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The 1700 °C isothermal section of the pseudoternary system TiC-ZrC-HfC
Journal of the Less-Common
Metals,
81 (1981)
173 - 179
173
THE 1700 ‘=C ISOTHERMAL SECTION OF THE PSEUDOTERNARY SYSTEM Tic-ZrC-HfC
P. MURRAY*
and J. E. WESTON?
Department of Metallurgy and Materials Science, Cambs. CB2 3QZ (Gt. Britain)
University
of Cambridge,
Cambridge,
(Received May 5,198l)
Summary The 1700 “C isothermal section of the pseudoternary phase diagram for the system Tic-ZrC-HfC was determined by X-ray analysis of annealed compacted specimens. The results indicate a smaller solubility of ZrC and HfC in TiC than has previously been found but are in general agreement with investigations of the pseudobinary Tic-ZrC, Tic-HfC and ZrC-HfC systems. The morphology of material in the miscibility gap was examined by scanning electron microscopy.
1. Introduction Transition metals of the subgroup IVa all form cubic monocarbides with carbon, which have the sodium chloride-type structure [l] .These carbides are characterized by a wide homogeneity range and their compositions are best described as Me&_, (Me = Ti, Zr, Hf) where x is in the range from 0 to as much as 0.6, depending on the temperature and the group IVa metal concerned. As all the group IVa carbides are isostructural, it was initially thought that the Tic-ZrC, Tic-HfC and ZrC-HfC systems would all show complete solid state miscibility [2] . However, subsequent work [ 31 has shown that there are solid state miscibility gaps in the phase diagrams of both the TIC-ZrC and Tic-HfC systems. These experimental results have been supported by theoretical calculations of the phase diagrams for Tic-ZrC, Tic-HfC and ZrC-HfC [4,5]. In this paper we describe the results of an investigation of the pseudoternary system Tic-ZrC-HfC.
*Present address: London and Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, Yorks. S60 lDL, Gt. Britain. ‘Present address: Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London SW7 2A2, Gt. Britain. 0022-5088/81/0000-00001$02.50
0 Else&r
Sequoia/Printed
in The Netherlands
174 TABLE 1 Carbide analyses data Compound
HfC ZrC TiC aCC=
Total C fTC)
Free C fFC)
Combined c” (CC)
Theoretical corn bined C
(wt.%)
(wt.%)
(wt.%)
VW (wt.%)
5.99 12.30 19.51
1.05 3.50 0.67
4.99 9.12 18.97
6.30 11.63 20.03
TC-FC 100 - FC
2. Experimental
0
N
(wt.%)
(wt.%)
0.35 0.60 0.44
0.001 0.006 0.12
x 100%
details
2.1. Sample preparation Almost stoichiometric ZrC and HfC (3~ = 0) were produced by the carbothermic reduction of the oxides ZrO, and HfO, [6]. Stoichiometric amounts of the oxide and carbon black were intimately mixed and cold pressed using a binder (10 wt.% gum arabic solution) to give pellets approximately 12 mm in diameter. The pellets were placed in a high purity graphite crucible in a laboratory scale vacuum induction furnace and fired under a dynamic vacuum of about lop2 Torr. The temperature of the hot zone of the furnace was measured using an optical pyrometer via a sighting window and glass prism, using standard allowances for absorption by the window and prism [ 71. This equipment enabled the reaction temperature to be reached within 5 - 10 min from the start of heating and allowed rapid cooling of the specimen at the end of the reaction time (e.g. from 2000 “C to less than 700 “C in 20 min). HfC could be obtained using a single reaction time of 2 h at 1950 “C whereas a two-stage process was necessary to provide ZrC of suitable quality. After an initial firing at 2000 “C for 2 h the product was cooled, reground, recompacted and refired for a further 2 h at 2000 “C. The TiC powder used in this study was obtained as the carbide (London and Scandinavian Metallurgical Co. Ltd.). Typical analyses of the starting carbides are given in Table 1. Although the work of Kieffer et al. [3] on the Tic-ZrC and TIC-HfC systems suggests that all compositions in the Tic-ZrC-HfC system should be single phase above approximately 2000 “C, it was found that temperatures greater than 2100 “C were required to produce single-phase materials for compositions near the middle of the miscibility gap. As the laboratory scale induction furnace had a maximum operating temperature of 2100 “C, it was not possible to obtain single-phase solid solutions of all the compositions studied. An alternative procedure was therefore adopted for this study. Powders of known Tic-ZrC-HfC compositions were intimately mixed with
175
2 wt.% Co and compacted as before. The compacts were fired at 1700 “C in the vacuum induction furnace and the products were cooled rapidly to less than 700 “c. Samples of the products were taken for X-ray analysis and selected specimens were then recompacted and refired for 4 h at 1700 “C to check that equilibrium had been achieved. 2.2. X-my analysis Samples of all the specimens prepared were ground in an agate pestle and mortar and analysed using a Philips PM730 powder dif~a~tometer with Cu Kcuradiation. Phase analysis was carried out by continuous scanning in 28 at a rate of 1 a mine1 and the lattice parameters of each phase were measured by step scanning the (111) and (200) diffraction peaks using a step interval of 0.01” (28 ) and a step count time of 40 s. 2.3. Scanning electron microscopy Compacts which had been fired at 1700 “C were mounted and polished using standard met~o~aphic techniques. The uncoated specimens were examined in an IS1 100 scanning electron microscope. The bias voltage on the secondary electron collector was adjusted to allow a proportion of the backscattered electrons to enter the detector. This provided good atomic number contrast in two-phase specimens. The microscope was fitted with a lithiumdrifted silicon detector which was connected to a Link energy-dispersive X-ray analysis system. This system was used to identify the phases detected by atomic number contrast in the viewing mode of the scanning electron microscope.
3. Results and discussion For all compositions containing less than approximately 80 mol.96 Tic it was found that 4 h at 1700 “C was sufficient to reach an equilibrium phase composition as detected by X-ray analysis. Compo~tions rich in TiC were very slow to come to equilibrium, even after repeated firings for 4 h at 1700 “c. This behaviour is in agreement with studies [ 11 on thecarbothermic reduction of titanium oxides where it has been found that it is very difficult to remove the last traces of oxygen and nitrogen from Tic and to achieve the equilibrium stoichiometry owing to the slow rates of diffusion in the TiC crystals. The results of the X-ray analyses of samples with HfC:ZrC ratios of 1: 1, I:3 and 3:l are shown in Figs. 1,2 and 3. Each series of compositions exhibits a two-phase region, as is to be expected from the work of Kieffer et al. [ 31 on the pseudobinary systems. The limits of the two-phase regions at the Tic-poor compositions are fairly well defined but the boundary for the TiCrich samples is less definite owing to the apparently small solubility of ZrC and HfC in TiC at this temperature and the difficulty in achieving equilibrium in the TiCrich compositions. The information from the X-ray analyses was
0--___o--_
_
_~~-__-x__---_
:j
__?,
o-_io
----__
--x-
_K-
- - ---
r _ o-
,-
7”
-a_$ I
I
I Ti C
c-
20
LO mol.%
60
Fig. 1. Lattice parameter us. composition rich phase; 0, HfC-ZrC-rich phase.
80
HfC-zrc
for Tic-(ZrC-HfC)
(1:l)
at 1700 “C!:X, Tic-
1.8 -
3 ; z 5
-*---o--_------_-__-__
_*_
0_ _ __*-_
-----
16-
*cc
I I
$ .-s t z
r-o
I I LL-
__X___-_-X-
__x__X___-_------
I
---;
I
i TiC
20
LO mol.%
Fig. 2. Lattice parameters us. com~sition rich phase; 0, HfC-3ZrC-rich phase.
60
80
for TX-(ZrC-HfC)
HfCBZrC
(3:l)
at 1700 “C:
X, Tic-
used to plot the isothermal section of the pseudotemary system TiC-ZrCHfC at 1700 “C (Fig. 4). Also shown in Fig. 4 are the miscibility limits found by experiment [ 31 and by calculation [S] for the systems Tic-ZrC and Tic-HfC at 1700 “C. It should be noted that calculation predicts no miscibility gap in the Tic-HfC system at 1700 “C [ 53. The estimated phase boundary from the present study is in reasonable agreement with the result of Kieffer et al. for the ZrC-rich end of the TiCZrC system but predicts a lower solubility of ZrC in TiC than has been found previously. Also, these results suggest that the miscibility gap in the Tic-HfC system is shifted towards higher TiC contents than that found by Kieffer et
177
_ _ 9.-o---
__---
X__--L-
XI_.----
Y-
-A---
-20
1I
x
--TIC
-o- -
-9_,-
__~___*__~~~~~-~___
rnQl.%
LO
3. Lattice parameters us. composition rich phase; 0 SHfC-ZrC-rich phase.
Fjg.
60
I
80
for TiC-(ZrC-HfC)
3HfC -2rC
(1:3) at 1700 “C:
X,
TiC-
TiC
Hf
rC
Fig. 4. Isothermal section of the pseudoternary system Tic-ZrC-HfC at 1700 “C: oI twophase compositions; 0, single-phase compositions; *, estimated phase boundary; *, experimental phase boundary [ 3 ] ; 0, calculated phase boundary [ 5 1.
al. 131. In order to check these results, two pseudobin~ compositions were prepared (90 mol.%TiC-10 mol.%ZrC and 90 mol.%TiC-10 mol.%HfC). Both these compositions gave two-phase alloys on firing at 1700 “C for 4 h, confirm-
178
Fig. 5. A scanning electron micrograph of 80mol.%TiC-lOmol.%ZrCannealed for 3 h at 1700 “C (frame width, approximately 80 /-&I).
lOmol.%HfC
ing the results for the pseudoternary compositions containing 90 mol.% Tic. However, the Tic-rich phase boundary found here must be regarded as being approximate owing to the difficulty experienced in obtaining equilibrium for these compositions. Figure 4 also indicates a wider miscibility gap for compositions rich in ZrC than that for those rich in HfC. This reflects the greater difference between the atomic radii of titanium and zirconium than between the radii of titanium and hafnium as, in this system, Tic, ZrC and HfC are all isostructural and have similar bonding. Figure 5 is a scanning electron micro~aph of a specimen (80 mol.%TiC10 mol.%ZrC-10 mol.%HfC) which had been fired at approximately 2300 “C for 3 h in a pilot plant vacuum induction furnace (by the London and Scandinavian Metallurgical Co. Ltd.) to produce a material which was single phase by X-ray analysis and scanning electron microscopy examination. The specimen was subsequently annealed for 3 h at 1700 “C in the laboratory scale furnace. The atomic number contrast revealed the presence of second-phase particles in the originally homogeneous grains. Energy-dispersive X-ray analysis confirmed that the darker matrix was the Tic-rich phase and that the lighter particles were rich in ZrC and HfC. The relative proportions of the two phases were found to be in approximate agreement with the estimated phase boundaries shown in Fig. 4. This tends to support the low solubilities of ZrC and HfC in TiC at 1700 “C found in this study. Acknowledgments The authors would like to thank the London and Scandinavian Metallurgical Co. Ltd. for their financial and practical support of this project,
179
Professor R. Kieffer for his valuable advice on all aspects of this work and Professor R. Honeycombe for the provision of facilities in the Department of Metallurgy and Materials Science, University of Cambridge. References 1 E. K. Storms, The Refractory Carbides, Academic Press, New York, 1967. 2 R. Kieffer, F. Benesowski and K. Messmer, Metallforschung, 18 (1959) 919. 3 R. Kieffer, H. Nowotny, A. Nekel, P. Ettmayer and L. Usner, Monatsh. Chem., 99 (1968) 1020. 4 A. I. Gusev and G. P. Shveikin, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 13 (1977) 67. 5 V. V. Ogorodnikov and A. A. Ogorodnikova, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 13 (1977) 658. 6 P. Murray, M.Phil. Thesis, University of Cambridge, 1980. 7 W. S. Coxon, Temperature Measurement and Control, Heywood, London, 1960.
Metals,
81 (1981)
173 - 179
173
THE 1700 ‘=C ISOTHERMAL SECTION OF THE PSEUDOTERNARY SYSTEM Tic-ZrC-HfC
P. MURRAY*
and J. E. WESTON?
Department of Metallurgy and Materials Science, Cambs. CB2 3QZ (Gt. Britain)
University
of Cambridge,
Cambridge,
(Received May 5,198l)
Summary The 1700 “C isothermal section of the pseudoternary phase diagram for the system Tic-ZrC-HfC was determined by X-ray analysis of annealed compacted specimens. The results indicate a smaller solubility of ZrC and HfC in TiC than has previously been found but are in general agreement with investigations of the pseudobinary Tic-ZrC, Tic-HfC and ZrC-HfC systems. The morphology of material in the miscibility gap was examined by scanning electron microscopy.
1. Introduction Transition metals of the subgroup IVa all form cubic monocarbides with carbon, which have the sodium chloride-type structure [l] .These carbides are characterized by a wide homogeneity range and their compositions are best described as Me&_, (Me = Ti, Zr, Hf) where x is in the range from 0 to as much as 0.6, depending on the temperature and the group IVa metal concerned. As all the group IVa carbides are isostructural, it was initially thought that the Tic-ZrC, Tic-HfC and ZrC-HfC systems would all show complete solid state miscibility [2] . However, subsequent work [ 31 has shown that there are solid state miscibility gaps in the phase diagrams of both the TIC-ZrC and Tic-HfC systems. These experimental results have been supported by theoretical calculations of the phase diagrams for Tic-ZrC, Tic-HfC and ZrC-HfC [4,5]. In this paper we describe the results of an investigation of the pseudoternary system Tic-ZrC-HfC.
*Present address: London and Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, Yorks. S60 lDL, Gt. Britain. ‘Present address: Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London SW7 2A2, Gt. Britain. 0022-5088/81/0000-00001$02.50
0 Else&r
Sequoia/Printed
in The Netherlands
174 TABLE 1 Carbide analyses data Compound
HfC ZrC TiC aCC=
Total C fTC)
Free C fFC)
Combined c” (CC)
Theoretical corn bined C
(wt.%)
(wt.%)
(wt.%)
VW (wt.%)
5.99 12.30 19.51
1.05 3.50 0.67
4.99 9.12 18.97
6.30 11.63 20.03
TC-FC 100 - FC
2. Experimental
0
N
(wt.%)
(wt.%)
0.35 0.60 0.44
0.001 0.006 0.12
x 100%
details
2.1. Sample preparation Almost stoichiometric ZrC and HfC (3~ = 0) were produced by the carbothermic reduction of the oxides ZrO, and HfO, [6]. Stoichiometric amounts of the oxide and carbon black were intimately mixed and cold pressed using a binder (10 wt.% gum arabic solution) to give pellets approximately 12 mm in diameter. The pellets were placed in a high purity graphite crucible in a laboratory scale vacuum induction furnace and fired under a dynamic vacuum of about lop2 Torr. The temperature of the hot zone of the furnace was measured using an optical pyrometer via a sighting window and glass prism, using standard allowances for absorption by the window and prism [ 71. This equipment enabled the reaction temperature to be reached within 5 - 10 min from the start of heating and allowed rapid cooling of the specimen at the end of the reaction time (e.g. from 2000 “C to less than 700 “C in 20 min). HfC could be obtained using a single reaction time of 2 h at 1950 “C whereas a two-stage process was necessary to provide ZrC of suitable quality. After an initial firing at 2000 “C for 2 h the product was cooled, reground, recompacted and refired for a further 2 h at 2000 “C. The TiC powder used in this study was obtained as the carbide (London and Scandinavian Metallurgical Co. Ltd.). Typical analyses of the starting carbides are given in Table 1. Although the work of Kieffer et al. [3] on the Tic-ZrC and TIC-HfC systems suggests that all compositions in the Tic-ZrC-HfC system should be single phase above approximately 2000 “C, it was found that temperatures greater than 2100 “C were required to produce single-phase materials for compositions near the middle of the miscibility gap. As the laboratory scale induction furnace had a maximum operating temperature of 2100 “C, it was not possible to obtain single-phase solid solutions of all the compositions studied. An alternative procedure was therefore adopted for this study. Powders of known Tic-ZrC-HfC compositions were intimately mixed with
175
2 wt.% Co and compacted as before. The compacts were fired at 1700 “C in the vacuum induction furnace and the products were cooled rapidly to less than 700 “c. Samples of the products were taken for X-ray analysis and selected specimens were then recompacted and refired for 4 h at 1700 “C to check that equilibrium had been achieved. 2.2. X-my analysis Samples of all the specimens prepared were ground in an agate pestle and mortar and analysed using a Philips PM730 powder dif~a~tometer with Cu Kcuradiation. Phase analysis was carried out by continuous scanning in 28 at a rate of 1 a mine1 and the lattice parameters of each phase were measured by step scanning the (111) and (200) diffraction peaks using a step interval of 0.01” (28 ) and a step count time of 40 s. 2.3. Scanning electron microscopy Compacts which had been fired at 1700 “C were mounted and polished using standard met~o~aphic techniques. The uncoated specimens were examined in an IS1 100 scanning electron microscope. The bias voltage on the secondary electron collector was adjusted to allow a proportion of the backscattered electrons to enter the detector. This provided good atomic number contrast in two-phase specimens. The microscope was fitted with a lithiumdrifted silicon detector which was connected to a Link energy-dispersive X-ray analysis system. This system was used to identify the phases detected by atomic number contrast in the viewing mode of the scanning electron microscope.
3. Results and discussion For all compositions containing less than approximately 80 mol.96 Tic it was found that 4 h at 1700 “C was sufficient to reach an equilibrium phase composition as detected by X-ray analysis. Compo~tions rich in TiC were very slow to come to equilibrium, even after repeated firings for 4 h at 1700 “c. This behaviour is in agreement with studies [ 11 on thecarbothermic reduction of titanium oxides where it has been found that it is very difficult to remove the last traces of oxygen and nitrogen from Tic and to achieve the equilibrium stoichiometry owing to the slow rates of diffusion in the TiC crystals. The results of the X-ray analyses of samples with HfC:ZrC ratios of 1: 1, I:3 and 3:l are shown in Figs. 1,2 and 3. Each series of compositions exhibits a two-phase region, as is to be expected from the work of Kieffer et al. [ 31 on the pseudobinary systems. The limits of the two-phase regions at the Tic-poor compositions are fairly well defined but the boundary for the TiCrich samples is less definite owing to the apparently small solubility of ZrC and HfC in TiC at this temperature and the difficulty in achieving equilibrium in the TiCrich compositions. The information from the X-ray analyses was
0--___o--_
_
_~~-__-x__---_
:j
__?,
o-_io
----__
--x-
_K-
- - ---
r _ o-
,-
7”
-a_$ I
I
I Ti C
c-
20
LO mol.%
60
Fig. 1. Lattice parameter us. composition rich phase; 0, HfC-ZrC-rich phase.
80
HfC-zrc
for Tic-(ZrC-HfC)
(1:l)
at 1700 “C!:X, Tic-
1.8 -
3 ; z 5
-*---o--_------_-__-__
_*_
0_ _ __*-_
-----
16-
*cc
I I
$ .-s t z
r-o
I I LL-
__X___-_-X-
__x__X___-_------
I
---;
I
i TiC
20
LO mol.%
Fig. 2. Lattice parameters us. com~sition rich phase; 0, HfC-3ZrC-rich phase.
60
80
for TX-(ZrC-HfC)
HfCBZrC
(3:l)
at 1700 “C:
X, Tic-
used to plot the isothermal section of the pseudotemary system TiC-ZrCHfC at 1700 “C (Fig. 4). Also shown in Fig. 4 are the miscibility limits found by experiment [ 31 and by calculation [S] for the systems Tic-ZrC and Tic-HfC at 1700 “C. It should be noted that calculation predicts no miscibility gap in the Tic-HfC system at 1700 “C [ 53. The estimated phase boundary from the present study is in reasonable agreement with the result of Kieffer et al. for the ZrC-rich end of the TiCZrC system but predicts a lower solubility of ZrC in TiC than has been found previously. Also, these results suggest that the miscibility gap in the Tic-HfC system is shifted towards higher TiC contents than that found by Kieffer et
177
_ _ 9.-o---
__---
X__--L-
XI_.----
Y-
-A---
-20
1I
x
--TIC
-o- -
-9_,-
__~___*__~~~~~-~___
rnQl.%
LO
3. Lattice parameters us. composition rich phase; 0 SHfC-ZrC-rich phase.
Fjg.
60
I
80
for TiC-(ZrC-HfC)
3HfC -2rC
(1:3) at 1700 “C:
X,
TiC-
TiC
Hf
rC
Fig. 4. Isothermal section of the pseudoternary system Tic-ZrC-HfC at 1700 “C: oI twophase compositions; 0, single-phase compositions; *, estimated phase boundary; *, experimental phase boundary [ 3 ] ; 0, calculated phase boundary [ 5 1.
al. 131. In order to check these results, two pseudobin~ compositions were prepared (90 mol.%TiC-10 mol.%ZrC and 90 mol.%TiC-10 mol.%HfC). Both these compositions gave two-phase alloys on firing at 1700 “C for 4 h, confirm-
178
Fig. 5. A scanning electron micrograph of 80mol.%TiC-lOmol.%ZrCannealed for 3 h at 1700 “C (frame width, approximately 80 /-&I).
lOmol.%HfC
ing the results for the pseudoternary compositions containing 90 mol.% Tic. However, the Tic-rich phase boundary found here must be regarded as being approximate owing to the difficulty experienced in obtaining equilibrium for these compositions. Figure 4 also indicates a wider miscibility gap for compositions rich in ZrC than that for those rich in HfC. This reflects the greater difference between the atomic radii of titanium and zirconium than between the radii of titanium and hafnium as, in this system, Tic, ZrC and HfC are all isostructural and have similar bonding. Figure 5 is a scanning electron micro~aph of a specimen (80 mol.%TiC10 mol.%ZrC-10 mol.%HfC) which had been fired at approximately 2300 “C for 3 h in a pilot plant vacuum induction furnace (by the London and Scandinavian Metallurgical Co. Ltd.) to produce a material which was single phase by X-ray analysis and scanning electron microscopy examination. The specimen was subsequently annealed for 3 h at 1700 “C in the laboratory scale furnace. The atomic number contrast revealed the presence of second-phase particles in the originally homogeneous grains. Energy-dispersive X-ray analysis confirmed that the darker matrix was the Tic-rich phase and that the lighter particles were rich in ZrC and HfC. The relative proportions of the two phases were found to be in approximate agreement with the estimated phase boundaries shown in Fig. 4. This tends to support the low solubilities of ZrC and HfC in TiC at 1700 “C found in this study. Acknowledgments The authors would like to thank the London and Scandinavian Metallurgical Co. Ltd. for their financial and practical support of this project,
179
Professor R. Kieffer for his valuable advice on all aspects of this work and Professor R. Honeycombe for the provision of facilities in the Department of Metallurgy and Materials Science, University of Cambridge. References 1 E. K. Storms, The Refractory Carbides, Academic Press, New York, 1967. 2 R. Kieffer, F. Benesowski and K. Messmer, Metallforschung, 18 (1959) 919. 3 R. Kieffer, H. Nowotny, A. Nekel, P. Ettmayer and L. Usner, Monatsh. Chem., 99 (1968) 1020. 4 A. I. Gusev and G. P. Shveikin, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 13 (1977) 67. 5 V. V. Ogorodnikov and A. A. Ogorodnikova, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 13 (1977) 658. 6 P. Murray, M.Phil. Thesis, University of Cambridge, 1980. 7 W. S. Coxon, Temperature Measurement and Control, Heywood, London, 1960.