The System Zirconium-Carbon by R. V. SARA Research Laboratory, Carbon Products Division, Union Carbide Corporation, Parma, Ohio Phase relations in the system zirconium-carbon were established by DTA, X-ray, and metallographic methods. Only one phase, a monocarbide with the NaCl structure, was found. The low-carbon phase boundary for the monocarbide contains 38.5 at.7, carbon between 1900' and 3300OC. The carbon-rich boundary between 2850" and 330OoC is at 48.9 at.% carbon. ZrC melts congruently a t 342OoC and 46 at.% carbon. T h e lattice parameter versus composition curve shows a maximum a t 46 at.% carbon. The Zr-ZrC eutectic is at 186OoC and 1 at.% carbon; the Zr-ZrC eutectic is at 285OoC and 65 at.% carbon. Liquidus curves are inferred from the invariant points. tions containing 35.4 and 48.7 at.% respectively. Beneiovsky and Rudy' reported 4.675 and 4.694 A for 35 and 50 at.% carbon, respectively. Zirconium is reported to melt a t 1852' =t2'C8 and can dissolve a maximum of 0.35 wt% carbon near the melting temperature.F 11. Experimental (1) I. Introduction of idormation cxists for thc binary systems containing group I V metals with carbon. This lack of information can he attributed largcly to the experimental A Materials The starting materials for the present study were Union Carbide spectroscopic graphite powder, grade SP- 1 and zirconium hydride powder supplied by Metal Hydrides, Inc., Beverly, Massachusetts. The graphite contained impurities at levels of 0.5 ppm or less. The hydride, according to the manufacturer, had the following analysis: hafnium, 94 ppm; nitrogen, 80 ppm; and hydrogen, 2.12%. Hydrogen content was verified by analysis in this laboratory. PAUCITY ( 2 ) Methods and Apparatus Samples with controlled compositions between 0 and 90 at.'% carbon were prepared by tumbling graphite and zirconium hydride powders overnight. Small pellets, 3//16 to in. in diamctcr and of similar length, were formed by coldpressing without a binder. Presintering these pellets at about 800°C in vacuo provided sufficient strength to permit the drilling of a small axial hole in each sample. This procedure simplified the problem of suspending samples in the €urnace. Phase relations in thc system zirconium-carbon were established by DTA, annealing of samples followed by rapid cooling or quenching, and subsequent analysis by chemical, metallographic, and X-ray techniques. (A) Furnace Equipment: Two types of heating assemblies were used for preparing samples for phase analysis. Studics to approximately 2850OC were carried out in a vacuum resistance furnace12 containing tungsten, tantalum, or graphite tubular heating elements. The graphite heater assembly was used only for the study of high-carbon alloys or for DTA experiments. Induction heating with a flux ~ o n c e n t r a t o rwas ~ ~ used for studies on zirconium-carbon alloys above 2850°C. The copper concentrator linked the field, or energy from the induction coil, to the sample or crucible. A Pyrex-glass mantle was close-fitted over the concentrator assembly and either vacuum or suitable gaseous environments could be coiiiplcxitics associated with achieving the controlled high temperatures and environments rcquired for these evaluations. A second factor has been the limited availability of very high purity metals, particularly hafnium and zirconium. Several hypothetical configurations of the system zircoiiiun-carbon have been published', but thcse are based on insufficicnt expcrimcntal data. One fact which seems to be reasonably well established in the literature is that ZrC is face-centered cubic and constitutes the only compound in the system. Considerable variation exists in reported data for the systcm zirconiurn-carbon (Table I). Similarly, numerous cell constants have bcen reported For ZrC, but thcse have little significance bccause of the absence of corresponding chemical analyses or bccausc the low values reflect contamination by oxygcn or nitrogen. Recently, however, lattice parameters ol 4.(i!>I and 4.702 A were determined by Farr5 for composiPrcsciitcd a t the F i l l Mccting of the Basic Sciencc Division, The American Ceramic Society, Washington, D. C., October 8, 1903 (Paper No. 13-13-63), Received May 27, 1964; revised copy rcccivctl November 2, 1964. This work was sponsored by the United States Air Force under Contract Nos. A F 33(616)-6286 and A F 33(65i)--8025. Tlic writcr is ceramist, IZesearch Laboratory, Carbon Products Division. Union Carbidc Corporation. Table I. Literature Data on t h e System Zirconium-Carbon Refet ciice Z I C melting temp ( " C ) 3 3100-3200 4 5 1 3530rt 125 3400 ZrC/ZrC f C at o/o C 49.1-2850°C 49 .4-240OoC 50.0-1400°C + ZrC/ZrC Zr at.% C 35.4 35.0-1400°C A - ZrC-C ZlC eutectic -~ ZI solidus 2850 f 50°C Eutectic 2800°C. 64.3 at.%C 2920 f'50"C Eutectic I " 7 2 9, 10, 11 24.3 Eutectic Peritectic Vol. 48, No. 5 Journal of l'he American Ceramic Society-Sara 244 0.080" DIA. To /SPLIT RING SLOTTED PYROLYTIC TUBE SAMPLE OR CRUCIBLE CONENTRATOR DONUT" TaC crucibles with lids containing blackbody holes were used to hold ZrC samples heated by induction to above 2850°C. The samples rested on TaC or ZrC pedestals in the crucible. Rapid absorption of carbon by the ZrC above this temperature limited the higher-temperature usefulness of this combination. High-zirconium alloys were usually quenched into molten tin from peak temperatures, whereas ZrC compositions and those higher in carbon were cooled a t furnace rate. No precipitation, anomalous microstructures, or unsuspectcd DTA heat effects were encountered which would require consideration of other cooling conditions. ( C ) Differential Thermal Analysis: Samples representative of the Zr ZrC and ZrC C fields were studied by DTA to verify the absence of high-temperature effects which might otherwise escape detection. A graphite/boronatcd graphite dcviccl5 was used for this analysis in conjunction with a Leeds & Northrup 9835-B dc amplifier and a Speedomax (AZAR) indicating recorder. Heating and cooling rates of approximately 10" per minute were maintained by power-driven Variacs. (D) Analytical Methods: Samples annealed under various conditions of time and temperature were evaluated by chemical, metallographic, and X-ray methods. Chemical analysis was confined primarily to determination of zirconium and total carbon. The alloy was oxidized in pure oxygen a t 1000" to ll00OC for I hour and the Zr was determined from the residual ZrOa by weight change. The carbon content was determined by COz absorption in Indicarb. Specimens for metallographic analyses were prepared in the conventional manner. Etching was not required in most instances because of the ease in differentiating among the various phases. Diffraction studies were concerned with phase identification and lattice parameter measurements of zirconium carbide as a function of carbon content. A wide-angle North American Philips spectrometer was used for this purpose. For determining the cell constant, consideration was given only to the four maximums, (422), (511, 333), (440), and (531), which are located farthest in the back reflection region for Cu K a radiation and which provide the highest accuracy of measurement. A cell constant was computed independently from each of the four d values and then was averaged to provide the reported lattice parameter. A maximum deviation of *0.0008 A was encountered Tor the cell constant within a given scan and between repeat runs. + Fig. 1. Concentrator cavity arrangement. maintainccl. The glass mantlc also insulated the induction coil from the barrcl. The concentrator cavity arrangement illustratcd in Fig. 1 further increased thc power-temperature efficiency and also minimized radiation to the surrounding glass mantle. Graphitized felt" has cxccllent thermal insulation charactcristics and is inert to an rf Geld a t 460 ltc. Felt has also been used successfully in place of the slotted pyrolytic tube to operating tcmpcratures of 3300" to 3400°C. The tatitalum split ring on the top felt washer prevented misalignment o f the temperature-monitoring sight hole with rcspect to the sample blackbody hole when gases were introduced intd thc cvacuatcd chamber. Thc powcr source was a General Elcctric Company RF generator rated a t 15 Itw output with a frequency ol 460 kc per sccoiid. The optical pyrometers (Leeds & Northrup and PyroMicro-Optical disappearing filament types) had been calibratcd to 3500°C through the prism and window using a calibrated National Bureau of Standards tungsten-ribbon lamp and a standard arc with sectored disks as radiation sources. Temperaturcs rcportcd in this study are bclicvcd to be accurate to within =tlT]. A net accuracy of =t0.7% is reasonable up t o the mclting temperature of tantalum carbide (-3900OC) according to calculations by Zalabak, l 4 who considered the accuracy of the standard pyrometer, deviations expcctcd on the basis of data obtained during calibration, and absorption by auxiliary optical elements in the sight path. (H) Thermal Treatment of Samples: Samples of zirconium hydride and graphite in situ, or 800°C presintered samples, werc hcatcd in vacuo to approximately 1500OC for a few hours or until outgassing ceased. Experiments to high temperatures were usually conducted in pure argon. Vacuum runs were occasionally uscd to the completion of the cxpcriment but only whcn the temperature did not exceed ')ooooc. + Samples rcprcscntative of the Zr ZrC field were suspended within the heater elements by graphite threads drawn through axial holcs drilled in the samples. Refractory metal support wircs invariatdy formed a low-melting eutectic with the zirconium. Samples in the ZrC C Geld wcre supported in solid graphite cubes measuring a/s in. on a side. The cylindrical samples were fitted into holcs of comparable diameter, with approximately one half of each sample projecting from the holder. The part of the sample which was free from contact with the holder was used for phase identification. + * Product of Union Carbide Corporation. + 111. Results and Discussion The phase diagram for the system zirconium-carbon, presented in Fig. 2, is based on the evaluation of approximately 107 alloys and on DTA studies. The experimentally determined features of the system closely resemble the predicted version by Beneiovsky and Rudy' and certain details are in good agreement with data obtained by Farr5 and by the Bureau of Mines.G ( 1 ) Zr-ZrC Solidus According to DTA heating and cooling data, the Zr-ZrC soliclus is located at approximately 1860°C. A heat effect observed at 885°C on cooling apparently corresponds to the 8-Zr + a-Zr transformation. Melting experiments, conducted by monitoring the temperature of liquid formation in blackbody holes in various samples, agreed with the DTA results. Seven samples covering the composition range from 2 to 30 at.% carbon melted in the range 1840' to 1900°C. In general, the samples with higher carbon content appeared to melt a t higher temperatures and to contain less liquid. The melting temperature of pure zirconium was 1860OC. From these experiments it was not possible to determine the nature of the invariant point between Zr and ZrC; however, the microstructure in Fig. 3 showing the primary grains of ZrC suggests that the invariant point probably occurs a t less than 2 at.% carbon. The System Zirconium-Carbon May 1965 245 Fig. 3. Microstructure of samples containing 2 at.% carbon at 1865°C revealing primary grains of ZrC. (HF-HNOZ-HQO etch; x 1 8 0 . ) Fig. 2. Zr-C phase diagram constructed from data obtained from this investigation and including several ZrC/ZrC C boundary limits according to Farr, Ref. 5. + (2) ZrC-C Solidus The eutectic temperature between ZrC and C was determined in a manner similar to the one described above for Zr-ZrC alloys. Liquid formation was consistently observed at 2850°C for this two-phase region. A eutectic composition of 65.0 at.% carbon was established on the basis of the photomicrograph shown in Fig. 4. This information correlated well with results obtained a t the U. S. Bureau of Mines6 wherc a temperature of 2800" f 50OC for a composition containing 64,3y0 carbon was noted. Absorption 01 carbon below 2850OC by alloys is virtually negligible but above this temperature compositions shift very rapidly toward the 65.0% cutcclic composition. No heat effects indicative of phase change or new compounds were observed in the subsolidus rcgions of the ZrC C phase field by DTA. + ( 3 ) ZrC Boundary Limits Fig. 4. ZrC-C eutectic from samples containing 65.0 at.% carbon at 2870'C. (As polished; X 180.) + Analyses of the ZrC/ZrC liquid boundary were undertaken a t 1900", 2600", 3030", and 3300°C. The low-carbon terminal member at each of these temperatures was determined by metallographic studies, since this method is more cffcctive lor observing trace quantities of Zr than lattice parameter analyses. For example, microstructure (Fig. 5 ) of a sample which had been heated to 3300°C and which contained 38.5 at.% carbon clearly shows the prior existence of a small quantity of Zr-rich liquid. Samples containing larger quantities of carbon prepared at the same temperature were free of liquid. Bracketing of the boundary in this manner, at the three lower temperatures cited in the foregoing, established absence of variation for a terminal composition approximating 38.5 at.% carbon. Furthermore, no precipitation was observed in the ZrC particles, a fact which implies no marked temperature dependency of the ZrC/ZrC Zr boundary in the subsolidus region. Porosity in samples near the stoichiometric composition hampered attempts to determine free carbon content by metallographic means and, in fact, rendered this method useless for defining the carbon rich terminal members of the ZrC field below 2850°C. the eutectic temperature. Above this temperature? the densify to a greater degree because of the liquid phase; the free carbon is thus retained and may be recognized quite readily in microstructures as a + Fig. 5 . Microstructure depicting small quantity of Zrmrich liquid in samples containing 38.5 at.% caibon at 3 3 0 0 0 ~ . (As polished; X 11 5.) Journal of 2 'he American Ceramic Society-Sara 246 Pig. 6. Evidence of a two-phase equilibrium in samples containing 49.0 at.% carbon at 3300'c. Carbon is present as a component of the eutectic outlining ZrC grains. (As polished; Fig. Vol. 48, No. 5 ,. Single-phase ZrC in samples containing 48.8 at.%o carbon at 3300°C. (HF-HNOs-H20etch; X 1 1 5 . ) Xll5.) component o f the cutcctic. The high-carbon boundary a t :i300°C was determined to bc 48.9 at.% carbon. The trace of eutectic in the grain boundarie indicates a two-phase rcgion for a sample containing 49.0 at.% carbon (see Fig. 6). As demonstratcd in Fig. 7 , a sample containing 48.8 at.% carbon is I'rce oE secondary components. According to Farr," the carbidc phase coexisting with liquid a t 2850°C contains 49.1 at.% carbon; and a t 2400°C the carbon content is raiscd to a t least 49.4 at.%. (4) Melting Point Determinations Since ZrC has a broad compositional field, it is appropriate that the same spcetruin of compositions uscd lor defining the field at :K30OoC should be considered for determining the melting point. The thcrmal stabilities of the compositions considered in this study are listed in Table 11. Of a total of 15 samples heated above 3300cC, six developed liquid, but two iiieltcd completely and could not be rctrieved from the crucibles for analysis. Four additional analyses are absent for samples which are irrelevant to positioning the solidus. The solidus shown in Fig 2 is based on the analysis of samples which have shown signs of melting, as observed eithcr by metallographic or visual means, and those which have not. These observations in conjunction with the analytical results of Table I1 suggcst that the highcst melting temperature is 3420OC for a composition containing approximately 40 at.% carbon. It can be seen, however, that the solidus is rclatively flat with composition. From the values listed in Table I1 i t is seen that 47 at.% carbon is an approximate congruent vaporization composition. Compositions containing less than 47 at.% carbon vaporize zirconium a t a higher rate than carbon, and compositions conpaining more than 47 at.% carbon vaporize carbon at a higher rate. ( 5 ) ZrC1-, Lattice Parameter Variation Two series of samples, based on annealing temperatures of 3300" and 2(iOO°C, were prepared for lattice parameter mcasurements. Thc 260OoC alloys, studied initially, exhibited successive lowering in lattice parameter with repeated samplc crushing, pressing, and reannealing. This behavior was very apparent in samples with carbon contents beIow approximately 46.0 at.%. Another series of samples was prepared a t 3300°C to eliminate the possibility of oxidation which could have resulted from the powdering operation. These higher-temperature samples, after only one heat cycle, were decidedly superior in chemical homogeneity to those Table 11. Carbon Content of ZrC Melting Point Specimens Carbon c o n t c n t (at Yo) I . 1 CmrJ. ( O C ) 3350 3400 (melted) 3330 3375 3410 3430 (melted) 3390 3400 3425 (melted) 3400 343.5 (melted) 7 Nominal Fi nal 40 42 42 42.6 43.0 44 44 44 46 46 46 48 48 3.171) m _- 3430 (melted) 50 50 . G o (some melting) * 41 * Appreciable melting prevented retrieval of 44.5 45 4 46 8 47 2 46 8 47 ti 48 4 * sample for analy- sis, prepared a t 2GOO"C. This judgment was made principally from an examination of the symmetry of the X-ray diffraction maximums. Lattice parameters and the corresponding chemical composition for the series of samples annealed at 3300cC are listed in Table 111 and are plotted in Fig. 8. Included are cell data for samples containing carbon substantially in cxcess of the stoichiometric requirements. The samples had been previously heated to approximately 2850°C in determining the ZrC-C eutectic. The average cell constant of 4.6983 A derived from these compositions is the same as that determined for the 49 at.% carbon sample. Figure 6, depicting a trace of eutectic in the grain boundaries, indicates that this composition is in the two-phase region. The similarity of the cell constant, for the carbide phase coexisting with liquid a t 2850" and a t :3300°C, suggests invariancc of the ZrC/(ZrC liquid) boundary in this temperature interval. A cell constant of 4.6941 A was determined for the carbide phase coexisting with liquid on the zirconium-rich side of the field. Figure 8 shows that the lattice parameter reaches a maximum a t 4.7021 A for a composition containing 45.5 at.'% carbon. There were symptoms of this behavior in the series of samples studied at 2600OC. Such a trend is quite unusual when a comparison is made with other face-cen- + The System Zirconium-Carbon May 1965 Table 111. Variation of ZrC Lattice Parameter with Composition Annealing temp. ( “ C ) Lattice varameter (A) 38 38 40 44 45 47 47 48 48 48 49 5 5 7 4 5 4 3 1 9 8 0 3300 3300 3300 3300 3300 3300 3300 3300 3300 3300 3300 4 6941 4.6956 4.6998 4.7017 4.7021 4.7016 4.7007 4.6995 4.6991 4.6986 4.6983 55 60 65 65 65 0 0 0 2850 2850 2850 2850 2850 Avg for 2850°C series 4 6982 4 6985 4.6985 4.6982 4 6980 4 6983 0 0 tered-cubic carbides (those which show a linear or quadratic expansion of the lattice with carbon content). Lattice paramcter measurements on several zone-refined ZrC samplcs” support the d a t a obtained in this study. T h e carbon contents and cell constants of these boules are as follows: 41.2 at.% carbon, 4.6984 A ; 51.8 at.’% carbon, 4.63979 A. Recent experiments confirmed the lattice parameter d a t a presented in Fig. 8.IG Two ZrC compositions prepared by arc melting had carbon contents of 46.0 and 48.2 at.% and lattice parameters of 4.7009 and 4.6983 A, respectively. The latter sample also containcd 18% free carbon. IV. Conclusions Analysis of t h e system zirconium-carbon by DTA, X-ray, and rnetallographic methods confirmed face-centered-cubic ZrC as the only intermediate phase in this binary. A lattice parameter variation for ZrCl- that has a maximum suggests a n atomic distribution scheme somewhat more complicated t h a n t h e commonly accepted simple subtractive model. T h e remaining features of this binary system are essentially the same a s were proposed as hypothetical configurations. Acknowledgments The writer wishes to thank R. T. Dolloff for invaluable aid in discussions of the experimental details and for helpful suggestions in preparing this manuscript. H e also wishes to thank J. Weigel for all aspects of the experimental work, and L. A. McClaine and G. Feich of Arthur 1). Little, Inc., for making available samples of zone-rcfincd ZrC. * Obtained from Arthur D. Little, Inc., Cambridge, Massachusetts. 247 s L 5 4.700 a 0 0 3300 OC -I 4.695 30 o 285OOC 40 50 ATOMIC PERCENT CARBON Fig. 8. Variation of ZrC lattice parameter with composition. Large circles pertain to samples initially heated to 3300”C, whereas the smaller circles refer to measurements made on samples heated to 285OOC. References F. BeneBovsky and E. Rudy, “Contribution t o Structure of the Systems Zirconium-Carbon and Hafnium-Carbon,” Planseeber. Pulvermet., 8,66-71 (1960). Paul Schwarzkopf and Richard Kieffer, Refractory Hard Metals; p. 95. The Maemillan Co., New York, 1953. 3 Ernst Fricderich and Lieselotte Sittig, “Preparation and Properties of Carbides,” 2.Anorg. Chem., 144, 169-89 (1925). C. Agte and H. Alterthum, “Researches on Systems with Carbides of High Melting Point and Contributions to the Problem of Fusion of Carbon,” Z. Tech. Physik., 11, 182-91 (1930). J. Farr, data reported by E. K. Storms, “Critical Review of Refractories: I,” U . S. At. Energy Comm., LAMS-2674, 87 pp., March 1962. G. L. Miller, Metallurgy of the Rarer Metals-2, Zirconium, 2d ed.; pp. 358-59. Edited by H. M. Finniston. Academic Press, New York, 1957. K. I. Portnoi, Yu. V. Levinskii, and V. I. Fadeeva, “Reaction with Carbon of Some Kefractorv Carbides and Their Solid Solutions,” Izv Akad. Nauk SSSR,dOtd. Tekhn. Nauk, Met. i T O ~ ~ Z1961 V O[2] , 147-49. Metals Handbook, Vol. I, Properties and Selection of Metals, 8th ed.; p. 1228. Edited by Taylor Lyman. American Societv for Metals. Noveltv. Ohio. 1961. PI Chiotti, “Some investigations into Zirconium Alloy Systems,” Iowa State College Report No. ISC-132, November 1950. l o M. W. Mallett, p. 496 in Reactor Handbook, Vol. 3. U . S . At. Energy Comm., Washington, D. C., 1955. l 1 A. R. Allen, Metallurgical Project. Massachusetts Institute of Technology Technical Project Report for April-June 1950. U . S. At. Energy Comm. Rept. No. MIT-1052, September 1950. J. M. Dickinson, “High Temperature Resistance Furnace for Operations a t 3000”C,” U . S. At. Energy Comm., AECU4131,1958. l 3 J. M. Leitnaker, M. G. Bowman, and P. W. Gilles, “HighTemperature Phase Studies in the Tantalum-Boron System Between Ta and TaB,” J . Electrochem. Soc., 108 [6] 568-72 (1961). l 4 C. F. Zalabak, “Melting Points of Tantalum Carbide and of Tungsten,” N A S A Tech. Note, D-761, 21 pp., 1961. l6 R. P. Goton, “High Temperature Differential Thermal Analyzer,” U. S. Pat. 3,084,534, April 1963. E. K. Storms; private communication, Los Alamos Scientific Laboratory, Los Alamos, New Mexico.
