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Creep behavior of a tungsten-3.6% rhenium-0.33% zirconium carbide alloy
Refractory Metals & Hard Materials 11 (1992) 97 103
Creep behavior of a tungsten-3.6 % rhenium-0.3 3 % zirconium carbide alloy Anhua Luo & Dean L. Jacobson Department of Chemical, Bio & Materials Engineering, Arizona State University, Tempe, Arizona 85287-6006, USA (Received 13 February 1992; accepted 28 April 1992)
Abstract: The creep behavior of an arc-melted tungsten-3"6% rhenium-0"33 % zirconum carbide, considered to be a potential emitter material for a thermionic energy converter, was evaluated at elevated temperatures (1900-2400 K) under stresses of 15-80 MPa in a vacuum below 1-3 x 10 6 Pa (1.0 × 10 ~ torr). The effects of temperature and stress on the steady-state creep rate were determined. The stress exponent and activation energy for creep deformation were also measured. Zirconium carbide particles were found to be very effective in strengthening tungsten at high temperatures. The temperature-compensated creep rate of a tungsten-3-6 % rhenium~).33 % zirconium carbide at temperatures above 0-5 T.1 was approximately one order lower than a tungsten-5.0 % rhenium, and was two orders lower than tungsten tested under identical conditions. The creep behaviour of a tungsten-3-6 % rhenium~l-33 % zirconium carbide was also correlated with the microstructure that developed during high temperature deformation. Transmission electron microscope (TEM) micrographs of the creep tested specimens illustrated that the high creep-resistance of this alloy was associated with the presence of zirconium carbide particles which retarded the movements of dislocations and subgrain boundaries, resulting in both direct particle strengthening and indirect particle strengthening.
INTRODUCTION
material for the emitter of the thermionic energy converter. Pure tungsten is susceptible to brittle fracture at room temperature. As a result, the use of tungsten has been severely limited due to difficulties encountered during the fabrication process. An effective way to lower the ductile-brittle transition temperature of tungsten is to alloy with rhenium at a composition of about 4.0 % rhenium.1 7 Previous studies have shown that solid-solution softened tungsten-rhenium alloys are ductile at room temperature if the fabrication history is carefully controlled. On the other hand, the dispersion of secondphase particles is an effective way of strengthening tungsten at temperatures above 0"5 T,,,. By studying the mechanical properties of particle-strengthened tungsten it was found that the addition of oxide or carbide particles with extremely high melting points
Future thermionic energy converters are expected to operate at elevated temperature for several years. Because of the ultrahigh working temperature of the emitter and an extremely small gap between emitter and collector, the creep resistance of emitter material is very important for the space power applications. It determines not only the survivability of space nuclear power systems during long-term operation but also the operation temperature of these power systems. The creep strain of an emitter is required to be less than a few per cent after operating at 2000 K (or even higher temperatures) for 7 years. Tungsten (W) possesses unique thermionic properties and the highest melting temperature among all metals, and thus is considered as an ideal 97
Refractor)' Metals" & Hard Materials 0263-4368/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
98
A. Luo, D. L. Jacobson
would significantly increase the high-temperature strength of tungsten. 8-12 It is therefore advantageous to develop particle strengthened tungsten-rhenium alloys for the space power applications. Zirconium carbide, a c o m p o u n d with a very high melting temperature and great formation energy, seems to be a potential high temperature strengthener in tungsten. The effect of zirconium carbide on the high temperature mechanical properties of dilute tungsten-rhenium has not been studied. The objectives of the present study were: (1) to study in detail the creep behavior of a tungstenrhenium-zirconium carbide alloy by determining the effects of stress, temperature and microstructure on creep rate; (2) to compare the creep strength and creep rate of a tungsten-rhenium-zirconium carbide alloy with tungsten and tungsten-rhenium, and (3) to determine the strengthening mechanism of dispersed zirconium carbide particles in tungsten at ultrahigh temperatures. EXPERIMENTAL PROCEDURES
The material used in the present study was an arcmelted tungsten-3.6w/o rhenium-0.33w/o zirconium carbide (W-3"6Re~).33ZrC) alloy provided by NASA Lewis Research Center, Cleveland, Ohio. Alloy was prepared by vacuum-consumable-arc melting of blended, pressed and sintered electrodes of tungsten and elemental alloying addition powders. Arc melting was performed by using directcurrent straight polarity (electrode negative) into a 50"8-mm-diameter water-cooled copper crucible. The arc-melted ingot was then extruded at 2473 K with a reduction ratio of 8. Flat-plate creep specimens with a gage section of 12"7 m m × 3"175 m m × 0"8 m m were prepared by an electric discharge machine (EDM). The creep specimens were first mechanically polished with 400 and 600 grit silicon carbide paper, and then chemically polished in a 10% N a O H solution to ensure optically smooth surfaces. The creep specimens were annealed at 2473 K for 2 h in order to produce a stable grain size prior to testing. After annealing, the average grain size was 110_+ 5 / t m as determined by the intercept method. The specimens were tested in tension by a step-load method in a vacuum below 1-3 x 10-6 Pa (1-0 × 10 -8 torr) on a custom-built ultrahigh vacuum (UHV) creep testing facility interfered with a computer data acquisition and analysis system. No contamination on the specimen surfaces was observed during the annealing and testing procedures.
The specimens were tested in a stress range of 15 to 80 MPa at a constant temperature of between 1900 and 2400 K. The load was applied to the specimen through a bellows which was attached to the U H V containment vehicle. The applied stress was maintained to _ 1% of the selected value by making periodical corrections. The corrections were implemented to compensate for the changes in specimen's crosssectional area during deformation and the changes in the bellows spring force due to extension of the load column. Elongation was measured using a linear variable differential transducer (LVDT) by which it was possible to measure the creep strains to _+0.03 %. The testing temperature was measured with a micro-optical disappearing filament pyrometer calibrated with a standard tungsten strip lamp provided by the National Institute of Standards and Technology, Gaitherburg, Maryland. The maximum uncertainty of the measured temperature was within 8 K in the entire temperature range employed in the present study. No measurable temperature gradient along the specimen gage length was determined. The growth behavior of ZrC particles and the dislocation substructures of deformed specimens were studied by transmission electron microscope (TEM). TEM samples, taken from the gage section of post-test creep specimens, were first thinned by electropolishing in a 2.0 % NaOH solution at a cell potential of 15 V. The thinned samples were then examined wih a JEOL 2000FX high-resolution transmission electron microscope operating at 200 kV. RESULTS
Normal three-stage creep was observed in the stepload creep tests of this alloy, i.e. primary, steadystate and tertiary creep. Figure 1 shows the typical creep strain versus time curves obtained from the step-load creep specimens tested at 1900 K and 2200 K. The curves illustrate that upon initial loading a primary creep occurs, followed by a steady-state creep region with a constant creep rate. After the steady-state creep rate was measured at a given stress, the stress was abruptly changed to a new level and the primary creep took place again. Eventually tertiary creep occurred with necking in the gage section followed by creep rupture. The steady-state creep, which was of primary interest in the present study, took a relatively long time for most of the test conditions employed in the present
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given temperature can be obtained by measuring the slope of each line in Fig. 2. The values of n obtained for W-3"6Re-0-33ZrC ranged from 5"2 to 5"8 in the temperature range of 1900 to 2400 K, indicating that the stress exponent of this alloy was not sensitive to the testing temperature. A leastsquares approximation of the creep data indicated that the n value of this alloy was approximately 5"4. The steady-state creep rate of a metal at a constant temperature can also be expressed as a function of the normalized stress ?3
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where E is the Young's modulus of matrix at a given temperature. The applied stresses were normalized by the Young's modulus of polycrystalline tungsten reported by Lowrie and Gonas. ]4 A linear relationship between the creep rate and the normalized stress was also noted (see Fig. 3). The normalized stress exponent of W-3"6Re-0"33ZrC was also determined to be approximately 5.4. The effect of temperature on the steady-state creep rate of the W-3.6Re-0.33ZrC alloy is shown in Fig. 4. This figure is a plot of steady-state creep rate (log scale) versus the reciprocal temperature. The apparent activation energy for the creep deformation of this alloy was determined experimentally at constant stress by applying the following expression :~ ~oc exp ( - R ~ )
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DISCUSSION
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In order to characterize the strengthening effect ZrC particles in tungsten at high temperatures, the creep properties of W-3.6Re-0-33ZrC were compared with those of tungsten and tungsten-rhenium. Figure 5 shows the creep strength of W-3.6Re0.33ZrC with tungsten is and a tungsten-5.0% rhenium 19 at a constant creep rate of 10-6/s. It can be seen from Fig. 5 that ZrC particles had a significant strengthening effect on the tungstenrhenium matrix. At 1900 K, the creep strength of W-3.6Re-0-33ZrC is approximately 44 MPa greater than W-5Re. However, the strengthening effect of ZrC particles decreases as temperature increases. At 2200K, the strength advantage of W-3.6Re0"33ZrC over W-SRe reduces to about 18 MPa. If the creep strength of W-SRe is extrapolated at higher temperatures, it can be shown that W-3.6Re-0.33ZrC and W-5Re alloys have almost the same creep strength for a creep rate of 10-8/s at approximately 2500 K, indicating that ZrC particles lose all of their strengthening effect on the tungstenrhenium matrix at this temperature. One of the most often used parameters for the comparison of creep properties in different materials is the temperature-compensated creep rate, Z, the Zener-Holloman parameter ~° which is calculated from the relation:
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10 4/TEMPERATURE (l/K) Fig. 4. The effect of temperature on the steady-state creep rate of W-3.6Re-O.33ZrC. slope of each line in Fig. 4 was determined. It was found that in the temperature region of 1900 to 2400 K the apparent activation energies of W-3.6Re-0.33ZrC ranged from 104 to 112 kcal/mol. The average was 109 kcal/mol. F r o m the results obtained in the present study, the dependence of the steady-state creep rate of W-3.6Re-0.33ZrC alloy on stress and temperature can be summarized by the following equation:
~ = C x (a/E)" x e x p ( - - ~ )
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where C is a constant and other parameters retain the definitions given above. This equation is in accordance with the equations for steady-state creep rate of c o m m o n metallic materials proposed by Mukherjee, Bird and Dorn a6 and by Weertman. a7 By substituting the known values of n and Qe in eqn (4), the value of the constant C was determined to be approximately 1.1 x 10~5/s. Therefore, eqn (4) becomes: = 1"1x1055x(a/E) a 4 x e x p
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By use o f e q n (5), it is possible to predict the steadystate creep rate of W-3-6Re-0.33ZrC alloy at a given stress of a and a temperature of T (1900 K ~< T ~< 2400 K) as long as the power-law relationship is valid. According to this equation, a thermionic emitter made by W-3.6Re-0-33ZrC alloy under a load of 10-4 MPa (1500 psi) at 2100 K would have a steady-state creep rate of 3 x 10-11/s, i.e. it would take about 10 years for such an emitter operating under these conditions to reach a strain of 1.0%. Therefore, W - R e - Z r C alloy seems to be a very
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NORMALIZED STRESS ( (~/E) Fig. 6. Creep rate comparison for W, W-5Re and W-3'6Re0'33ZrC. The temperature-compensated creep rate of W-3.6ReN)-33ZrC is about one order lower than W-5Re and about two orders lower than W.
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Fig. 7. Dislocation substructure in W-3.6Re~.33ZrC creep specimen tested at 1900 K.
By using this parameter, the creep resistance of W-3-6Re-0.33ZrC can be compared with those of tungsten TM and W-5Re 19 in a temperature-compensated creep rate versus normalized stress plot. In Fig. 6, the Z values for these three materials were plotted against the normalized stress. This figure clearly shows the strength advantage of W-3.6Re0-33ZrC over tungsten and W-5Re alloy in a wide range of testing temperature and applied stress. At any given stress, the temperature-compensated creep rate of W-3.6Re-0.33ZrC is approximately one order lower than W-5Re and is approximately two orders lower than tungsten. The superior creep resistance of W-3-6Re-0"33ZrC over tungsten and
Fig. 8. Subgrain microstructure near a grain boundary in W 3.6Re~)'33ZrC creep specimen tested at 2200 K.
W-5Re at ultrahigh temperatures is apparently due to the second-phase particle strengthening of ZrC in tungsten-rhenium. Typical transmission electron micrographs of creep tested W-3-6Re-0.33ZrC specimens are shown in Figs 7 and 8. The interaction between dislocations and ZrC particles can be clearly seen in Fig. 7. This figure illustrates that ZrC particles tend to retard the movement of dislocations. The dislocations were bent and constrained by the ZrC particles, indicating that the motion of the individual dislocation was impeded by ZrC. Figure 8 shows the dislocation substructure of a creep specimen tested at 2200 K. The figure reveals some of this alloy's interesting creep characteristics. Apparently, subgrains form and dislocation tangles no longer exist. Sub-boundaries pinned by spherical ZrC particles usually consist of either dislocation networks or dislocation walls (simple tilt boundaries). Zirconium carbide particles were located in the sub-boundaries as well as the subgrain interior. The dislocation density in the subgrain interior was relatively low and the dislocations in the subgrain interior were always pinned by ZrC particles. In the examination of the subgrain structure it was noted that the subgrain size was relatively small near the grain boundaries. In other words, the density of the subgrains was higher near the grain boundaries. High density grain boundary dislocations (GBDs) can also be observed in Fig. 8. From these microstructural examinations, it was concluded that the interactions between ZrC particles and dislocations and the formation of subgrains are the typical substructural features of creep tested
102
A. Luo, D. L. Jacobson
W-3.6Re-0.33ZrC specimens. The strengthening effect of dispersed ZrC particles in tungstenrhenium matrix is therefore attributed to both direct and indirect particle strengthening. Direct particle strengthening was due to the immobilization of dislocations by direct interaction with ZrC particles. The indirect particle strengthening was due to the stabilization effect of ZrC particles on grain and subgrain which in turn reduces the distance that mobile dislocations can move before being immobilized at sub-boundaries or grain boundaries. It has been shown that the strengthening effect of ZrC particles decreases as temperature increases. This was most likely due to the particle coarsening at ultrahigh temperatures. The transmission electron micrographs of post-tested specimens showed that the size of ZrC particles was stable up to 2000 K. Slight growth of particles was observed at 2100K. At 2200 K and above, ZrC particles experienced substantial coarsening, as shown in Fig. 8. The driving force for precipitate growth is the reduction of total surface energy of precipitates. A theory of diffusion controlled coarsening, in which large precipitates grow at the expense of small ones, has been derived by Lifshitz and Slyozov ~1 and by Wagner. 22 This theory predicts that the average particle radius is a function of time and temperature. It was concluded that the decreased strengthening effect of ZrC particles was associated with the particle growth. Because the total number of the particles available in the matrix decreased rapidly with increased particle size, the growth of the particles reduced not only the pinning effect of particles on dislocations but also the retarding effect of particles on the growth of grains and subgrains. Therefore, both the direct particle strengthening effect (interaction of dislocations and particles) and the indirect particle strengthening effect (stabilization of grain and subgrain sizes) decreased with increasing temperature. CONCLUSIONS The following conclusions are drawn from the present study on the creep behavior of W-3.6Re0.33ZrC alloy. 1. The stress exponent for the steady-state creep of W-3.6Re-0.33ZrC alloy is determined to be approximately 5"4 and is not sensitive to the testing temperature. 2. In the temperature range of 1900 to 2400 K, the activation energy for creep deformation of
this alloy is 109 kcal/mol and is not sensitive to the applied stress. 3. The temperature-compensated creep rate of W-3.6Re-0-33ZrC is about one order lower than W-5Re and two orders lower than tungsten at temperatures above 0-5 Tm. 4. The high creep resistance of W-3-6Re-0.33ZrC is associated with the presence of ZrC particles which retard the movements of dislocations and subgrain boundaries, resulting in both direct particle strengthening (interaction of dislocations and particles) and indirect particle strengthening (stabilization of grain and subgrain sizes). 5. The strengthening effect of ZrC particles in tungsten-rhenium matrix decreases with increasing temperature due to the particle growth. ACKNOWLEDGEMENTS This work was funded by The Wright Research and Development Center, Wright-Patterson Air Force Base, Ohio under the contract F-33615-91-C-2109. The contract manager is Mr Steve Adams. The authors would like to thank Mr J. Clark for the use of the analytical electron microprobe, which was purchased with the aid of NSF grant E A R 8408163, in the Chemistry Department of Arizona State University. The authors would also like to thank Mr D. R. Bosch for his comments on and assistance in preparing this manuscript. REFERENCES 1. Sims, C. T. & Jaffee, R. I., Properties of refractory alloy containing rhenium. Trans. ASM, 52 (1960) 929-41. 2. Ratliff, L., Maykuth, D. J., Ogden, H. R. & Jaffee, R. I., Tungsten sheet alloys with improved low-temperature ductility. Trans. AIME, 230 (1964) 490-501. 3. Klopp, W. D., Witzke, W. R. & Raffo, P. L., Mechanical properties of dilute tungsten-rhenium alloys. NASA Tech. Note D-3483, Lewis Research Center, Cleveland, Ohio, 1966, pp. 1-38. 4. Raffo, P.L., Yielding and fracture in tungsten and tungsten-rhenium alloys. J. Less-Common Metals, 17 (1969) 133-42. 5. Garfinkle, M., Witzke, W. R. & Klopp, W.D., Superplasticity in tungsten-rhenium alloys. Trans. AIME, 245 (1969) 303-16. 6. Stephens,J. R. & Witzke, W. R., Alloy softening in group VIA metals alloyedwith rhenium. J, Less-Common Metals, 23 (1971) 325-42. 7. Luo, A., Jacobson, D. L. & Shin, K. S., Solution softening mechanism of iridium and rhenium in tungsten at room temperature. J. Refractory Metals & Hard Materials, 10 (1991) 107-14. 8. Klopp, W. D. & Witzke, W. R., Mechanical properties of
Creep behavior of a W - R ~ Z r C
9. 10.
11. 12.
13. 14.
a tungsten-23.4 percent rhenium~).27 percent hafniumcarbide alloy. J. Less-Common Metals, 24 (1971) 427-43. Shin, K. S., Luo, A., Chen, B. L. & Jacobson, D. L., High temperature properties of particle-strengthened W-Re. J. Metals, 42 (1990) 12 15. Luo, A., Shin, K.S. & Jacobson, D.L., Ultrahightemperature tensile properties of a powder metallurgy processed tungsten-3.6w/o rhenium-l.0w/o thoria alloy. Scripta Metallurgica et Materialia, 25 (1991) 1811 14. Luo, A., Shin, K. S. & Jacobson, D. L., High temperature tensile properties of W Re-ThO~ alloys. Material Science & Engineering, A148 (1991) 219-29. Luo, A., Jacobson, D. L. & Shin, K. S., High-temperature strengthening mechanism of HfC in W-3'6Re alloy. Proc. 8th Symposium on Space Nuclear Power Systems, vol. 1, ed. M. S. El-Genk. American Institute of Physics, New York, (1991) pp. 193-204. Sherby, O.D., Factors affecting the high temperature strength of polycrystalline solids. Acta Metall., 10 (1962) 135-43. Lowrie, K. & Gonas, A.M., Dynamic properties of polycrystalline tungsten, 24-1800°C. J. Appl. Phys., 36 (1965) 2189 97.
alloy
103
15. Barrett, C. R., Ardell, A. J. & Sherby, O. D., Influence of modulus on the temperature dependence of the activation energy for creep at high temperatures. Trans. AIME, 230 (1964) 200-12. 16. Mukherjee, A. K., Bird, J. E. & Dorn, J. E., Experimental correlations for high-temperature creep. Trans. ASM, 62 (1969) 155-73. 17. Weertman, J., Steady-state creep through dislocation climb. J. Appl. Phys., 28 (1957) 362-76. 18. Vandervoort, R.R. & Barmore, W.L., Elevated temperature deformation and electron microscope studies of polycrystalline tungsten and tungsten-rhenium alloys. Proc. 6th Plansee Seminar, 20-24 May, 1969, Reutte, Tirol, Austria, Metallwerk Plansee GMBH, Reutte, Tirol, Austria, pp. 108-22. 19. Vandervoort, R. R., The creep behavior ofW-5Re. Metall. Trans., | (1970) 857-67. 20. Zener, C. & Hollomon, J. H., Effect of Strain Rate upon Plastic Flow of Steel. J. Appl. Phys,, 15 (1944) 22 31. 21. Lifshitz, I.M. & Slyozov, V.V., The kinetics of precipitation from supersaturated solid solutions. J. Phys. Chem. Solids, 19 (1961) 35 54. 22. Wagner, C. Z. Elektrochem., 65 (1961) 581-90.
Creep behavior of a tungsten-3.6 % rhenium-0.3 3 % zirconium carbide alloy Anhua Luo & Dean L. Jacobson Department of Chemical, Bio & Materials Engineering, Arizona State University, Tempe, Arizona 85287-6006, USA (Received 13 February 1992; accepted 28 April 1992)
Abstract: The creep behavior of an arc-melted tungsten-3"6% rhenium-0"33 % zirconum carbide, considered to be a potential emitter material for a thermionic energy converter, was evaluated at elevated temperatures (1900-2400 K) under stresses of 15-80 MPa in a vacuum below 1-3 x 10 6 Pa (1.0 × 10 ~ torr). The effects of temperature and stress on the steady-state creep rate were determined. The stress exponent and activation energy for creep deformation were also measured. Zirconium carbide particles were found to be very effective in strengthening tungsten at high temperatures. The temperature-compensated creep rate of a tungsten-3-6 % rhenium~).33 % zirconium carbide at temperatures above 0-5 T.1 was approximately one order lower than a tungsten-5.0 % rhenium, and was two orders lower than tungsten tested under identical conditions. The creep behaviour of a tungsten-3-6 % rhenium~l-33 % zirconium carbide was also correlated with the microstructure that developed during high temperature deformation. Transmission electron microscope (TEM) micrographs of the creep tested specimens illustrated that the high creep-resistance of this alloy was associated with the presence of zirconium carbide particles which retarded the movements of dislocations and subgrain boundaries, resulting in both direct particle strengthening and indirect particle strengthening.
INTRODUCTION
material for the emitter of the thermionic energy converter. Pure tungsten is susceptible to brittle fracture at room temperature. As a result, the use of tungsten has been severely limited due to difficulties encountered during the fabrication process. An effective way to lower the ductile-brittle transition temperature of tungsten is to alloy with rhenium at a composition of about 4.0 % rhenium.1 7 Previous studies have shown that solid-solution softened tungsten-rhenium alloys are ductile at room temperature if the fabrication history is carefully controlled. On the other hand, the dispersion of secondphase particles is an effective way of strengthening tungsten at temperatures above 0"5 T,,,. By studying the mechanical properties of particle-strengthened tungsten it was found that the addition of oxide or carbide particles with extremely high melting points
Future thermionic energy converters are expected to operate at elevated temperature for several years. Because of the ultrahigh working temperature of the emitter and an extremely small gap between emitter and collector, the creep resistance of emitter material is very important for the space power applications. It determines not only the survivability of space nuclear power systems during long-term operation but also the operation temperature of these power systems. The creep strain of an emitter is required to be less than a few per cent after operating at 2000 K (or even higher temperatures) for 7 years. Tungsten (W) possesses unique thermionic properties and the highest melting temperature among all metals, and thus is considered as an ideal 97
Refractor)' Metals" & Hard Materials 0263-4368/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
98
A. Luo, D. L. Jacobson
would significantly increase the high-temperature strength of tungsten. 8-12 It is therefore advantageous to develop particle strengthened tungsten-rhenium alloys for the space power applications. Zirconium carbide, a c o m p o u n d with a very high melting temperature and great formation energy, seems to be a potential high temperature strengthener in tungsten. The effect of zirconium carbide on the high temperature mechanical properties of dilute tungsten-rhenium has not been studied. The objectives of the present study were: (1) to study in detail the creep behavior of a tungstenrhenium-zirconium carbide alloy by determining the effects of stress, temperature and microstructure on creep rate; (2) to compare the creep strength and creep rate of a tungsten-rhenium-zirconium carbide alloy with tungsten and tungsten-rhenium, and (3) to determine the strengthening mechanism of dispersed zirconium carbide particles in tungsten at ultrahigh temperatures. EXPERIMENTAL PROCEDURES
The material used in the present study was an arcmelted tungsten-3.6w/o rhenium-0.33w/o zirconium carbide (W-3"6Re~).33ZrC) alloy provided by NASA Lewis Research Center, Cleveland, Ohio. Alloy was prepared by vacuum-consumable-arc melting of blended, pressed and sintered electrodes of tungsten and elemental alloying addition powders. Arc melting was performed by using directcurrent straight polarity (electrode negative) into a 50"8-mm-diameter water-cooled copper crucible. The arc-melted ingot was then extruded at 2473 K with a reduction ratio of 8. Flat-plate creep specimens with a gage section of 12"7 m m × 3"175 m m × 0"8 m m were prepared by an electric discharge machine (EDM). The creep specimens were first mechanically polished with 400 and 600 grit silicon carbide paper, and then chemically polished in a 10% N a O H solution to ensure optically smooth surfaces. The creep specimens were annealed at 2473 K for 2 h in order to produce a stable grain size prior to testing. After annealing, the average grain size was 110_+ 5 / t m as determined by the intercept method. The specimens were tested in tension by a step-load method in a vacuum below 1-3 x 10-6 Pa (1-0 × 10 -8 torr) on a custom-built ultrahigh vacuum (UHV) creep testing facility interfered with a computer data acquisition and analysis system. No contamination on the specimen surfaces was observed during the annealing and testing procedures.
The specimens were tested in a stress range of 15 to 80 MPa at a constant temperature of between 1900 and 2400 K. The load was applied to the specimen through a bellows which was attached to the U H V containment vehicle. The applied stress was maintained to _ 1% of the selected value by making periodical corrections. The corrections were implemented to compensate for the changes in specimen's crosssectional area during deformation and the changes in the bellows spring force due to extension of the load column. Elongation was measured using a linear variable differential transducer (LVDT) by which it was possible to measure the creep strains to _+0.03 %. The testing temperature was measured with a micro-optical disappearing filament pyrometer calibrated with a standard tungsten strip lamp provided by the National Institute of Standards and Technology, Gaitherburg, Maryland. The maximum uncertainty of the measured temperature was within 8 K in the entire temperature range employed in the present study. No measurable temperature gradient along the specimen gage length was determined. The growth behavior of ZrC particles and the dislocation substructures of deformed specimens were studied by transmission electron microscope (TEM). TEM samples, taken from the gage section of post-test creep specimens, were first thinned by electropolishing in a 2.0 % NaOH solution at a cell potential of 15 V. The thinned samples were then examined wih a JEOL 2000FX high-resolution transmission electron microscope operating at 200 kV. RESULTS
Normal three-stage creep was observed in the stepload creep tests of this alloy, i.e. primary, steadystate and tertiary creep. Figure 1 shows the typical creep strain versus time curves obtained from the step-load creep specimens tested at 1900 K and 2200 K. The curves illustrate that upon initial loading a primary creep occurs, followed by a steady-state creep region with a constant creep rate. After the steady-state creep rate was measured at a given stress, the stress was abruptly changed to a new level and the primary creep took place again. Eventually tertiary creep occurred with necking in the gage section followed by creep rupture. The steady-state creep, which was of primary interest in the present study, took a relatively long time for most of the test conditions employed in the present
99
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Fig. 3. The normalized stress dependence of the steady-state creep rate of W-3.6Re 0.33ZrC. The normalized stress exponent for creep was determined to be 5.4.
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given temperature can be obtained by measuring the slope of each line in Fig. 2. The values of n obtained for W-3"6Re-0-33ZrC ranged from 5"2 to 5"8 in the temperature range of 1900 to 2400 K, indicating that the stress exponent of this alloy was not sensitive to the testing temperature. A leastsquares approximation of the creep data indicated that the n value of this alloy was approximately 5"4. The steady-state creep rate of a metal at a constant temperature can also be expressed as a function of the normalized stress ?3
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where E is the Young's modulus of matrix at a given temperature. The applied stresses were normalized by the Young's modulus of polycrystalline tungsten reported by Lowrie and Gonas. ]4 A linear relationship between the creep rate and the normalized stress was also noted (see Fig. 3). The normalized stress exponent of W-3"6Re-0"33ZrC was also determined to be approximately 5.4. The effect of temperature on the steady-state creep rate of the W-3.6Re-0.33ZrC alloy is shown in Fig. 4. This figure is a plot of steady-state creep rate (log scale) versus the reciprocal temperature. The apparent activation energy for the creep deformation of this alloy was determined experimentally at constant stress by applying the following expression :~ ~oc exp ( - R ~ )
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In order to characterize the strengthening effect ZrC particles in tungsten at high temperatures, the creep properties of W-3.6Re-0-33ZrC were compared with those of tungsten and tungsten-rhenium. Figure 5 shows the creep strength of W-3.6Re0.33ZrC with tungsten is and a tungsten-5.0% rhenium 19 at a constant creep rate of 10-6/s. It can be seen from Fig. 5 that ZrC particles had a significant strengthening effect on the tungstenrhenium matrix. At 1900 K, the creep strength of W-3.6Re-0-33ZrC is approximately 44 MPa greater than W-5Re. However, the strengthening effect of ZrC particles decreases as temperature increases. At 2200K, the strength advantage of W-3.6Re0"33ZrC over W-SRe reduces to about 18 MPa. If the creep strength of W-SRe is extrapolated at higher temperatures, it can be shown that W-3.6Re-0.33ZrC and W-5Re alloys have almost the same creep strength for a creep rate of 10-8/s at approximately 2500 K, indicating that ZrC particles lose all of their strengthening effect on the tungstenrhenium matrix at this temperature. One of the most often used parameters for the comparison of creep properties in different materials is the temperature-compensated creep rate, Z, the Zener-Holloman parameter ~° which is calculated from the relation:
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10 4/TEMPERATURE (l/K) Fig. 4. The effect of temperature on the steady-state creep rate of W-3.6Re-O.33ZrC. slope of each line in Fig. 4 was determined. It was found that in the temperature region of 1900 to 2400 K the apparent activation energies of W-3.6Re-0.33ZrC ranged from 104 to 112 kcal/mol. The average was 109 kcal/mol. F r o m the results obtained in the present study, the dependence of the steady-state creep rate of W-3.6Re-0.33ZrC alloy on stress and temperature can be summarized by the following equation:
~ = C x (a/E)" x e x p ( - - ~ )
(4)
where C is a constant and other parameters retain the definitions given above. This equation is in accordance with the equations for steady-state creep rate of c o m m o n metallic materials proposed by Mukherjee, Bird and Dorn a6 and by Weertman. a7 By substituting the known values of n and Qe in eqn (4), the value of the constant C was determined to be approximately 1.1 x 10~5/s. Therefore, eqn (4) becomes: = 1"1x1055x(a/E) a 4 x e x p
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By use o f e q n (5), it is possible to predict the steadystate creep rate of W-3-6Re-0.33ZrC alloy at a given stress of a and a temperature of T (1900 K ~< T ~< 2400 K) as long as the power-law relationship is valid. According to this equation, a thermionic emitter made by W-3.6Re-0-33ZrC alloy under a load of 10-4 MPa (1500 psi) at 2100 K would have a steady-state creep rate of 3 x 10-11/s, i.e. it would take about 10 years for such an emitter operating under these conditions to reach a strain of 1.0%. Therefore, W - R e - Z r C alloy seems to be a very
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TEMPERATURE (K) Fig. 5. Creep strengths o f W, W - 5 R e and W-3-6Re4).33ZrC at a creep rate o f 10-6/s.
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NORMALIZED STRESS ( (~/E) Fig. 6. Creep rate comparison for W, W-5Re and W-3'6Re0'33ZrC. The temperature-compensated creep rate of W-3.6ReN)-33ZrC is about one order lower than W-5Re and about two orders lower than W.
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Fig. 7. Dislocation substructure in W-3.6Re~.33ZrC creep specimen tested at 1900 K.
By using this parameter, the creep resistance of W-3-6Re-0.33ZrC can be compared with those of tungsten TM and W-5Re 19 in a temperature-compensated creep rate versus normalized stress plot. In Fig. 6, the Z values for these three materials were plotted against the normalized stress. This figure clearly shows the strength advantage of W-3.6Re0-33ZrC over tungsten and W-5Re alloy in a wide range of testing temperature and applied stress. At any given stress, the temperature-compensated creep rate of W-3.6Re-0.33ZrC is approximately one order lower than W-5Re and is approximately two orders lower than tungsten. The superior creep resistance of W-3-6Re-0"33ZrC over tungsten and
Fig. 8. Subgrain microstructure near a grain boundary in W 3.6Re~)'33ZrC creep specimen tested at 2200 K.
W-5Re at ultrahigh temperatures is apparently due to the second-phase particle strengthening of ZrC in tungsten-rhenium. Typical transmission electron micrographs of creep tested W-3-6Re-0.33ZrC specimens are shown in Figs 7 and 8. The interaction between dislocations and ZrC particles can be clearly seen in Fig. 7. This figure illustrates that ZrC particles tend to retard the movement of dislocations. The dislocations were bent and constrained by the ZrC particles, indicating that the motion of the individual dislocation was impeded by ZrC. Figure 8 shows the dislocation substructure of a creep specimen tested at 2200 K. The figure reveals some of this alloy's interesting creep characteristics. Apparently, subgrains form and dislocation tangles no longer exist. Sub-boundaries pinned by spherical ZrC particles usually consist of either dislocation networks or dislocation walls (simple tilt boundaries). Zirconium carbide particles were located in the sub-boundaries as well as the subgrain interior. The dislocation density in the subgrain interior was relatively low and the dislocations in the subgrain interior were always pinned by ZrC particles. In the examination of the subgrain structure it was noted that the subgrain size was relatively small near the grain boundaries. In other words, the density of the subgrains was higher near the grain boundaries. High density grain boundary dislocations (GBDs) can also be observed in Fig. 8. From these microstructural examinations, it was concluded that the interactions between ZrC particles and dislocations and the formation of subgrains are the typical substructural features of creep tested
102
A. Luo, D. L. Jacobson
W-3.6Re-0.33ZrC specimens. The strengthening effect of dispersed ZrC particles in tungstenrhenium matrix is therefore attributed to both direct and indirect particle strengthening. Direct particle strengthening was due to the immobilization of dislocations by direct interaction with ZrC particles. The indirect particle strengthening was due to the stabilization effect of ZrC particles on grain and subgrain which in turn reduces the distance that mobile dislocations can move before being immobilized at sub-boundaries or grain boundaries. It has been shown that the strengthening effect of ZrC particles decreases as temperature increases. This was most likely due to the particle coarsening at ultrahigh temperatures. The transmission electron micrographs of post-tested specimens showed that the size of ZrC particles was stable up to 2000 K. Slight growth of particles was observed at 2100K. At 2200 K and above, ZrC particles experienced substantial coarsening, as shown in Fig. 8. The driving force for precipitate growth is the reduction of total surface energy of precipitates. A theory of diffusion controlled coarsening, in which large precipitates grow at the expense of small ones, has been derived by Lifshitz and Slyozov ~1 and by Wagner. 22 This theory predicts that the average particle radius is a function of time and temperature. It was concluded that the decreased strengthening effect of ZrC particles was associated with the particle growth. Because the total number of the particles available in the matrix decreased rapidly with increased particle size, the growth of the particles reduced not only the pinning effect of particles on dislocations but also the retarding effect of particles on the growth of grains and subgrains. Therefore, both the direct particle strengthening effect (interaction of dislocations and particles) and the indirect particle strengthening effect (stabilization of grain and subgrain sizes) decreased with increasing temperature. CONCLUSIONS The following conclusions are drawn from the present study on the creep behavior of W-3.6Re0.33ZrC alloy. 1. The stress exponent for the steady-state creep of W-3.6Re-0.33ZrC alloy is determined to be approximately 5"4 and is not sensitive to the testing temperature. 2. In the temperature range of 1900 to 2400 K, the activation energy for creep deformation of
this alloy is 109 kcal/mol and is not sensitive to the applied stress. 3. The temperature-compensated creep rate of W-3.6Re-0-33ZrC is about one order lower than W-5Re and two orders lower than tungsten at temperatures above 0-5 Tm. 4. The high creep resistance of W-3-6Re-0.33ZrC is associated with the presence of ZrC particles which retard the movements of dislocations and subgrain boundaries, resulting in both direct particle strengthening (interaction of dislocations and particles) and indirect particle strengthening (stabilization of grain and subgrain sizes). 5. The strengthening effect of ZrC particles in tungsten-rhenium matrix decreases with increasing temperature due to the particle growth. ACKNOWLEDGEMENTS This work was funded by The Wright Research and Development Center, Wright-Patterson Air Force Base, Ohio under the contract F-33615-91-C-2109. The contract manager is Mr Steve Adams. The authors would like to thank Mr J. Clark for the use of the analytical electron microprobe, which was purchased with the aid of NSF grant E A R 8408163, in the Chemistry Department of Arizona State University. The authors would also like to thank Mr D. R. Bosch for his comments on and assistance in preparing this manuscript. REFERENCES 1. Sims, C. T. & Jaffee, R. I., Properties of refractory alloy containing rhenium. Trans. ASM, 52 (1960) 929-41. 2. Ratliff, L., Maykuth, D. J., Ogden, H. R. & Jaffee, R. I., Tungsten sheet alloys with improved low-temperature ductility. Trans. AIME, 230 (1964) 490-501. 3. Klopp, W. D., Witzke, W. R. & Raffo, P. L., Mechanical properties of dilute tungsten-rhenium alloys. NASA Tech. Note D-3483, Lewis Research Center, Cleveland, Ohio, 1966, pp. 1-38. 4. Raffo, P.L., Yielding and fracture in tungsten and tungsten-rhenium alloys. J. Less-Common Metals, 17 (1969) 133-42. 5. Garfinkle, M., Witzke, W. R. & Klopp, W.D., Superplasticity in tungsten-rhenium alloys. Trans. AIME, 245 (1969) 303-16. 6. Stephens,J. R. & Witzke, W. R., Alloy softening in group VIA metals alloyedwith rhenium. J, Less-Common Metals, 23 (1971) 325-42. 7. Luo, A., Jacobson, D. L. & Shin, K. S., Solution softening mechanism of iridium and rhenium in tungsten at room temperature. J. Refractory Metals & Hard Materials, 10 (1991) 107-14. 8. Klopp, W. D. & Witzke, W. R., Mechanical properties of
Creep behavior of a W - R ~ Z r C
9. 10.
11. 12.
13. 14.
a tungsten-23.4 percent rhenium~).27 percent hafniumcarbide alloy. J. Less-Common Metals, 24 (1971) 427-43. Shin, K. S., Luo, A., Chen, B. L. & Jacobson, D. L., High temperature properties of particle-strengthened W-Re. J. Metals, 42 (1990) 12 15. Luo, A., Shin, K.S. & Jacobson, D.L., Ultrahightemperature tensile properties of a powder metallurgy processed tungsten-3.6w/o rhenium-l.0w/o thoria alloy. Scripta Metallurgica et Materialia, 25 (1991) 1811 14. Luo, A., Shin, K. S. & Jacobson, D. L., High temperature tensile properties of W Re-ThO~ alloys. Material Science & Engineering, A148 (1991) 219-29. Luo, A., Jacobson, D. L. & Shin, K. S., High-temperature strengthening mechanism of HfC in W-3'6Re alloy. Proc. 8th Symposium on Space Nuclear Power Systems, vol. 1, ed. M. S. El-Genk. American Institute of Physics, New York, (1991) pp. 193-204. Sherby, O.D., Factors affecting the high temperature strength of polycrystalline solids. Acta Metall., 10 (1962) 135-43. Lowrie, K. & Gonas, A.M., Dynamic properties of polycrystalline tungsten, 24-1800°C. J. Appl. Phys., 36 (1965) 2189 97.
alloy
103
15. Barrett, C. R., Ardell, A. J. & Sherby, O. D., Influence of modulus on the temperature dependence of the activation energy for creep at high temperatures. Trans. AIME, 230 (1964) 200-12. 16. Mukherjee, A. K., Bird, J. E. & Dorn, J. E., Experimental correlations for high-temperature creep. Trans. ASM, 62 (1969) 155-73. 17. Weertman, J., Steady-state creep through dislocation climb. J. Appl. Phys., 28 (1957) 362-76. 18. Vandervoort, R.R. & Barmore, W.L., Elevated temperature deformation and electron microscope studies of polycrystalline tungsten and tungsten-rhenium alloys. Proc. 6th Plansee Seminar, 20-24 May, 1969, Reutte, Tirol, Austria, Metallwerk Plansee GMBH, Reutte, Tirol, Austria, pp. 108-22. 19. Vandervoort, R. R., The creep behavior ofW-5Re. Metall. Trans., | (1970) 857-67. 20. Zener, C. & Hollomon, J. H., Effect of Strain Rate upon Plastic Flow of Steel. J. Appl. Phys,, 15 (1944) 22 31. 21. Lifshitz, I.M. & Slyozov, V.V., The kinetics of precipitation from supersaturated solid solutions. J. Phys. Chem. Solids, 19 (1961) 35 54. 22. Wagner, C. Z. Elektrochem., 65 (1961) 581-90.