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Deformation and failure mechanisms of niobium and tantalum during tensile testing in uranium at 1473 K
Scripta
METALLURGICA
V o l . 23, pp. 1 0 3 - 1 0 8 , 1989 Printed in the U.S.A.
Pergamon P r e s s
plc
DEFORMATION AND FAILURE MECHANISMS OF NIOBIUM AND TANTALUM DURING TENSILE TESTING IN URANIUM AT 1473 K J.S. Huang, G.F. Gallegos, M.P. Stratman, and E. Sedillo Lawrence Livermore National Laboramry, P.O. Box808, Livermo~,CA ( R e c e i v e d S e p t e m b e r 9, 1988) ( R e v i s e d O c t o b e r 17, 1988) Introduction It is well known that, when a higher-melting-point solid metal is exposed to the combination of a specific lower-melting-point liquid metal and stress, severe embrittlement can occur. This is known as liquid metal embrittlement (LME). Refractory metals with high melting points and good workability, such as the Group VB metals (V, Nb, and Ta) have been used to construct molds for casting other metals that have low melting points. One of these low-melting-pointmetals is U, which is used in nuclear power reactors as fuel. However, it was reported by Kuznietz et al. (1) that severe degradation occurred when Ta was exposed to liquid U. The kinetics of the penetration of U into Ta under stress-free conditions were studied. It was found that, between 1433 K and 1623 K, despite the low solubility of U in Ta, the Ta matrix recrystallized into elongated grains, and U penetrated between the grains. The penetrated U existed in front of the transformed grains as a separate layer and along grain boundaries. For conditions in which stresses exist, it is expected that the penetration of liquid uranium could be further accelerated and thus embritfle the Ta matrix. There is no detailed study about the LME mechanisms of Group VB metals in liquid U. We have initiated a research activity to study the mechanisms of embrittlement of Nb and Ta by liquid uranium. Our initial results, as reported in this paper, indicate a significant difference between Nb and Ta when exposed to liquid U and subjected to tensile stress. Ex~rimental We used metallurgical-grade Ta and Nb rods (both of 99.9 wt% purity) in this study. Chemical compositions of these metals are shown in Table 1. The Ta has an average grain size of 100 ima; the Nb has 20 ima. TABLE 1 Chemical Compositions(by wt%) of the Niobium and Tantalum Studied in This Work C H O N C___uu. Zr M._~o W ~ N._b_b N__bb <0.001 0.0003 0.017 0.007 <0.008 0.02 <0.001 <0.010 0.01 Bal. Ta 0.004 0.0001 0.002 0.001 0.008 <0.015 <0.015 <0.015 <0.002 ---
Ta --Bal.
The U used for this study (D-38) was depleted of the 235 isotope, most of it being the U-238 isotope with a chemical purity of 99.85 wt%. Tensile testing was conducted in vacuum: 1.3x10-5 Pa at room temperature, 2.7×10-3 Pa at the test temperature of 1473 K. The strain rates used for the tensile testing were 3x10-4 and 3x10"5 per second. For testing in U, samples were installed in an open-end Ta crucible that was heavily oxidized. The crucible with the tensile sample was then installed in the vacuum chamber on the pull rods of the tensile-testing machine. The tensile samples have a cylindrical gauge section with 4.06-ram diameter and 25.4-mm length. To ensure temperature uniformity, we heated the sample slowly (at a rate of about 40 K/min) from room temperature to the test temperature and held at the test temperature for 5 minutes before applying a tensile load. The sample's total contact time with liquid U before tensile loading was about 7 minutes. Tensile load was applied by an electromechanical Instron TT-CL universal testing machine. The cross-head movement was monitored with a linear voltage-displacement transducer. The stress reported is the monitored load divided by the original sample cross section; the elongation is the monitored cross-bead movement divided by the original gauge length. This use of engineering stress and strain is limited, but it is adequate for understanding the effects of liquid U on the tensile behavior of materials. After tensile testing, the samples were studied for their failure mechanisms by investigating their cross sections with conventional optical metallography, scanning electron microscopy (SEM), and x-ray WDS (wave-length dispersive spectroscopy) microanalysis. The cross-section samples were prepared by regular mechanical polishing and chemical etching with an acid solution (20ml HF~a0ml Lactic-20ml Nitric).
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Re~llts and Discussion Stress-Strain Behavior Figure 1 shows the stress-vs-elongationcurves for Nb in vacuum and in D-38 at two different strain rates. Both ultimate tensile strength and total elongation are reduced when in contact with D-38; the reduction is larger at the slower strain rate. An inspection of tested samples showed that all samples failed because of a mechanism of combined creep and plastic deformation and that the samples necked down completely to sharp tips. Figure 2 shows a longitudinalcross section of the sample tested in U at strain rate 3x10-4/s. For the sample tested at strain rate 3x10-5/s, the cross section is similar but more severely corroded. As discussed below, the reduction of tensile strength and elongation for tests in U can be explained in terms of corrosion and dissolution of Nb by U. The stress-vs-elongationcurves for Ta in vacuum and in D-38 at strain rate 3x10-4/s are shown in Fig. 3. The ductility of Ta in D-38 was very low and the material failed in the elastic regime. Figure 4 shows a photo,graph of a sample which failed by brittle fracture with negligible plastic deformation. For tests at strain rate 3×10-~/s, the •stress-vs-elongation curve and the mode of failure were identical. 4O 3O -5 30
vacuum
Q.
| e
~
20
;R
20 ! 10 I
I
1473 K
~
strain rate = 3"10 Is 0 0.0
O Hm
0.0
10
0.2
0.4
0.6
0.2
Elongation (m/m)
0.4
0.6
Elongmlon (m/m)
(a) (b) FIG. 1. Stress-vs-elongationcurves for Nb tested in U at 1473 K and at strain rates: (a) 3×10-4/s and (b) 3×10-5/s.
473K
6O
2
I)-38, point of failure
20
.4 strldn rate = 3"10 Is
I~. 0 0.(
FIG. 2. Photograph showing longitudinal cross section of Nb sample tested in U at swain rate 3x10"4/s.
i
I
i
i
0.2 0.4 Elongation (m/m)
i
0.6
FIG. 3. Stress-vs-elongation curves for Ta tested in vacuum and in U at 1473 K and strain rate 3x 10-4Is.
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FIG. 4. Photograph showing Ta sample failed in U at 1473 K and strain rate 3x10-4/s. Metallo~raohv. Scannin~ Electron Microscoov. and X-Ray Microanalysis Figs. 5a and 5b, respectively, show the op,fi,cal micm,,graphs of the transverse cross sections of Nb samples tested at strain rates 3×10-4 and 3x10-S/s. The particles appearing on the micrographs are etch pits and have no compositional difference. Note that several grooves exist on the intersections of surfaces and grain boundaries; these grooves are more heavily etched. WDS x-ray analyses showed that they are enriched with U. They probably result from diffusion or penetration of U along grain boundaries and subsequent dissolution of Nb in U. Although not shown here, the depths of grooves on the longitudinal cross section are about the same.
(a)
(b)
FIG. 5. Optical micrographs showing transverse cross sections of Nb samples tested in U at 1473 K and at swain rates (a) 3x10-a/s and (b) 3x10-5/s. Figure 6 shows an optical micrograph of the transverse cross section of a Ta sample tested in U at strain rate 3x10-4/s. Note that severe grain boundary penetration had occurred. When investigated with scanning electron
FIG. 6. Optical micrograph showing transverse cross section of Ta sample tested in U at 1473 K and strain rate 3x10-4/s.
microscopy and WDS x-ray analysis, these penetrated regions are U-enriched and more heavily etched from that enrichment. The total time of exposure to liquid U for this sample is about 7 minutes, implying a penetration rate of about 8.5x10"8 m/s. We also investigated the longitudinal cross section of this sample (Fig. 7) and found that the penetration depths of U for most places were the same except near the fracture section, where penetration had
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occurred through the entire diameter (explaining the failure location). It should be noted that most of these grain boundary penetrations occurred during the soak period prior to loading, as the duration under load was less than 30 seconds, whereas the soak time was about 7 minutes.
FIG. 7. Optical micrograph showing longitudinal cross section of Ta sample tested in U at 1473 K and at strain rate 3x10-4/s.
The obvious question is, why was more penetration observed near the fractured area.'? One explanation is that the tensile testing system we used had a slight misalignment between the upper and lower pull-rods. A preexisting bending stress on the sample enhanced the penetration of U. Further work is needed to verify this hypothesis and to investigate quantitatively the effect of stress on grain boundary penetration of U in Ta. The Mcfhanism of Tensile Prot~-nv Deeradation for Nb The major effects of U on the stress-elongation relation for Nb is the reduction of ultimate tensile strength and total elongation. In a tensile test, the ultimate tensile strength is usually related to the onset of necking instability. It was observed that this onset of necking instability occurred earlier in U than in vacuum This effect of tensile instability is explained with the following: dP=odA +A do=0 [1] or -(dA/A) = do/g [2] where P is the load, o is flow stress, A is the cross-section area, and do[o is the material's hardening characteristic. When a chemical medium exists that causes corrosion or dissolution, the area reduction term, -(dA/A), can be expressed as the sum of two terms, one caused by chemical and one by mechanical effects. The instability criterion then becomes: -(d/VA)total = -(dA/A)chcm - (dA/A)mechanical= do/o [3] or -(dA/A)mechanical= -(-dA/A)ehem + d6[o [4] Therefore, any corrosion or dissolution can result in a decreasing -(dA/A)mechanical and earlier onset of tensile instability. Furthermore, since the area reduction resulting from corrosion is greater at lower strain rates, it is expected that the onset of tensile instability will occur earlier, resulting in lower strength and ductility. This is consistent with our observations for Nb. Embrittlement of Tantalum by Liuuid Uranium At least four kinds of LME phenomena have been cited in the literature (2,3,4): (a) classical LME, in which a pure metal or alloy can suffer fast brittle fracture when strained in contact with a low-melting-point metal; Co) grain boundary penetration or dissolution by the liquid, sometimes in the absence of stress; (c) selective attack of phases or precipitates in the solid; (d) reaction between solid and liquid. The classical LME phenomenon draws most attention in the LME research community. Examples of this process type arc Zn cmbrittled by In, Sn, or Ga and brass embrittled by Hg. It is normally observed with this kind of LME that a plot of ductility loss versus temperature shows a ductile-to-brittle transition and that the recovery of ductility occurs at higher temperature for loading at higher strain rate. Several theories have been proposed for the mechanism of this kind of LME: stressassisted dissolution (5); bond breaking from adsorption-induced reduction of bond strength (6), cohesion (4), and surface energy (3); and adsorption-assisted nucleation and movement of dislocation motion at the crack tip (7). For the Ta/U system, it appears that LME is the second type of phenomenon since grain boundary penetration can occur without stress. To confirm this, we immersed a Ta sample in the D-38; grain boundary penetration of U indeed occurred, as shown in Fig. 8. For a 30-minute period, the average penetration depth was about 260 ~tm.
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However, one should not conclude at this stage that the classical LME mechanisms are not operative in the system. Future work is needed to address this possibility under different testing conditions.
FIG. 8. Optical micrograph showing the penetration of U in Ta at 1473 K for 30 minutes under no mechanical stress.
The difference between grain boundary penetration of U in Nb and that in Ta is rather surprising and worth further discussion since Nb and Ta belong to the same group of metals in the periodic table. Grain boundary penetration can occur by at least two types of mechanisms: grain boundary diffusion and grain boundary wetting (in which liquid penetrates by capillary effect). Whipple (8) has analyzed the grain boundary diffusion problem and concluded that sign~_c_c~tadditional grain boundary penetration will occur by diffusion when the parameter ~, defined as (DbS)/(2D~/I~t), is equal to or greater than 1. In this representation, Db is the grain boundary diffusion coefficient, 13x is the lattice diffusion coefficient, t is time, and 8 is grain boundary thickness (typically = 3x10 "]° m). Gjostein (9) has deduced the diffusivity spectrum based upon literature and given the following equations for BCC metals: Db = DI exp(-20 Tm/RT) [5] D~ = I)2 exp(-34 Tm/RT) [6] where DI is 2x10 -5 m2/s, 1)2 is 5x10"5 m2/s, R is the universal gas constant (1.987 cal/K-mol), and Tm is the melting point. Since the grain boundary diffusion data of U into Nb and Ta are not available, we used Eqs. [5] and [6] for our analysis and concluded that grain boundary penetration should occur for both Nb and Ta. Fisher (7) also analyzed the grain boundary diffusion problem and mathematically expressed the concentration, C, as a function of position, y, and time, t, as follows: C(y,t) = CO x {
-Y~/2 "~ e pk(nDkt)ll4(SDb/Dk)ll2 fl
[7]
where CO is the maximum concentration at the outer surface. Taking the concentration at the penetration tip as 50% for approximation and using Eqs. [5], [6], and [7], we calculated that the grain boundary penetration distance is 1-3 lam for Nb, 1-2 lam for Ta. The calculated value apparently agrees with the experimental observation for Nb but is significantly different from that for Ta. The mechanism of grain boundary penetration by wetting occurs when the grain boundary energy exceeds the energy of the solid-liquid interface by a factor of two. It is a common mode of intergranular attack in systems in which the lower-melting-point components are relatively insoluble in the solid but the solid has an appreciable solubility in the liquid. The more severe grain boundary penetration of U in Ta than in Nb agrees with the fact that U is highly soluble in Nb but not in Ta (11). The lack of experimental data on solid-liquid interfacial energies has prevented us from fully exploring the difference between Nb and Ta; our analyses are based on predictive models from literature. Miedema and den Broeder (12) have suggested that the solid-liquid interracial energy can be calculated from the contributions of three energy terms: a fraction of the heat of fusion of the solid metal; an entropy term that represents the decrease in atomic disorder of surface-layer atoms in the liquid; an energy term related to the heat of solution of the two metals. This is expressed mathematically as: A AHAo¿ AB I II III AHf T 7SL = (YSL)A +(TSL)B + 7SL = 2"5x10-9 (Vm2/3)A+ 0"52x10-7 (V2m~)B+ 2"5x10-9 (Vm2/3)A
[8]
A. where M-If is the molar heat of fusion of solid metal A and V A and V B are the respective molar volumes of metals
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A and B. T is temperature, and 6t-I_A_1is the heat of solution of liquid metal A in liquid metal B. For Ta-U and Nb-U systems, Table 2 gives the res~,ctive values of 6I-If,Vm, td-Isol, calculated liquid-solid interfacial energy('rsl), and calculated grain boundary energy ('tgb)- We obtained values for the heat of solution from Miedema,-Boer and Boom (13), calculated using a ee-llular-atom model; we estimated grain boundary energy as 36% (14) of the respective surface energy, 2260 for Nb and 2500 mJhn2 for Ta (15), at melting point. TABLE 2 Molar Volume, Heat of Fusion, Grain Boundary Energy, Solid-Liquid Interfacial Energy, Heat of Solution for Ta and Nb in Liquid U at 1473 K "fsI AHf AHsol (mJ?m2) (kJ/mol) (kJ/mol~ (,,,,,,,,2) Ta 1.1xl0 -5 31.6 6 900 322 Nb 1.1xl0 -5 26.3 4 813 286 These results indicate that the grain boundary energies are greater than twice the solid-liquid interfacial energies for both Ta-U and Nb-U systems. Therefore, it seems that grain boundary penetration of U by wetting should occur in both Ta and Nb. An inspection of Fig. 6 gives some support for Ta, as the second grain boundary from the right side of this micrograph shows that the angle between the boundary and the two U-Ta interfaces is near zero. By contrast, in Nb this angle is very large, and grain boundary wetting did not play an important role. The discrepancy between prediction and the experimental results for Nb calls for further study. It is possible that the segregation of U on the grain boundaries of Nb and Ta may modify their free energies. Concl0sions Tensile testing was conducted for Ta and Nb at 1473 K with strain rates from 10-5 to 10-4Is. We concluded: 1. No embrittlement was observed for Nb. The failure in tension was by plastic deformation. The presence of molten U dissolved the sample outer surfaces, which lead to accelerated reduction of areas and earlier onset of tensile instability. 2. The presence of liquid U resulted in zero or near-zero ductility for Ta. The embrittlement was caused by grain boundary penetration of liquid U, which can occur without the presence of stress. 3. Both grain boundary diffusion and wetting are operative for penetration of U into Ta, but the wetting mechanism is not operative for the penetration of U into Nb. Future work is needed to quantify these effects. Acknowledmaaents We wish to express our appreciation to Mr. James M. Yoshiyama and Mr. Donald D. McCoy for their assistance in the areas of scanning electron microscopy and x-ray microanalysis. We would also like to acknowledge Dr. Martyn Adamson for stimulating discussions on this work. ~mq~)
Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
References Moshe Kuznietz, Zvi Livne, and Carlos Coffer, High Temp. High Pressures 18, 1 (1986). M.G. Nicholas and C. F. Old, J. Mater. Sci. 14, 1 (1979). W. Rostoker, J. M. McCaughey, and H. Markus, Embritffement by Liquid Metals, p. 131, Rheinhold, New York, N. Y. (1960). M.H. Kamdar, Prog. Mat. Sci. 15, 289 (1973). W.M. Robertson, Trans. AIME 236, 1478 (1966). N.S. Stoloff and T. Johnston, Acta Met. 11,251 (1963). S.P. Lynch, Fracture 1977, D. M. R. Taplin, Ed., p. 859, University of Waterloo Press, Waterloo, Ontario, Canada (1977). R.T. Whipple, Phil. Mag. 45, 1225 (1954). N.A. Gjostein, Diffusion, p. 241, American Society for Metals, Metals Park, Oh. (1973). J. C. Fisher, J. Appl. Phys. 22, 74 (1951). T. B. Massalski, Ed., Binary Alloy Phase Diagrams 2, p. 1704 & 2103, American Society for Metal, Metals Park, Ohio (1986). A. R. Miedema and F. J. A. den Broeder, Z. Metallkd. 70, 10 (1979). A. R. Miedema, F. R. de Boer, and R. Boom, CALPHAD 1:4, 341 (1977). L.E. Murr, Inteffacial Phenomena in Metals and Alloys, p. 76, Addison-Wesley, Reading, Mass. (1975). H. Jones, Metal Sci. J. 5, 15 (1971).
1
METALLURGICA
V o l . 23, pp. 1 0 3 - 1 0 8 , 1989 Printed in the U.S.A.
Pergamon P r e s s
plc
DEFORMATION AND FAILURE MECHANISMS OF NIOBIUM AND TANTALUM DURING TENSILE TESTING IN URANIUM AT 1473 K J.S. Huang, G.F. Gallegos, M.P. Stratman, and E. Sedillo Lawrence Livermore National Laboramry, P.O. Box808, Livermo~,CA ( R e c e i v e d S e p t e m b e r 9, 1988) ( R e v i s e d O c t o b e r 17, 1988) Introduction It is well known that, when a higher-melting-point solid metal is exposed to the combination of a specific lower-melting-point liquid metal and stress, severe embrittlement can occur. This is known as liquid metal embrittlement (LME). Refractory metals with high melting points and good workability, such as the Group VB metals (V, Nb, and Ta) have been used to construct molds for casting other metals that have low melting points. One of these low-melting-pointmetals is U, which is used in nuclear power reactors as fuel. However, it was reported by Kuznietz et al. (1) that severe degradation occurred when Ta was exposed to liquid U. The kinetics of the penetration of U into Ta under stress-free conditions were studied. It was found that, between 1433 K and 1623 K, despite the low solubility of U in Ta, the Ta matrix recrystallized into elongated grains, and U penetrated between the grains. The penetrated U existed in front of the transformed grains as a separate layer and along grain boundaries. For conditions in which stresses exist, it is expected that the penetration of liquid uranium could be further accelerated and thus embritfle the Ta matrix. There is no detailed study about the LME mechanisms of Group VB metals in liquid U. We have initiated a research activity to study the mechanisms of embrittlement of Nb and Ta by liquid uranium. Our initial results, as reported in this paper, indicate a significant difference between Nb and Ta when exposed to liquid U and subjected to tensile stress. Ex~rimental We used metallurgical-grade Ta and Nb rods (both of 99.9 wt% purity) in this study. Chemical compositions of these metals are shown in Table 1. The Ta has an average grain size of 100 ima; the Nb has 20 ima. TABLE 1 Chemical Compositions(by wt%) of the Niobium and Tantalum Studied in This Work C H O N C___uu. Zr M._~o W ~ N._b_b N__bb <0.001 0.0003 0.017 0.007 <0.008 0.02 <0.001 <0.010 0.01 Bal. Ta 0.004 0.0001 0.002 0.001 0.008 <0.015 <0.015 <0.015 <0.002 ---
Ta --Bal.
The U used for this study (D-38) was depleted of the 235 isotope, most of it being the U-238 isotope with a chemical purity of 99.85 wt%. Tensile testing was conducted in vacuum: 1.3x10-5 Pa at room temperature, 2.7×10-3 Pa at the test temperature of 1473 K. The strain rates used for the tensile testing were 3x10-4 and 3x10"5 per second. For testing in U, samples were installed in an open-end Ta crucible that was heavily oxidized. The crucible with the tensile sample was then installed in the vacuum chamber on the pull rods of the tensile-testing machine. The tensile samples have a cylindrical gauge section with 4.06-ram diameter and 25.4-mm length. To ensure temperature uniformity, we heated the sample slowly (at a rate of about 40 K/min) from room temperature to the test temperature and held at the test temperature for 5 minutes before applying a tensile load. The sample's total contact time with liquid U before tensile loading was about 7 minutes. Tensile load was applied by an electromechanical Instron TT-CL universal testing machine. The cross-head movement was monitored with a linear voltage-displacement transducer. The stress reported is the monitored load divided by the original sample cross section; the elongation is the monitored cross-bead movement divided by the original gauge length. This use of engineering stress and strain is limited, but it is adequate for understanding the effects of liquid U on the tensile behavior of materials. After tensile testing, the samples were studied for their failure mechanisms by investigating their cross sections with conventional optical metallography, scanning electron microscopy (SEM), and x-ray WDS (wave-length dispersive spectroscopy) microanalysis. The cross-section samples were prepared by regular mechanical polishing and chemical etching with an acid solution (20ml HF~a0ml Lactic-20ml Nitric).
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D E F O R M A T I O N AND FAILURE OF Nb AND Ta IN U
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Re~llts and Discussion Stress-Strain Behavior Figure 1 shows the stress-vs-elongationcurves for Nb in vacuum and in D-38 at two different strain rates. Both ultimate tensile strength and total elongation are reduced when in contact with D-38; the reduction is larger at the slower strain rate. An inspection of tested samples showed that all samples failed because of a mechanism of combined creep and plastic deformation and that the samples necked down completely to sharp tips. Figure 2 shows a longitudinalcross section of the sample tested in U at strain rate 3x10-4/s. For the sample tested at strain rate 3x10-5/s, the cross section is similar but more severely corroded. As discussed below, the reduction of tensile strength and elongation for tests in U can be explained in terms of corrosion and dissolution of Nb by U. The stress-vs-elongationcurves for Ta in vacuum and in D-38 at strain rate 3x10-4/s are shown in Fig. 3. The ductility of Ta in D-38 was very low and the material failed in the elastic regime. Figure 4 shows a photo,graph of a sample which failed by brittle fracture with negligible plastic deformation. For tests at strain rate 3×10-~/s, the •stress-vs-elongation curve and the mode of failure were identical. 4O 3O -5 30
vacuum
Q.
| e
~
20
;R
20 ! 10 I
I
1473 K
~
strain rate = 3"10 Is 0 0.0
O Hm
0.0
10
0.2
0.4
0.6
0.2
Elongation (m/m)
0.4
0.6
Elongmlon (m/m)
(a) (b) FIG. 1. Stress-vs-elongationcurves for Nb tested in U at 1473 K and at strain rates: (a) 3×10-4/s and (b) 3×10-5/s.
473K
6O
2
I)-38, point of failure
20
.4 strldn rate = 3"10 Is
I~. 0 0.(
FIG. 2. Photograph showing longitudinal cross section of Nb sample tested in U at swain rate 3x10"4/s.
i
I
i
i
0.2 0.4 Elongation (m/m)
i
0.6
FIG. 3. Stress-vs-elongation curves for Ta tested in vacuum and in U at 1473 K and strain rate 3x 10-4Is.
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FIG. 4. Photograph showing Ta sample failed in U at 1473 K and strain rate 3x10-4/s. Metallo~raohv. Scannin~ Electron Microscoov. and X-Ray Microanalysis Figs. 5a and 5b, respectively, show the op,fi,cal micm,,graphs of the transverse cross sections of Nb samples tested at strain rates 3×10-4 and 3x10-S/s. The particles appearing on the micrographs are etch pits and have no compositional difference. Note that several grooves exist on the intersections of surfaces and grain boundaries; these grooves are more heavily etched. WDS x-ray analyses showed that they are enriched with U. They probably result from diffusion or penetration of U along grain boundaries and subsequent dissolution of Nb in U. Although not shown here, the depths of grooves on the longitudinal cross section are about the same.
(a)
(b)
FIG. 5. Optical micrographs showing transverse cross sections of Nb samples tested in U at 1473 K and at swain rates (a) 3x10-a/s and (b) 3x10-5/s. Figure 6 shows an optical micrograph of the transverse cross section of a Ta sample tested in U at strain rate 3x10-4/s. Note that severe grain boundary penetration had occurred. When investigated with scanning electron
FIG. 6. Optical micrograph showing transverse cross section of Ta sample tested in U at 1473 K and strain rate 3x10-4/s.
microscopy and WDS x-ray analysis, these penetrated regions are U-enriched and more heavily etched from that enrichment. The total time of exposure to liquid U for this sample is about 7 minutes, implying a penetration rate of about 8.5x10"8 m/s. We also investigated the longitudinal cross section of this sample (Fig. 7) and found that the penetration depths of U for most places were the same except near the fracture section, where penetration had
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occurred through the entire diameter (explaining the failure location). It should be noted that most of these grain boundary penetrations occurred during the soak period prior to loading, as the duration under load was less than 30 seconds, whereas the soak time was about 7 minutes.
FIG. 7. Optical micrograph showing longitudinal cross section of Ta sample tested in U at 1473 K and at strain rate 3x10-4/s.
The obvious question is, why was more penetration observed near the fractured area.'? One explanation is that the tensile testing system we used had a slight misalignment between the upper and lower pull-rods. A preexisting bending stress on the sample enhanced the penetration of U. Further work is needed to verify this hypothesis and to investigate quantitatively the effect of stress on grain boundary penetration of U in Ta. The Mcfhanism of Tensile Prot~-nv Deeradation for Nb The major effects of U on the stress-elongation relation for Nb is the reduction of ultimate tensile strength and total elongation. In a tensile test, the ultimate tensile strength is usually related to the onset of necking instability. It was observed that this onset of necking instability occurred earlier in U than in vacuum This effect of tensile instability is explained with the following: dP=odA +A do=0 [1] or -(dA/A) = do/g [2] where P is the load, o is flow stress, A is the cross-section area, and do[o is the material's hardening characteristic. When a chemical medium exists that causes corrosion or dissolution, the area reduction term, -(dA/A), can be expressed as the sum of two terms, one caused by chemical and one by mechanical effects. The instability criterion then becomes: -(d/VA)total = -(dA/A)chcm - (dA/A)mechanical= do/o [3] or -(dA/A)mechanical= -(-dA/A)ehem + d6[o [4] Therefore, any corrosion or dissolution can result in a decreasing -(dA/A)mechanical and earlier onset of tensile instability. Furthermore, since the area reduction resulting from corrosion is greater at lower strain rates, it is expected that the onset of tensile instability will occur earlier, resulting in lower strength and ductility. This is consistent with our observations for Nb. Embrittlement of Tantalum by Liuuid Uranium At least four kinds of LME phenomena have been cited in the literature (2,3,4): (a) classical LME, in which a pure metal or alloy can suffer fast brittle fracture when strained in contact with a low-melting-point metal; Co) grain boundary penetration or dissolution by the liquid, sometimes in the absence of stress; (c) selective attack of phases or precipitates in the solid; (d) reaction between solid and liquid. The classical LME phenomenon draws most attention in the LME research community. Examples of this process type arc Zn cmbrittled by In, Sn, or Ga and brass embrittled by Hg. It is normally observed with this kind of LME that a plot of ductility loss versus temperature shows a ductile-to-brittle transition and that the recovery of ductility occurs at higher temperature for loading at higher strain rate. Several theories have been proposed for the mechanism of this kind of LME: stressassisted dissolution (5); bond breaking from adsorption-induced reduction of bond strength (6), cohesion (4), and surface energy (3); and adsorption-assisted nucleation and movement of dislocation motion at the crack tip (7). For the Ta/U system, it appears that LME is the second type of phenomenon since grain boundary penetration can occur without stress. To confirm this, we immersed a Ta sample in the D-38; grain boundary penetration of U indeed occurred, as shown in Fig. 8. For a 30-minute period, the average penetration depth was about 260 ~tm.
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However, one should not conclude at this stage that the classical LME mechanisms are not operative in the system. Future work is needed to address this possibility under different testing conditions.
FIG. 8. Optical micrograph showing the penetration of U in Ta at 1473 K for 30 minutes under no mechanical stress.
The difference between grain boundary penetration of U in Nb and that in Ta is rather surprising and worth further discussion since Nb and Ta belong to the same group of metals in the periodic table. Grain boundary penetration can occur by at least two types of mechanisms: grain boundary diffusion and grain boundary wetting (in which liquid penetrates by capillary effect). Whipple (8) has analyzed the grain boundary diffusion problem and concluded that sign~_c_c~tadditional grain boundary penetration will occur by diffusion when the parameter ~, defined as (DbS)/(2D~/I~t), is equal to or greater than 1. In this representation, Db is the grain boundary diffusion coefficient, 13x is the lattice diffusion coefficient, t is time, and 8 is grain boundary thickness (typically = 3x10 "]° m). Gjostein (9) has deduced the diffusivity spectrum based upon literature and given the following equations for BCC metals: Db = DI exp(-20 Tm/RT) [5] D~ = I)2 exp(-34 Tm/RT) [6] where DI is 2x10 -5 m2/s, 1)2 is 5x10"5 m2/s, R is the universal gas constant (1.987 cal/K-mol), and Tm is the melting point. Since the grain boundary diffusion data of U into Nb and Ta are not available, we used Eqs. [5] and [6] for our analysis and concluded that grain boundary penetration should occur for both Nb and Ta. Fisher (7) also analyzed the grain boundary diffusion problem and mathematically expressed the concentration, C, as a function of position, y, and time, t, as follows: C(y,t) = CO x {
-Y~/2 "~ e pk(nDkt)ll4(SDb/Dk)ll2 fl
[7]
where CO is the maximum concentration at the outer surface. Taking the concentration at the penetration tip as 50% for approximation and using Eqs. [5], [6], and [7], we calculated that the grain boundary penetration distance is 1-3 lam for Nb, 1-2 lam for Ta. The calculated value apparently agrees with the experimental observation for Nb but is significantly different from that for Ta. The mechanism of grain boundary penetration by wetting occurs when the grain boundary energy exceeds the energy of the solid-liquid interface by a factor of two. It is a common mode of intergranular attack in systems in which the lower-melting-point components are relatively insoluble in the solid but the solid has an appreciable solubility in the liquid. The more severe grain boundary penetration of U in Ta than in Nb agrees with the fact that U is highly soluble in Nb but not in Ta (11). The lack of experimental data on solid-liquid interfacial energies has prevented us from fully exploring the difference between Nb and Ta; our analyses are based on predictive models from literature. Miedema and den Broeder (12) have suggested that the solid-liquid interracial energy can be calculated from the contributions of three energy terms: a fraction of the heat of fusion of the solid metal; an entropy term that represents the decrease in atomic disorder of surface-layer atoms in the liquid; an energy term related to the heat of solution of the two metals. This is expressed mathematically as: A AHAo¿ AB I II III AHf T 7SL = (YSL)A +(TSL)B + 7SL = 2"5x10-9 (Vm2/3)A+ 0"52x10-7 (V2m~)B+ 2"5x10-9 (Vm2/3)A
[8]
A. where M-If is the molar heat of fusion of solid metal A and V A and V B are the respective molar volumes of metals
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A and B. T is temperature, and 6t-I_A_1is the heat of solution of liquid metal A in liquid metal B. For Ta-U and Nb-U systems, Table 2 gives the res~,ctive values of 6I-If,Vm, td-Isol, calculated liquid-solid interfacial energy('rsl), and calculated grain boundary energy ('tgb)- We obtained values for the heat of solution from Miedema,-Boer and Boom (13), calculated using a ee-llular-atom model; we estimated grain boundary energy as 36% (14) of the respective surface energy, 2260 for Nb and 2500 mJhn2 for Ta (15), at melting point. TABLE 2 Molar Volume, Heat of Fusion, Grain Boundary Energy, Solid-Liquid Interfacial Energy, Heat of Solution for Ta and Nb in Liquid U at 1473 K "fsI AHf AHsol (mJ?m2) (kJ/mol) (kJ/mol~ (,,,,,,,,2) Ta 1.1xl0 -5 31.6 6 900 322 Nb 1.1xl0 -5 26.3 4 813 286 These results indicate that the grain boundary energies are greater than twice the solid-liquid interfacial energies for both Ta-U and Nb-U systems. Therefore, it seems that grain boundary penetration of U by wetting should occur in both Ta and Nb. An inspection of Fig. 6 gives some support for Ta, as the second grain boundary from the right side of this micrograph shows that the angle between the boundary and the two U-Ta interfaces is near zero. By contrast, in Nb this angle is very large, and grain boundary wetting did not play an important role. The discrepancy between prediction and the experimental results for Nb calls for further study. It is possible that the segregation of U on the grain boundaries of Nb and Ta may modify their free energies. Concl0sions Tensile testing was conducted for Ta and Nb at 1473 K with strain rates from 10-5 to 10-4Is. We concluded: 1. No embrittlement was observed for Nb. The failure in tension was by plastic deformation. The presence of molten U dissolved the sample outer surfaces, which lead to accelerated reduction of areas and earlier onset of tensile instability. 2. The presence of liquid U resulted in zero or near-zero ductility for Ta. The embrittlement was caused by grain boundary penetration of liquid U, which can occur without the presence of stress. 3. Both grain boundary diffusion and wetting are operative for penetration of U into Ta, but the wetting mechanism is not operative for the penetration of U into Nb. Future work is needed to quantify these effects. Acknowledmaaents We wish to express our appreciation to Mr. James M. Yoshiyama and Mr. Donald D. McCoy for their assistance in the areas of scanning electron microscopy and x-ray microanalysis. We would also like to acknowledge Dr. Martyn Adamson for stimulating discussions on this work. ~mq~)
Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
References Moshe Kuznietz, Zvi Livne, and Carlos Coffer, High Temp. High Pressures 18, 1 (1986). M.G. Nicholas and C. F. Old, J. Mater. Sci. 14, 1 (1979). W. Rostoker, J. M. McCaughey, and H. Markus, Embritffement by Liquid Metals, p. 131, Rheinhold, New York, N. Y. (1960). M.H. Kamdar, Prog. Mat. Sci. 15, 289 (1973). W.M. Robertson, Trans. AIME 236, 1478 (1966). N.S. Stoloff and T. Johnston, Acta Met. 11,251 (1963). S.P. Lynch, Fracture 1977, D. M. R. Taplin, Ed., p. 859, University of Waterloo Press, Waterloo, Ontario, Canada (1977). R.T. Whipple, Phil. Mag. 45, 1225 (1954). N.A. Gjostein, Diffusion, p. 241, American Society for Metals, Metals Park, Oh. (1973). J. C. Fisher, J. Appl. Phys. 22, 74 (1951). T. B. Massalski, Ed., Binary Alloy Phase Diagrams 2, p. 1704 & 2103, American Society for Metal, Metals Park, Ohio (1986). A. R. Miedema and F. J. A. den Broeder, Z. Metallkd. 70, 10 (1979). A. R. Miedema, F. R. de Boer, and R. Boom, CALPHAD 1:4, 341 (1977). L.E. Murr, Inteffacial Phenomena in Metals and Alloys, p. 76, Addison-Wesley, Reading, Mass. (1975). H. Jones, Metal Sci. J. 5, 15 (1971).
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