High temperature diffusion of hafnium in tungsten and a tungsten-hafnium carbide alloy

Pergamon

ScriptaMetallurgicae t Materialia,Vol. 30, No. 10, pp.1263-1267, 1994 Copyright © 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00

H I G H T E M P E R A T U R E D I F F U S I O N O F H A F N I U M IN T U N G S T E N AND A TUNGSTEN-HAFNIUM

CARBIDE ALLOY

Yo Ozaki and Ralph H. Zee Materials Engineering Program Auburn University Auburn, AL 36849

(Received November 9, [993) (Revised January 25, 1994) Introduction Refractory metals and ceramics are used extensively in energy systems due to their high temperature properties (1-4). This is particularly important in direct conversion systems where thermal to electric conversion efficiency is a direct function of temperature. Tungsten, which has the highest melting temperature among elemental metals, does not possess sufficient creep resistance at temperatures above 1600 K. Different dispersion strengthened tungsten alloys have been developed to extend the usefulness of tungsten to higher temperatures. One of these alloys, tungsten with 0.4 mole percent of finely dispersed HfC particles (W-HfC), has the optimum properties for high temperature applications (5-7). Hafnium carbide is used as the strengthening agent due to its high chemical stability and its compatibility with tungsten. The presence of HfC particles retards the rate of grain growth as well as restricting dislocation motion, both of which are beneficial for creep resistance. The long term behavior of this alloy depends largely on the evolution of its microstructure which is governed by the diffusion of its constituents (8). Data on the diffusion of carbon in tungsten and tungsten self-diffusion are available (9-12), but no direct measurements have been made on the diffusion of hafnium in tungsten. The only diffusion data available are estimated from a coarsening study (13) and these data are highly unreliable. In this study, the diffusion behavior of hafnium in pure tungsten and in a W-HfC alloy was directly measured by means of Secondary Ion Mass Spectroscopy (SIMS) (14). The selection of the W-HfC alloy is due to its importance in high temperature engineering applications, and its higher recrystallization temperature. The presence of HfC particles in tungsten restricts grain growth resulting in better high temperature creep resistance. More importantly for this study, the higher recrystallization temperature allows measurements to be made over a wider range of temperatures at a relatively constant grain size.

Experimental Procedures Tungsten samples were cut from a 1 mm thick sheet obtained from Johnson Matthey AESAIL and samples of tungsten with 0.4 mole percent of hafnium carbide were cut from a 15 mm diameter rod fabricated by the Westinghouse Corporation. The samples were coated with a hafnium layer with a thickness on the order of 1000 angstroms by means of electron beam evaporation. These samples were processed under different conditions, with the objective of isolating the effects of grain boundaries and hafnium carbide on the diffusion process. Three types of substrates were used: large grain size (100 I~m) pure tungsten by preannealing the material at 2373 K for one hour prior to diffusion annealing (denoted as W/preanneal); fine grain size (10-20 gin) pure tungsten (denoted as W); and fine grain size (10 lain) tungsten with 0.4 mole percent HfC (denoted as W-HfC). Table 1 summarizes the diffusion annealing conditions for the three types of samples. The durations of the annealing were selected so that similar diffusion lengths were attained. The diffusion annealing temperature for tungsten was limited to 2123 K due to grain growth at higher temperatures. One sample (W-HfC diffusion annealed at 2573 K for one hour) was also included. This high temperature annealing was conducted for a coarsening study where hafnium carbide

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particles near the surface were found to be dissociated, resulting in enhanced surface grain growth. The diffusion constant was extracted from this sample based on surface depletion rate of hafnium. In all the experiments, the vacuum was maintained at below 10~ Pascal at all times using an all-metal system evacuated by an ion pump and a titanium sublimation pump. Sample heating was achieved by electron beam heating and the temperature was monitored by thermocouples and an optical pyrometer. Hafnium profiles in tungsten and the alloy were directly measured by SIMS using a 12 keV oxygen ion beam as the probe. A quadrupole SIMS manufactured by Atomika Technische Physik GmbH was used. The sputtering rate was calibrated by measuring the depth of the crater created by the scan using a profilometer (Alpha-Step 200 by Tencor Instruments).

TABLE 1. Experimental Conditions

Sample Substrate

Grain Size (I.tm)

W/Preanneal

100

W

10-20

W-HfC

Diffusion Anneaiing Temperature (K)

Diffusion Time (h)

1773 1973 2123

8 1 1

10

1773 1923 1973 2123 1773 1973 2123 2273 2573

8 1 1 1/3 1

Results and Discussion

Figure 1 shows the SIMS profiles (as a function of depth) for tungsten and hafnium measured from a W-HfC substrate prior to surface coating of hafnium and diffusion anneal. The hafnium signal is this case is from the 0.4 mole percent of HfC particles. It illustrates that the composition of the substrate is uniform prior to the diffusion process. Similar uniform profiles, but with the Hf signal in the noise range, were obtained for the pure tungsten material. Figure 2 shows the SIMS profiles for a W/preanneal sample after diffusion annealed at 2123 K for 1 hour. These profiles are typical of those obtained from other specimens in this study. Results from the analysis of the diffusion annealed samples show a hafnium layer on the surface indicating that the condition resembles that of an infinite source. Therefore the hafnium diffusion coefficients were obtained using the equation C x = C , - (C, - C0) erf{x/(4Dt) in}

[1]

where C~ is the concentration at position x and time t, C, is the surface concentration, COis the initial concentration, and D is the diffusion coefficient. The diffusion coefficients obtained as a function of temperature for the three groups of substrate materials examined are plotted in Figures 3 to 5 in the Arrhenius manner. In the case of the preannealed tungsten (Figure 3), the grain size was about 100 l~m and was found to remain constant even after annealing at 2123 K for 1 hour. Since the size of the scan gate of SIMS was 30 gm and an effort was made to ensure that the scan area was within one grain area, the diffusion coefficients obtained using this substrate should correspond to the lattice (or bulk) diffusion process. From the analysis, the equation of hafnium lattice

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diffusion coefficient into tungsten was found to be D (cm2/sec)

=

[2].

10"~ exp(-Q/RT)

with a Q value of 73 kcal/mole. This energy for diffusion of 73 kcal/mole corresponds to the barrier required for hafnium diffusion through the tungsten lattice. The activation energy values for tungsten self diffusion has been reported to be between 116.4 kcal/mole and 153.1 kcal/mole (13) which are considerably larger than the value obtained in this study. A study by Braun and Rudy (15) indicates that hafnium solute in a tungsten matrix has a linear misfit factor of about +10 % (hafnium solute is larger than tungsten). The misfit strain due to the presence of hafnium would result in a reduction in the diffusion barrier for hafnium diffusion consistent with the observation that the diffusion energy of hafnium in tungsten is less than that for tungsten self diffusion. In addition, the pre-exponential term (106cm2/s) is smaller than that for self diffusion (on the order of 1 cm2/s). Again the size misfit of hafnium leads to an attractive potential between tungsten lattice vacancies and hafnium solutes. This strong correlation suppresses the effectiveness of the hafnium solutes to diffuse long distances. The diffusion experiments conducted with tungsten substrate that did not undergo the preanneal process possessed smaller grain size. Results from the microstructure analysis show that the average grain size in these specimens is on the order of 10 p.m, which is smaller than the diameter of the scan gate used (60 lam). This implies that the diffusion process measured will be dominated by grain boundaries. The effort to limit grain growth restricted the experiments to low temperatures (below 2123 K). Figure 4 shows the results from this group of samples from which an activation energy for grain boundary diffusion of 45 kcal/mole is obtained. This is approximately half of the bulk diffusion energy, which is consistentwith the general observation that motion through grain boundaries requires less energy than through the lattice. Results from the diffusion study of hafnium in W-HfC are shown in Figure 5. The recrystallization temperature of W-HfC is well above 2573 K. The grain size of this group of samples remained at approximately 10/am even after a high diffusion annealing temperature of 2273 K. Since the scan gate used for the SIMS analysis was 60 Ixm, the diffusion process determined in this set of samples corresponds largely to grain boundary diffusion, at least at low temperatures. According to the Arrhenius plot, the low temperature diffusion is governed by a process with an activation energy of approximately 40 kcal/mole (grain boundary diffusion), whereas at temperatures above 2123 K, a higher activation of 80 kcal/mole (lattice diffusion) is required for solute migration. This is illustrated in the two lines shown in Figure 5. The data at 2573 K was obtained based on the outdiffusion of hafnium dissociated from the HfC particles at this high temperature. In this sample, a depletion zone was detected and the thickness of this depletion layer was used to estimate the diffusion coefficient The ratio of activation energy for the two processes (lattice versus boundary) is approximately 2:1, which is consistent with the general observation in most metals. The diffusion data obtained from this research using both pure tungsten and W-HfC are shown in Figure 6. This direct comparison shows the similarity in the diffusion rates observed for both grain boundary and bulk diffusion in these two types of materials. This suggests that the presence of HfC does not significantly affect the hafnium diffusion behavior in tungsten. Although hafnium is highly soluble in tungsten, the presence of carbon has been demonstrated to result in a large reduction in the solubility of hafnium due to the formation of carbides. This implies that the concentration of hafnium in solid solution is very low (in the parts per million range) and therefore has little influence on the diffusion mechanism. Conclusions

The activation energies for hafnium diffusion into pure tungsten was found to be 73 kcal/mole for lattice diffusion and 45 kcal/mole for grain boundary diffusion respectively. The presence of HfC restricts grain growth, thereby enabling diffusion measurements to be made over a wider range of temperature while maintaining a relatively constant grain size of l0 p.m. The diffusion process was found to be unaffected by the presence of HfC (80 kcal/mole for lattice and 40 kcal/mole for boundary). Grain boundaries were found to enhance the observed diffusion at temperatures below half the absolute melting point as expected. These findings can be used to predict long term microstructure stability in tungsten alloys.

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Acknowledgements

This work was supported by the U.S. Air Force's Phillips Laboratory at Kinland Air Force Base in New Mexico through the Auburn University's Space Power Institute. The authors would also like to thank Drs. Schuller and Murray and Capt. Hidinger of the Air Force for their continuing support and guidance. References

1.

Y. Ozaki, P. Gao and R.H. Zee, Proceedings of the 10th Symposium on Space Nuclear Power and Propulsion, American Institute of Physics, ed. M. EI-Genk, Part I, 63 (1993). R.H. Zee, W.Y. Wu and B.A. Chin, Journal of Nuclear Materials 191-194, 571 (1992) S. Chen, W. Yu, R.H. Zee and B.A. Chin, Journal of Nuclear Materials, to be published. RH. Zee, C.C. Yang, Y. Lin and B.A. Chin, Journal of Materials Science 26, 3853 (1991). R.H Titran, J.R. Stephens, and DW. Petrasek, NASA TM-101364, DOE/NASA/16310-8 (1988). WD. Klopp, P.L Raft'o, and W.R. Witzke, Journal of Metals, 6, 27 (1971). K.S. Shin, A. Luo, B. Chen, D.L Jacobsen, Journal of Metals, 8, 12 (1990). AJ. Ardell, Acta Metall., 15, 1772 (1967). A. Shepela, Journal of Less-Common Metals, 26, 33 (1972). A.A. Korolev, L.V. Pavlinov, and M.I. Gavrilyuk, Fiz. metal, metalloved., 33, 295 (1972). R.E. Pawel and T.S. Lundy, Acta Metall., 17, 979 (1969). J.N. Mundy et al., Phys. Rev. (B), 18, 6566 (1978). M. Hasaka et al., Scripta Metall., 29, 959 (1993). K.B. Povarova, Russian Metall., 2, 134 (1981). H. Braun and E. Rudy, Z. Metallk., 51,360 (1960).

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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DEPTH (nm)

DEPTH (rim)

HG. I. Depth distribution of tungsten and hafnium, measured by SIMS, in a W-HFC sample prior to the diffusion process showing uniform substrate composition. anneal at 2123 K for 1 hour.

.

FIG. 2.

Depth distribution of tungsten and hafnium, measured by SIMS, in a W/preanneal sample after diffusion annealed at 2123 K for 1 hour.

Vol. 30, No. 10

10 -~3

HIGH TEMPERATUREDIFFUSION

1267

10 -13

1

Q=45 kcal/mol

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FIG. 4.

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o.o

Arrhenius plot of the diffusion coefficient of hafnium in pure tungsten without the preanneal (substrate grain size approximately 10 I.tm).

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Q=80 kcal/mol

i0 -13

10 -13 E

E (.i 0

s.s

1/T (IO-?K)

(lo-?K)

FIG. 3. Arrhenius plot of the diffusion coefficient of hafnium in pure tungsten preannealed at 2373 K for 1 hour (substrate grain size of 100 lain).

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(tO-?K)

FIG. 5. Arrhenius plot of the diffusion coefficient of hafnium in W-HfC with a grain, size of 10 gm.

I

4.0

FIG. 6.

Arrhenius plot of the hafnium diffusion coefficient observed in all the samples examined in this research.