T. C. WALLACE, C. P. GUTIERREZ, AND P.L. STONE
796
TABLEI11 HEATSOF COMBL+STION AND HEATSOF FORMATIOX OF TAWALUM CARBIDES
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5,in
H e a t of combustion cor. for impurities, % change from cal./g. uncor. value
TaCI
Metal
1343
$0.25
0.485 ,724 ,749 ,802 ,821 ,838 .904 ,936 ,958 ,982 .998 1. 00 (TaC, linear extrapolation)
1426 1498 1500 1506 1526 1521 1543 1544 1567 1565 1,567
+ + + + + + + + + +
..
.12 .41 .22 .08 .28 .05 .14 .12 .29 .10 .12
....
H e a t of formation, - A f f f , kcal./mole
487.7 i 0 . 9 (for Ta20j) 22.9 i 1.7 27.1 zt 1 . 3 28.7 =k 0 . 5 31.5 It 1 . 2 29.0 & 1.1 31.3 zt 0 . 5 32.2 & 0 . 6 34.4 i 1 . 3 31.5 rt 1 . 7 33.8 i 0 . 6 34.6 & 1.2 34.6 zt 0 . 9
The values for the heats of formation for the ten compositions in the homogeneous phase region7,11 were fitted to both a quadratic and a linear equation by the method of least squares by means of ail IBM 704 computer. Each value was weighted inversely proportional to the square of its uncertainty. The resulting equations are AH = 22.81 - 103.78~$- 4 6 . 8 8 ~ ~ AH = -11.92
- 22.67~
When these equations are extrapolated to z (11)
=
1, the
F.H. Ellinger, Trans. A m . Soc. Metals, 51, 89 (1943).
Vol. 67
values for the heat of formation of stoichiometric TaC are found to be -34.1 i 1.2 and -34.6 + 0.9 kcal./ mole, respectively. There is no statistically significant difference between the quality of the fit of the two curves to the experimental data. The calculated values of A H for the TaC phase are plotted as a function of composition in Fig. 1. The values of McKenna,2 H ~ m p h r e y , and ~ Smirnova and Ormontj are shown for comparison. If it is assumed that the samples of Humphrey were actually 0.05 mole free carbon, then his data lead to a heat of formation of -33.8 kcal./mole for TaCo.96, in excellent agreement with the present work. Such a free carbon coiiteiit has been observed iii commercial tantalum carbide samples. .A similar argument could be applied to the data to McKenna.2 Since most of the samples of Smiriiova and Ormontj appear to be inhomogeneous, and since their lattice l2 parameter-composition data are seriously in no correlation should be expected between their calculated heats of formation and the present work. The range of homogeneity of the T a l c phase is very narrow at the temperature of preparation of the samples (1850°)'1~13; hence, the heat of formation per mole of carbon should be expected to be nearly constant. Thus the heat of formation of TazC may be taken as 3.4 kcal./mole by linear extrapolation. -47.2 Acknowledgments.-Valuable assistaiice was rendered by F. H. Ellinger, X-ray analysis; H. &I. Burnett, spectrochemical analysis; and G. C. Heasley, chemical analysis. Thanks are due R. K. Zeigler for programming the data for the IBM 704 computer.
+
(12) R . Lesser a n d G. Rrauer, Z. .MetaZllc., 49, 622 (1968). (13) A. L. Bowman, t o be published.
THE MOLYBDENUM-ZIRCOXIUM-CLXEtBON SYSTEM' BY T. C. WALLACE, C. P. GUTIERREZ, AND P. L. STONE Universzty of California, Los Alainos ScientlfLc Laboratory, Los Alamos, A7ew Mexico
Received September 16, 1966 The approximate phase boundaries of the solid portion of the 2100" isothermal section of the No-Zr-C system were determined by chemical and X-ray techniques. Two of the more interesting features are: (1)an extensive horn shaped solid solution region formed between ZrC and Mo3C2that extends from the Zr-C boundary to Moo.bsZro.o&o.40;and (2) the highest carbide of the Mo-C system, Mo8Cz(ao = 3.010 0.002, co = 14.62 f 0.01 A.) lies a t 38 at. 70C. AFrO of MoCo.cl M o ~ Cis~ estimated ) t o be -2.6 f 1.5 kcal./mole. In addition, the phases in equilibrium in the low-carbon portion of the 1500" isothermal section were established, and melting portion of the temperatures were determined along the Mo-C boundary and in the high-carbon ( C > 50 at. YO) ternary.
+
Introduction boundary systems are well known. The M0-C7-11 boundary system has been investigated by a number Increasing int'erest has been shown in recent years of workers, but until recently there has been little in the refractory metal carbides for use as high temagreement about the composition and structure of the perature materials. The binary systems of these higher carbide. hppareiit'ly, the only work 011 the metals with carbon have been extensively studied and ternary is that of Nowotny and Kieffer,12 who studied recently have been. critically reviewed$; however, very little research has been reported on t'ernary systems ( 5 ) R. F.l)oma,oala, D. J. McPherson, a n d LI. Hansen, Trans. Am. I n s t . Xining, M e t . Petrol. khgrs., 197, 73 (1953). consisting of two transition metals and carbon. I n (6) E. Pipita and R . Kieffer, 2.Metallk., 4 6 , 187 (1955). the Zr-Mo-C ternary, the Zr-C3 and Z ~ - M O ~ - ~ (7) W. P. Sykes, K . R. "an Horn, and C. PL. Tucker, Trans. A m . Inst. (1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) E. K. Storms, "A Critical Review of Refractories. P a r t I. Selected Properties of Group 4a, -6a, and -6a Carbides," Los Alamos Scientific Labor a t o r y Report LAMS-2674, Feb. 1, 1962. (3) J. D. Farr, J. Phys. Chem., in press. (41 P. Duwez and C. R. .Jordan, J . Am. Chem. SOC.,75, 5509 (1961).
Mining, M e t . Petrol. Engrs., 117, 173 (1936). (8) W. Few a n d G. Manning, ibid.. 194, 271 (1952). (9) H. Nowotny, E. Partlib, R . Kieffer, a n d F. Benesovsky, Monatsh. Chem., 65, 255 (1954). (10) E. Rudy, F . Benesovsky, and K . Sedlatschek, ibid.,92, 841 (1961). (11) E. R u d y , El. Rudy, and F. Benesovsky, Planseeber. Puluernzet., 10, 42 (1962).
MOLYBDENUM-ZIRCONIUM-CARBOX SYSTEM
April, 1963
the ZrC-MonC section, and U m a n ~ k i , l 3who , ~ ~ studied the ZrC-MoC section. This work was concerned with three areas of the Mo-Zr-C ternary : (1) establishment of the approximate solid-phase boundaries in the 2100' isothermal section; ( 2 ) determination of the phases in equilibrium in the low-casbon Zr-Mo portion of the 1500' isothermal section; and (3) determination of some C region and along melting temperatureci in the y the Mo-C boundary. Experimental
TABLE I:
AT
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X-RAYDATA
O F sAMPLI3S ANSEALED
2100 ASD 1500'
NO. Compn. b y chemical analysis
of phases
1 2
2 1 1 1 2 1 2 2 2 2 3 2 2 2 2 3 2 2
+
Starting materials were reactor grade zirconium sponge from Wah Chang Corporation, high-purity molybdenum rod, and Sational Carbon spectrographic grade carbon rod. The chemical purity of the zirconium was 99.97,, and spectrochemical analysis showed that the major metal impurities were Al, Si, Ca, Ti, V, Co, Xi, Mo, and Hf, each of which was present in quantities of less than 100 p.p.m. The chemical purity of the molybdenum was 99.80/,, and the major metal impurities were found to be W < O.l%, Ta < 300 p.p.m., and Fe < 200 p.p.m. The spectrochemical analysis of the carbon rod indicated that B, Mg, and Na, were present in qu.antities of less than 50 p.p.m., while Hz, and HzO to be present in chemical analysis showed 0 2 , SZ, quantities of 0 68, 0.17, 0.15, and 0.08 wt. 70, respectively. The samples were prepared by arc-melting the components on a water-cooled copper hearth in an atmosphere of purified helium, using a carbon electrode. Five to ten gram portions of the pulverized melt (-325 mesh) were pressed, without binder, in a in. hole was drilled in. steel die a t 100,000 p.s.i. A 0.040 by in the top of the cylinder to give blackbody conditions for pyrometric temperature measurements. The heating was done inductively with an eddy-current concentrator, which has been described previously in detail.16 The eddy-current concentrator was contained in a vacuum system capable of maintaining pressures of 10-5 torr or lower. Those samples which were not to be in equilibrium with carbon were supported on a molybdenum tripod, whereas samples containing free carbon were heated in graphite crucibles. After the pressure had torr, the temperature was slowly been reduced t o less than raised to the appropriate value and maintained there. During most of the heatings, the pressure was below 10-6 torr. After they were heated, the isamples were cooled by radiation, and dropped t o 900" in about 1 min. Samples were heated for 16-30 hr., cooled, pulverized t o -325 mesh powder, re-pressed, and heated for an additional 15-30 hr. After the final heating, the samples were again pulverized to -325 mesh powder, an X-ray powder pattern was taken, and the remaining material was analyzed separately for Zr, $10,total C, and free C.16 The sum. of the percentages of the individual analyses of all the samples lay within the interval 99.3-100.270. The X-ray powder patterns were made in a 114.6 mm. DebyeSchemer camera, using nickel-filtered copper radiation. Lattice parameters were obtained from the back-reflection lines by applying the least-squares extrapolation of Cohen17 as modified by Hess18 and calculating the results on an IBM 704 computer. Standard deviations were calculated for e3ch lattice-parameter. These deviations were less than f 0 . 0 0 3 A . in the cubic phases (a-Mo, 7 , and u ) , less than &0.005 in the p-phase and less than d~0.005for a. and 1 0 . 0 2 for cointhe ?'-phase. Table I gives the analytical chemical composition of the annealed samples, number of phases that were found by X-ray techniques to be present, and the lattice parameters of these phases when they were present in sufficient quantity to show lines in the back-reflection region. The standard deviations are not presented
2
1 2 3 3 3 2 2 3 2 2 2 2 2 3 2 2 2 2 2 2
3
a 3 2 1 2 1
2 2 1
2 2
1 2
1 2 1
1 1
2 3 2 2 3 2 2 3 3 3 2 3 3 2 2 2
I
(12) H. Nowotny and R ieBer, MetaZZJorsehuno, 2, 257 (1947). (13) Ya. S. Urnanski, Iznest. Sektora Piz.Khim. Anal. Inst. Obshchei N e o w . Khim. Akad. ?Tauk S S S R , 16, No. I , 127 (1943). (14) Ya. 8. Umanski a n d G. V. Samsonov, "Hard Alloys of the Refractory Metals," Metallurgical Publishing House, Illoscow, 1957 (in Russian). (15) J. If. Leitnaker, >l. C:. Bowman, and P. Gilles, J. Electrochem. Soe., 108, 568 (1961). (16) 0. H. Kriege, "The Analysis of Refractory Borides, Carbides, Nitrides a n d Silicides," Los Alamos Scientific Laboratory Report LA-2306, March, 1969. (17) M. U. Cohen, Rev. Sc?;. Instr., 6 , 68 (1935); Z. Krist., 94A, 288 and 306 (1936). (18) J. B. Hess, Acta Crust., 4, 209 (1951).
h A L Y T I C A L AND
797
1
1 1 2 a
Phases present by X-ray and lattice parameters, A.' 2100° data a-nIo ao = 3.147 a-Rlo (3.147 (2.993, 4.724) a-&Io 4- P (2.994, 4.722) p, ao = 2.994, co = 4.725 p , ao = 3.000, eo = 4.727 P , ao = 3.015, co = 4.739 4 4- y' (3.012, 14.62) y'. ao = 3.008, eo = 14.63 C f y' (3.008, 14.62) y' (3.011, 14.63) C C 4- y' (3.009, 14.62) y' (3.009, 14.62) C a-Mo (3.147) f y a-Rlo (3.151) y or-MO (3.159) y a-Mo (3.150) f y (4.672) a-hfo (3.151) f y (4.682) a-RIo (3.148) f p f y (4.649) a-&lo (3.149) f 2 (4.669) a-Mo (3.154) y (4.684) a-Mo p (3.002, 4.751) 0,ao = 3.010, cu = 4.747 p (3.009, 4.746) y a-Mo p (3.002, 4.754) y a - M O p (3.002, 4.753) f y
+
+ +
+
+
+
+
+
+
+ +
+
P+r+r'
p (3.015, 4.769) f
y (4.492) y (4.267) y' (3.017, 14.66) y (4.267) y' C
+ C +
+
+
(4.282) C f y (4.287) C f y (4.285) y (4.306) y' P y (4.362) C y (4.261) f y' (3.012, 14.63) y (4.293) C C f y (4.295) y (4.329) C C f y (4.348) P (3.018, 4.758) -t (4.323) p (3.005, 4.749) y (4.635) a - M o (3.148) f p (3.002, 4.754) f y (4.661) c y (4.455) a-Mo (3.147) f 6 (3.001, 4.756) y (4.652) p (3.011, 4.758) y (4.576) y , ao = 4.510 y (4.517) C y , ao = 4.557 P f Y (4.613) a - M o (3.162) f y (4.687) y, ao = 4.606 a-Mo 4- y (4.696) C f y (4.624) y , a0 = 4.623 a-hIo y (4.680) y , ao = 4.660 a-Mo f y (4.692) 2 , aa = 4.676 y , ao = 4.698 y , ao = 4.699 1500' data, a-Mo (3.173) y a - h l o (3.176) f y (4.692) f u (7.583) y (4.689) f u (7.604) y (4.690) f u (7.608) y (4.690) u (7.608) f p-Zr y (4.691) f p-Zr y (4.688) p-Zr y (4.690) u (7.586) a-Mo (3.1751 y (4.888) f u (7.582) a-MO (3.174) y (4.690) u (7.588) a - M o (3.176) y (4.688) u (7.592) 8-Zr y (4.690) 4- u y (4.689) f u f ~ - M o y (4.686) f u (7.605) y (4.698) a-Mo y (4.690) f u y , ao = 4.699 y , ao = 4.693 y , ao = 4.689 a-Zr f y (4.689) y
+
++ + +
+
+
+
+
+
+
+
+
+ + + + +
+ + +
+
The free carbon was determined by chemical analysis.
T. c . PVCVALLACE, c. P. GUTIERREZ, AKD P. L. STOKE
79s
Vol. 67
C
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Zr INCREASING At.
Zr
20
C INCREASING At.%
40
Fig. 1.-Isothermal
sections a t 1500 (lower left portion) and 2100' (upper right portion): three-phased points.
Melting temperatures were determined by two methods. Method 1: The temperature of the sample was slowly raised ( 1O-2Oo/min.) until the blackbody hole filled with liquid. Ordinarily this occurred over a 10" temperature interval. The determination was made under an atmosphere of helium to prevent composition shifts. Method 2: A thermal arrest apparatusxg was used to determine the solidification temperature. Briefly, the method consists of taking light from a blackbody hole in a sample or crucible and focusing it on a photomultiplier tube connected to the y-axis of a time-sweep oscilloscope. Bbrupt changes in the smoothly changing slope of the display trace as the sample cools indicate a heat effect that would normally be present during a phase change. With proper calibration, it is then possible to ascertain the temperature a t which the transition took place. Bgain the determination was made in a helium atmosphere. Temperature measurements were made with a Pyro MicroOptical pyrometer that had been compared with a standard pyrometer calibrated a t the National Bureau of Standards. In addition, the pyrometer and optical system were calibrated against the melting points of Co(1495'), Pt (1773'), Rh (1964'), Nb (2468"), Mo (2620°), T a (2996"), and W (3410'). The purity of these metals was better than 9970.
Results and Discussion 2 100" Isothermal Section.-The upper left- and right-hand portions of Fig. 1 show the solid phase field (19) G. N. Rupert, t o be published.
80
60
Ma INCREASING At%-
., single phased;
X , two-phased;
A,
distribution that was found a t 2100". The y-phase (ZrC solid solution), ?'-phase (Mo3C2),p-phase (Mo2C), a-Mo-phase and some thermodynamic estimations will be discussed below. a-Mo-Phase.-The solubility of carbon in molybdenum* isoquite low (approximately 0.16 at. % ' with a. = 3.148 A.) a t 2200", whereas the solubility of zirconium a t 1000" is reported6 to be about 7 at. %. The lattice parameters of the a-Rlo phase in samples Riloo.,~Co.2s, Moo.96Zro.01Co.03, Moo.93Zr0.04C0.03, and Moob90Zro.a7Co.03 were 3.147, 3.147, 3.151, and 3.159 A,, respectively. The low carbon content of the last three samples and the variation of the lattice parameter with zirconium content seem to indicate that the solubility of carbon is quite small in the a-Mo (Zr) region. From considerations of the &lo-Zr boundary systein6a6the a-Mo (Zr) phase must pass into a two phase region (liquid solid) with increasing Zr. y However, all the samples examined in the a-Mo region showed no signs of melting, hence the multiphased regions containing liquid must lie to the left of those samples studied.
+
+
MOLYBDEXUM-ZIRCONIUM-CARBOX SYSTEM
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April, 1963
p-Phase.-The p-phase has the hexagonal Mo2C crystal structure. On the basis of variation of lattice parameter with composition, the p-phase has an appproximate range of homogeneity lying between 30.8 and 33.3 at. yo C, which is in agreement with previous There appears to be a slight solubility of the y-phase in the p-phase, which extends the p-phase slightly into the interior of the ternary. The exact location of the interior boundary of the p-phase as determined by Xray techniques is limited by the amount of the new phase that has to be present to show in the diffraction pattern (2-10 at. %). The variation of the lattice parameters along the 33.3 at. % C line indicates that the e-axis is expanding and the a-axis is shrinking with increase of Zr. From consideration of the lattice parameter variation, it would appear that probably less than 3 at. yo y-phase is dissolved by the p-phase. y '-Phase.-In older investigations reviewed by Storms12a compound, MoC, is described; but there is little agreement as to its crystal structure and composition. The various structures reported appear to be related to the method of preparation. Nowotny and co-workersg were the first investigators to solve successfully the complex crystal structure of the compound. They reported that two higher carbide phases were probably present ; a ocomplex hexagoonal structure (D6h4-type,a. = 3.01 A , , eo = 14.61 4.) and a facecentered cubic structure (ao = 4.28 A.). Unfortunately, there were no chemical analyses given for the compounds. Recently Rudy and co-workersll have reported that the cubic structure (a0 = 4.281 8.)is a high temperature modification of the hexagonal structure (ao = 3.013, eo = 14.64 A.). They were able to prepare the cubic modification by very rapid quench from the molten stake. The composition of the cubic and hexagonal modifications was foundlotl1 to lie within the range from 40.0 to 40.8 at. % C and they assigned a formula of ;l'IoaCz. The higher carbide phase that we found was successfully indexed with the hexagonal structure (Doh4), however we were unable to prepare the pure cubic modification. Along the Mo-C boundary, the lattice parameters of the ?/-phase mere the same within exy', y', and y' C regions, perimental error in the p with the average value of all determinatioizs being a. = 3.010 f 0.002 8. and eo = 14.63 f 0.01 A. The back-reflection lines of the y '-phase were always somewhat diffuse. Ana1,ysis of arc-melted samples with 43, 46, and 50 at. % C showed the combined carbon to be 40.8 f 0.5 at. %, which is in agreement with Rudy and co-workers.lO*ll However, the analysis of samples annealed a t 2100' with 40, 43, 46, and 50 at. % C showed combined carbon to be 38.2 f 0.5 at. %. Hence, with increase in temperature (2100O to melting temperature), the carbon-rich phase boundary would appear to move toward higher carbon. Since the sample MoO,~~CO.~~ showed the p-phase and ?'-phase to be present, the homogeneity range of Mo3C2must be quite narrow a t 2100' and lies a t approximately 38 at. Yo C. The presence of sigpificantly larger lattice parameters (3.017 A., 14.66 A,) in the sample Zro.asCo.,8indicates a slight solubility of the y-phase in the ?'-phase. y-Phase.-The y-phase crystallizes in the NaCl type tructure. Along the Zr-C boundary, the y-phase ex-
+
+
35
I
I
I
I
799
I
I
I
I
I
I
1 UMANSKI NOWOTNY AND KIEFFER THIS WORK
-
0.2
0.4
0.6
0.8
1.0
XYO.
Fig. 3.-Variation of the lattice parameter of the face-centered cubic ?-phase with the molar ratio, X%Ila,of Ma to total metal: 0 , single-phased (7); X, two-phased (y C,; A, three-phased (Y Y' C).
+
+ +
tends from approximately 35.5-50.0 at. % ' C, with the lattice parameter remaining essentially constant over the interval 37.5-50.0 at. ?& C (4.699 f 0.002 8.)and 0.001 A. at the lower limit.3 decreasing to 4.691 The carbon-rich side of the y-phase was determined by chemical analyses. Figure 2 shows the variation of combined carbon with the molar ratio, X M ~of, Mo to total metal. The samples for XM, < 0.96 were twoy, and established the phase boundary, phased, C whereas for 0.96 < X M
