0.02% C 0.1 Hydrogen 49 vxm. 65 ppm. Ti 0.04 Carbon 0.023% Si 0.02 Fe .042% Fe 0.01 Mg .023% CI .037% II. PROCEDURE AND RESULTS A. Alloy Materials. The zirconium used in this investigation was available in two grades; namely, low-hafnium reactor grade sponge in the form of 5/8'" diameter rod and iodide crystal bar as produced by the Foote Mineral Company. The analysis of the Foote crystal bar is given in Table 5 but no analysis for the reactor grade sponge was received. It is most certain that this latter material was fabricated from arc-melted sponge. Its impurity content would be therefore similar to that of the crystal bar except with regard to hafnium and gas content. The hafnium content of reactor grade sponge is about 0.01%. Although a gas analysis was not given it may be estimated"*" that the oxygen and nitrogen content of the sponge is about 0.12 and 0.005% respectively and that the oxygen content of the crystal bar is about 0,01%. The niobium used in this work was obtained as 4.7 mm diameter rod from Johnson, Matthey and Company. Their analysis is given in Table 6. Although no gas analysis is given, the gas content may be estimated from the data of Table 7 if, as was probably the case, this material was prepared by the vacuum sintering of niobium powder^. Table. 5. Analysis of Foote Crystal Bar Zirconium Impurity Element Weight Percent Si 0.005 Al 0.004 Mn 0.001 Mg 0.002 Fe 0.002 Cr 0.001 Sn nil Ti 0.004 Ni trace Ca 0.005 Cu 0.0005 Mo nil Hf approx. 2„17 Table 6 Spectrographs Analysis of Niobium Impurity Element Weight Percent Ta 0„5 Ni 0.0007 Fe . 0.004 ti 0.012 Table 7 Decrease in Gas Content of Niobium on Vacuum Sintering- Raw Powder After Vacuum Sintering Oxygen 0.9 wt. 1o 0.02 wt. % Nitrogen 0.1 0.01 Hydrogen 0.27 0.001 - 16 - B. Preliminary Work The first problem which required solution was the preparation of alloyso The pronounced tendency for zirconium (and niobium), to take UP large amounts of oxygen, nitrogen, and hydrogen in solid solution required that melting be performed in vacuum or an inert atmosphere. The accepted method for preparing alloys of reactive metals involves arc-melting in a water-cooled copper hearth. Such facilities were not available. However, the levitation 17 melting apparatus of Polonis et al with which the above authors had success• fully prepared titanium-base alloys was available. This melting technique has been amply described1^ and therefore only a diagram of the apparatus will be included here (see Figure 11). It was found that whereas solid zirconium could be levitated with the coil of Polonis, the liquid metal could not, even with a larger source of high frequency power as delivered by a Lepel valve oscillator r rated at 23»5 KVA. A coil design was eventually found which was successful in this regard (see Table 8) and six alloys were prepared. These alloys represent those which were the result of optimum operation, that is, in these cases the metal levitated well, no arcing occurred between coil and specimen, the liquid metal was held in levitation for about 30 seconds, and no contact occurred between the metal and coil during casting. .It may be stated that the improved coil design was only 50% successful - only 8 out of 16 charges to the apparatus yielded ingots which were considered to be satisfactory. These alloys were made from the reactor-grade zirconium and the niobium described above. Homogeneous ingots were obtained by inserting the niobium in a hole drilled in the zirconium specimen. The compositions were calculated simply from the relative weights of the components charged to the apparatus since weighing before and after melting showed a loss of less than one part in 2000. Tree machining brass e/)d plate 'fm i5m . Vie in thick • V in. diom. rubber O ring gasket ( 'Lucite 'cylinder - v in. 0.0. \ (set in groove in brass pJate) • - Vs in. thick 7 in. long. Ve \.n..diom steel rods Induct/on coil Vs tn. GO. copper tubing 0 03H inf wall \ Copper mould- 0. D. 3Ai in | //2'/«in. Tubing to pipe connector \ f in diom. rubber Qringgasket lon-er brass p/ote Sin... sin. *5/-i6Xn, thick Power leads -. '•Wooden platform T To pressure gouge. Argon inlet and vacuum pump Scale Vu in= 1 in. Figure 11. Diagram of the levitation melting apparatus of Polonis et al 17. Table 8 Data Pertinent to the Coil Design which was Successfully- Used to Prepare Zr-Nb Alloys by Levitation Melting Turn No. Coil Diameter 0,D. inches 1 (top) (Reverse turn) 1.618 2 1.610 3 1.450 4 1.322 5 . 1.144 6 1.000 7 0.859 8 0.780 9 0.755 10 (bottom) 0.721 Note: Coil is made from 1/8*» O.D. Conner tubing - wound on T" suitable conical mandrell. Overall height of coil is .1.500 inches. - 18 - Melting was done under argon which had been purified by passing it over P2O5 beads and through a calcium train which was held at 550°C. Just prior to melting this argon was gettered with a hot zirconium filament in the form of o00588 diameter wire. The alloys prepared are listed in Table 9 together with their as cast Vickers Hardness. A plot of as cast hardness vs composition is given in Figure 12. Micro hardness values were obtained with a Bergsman Microhardness Tester mounted on a Leitz Metallograph. A load of 100 grams was used. Table 9. Composition and As Cast Vickers Hardness of Zr-Nb Alloys Prepared by Levitation Melting Wto % Mb As Cast Hardness VPN 0 179 0 175 4o 16 311 4.62 334 5.94 317 13.0 281 14.2 • 19 063 216 | Heat treatments were carried out on some of the alloys by sealing portions of them in evacuated Vycor capsules. The specimens were wrapped in molybdenum coil and zirconium turnings were packed into both ends of the capsule. Representative microstructures of as-cast and heat-treated alloys are shown in Figures 13 through 20„ Powder patterns were also taken of as-cast filings and heat-treated powder. A 11.54 cm powder camera was used with filtered copper Ka radiation. The X-ray data are shown in Table 10 together with the NBS published values1'' for a-zirconium0 - 19 - - o L — g ^ 15 20 Wt. % Nb. Figure 12. Vickers Hardness vs wt. % Nb for as-cast Zr-Nb alloys o - 20 - Figure 13. Pure Zr - as-cast Figure 14. Zr - 4.16% Nb - as- Widmanstatten a. cast. Etch HN03 + HF in Transformed 3. lactic acid X300. Etch as above. X300. Figure 15. Zr - 4.62% Nb as-cast Figure 16. Zr - 14.2% Nb as-cast. Retained B.^r*^5^^^ Retained 8. Etch as above. X300. Etch as above. X300. - 21 - Figure 17, Zr - 13% Nb as-cast. Figure 18. Zr - 19.6% Nb as-cast. Retained 3 + needles. Retained 3 + needles. Etch HN03 + HF in Etch as above. X300 glycerine X300. Figure 19. Zr - 4.6% Nb. Figure 20. Zr - 19.6% Nb 850°C -* 12 hr. 620°C 850°C — 12 hr.620°C —* furnace cooled. —*• furnace cooled. Predominantly a. Eutectoid. Etch as above. X300. Etch as above. X300. - 22 - Table 10 D-Spacings (Angstroms) from X-ray Measurements for Alloys Made from Sponge Zirconium NBS As-Cast As-Cast As-Cast As-Cast Zr-19.6% Nb 3 h.800°C Hf-free Hf-free Zr Zr-6% Nb Zr-13#Nb Zr-19,6# Nb -* l6h.620°O^Fce.cooled a-Zr 2.798 2.793 2.771 2.759 2.573 2.561 2.537 2.494 2.490 2.563 2.459 2 o 452 2.437 2.459 1.894 1.883 2.012 2.350 1.616 1.609 1.879 2.215 1.463 1.456 1.772 1.766 1.757 1.891 1.399 1.681 1.743 1.368 1.356 1.601 1.539 1.608 1.350 1.447 1.433 1.436 1.456 1.287 1.281 1.356 1.313 1.427 1,229 1.226 1.245 I.245 1.390 1.169 1.163 1.164 1.117 1.111 1.363 1.084 1.080 1.343 1.059 1.284 1.036 1.035 1.236 1.006 1.004 1.002 1.108 0.978 0.978 1.081 0.966 0.964 1.055 0.947 0.945 0.942 1.034 0.933 0.930 1.005 0.900 0.898 0.989 0.877 0.876 0.975 0.857 O.966 0.829 0.944 0.82Q 0.935 0.899 0.887 0.875 0.829 0.818 0.809 C. Isothermal Transformation Study 1. Alloys It was decided to follow isothermal transformations in zirconium- niobium alloys of compositions near to the eutectoid value as given by Rogers and Atkins. For'the base material, the iodide crystal bar was chosen because of its assumed low oxygen content. Since oxygen has such a tremendous effect on the physical properties of zirconium it was felt that the advantages gained by using low-oxygen zirconium would greatly outweigh any deleterious effects due to the larger hafnium content of the crystal bar. Hafnium forms continuous solid solutions with zirconium in both the a and 3 phases. Three alloy charges were successfully melted and cast by the levitation method using the coil.of dimensions given in Table 11. Unlike the procedure used previously and by Polonis, where the specimen was simply placed in the-coil, melting was accomplished by supporting the charge in the top third u of the coil on a 0.005 diameter zirconium wire which was attached to a glass, hook in the top plate of the.apparatus. Purified argon was used as before at a positive pressure of 5 psig and was gettered with a hot zirconium filament prior to the melting of the charge. One of the ingots produced is pictured in Figure 21. The data pertaining to the three satisfactory ingots is given in Table 12. Table 12 Weight Data for Alloys of Crystal Bar Zr Base. Alloy Wt. Zr Wt. Nb Total Wt. Ingot Wt. Wt. increase Wt. % in gms. in gms. in - gms. in gms. in gms. Nb Z-17 • 5.1415 1.0100 6.1515 6.1605 0.0090 16.39 Z-18 5.1750 1.0915 6.2665 6.2735 0.0070 17.40 Z-20 5.3885 1.0305 6.4190 6.4245 0.0055 16.04 - 24 - Figure 21. Typical ingot produced by the levitation method. Table 11 Dimensions of Coil Used to Prepare Alloys of Crystal-Bar Zirconium Base. Turn O.D.'» I.D." 1 (bottom) 0.738 0.494 2 0.738 3 0.808 4 0.871 5 0.888 6 0.942 7 1.015 8 1.119 9 1.211 10 1.375 11 1.563 1.326 12 (top - reverse turn) 1.632 Height of coil = 1770•» Coil made of 1/8*' Conner tubing. The increase in weight was assumed to be due to the addition of zirconium wire which was wrapped around the charge (a length of 3 inches of wire - representing the amount believed added, weighs 0.0065 gm). This additional weight of zirconium was included in the calculation of the alloy- composition. The fabrication of these alloys to wires was then attempted. The first alloy tried (16.4% Nb) was wire drawn from a diameter of 0.185*' down to 0.040" diameter. No intermediate annealing was required. Soap was used as a lubricant. However, it was noticed that the wire surface showed copper smears due to the wire dies having been previously used to draw a considerable length of copper wire. The second alloy could only be drawn to 0.110" diameter, the third to 0.175" diameter before severe cracking occurred. It was therefore obvious that the original copper coating in the wire dies had prevented galling of the first alloy and had thereby facilitated the drawing of this alloy to wire. It was decided at this stage to follow two courses; to use slices of the 17.4% Nb alloy, heat-treat them in evacuated vycor capsules, to gather isothermal transformation data, and apart from this to design and construct apparatus wherein the wires of the 16.4% Nb alloy could be heated and their resistance measured with time at different isothermal transformation temperatures. 2. Transformations in Vycor Capsules. Slices of the 17.4% Nb alloy wire (.110" diameter and .05" thick) were wrapped in molybdenum foil and individually sealed in .evacuated Vycor capsules. Zirconium chips were also sealed in with the specimen to getter the atmosphere when at temperature. Each specimen was held at 900°C for 1 hour in a 1" tube furnace and then quickly transferred to another furnace which was held at a lower temperature. Each specimen was held at this temperature for a specific time and then rapidly cooled to room temperature by plunging the capsule (a) 1 hr. 900°C -rt 15 min.630°C -> WQ. (b) 1 hr. 900°C — 1 hr. 630°C — WQ 3 + needles. Anomalous structure - no needles. (c) 1 hr. 900°C--2 1/2 hr, 630°C (d) 1 hr. 900°C — 8 hrs.630°C -* WQ -+ WQ. Precipitation of a + Eutectoid + a. eutectoid. Figure 22. Microstructures of a Zr - 17.4% Nb alloy isothermally transformed at 630°C. Etch HN03 + HF in glycerine. X30O. Figure 23. Microstructures of a Zr - 17.4% Nb alloy isothermally transformed at 354°C. Etch HN03 + HF in glycerine. X300 (a) 1 hr.900°C — 30 min.630°C — WQ. (b) 1 hr. 900°C -* 1 1/2 hr.630°C 3 + needles. -•WQ, 3 + needles + spheroids. Figure 24. Microstructures of a Zr - 17.4% Nb alloy isothermally transformed at 630°C. Etch HN03 + HF in glycerine. X1600 - 29 - into cold water. The capsule was not broken, to increase the cooling rate, because of the small size of the specimen. Other workers^1 obtained more rapid cooling by smashing the capsule on quenching into water but they were dealing *ith specimens 3/8" in diameter by 2»' long and could subsequently grind away any surface contamination. The specimens were mounted in lucite, polished and etched, and examined metallographically. Some of the structures observed are pictured in Figures 22 through 2i+. - Microhardness measurements were taken on each specimen with a Bergsman Microhardness Tester mounted on a Leitz Metallograph. These results are plotted in Figure 25. 280,— ft. , ! 1 i r 1 0V1 10 100 1000 10,000 Time-Minutes Figure 25 Change in Vickers hardness for the 17.U% Nb alloy isothermally transformed at the tempera• ture shown. X-ray patterns were obtained, using a powder camera and filtered copper Ka radiation, of as-cast and heat-treated powder of the zirconium -17.4% niobium alloy. Data so obtained is presented in Table 13. From the hardness data a T-T-T curve was plotted (see Figure 26,) which, 15 it may be seen, is similar to that obtained by Domagala for a zirconium -14.6% niobium alloy (see Figure 10). 3. Resistance Measurements. The apparatus sketched in Figure 27 and pictured in Figures 28, 29, and.31 was constructed to facilitate the measurement of specimen resistance by the use of the circuit of Rogers and Atkins shown in Figure 30. Initially, a zirconium element was used for the self-gettering vacuum furnace winding. However, the poor creep strength of zirconium lowered the furnace efficiency. A tantalum winding was found to be more satisfactory in that its life was much longer than the zirconium winding and its self-regulating property ensured better furnace control at temperature. The furnace was controlled by a Honeywell Circular Scale Controller using a Pt-Pt-lORh thermocouple and was operated on a 220 v circuit. Temperatures were controlled to ±0.5°C. The vacuum system consisted of a mechanical pump, an oil diffusion pump, and a liquid air trap. Pressures of 5 x 10~5 mm Hg or better were achieved at temperature. In the potentiometric circuit a standard 0.1000 ohm resistor was used. The potentials were measured with a Pye Precision Potentiometer in conjunction with a Pye Scalamp Galvanometer. The accuracy of the potentiometer on the range used £0.04%. With reference to the circuit diagram, if connections are made as - 31 - Table 13 D-spacings (Angstroms) for the 17.4% Nb alloy isothermally transformed at 514°C. Values for the as-cast alloy and for a a-Zr (NBS) are included for comparison. a-Zr (NBS) As-Cast 900°C — WQ . 900°C -+ 1 hr, 900°C — 5 hr. 514° C WQ 514°C — WQ 2.798 2.776 2.762 2.671 2.655 2.651 2.573 2.544 2.547 2.459 2.495 2.496 2.463 2.453 2.328 2.354 2.324 2.319 1.894 1.902 1.885 1.879 1.881 1.766 1.763 1.751 1.743 1.664 1.638 1.639 1.616 0 1.613 1.613 1.463 1.466 1.467 1.458 1.441 1.442 1.434 1.428 1.399 1.403 1.396 1.397 1.388 1.368 1.377 1.371 1.362 1.350 1.342 1.346 1.287 1.281 1.229 1.248 1.249 1.243 1.233 1.169 1.168 1.121 1.117 1.112 1.106 1.084 1.084 1.079 1.064 1.061 1.066 1.059 1.053 1.036 1.039 1.036 1.034 1.006 1.016 1.020 1.017 1.005 0.978 0.977 0.966 0.964 0.964 0.947 0.943 0.945 0.942 0.934 0.933 0.933 0.900 0.899 0.903 0.898 0.877 0.893 0.878 0.876 0.857 0.883 0.829 0.829 0.833 0.832 0.817 0.820 0.791 0.788 0.809 800 700 600 500 400 1 1 lb too^" mm mmb Time - minutes. Figure 26. T-T-T curve for a Zr - 17.4% Nb alloy based on hardness changes. - 33 - 4.00' ,< 3.00 f t 7 JUL *—r c o 2.00" 6.00" 4.15' 1.625" C) o -1.52' Figure 27. Diagram of Furnace and Vacuum,Chamber Assembly. 1. Brass can 4. Specimen 7. Measuring 2. Brass lid 5. Lava block thermocouples. 3. Radiation shields 6. Furnace control 8. Self-gettering thermocouple. furnace. - 34 - Figure 28. General view of apparatus showing furnace control elements, vacuum system, transformation chamber, potentiometric measuring equipment. Pressure measuring apparatus is not shown. Figure 29. View of main vacuum furnace elements, Butter's self- gettering furnace, water cooled can, lid showing the thermocouples. - 35 - I Specimen -0 Specimen Temperature -O -O Specimen Potential -0 0 nn 0.1000 ohm Standard Potential v - o 0-100 V ohm '\ 40 ohm 0-100 ma 12 volt Figure 30. Diagram of .resistance.measuring- circuit, (a) Pt, (b) Pt-lORh. - 36 - Figure 31* Vacuum furnace assembly showing position of furnace and connections - with lid and upper radiation shields removed. as shown and a steady current is maintained in the circuit, the resistances of the specimen and standard are in the same ratio as the voltage drops across them. A wire specimen of the 16.4% Nb alloy, 4 cm. long, which had been annealed for 48 hours, in an evacuated Vycor capsule, at 800°C, was attached to the Pt-Pt 10 Rh couples via a «Lava« block connector as shown in Figure 32. Zirconium washers (0.010*• thick) were so placed that one washer rested on the Lava block, the thermocouple on the washer, one end of the alloy wire on the thermocouple, and another zirconium washer between the alloy and the nut. All were held firmly in contact by tightening the nut on the steel bolt. The specimen was placed in the apparatus, connections made, and the system pumped down. The first specimen was heated to 800°C and after 8-10 minutes at this temperature, it was quenched to 455°C by simply cutting the furnace power. The specimen reached 455°C in less than 60 seconds. It was held at this temperature and simultaneous measurements of potential and temperature were taken. After several hours at 455°C it was cooled to room temperature and its resistance measured. The change of the ratio of resistance at time t to the initial resistance at 455°C is shown in Figure 33 together with that for another specimen of the 16.4% Nb alloy which started in the same quenched condition but which was transformed at 507°C. The first specimen was heated again slowly to 800°C and the plot of resistance versus temperature obtained is shown in Figure 34. It was held at 800°C and then quenched to a lower temperature, held for several hours and the change of resistance noted. It was thereafter cycled to 800°C and transformed at lower temperatures several times.This procedure was repeated for the second specimen. Also, the measurements on the second specimen included feeding the potential into a Honeywell Strip Chart Recorder. The resistance changes could - 38 - Figure 32. Close-up showing method of attaching specimen to thermocouples via a small refractory block. Zr washers not shown. - 39 - Figure 33* The change of the ratio of resistance to initial resistance for isothermally transformed specimens. - 40 Figure 34. Change of resistance on heating the near-equilibrium structure of a Zr - 16.4% Nb alloy. be seen more clearly in ,this case but the accurate determination of temperature was sacrificed. Some representative curves for isothermal change of resistance with time are shown in Figure 35. Complete resistance data is given in Appendix 1. A tentative T-T-T curve, for the 16.4% Nb alloy, based on resistance data is shown in Figure 36. In conjunction with the resistance measurements,-a few pieces of the 16.4% Nb alloy wire were sealed in Vycor capsules and transformed at 515°C as has been described previously. These specimens were examined metallographically, microhardness taken, and "powder' x-ray patterns obtained by mounting the wire specimens in a powder camera. Filtered copper Kct radiation was' used. Also, back reflection Lauepictures were taken of these wire specimens with the wire axis perpendicular to the x-ray beam. Filtered cobalt Ka radiation was employed. X-ray data from the 'powder' patterns are presented in Table 14, the change of hardness is plotted in Figure 37, microstructures are shown in Figures 38 and 39, and the back reflection patterns are reproduced in Figure 40. - 42 - 3.6, Time - minutes Figure 35. Some typical resistance-time curves obtained on a Zr - 16.4% alloy. Figure 36. Tentative T-T-T curve for a'Zr - 16.4% Nb alloy, based on resistance data. 240r 0 v I 10 100 1000 10000 Time - minutes Figure 37. Hardness change in a Zr - 16.4% Nb alloy isothermally transformed at 515°C. Table 14 D-apacings (Angstroms) for a Zr-16.4# Nb alloy isothermally transformed at 515°C. Values for the as-cast alloy and for pure a-Zr (NBS) are included for comparison. ' NBS As-Cast 850°C — WQ 850°C — 1 hr. 850°C ^2 1/2 hr. 850°C — 4 hr. a-Zr 515°C -» WQ 515°C —• WQ 515°C ^WQ 2.798 2.911 2.573 2.459 2.486 2.465 2.003 • 2.042 1.894 1.757 1.752 1.734 1.871 1.616 1.835 1.463 1.438 1.433 1.429 1.713 . IL.399 1.572 1.368 1.498 1.350 1.418 1.287 1.229 I.246 1.243 1.241 L.169 1.113 1.114 1.111 1.107 -1.084 1.080 1.076 1.059 1.066 1.036 1.016 1.016 1.017 1.037 1.011 1.006 1.002 1.001 0.978 0.966 0.957 0.947 0.943 0.942 0.941 0.949 0.933 0.939 0.940 0.900 0.883 0.896 0.896 0.896 6.877 0.882 0.880 0.874 0.857 0.829 0.831 0.832 0.832 0.830 0.829 0.820 0.790 0.790 0.789 0.788 - 45 (a) WQ from 850°C. f> + needles (b) 850°C — lhr.520°C -* WQ. 3 + a + needles. (c) 850°C —2 1/2 hr.520°C -* WQ. (d) 850°C —• 4 hrs.520°C —• WQ, a + eutectoid Eutectoid. Figure 38 Microstructures of specimens of the 16.4% Nb alloy annealed 48 hours at 850°C, quenched to 520°C and held at this tempera• ture for the indicated tine. Etch HN03-HF in glycerine. X300. Figure 39. Needles in a Zr - 16.4% Nb alloy water quenched after 48 hours at 800°C. Etch HN03 + HF in glycerine. X2200 (a) WQ from 800°C. (b) 800°C — 1 hr. 515°C — WQ. Figure 40, Back reflection Laue pictures of a Zr - 16.4% Nb alloy wire isothermally transformed at 515°C. Filtered Co Ka radiation - perpendicular to wire axis. (cont'd.) - 48 - (c) 800°C — 2 1/2 hr. 515°C — WQ (d) 8O0°C — 4 hrs.515°C — WQ. Figure 40 (cont'd.) III. DISCUSSION OF RESULTS AND CONCLUSIONS The work of Domagala on a zirconium - 14.6% niobium alloy can be used as a guide in the interpretation of some of the results of the present investigation. The room temperature measurements, such as hardness, on specimens heat-treated in Vycor capsules gave data with which a T-T-T curve could be drawn for the 17.4% niobium alloy. This curve is similar to the one obtained by Domagala for a 14.6% Nb alloy except that it is displaced to longer times. This displacement can be considered to be due to the higher niobium content of the 17.4 weight percent alloy and the different technique of heat treatment employed. The methods used by Domagala would likely result in gas contamina• tion of the specimen. It.is therefore believed that the higher niobium content of the present alloy in addition to its lower gas impurity content stabilized the beta phase to lower temperatures and longer times and thereby accounts for the present T-T-T curve being displaced to the right of Domagala's on the time scale. The needle structure observed in partially transformed specimens was also observed by Domagala and, of course, much earlier by Rogers and Atkins. However, Domagala mentions that this structure was not always evident on metallographic observation. He also states that the structure could be the result of etching. The introduction of hydrogen during the pickling of 20 Titanium-base alloys has been observed to form hydrides whxch are very similar to the black needles seen in quenched zirconium alloys which have been chemically polished and etched. X-ray methods would not be too reliable in establishing a hydride phase because of the low scattering power of the hydrogen atom. The extra lines which appeared on the powder patterns taken of several - 50 - specimens could not be indexed as hexagonal or tetragonal. It was observed, however, that these lines could be indexed to represent either as simple cubic or face-centred-cubic structure and that the calculated d-spacings were quite similar to those quoted by the ASTM for ZrN (see Appendix 3). The possibility that a complex Zr-Nb-H or Zr-Nb-N compound is being formed should not be overlooked. This could explain perhaps the lack of complete agreement between the observed lines and the lines noted by the ASTM for the simpler ZrN and ZrH compounds. The rings obtained on the back reflection Laue pattern of a specimen of 16.4% Nb quenched from 800°C were calculated to be high angle lines of a-zirconium. Although the calculated d-spacings for the extra lines on the powder patterns closely match those for ZrN, it is not believed that sufficient nitrogen could have been absorbed during the wire-drawing operation or during the subsequent heat treatment to result in the formation of a nitride. Another possibility is that what we are seeing is the. result of a shear transformation which produces an intermediate structure. This structure subsequently breaks down into the equilibrium products on holding at temperature as might be interpreted from the microstructure in Figure 24b. That this structure therefore represents the result of a martensitic process is most probable but the possibility that the needle structure is produced by etching should not be discarded. The results from resistance measurements were disappointing in that, although they showed a high sensitivity in observing transformation and indicated a trend to longer times with lower temperatures for the initiation of transformation, successive quenches to the same isothermal transformation - 51 - temperature did not yield the same data. However, it was noted that above 500°C the data showed that initiation of transformation was occurring at approximately the times given by the curve based on room temperature hardness measurements. Around 500°C there was a region of uncertainty as to when transformation was occurring. Below 500°C the resistance measurements indicated a trend to longer times with decreasing temperature in contrast to the room temperature hardness measurements. Records of the change in specimen potential with time as obtained from a strip-chart recorder showed a sharp peak occurring at about 500°C on quenching the specimen from 800°C. This peak occurred within the -60 seconds required to reach the desired transformation temperature. An experiment with a copper wire specimen showed no peak on quenching. It is possible that some a or a* phase is being formed as the specimen crosses the 3/?-+ 3 transus and the tempering of this phase, if a*, is masking changes due to the subsequent decomposition of the retained 3 phase, or else, as has been mentioned above, an intermediate, phase is being formed in this region of temperature and subsequently decomposing. It may be concluded, therefore, that the apparatus which has been designed and constructed offers a sensitive means whereby phase transformations in alloys may be studied. However, certain modifications to the apparatus and technique are required. The apparent structural sensitivity of zirconium alloys to the variables of prior heat-treatment, degree of cold work, tempera• ture cycling, and high temperature soaking time requires that a specimen be used for only one cycle to the isothermal transformation temperature. Although the contact between the specimen and thermocouples was considered to be satisfactory, this contact could perhaps be improved. This could be achieved most efficiently by pressure-welding. The spot-welding of the specimen to the thermocouples is not believed to be a satisfactory solution to the problem. Great difficulties would beinvolved in the design and operation of the requisite spot-welding apparatus which would keep gas contamination in such a small specimen to a minimum It is also apparent that structural changes on quenching as measured potentiometrically must be followed by a high-speed recorder, and that facility must be provided for simultaneous temperature measurement. The strip recorder should also be used for following isothermal transformations above 500°C to resolve whether a martensite process is involved. Rather than substantiate the resistance measurements with room- temperature observations of metallography, hardness, and crystal structure as determined by x-ray methods, it is felt that x-ray measurements at high temperature, using a needle-shaped specimen, would give better correlation. The needle-shaped specimen would present less surface area than powder and thereby minimize gas contamination. This work has therefore indicated the complexity of the phase trans• formations occurring in zirconium-niobium alloys and has described the techniques by which these processes may be studied to eventually determine, by thermodynamic analysis, after the accumulation of considerable data, the mechanisms of the reactions involved. APPENDIX 1 Resistance Data: Temperatures from left to right are in the experimental sequence Resistance values are in OHMS x 10~2. Time is in hours. F i r s t s P) e c i m e n 450 °C 458°C 448° C 546°C 418°C . 515 °C 494° C 538°C 568°C t R t R t R t R t R t R t R t R t R 0.0 3.33 0.050 2.41 0.033 3.23 0.083 3.22 0.017 2.72 0.083 2.86 0.067 2.80 0.067 2.90 0.067 2.98 0.167 3.33 0.067 2.41 0.067 3.21 0.167 3.19 0.100 2,72 0.250 2.86 0.200 2.80 0.133 2.90 0.183 2.99 0.333 3.34 0.617 2,40 0.250 3.22 0.250 3.20 0.500 2.72 0.433 2.81 0.350 2.80 0.233 2.91 0.300 2.99 0.583 3.34 1.680 2.41 0.583 3.21 0.400 3.20 1.116 2.72- 0.517 2.76 0.533 2.72 0.317 2.92 0.380 3.00 0.830 3.32 -2.750 2.31 1.920 3.22 0.517 3.21 2.370 2.72 0.683 2.73 0.650 2.65 0.417 2.95 0.480 3.02 0.867 3.33 3.260 2.28 2.750 3.19 0.983 3.46 3.330 2.72 0.933 2.93 1.133 2.71 0.600 3.13 0.600 3.19 1.116 3.33 3.850 2.29 4.150 3.16 1.230 3.61 3.660 2.71 1.050 2.92 1.480 2.71 0.780 3.23 0.720 3.30 1.450 3.34 4.260 2.27 6.670 3.02 1.450 3.90 4.020 2.71 1.500 2.91 1.000 3.27 0.830 3.34 1.920 3.36 4.760 2.26 16.920 2.58 1.970 4.03 4.410 2.70 2.000 2.91 1.230 3.27 1.000 3.35 2.330 3.38 5.180 2.26 17.780 2.57 3.060 4.10 4.950 2.68 2.780 3.29 1.167 3.42 2.660 3.39 7.760 2.26 3.450 4.11 5.360 2.67 11.080 3.49 2.920 3.39 8,460 2.26 4.080 4.19 5.780' 2.66 12.450 3.42 3.000 3.40 8.860 2.25 4.230 4.18 6.250 2.66 11.750 3.45 9.380 2.25 4.600 4.21 6.450 2.65 L2.410 3.45 21.710 2.26 5.580 4.36 7.500 2.63 13.660 3.45 22.930 2.26 5.710 4.20 10.660 2.60 14.660 3.45 5.920 4.22 11.180 2.60 16.160 3.45 6.400 4.26 11.660 2.58 7.230 4.34 22.310 2.53 7.630 4.34 23.830 2.53 I I Appendix 1 (cont'd.) Sec 0 n d S p e c i p e n 507°C 542°C 476 °C 431°C 493°G t R - t R • t R t R t R 0.050 3.35 0.033 4.83 0.033 4.42 0.067 4.30 0 4.62 0.183 3.36 0.100 4.83 0.200 4.41 0.100 4.29 0.083 4.56 0.233 3.37 0.417 4.89 0.533 4.43 O.250 4.31 0.133 4.55 : 0.417 3.39 0.583 4.90 0.720 4.43 0.400 4.29 0.300 4.63 0.583 3.39 1.000 4.84 1.230 4.45 0.650 4.29 0.833 4.66 0.780 3.40 •1.500 4.81 1.500 4.47 0.830 4.29 0.933 4.55 0.933 3.41 2.370 4.77 2.750 4.23 1.680 4.28 1.000 4.54 1.000 3.42 . 3.450 4.76 3.000 4.21 2.000 4.28 1.500 4.47 1.450 3.45 4.080 4.75 3.660 4.19 2.330 4.28 1.680 4.45 1.680 3.47 4.600 4.72 4.950 4.14 2.780 4.28 2.000 3.48 4.950 4.71 5.500 4.13 3.45 4.27 2.330 3.49 5.920 4.69 5.920 4.12 4.26 4.28. 5.580 3.53 6.670 4.69 6.450 4.11 5.18 4.27 5.920 3.53 9.380 4.67 8.860 4.09 10.660 4.28 6.400 3.53 10.660 4.65 10.660 4.07 23.830 4.23 6.670 .3.54 7.760 3.54 17.780 3.57 21.710 3.57 APPENDIX II Related Phase Diagrams. Nitrogen At. % 25 40 0 5 10 13 Nitrogen Wt. % 1. The zirconium-nitrogen system. Oxygen, Wt. % 2 5 15 25 Oxygen, At'.. % 2. The zirconium-oxygen system. 3. The zirconium-hydrogen system. Tantalum - Atomic %' 2800 - Tantalum - Weight % 4. The niobium-tantalum system. 0 20 40 60 80 100 Tanatalum - Atomic % 5. The zirconium-tantalum system. - 59 - -APPENDIX 3 D-spacings (Angstroms) from ASTM card index for related zirconium compounds. ZrN Zr02 ZrH (approx.) 5 - ZrH £- ZrH 2.64 2.93 2.75 2.76 2.76 2.28 2.52 2.38 2.39 2.49 1.61 1.81 1.69 1.69 2.22 1.38 1.79 1.44 ' 1.44 1.76 1.32 1.55 1.38 1.38 1.66 . 1.14 1.53 1.19 1.20 1.48 1.05 1.47 1.09 1.10 1.38 1.02 1.29 1.07 1.07 1.37 0.93 1.27 0.97 1.24 1.17 0.92 1.13 1.13 1.11 1.10 1.09 1.05 1.08 1.04 1.02 6.99 0.99 0.98 cubic tetragonal cubic cubic tetragonal 4.76 aQ =4.56 a0 = 5.07 aQ = aQ - varies a0 = 4.97 cc = 5.16 between cQ = 4.45 4.77 and 4.78 - 60 - REFERENCES 1. Lustman, B., and Kerze, F., editors, 'The Metallurgy of Zirconium', National Nuclear Energy Series, McGraw-Hill 1955. . 2. 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