Elliott, R.O.

Giessen, B.C.

Struebing, V.O.

UNT Digital Library

Z:^77l?%5gS?!KK^^ i'-X&L i»(:;?.^:;i.i:vmf:iJJ3i(C Jfc^i?- -^ .* ->&.-- /r.'.VEItJ'flR! lA.UR-74»97 8 ^ V - 7&44Z — 3 - N O T I C E -Thls report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic fc'nergy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. METASTABLE FCC SOLID SOLUTIONS IN Ce-Pu ALLOYS QUENCHED RAPIDLY FROM THE MELT* by B. C. Giessen Solid State Chemistry Laboratory Department of Chemistry Northeastern University, Boston, Mass. and V. O. Struebing and R. O. Elliott"1, University of California Los Alamos Scientific Laboratory Los Alamos, New Mexico 87544 Introduction; Two complete equilibrium phase diagrams have been pub-lished for Ce-Pu alloys * . These a re in general agreement and the dia-gram shown in Fig. l a is a composite based on the results of both investi-gations . There a re no intermediate phases present in. this system and the maximum terminal solid solubilities are: ~ 25 at. % Ce in 6-Pu and ~ 38 at.% Pu in y-Ce. Both 6-Pu and y-Ce have the Cu-Al type structure (face-centered cubic). Pure cerium undergoes a phase transition from the y-form to the a-form either at ~ 100 K and atmospheric pressure or at ~ 8000 atm and room temperature . A very large decrease in volume (14 to 17%) accompanies this transition and there i s no change in the crystal s t ructure. Gschneidner, e t a l . 5 found that alloying additions of plutonium increased the stability of the a-form with respect to the y-forin. The y-a t r a n s -formation temperature was raised to ~ 215°K by the addition of 20 at.% *Work performed under the auspices of the U. S. Atomic Energy Commission t Speaker DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED G0 :;j*r:.--i:z.vz3-r rasa^ttttajsufit'flJgairefii J Pu in solid solution and the transformation pressure at room temperature was lowered to ~- 2200 atm. However, a-Ce alloys were not retained to room temperature at atmospheric pressure in this or any other study, to our knowledge. Ellinger, et ajL found that cerium exists in its higher valent a-form in 6-Pu solid solutions, while plutonium exists in its lower valent 6-form in y-Ce. In this way, the two elements tend to reduce their dif-ference in valence when forming solid solutions. The size difference between &-Pu and y-Ce is 10%, and that between 6-Pu and a-Ce is only £%. All three phases have the same fee crystal structure and there is relative-ly little difference in electronegativity. Thus, the Hume-Rothery criteria for complete solid solubility are almost satisfied, except for valence, and, as will be shown, there is even some tendency to reduce the valence dif-ference. Accordingly, the objective of the present work was to extend the limits of terminal solid solubility in the Ce- Pu system by rapidly qaenching alloys from the melt, and to see if a continuous series of metastable fee Ce-Pu solid solution alloys could be obtained. Experimental: Pure cerium and eleven binary alloys containing 5, 15, 22-1/2, 30, 40, 53, 60, 70, 75, 80 and 90 at.% Ce were studied. Master alloys were prepared by arc-melting weighed amounts of the two compo-nents, 99.98 wt% electrorefined Pu and 99.85 wt% pure Ce. The alloys were quenched by the "gun-type" splat-cooling technique; the apparatus and procedure have been described elsewhere6 '7 »8. Each sample to be splat cooled consisted of a bead from the master alloy which weighed from 25 to 50 mg depending on the alloy composition, with the weight adjusted to maintain approximately constant volumes of the splats. Quenching was carried out on copper target plates and the splatted deposits were sealed in polyethylene envelopes and examined on a G.E. XRD-5 diffractometer with Culto radiation. Lattice parameters were evaluated by internally calibrating the patterns with diffraction peaks from the copper target plate as previously described®. The lattice parameter for y-Ce obtained in this way is too low by 0,009A, as compared to the handbook value a = 5.1612A . This discrep-ancy may be in small part due to experimental error such as the absence of a specimen thickness correction; however, other current work and literature data11»^z indicate that well-prepared splat cooled alloys con-tain vacancy concentrations high enough to decrease the lattice parameters substantially. Since this effect could not be verified directly for the metastable alloys, it was assumed in the present study that under identical quenching conditions identical lattice parameter corrections can be used for all compositions. Inasmuch as the melting points of all Ce-Pu alloys vary only by < 200°C (Fig. la), the quenched-in vacancy concentrations could be assumed to be of similar magnitude. The resulting, excellent agreement of the values for splat-cooled alloys with those measured on equilibrium phases on both sides of the diagram justifies this approach. Lattice parameters have a maximum fractional error of ± 10 , correspond ing to Aa ss ± . 005 A. Results; The phases present after splat cooling are represented in the quenched-phase plot, Fig. lb; lattice parameter data are given in Table 1 and Fig, 2. It can be seen that solid solutions of the fact.—centered-cubic Cu-Al type were retained over the whole composition rauge; they were single phase with the exception of the alloy at 75 at.% Ce. The present work thus closes the gap between 20 and 70 at.% Ce in which Ellinger _et al. did not retain single-phase alloys by conventional methods. On the bases of the magnitude of the lattice parameters and the slopes of the curves in Fig. 2, three Al-<ype solid solution ranges can be distinguished, which are described according to increasing Ce content: 1. 6-Pu(Ce) solid solution, between 0 and 20 at.% Ce. Although only two alloys were studied by us in this range, this region had been carefully determined by Ellinger et ah . After mak-ing the lattice parameter corrections described £bove, the data for the splat-cooled alloys exactly follow those for the heat-treated alloys1 and extrapolate to the closely similar values for 6-Pu of aQ = 4.64 ± 0.005 A (a = 4.642 A1^ and for Ce of aQ = 4.84 ± . 01 A <a0 « 4.84 A3). On the basis of the qualitative agreement of the latter value with that reported for a-Ce , Ellinger et al. assumed that in this solid solution Ce atoms are present in the "tetravalent" state (valence 3.67 ± . 09 according to Gschneidner and Smoluchowski ) of a-Ce. To facilitate relation of tUe present results to a-Ce a probable room temperature value for ao of o;-Ce was obtained from the data of Mcllargue and Yakel^ by extrapolation, using coeffi-cients of expansion intermediate between those of trivalent rare earth metals (La, Gd) and tetravalent metals (Hf, Th) to arrive at a (a-Ce, 20 C) = 4.857 ± . 003 A. 2. m Ce(Pu) solid solution, between ~ 20 and 75 at.% Ce. The lattice parameters of this solid solution are continuous with 6-Pu(Ce), but have a distinctly different slope; the identifi-cation of a , as a phase separate from 6 is based on this fact. Data for metastable solid solutions over the range ~ 20 to 75 at.% Ce extrapolate to a = 4.91 ± 0.005 Aat 100% Ce. This value is larger than that of a -Ce and the form corresponding with this size has been designated a Ce. The transition in slope of the lattice parameter /composition curve between a- Ce(Pu) and 6Pu(Ce) appears to lie between 15 and 25 at.% Ce, as may be seen in Fig. 2. The probably valence state of aJCe will be discussed below. 3 . y-Ce(Pu) solid solution, between 75 and 100 at.% Ce. The lattice parameter sJope of this solid solution agrees with the values ol Ellinger et a l . 1 shown in Fig, 2. There is a discon-tinuous change of lattice parameter and slope in the composition range 75 ± 5 at.% Ce, with the measured lattice parameters changing from 5.016 to 4.844 A. Quenched alloys with 75 at.% Ce showed both or. and Y phases , and traces of a third phase. As noted already , a for y-Ce(Pu) extrapolates exactly to a for 6-Pu; Ce is therefore regarded as being i c j t s "trivalenf" state in this solid solution valence (3.06 ± . 06 ) with no devia-tion from a Vegard's Law straight line for v-Ce(Pu). Discussion: The postulated existence of a, (Ce-Pu) as a phase separate from 6-Pu(Ce) is suggested by the data, especially the straight-line lattice parameter values in the a.Ce(Pu) region and the change of slope between 15 and 25 at.% Ce. Although less likely, it is also possible that "a" is merely an extension of 6 -Pu(Ce), with a negative deviation from a vegard 's Law straight line connecting 6 -Pu to an unknown modification of Ce. How-ever , in size and valence, this modification would be identical with o^; the following consideration concerning a. applies therefore in either case . The valence state of Ce in a is assessed from the lattice parameters of aCe(25Pu), a Ce(25Pu), 6Ce(25Pu), and pure aCe and yCe (see Table 1). Lattice contractions for the following transitions a re : yCe(25Pu) -> aCe(25Pu) A a = 5„030-4.789 = 0.241 A (1) YCe(25Pu) - a Ce(25Pu) A a = 5.030-4.838 = 0.192 A (2) YCe - cuCe1 A a = 5.161-4.84 = 0.321 A (3) YCe - a C e 1 8 A a = 5.161-4.857 = 0.304 A (4) The size change for the transition given in eq'.n (1), although not actually observed, can be assumed to be due to a valence change of the Ce atoms in the alloy similar in magnitude to that in pure Ce. The contraction in eq'n (2) is 80% of that in eq'n (1). With this as a bas i s , it can be assumed that 80% of the total valence change from pure \Ce to pure aCe corresponds to the valence change for the transition in eq'n (2). Thus the valence change (Av) of Ce for the transition specified in eq'n (2) can be estimated as follows: Av = 0.8 (3.67-3.06) = 0.49 (5) The remaining 20% of the valence change (0.12) occurs gradually between a- Ce(25Pu) and a . Ce(~* 80Pu)„ This argument is based on the valence values of Gschneidher and Smoluchowski; 7 only slightly higher numbers result if the values of Zachariasen19 are used. (It is recognized that neither of the schemes " ' agrees with the numbers of valence (non-f) electrons computed for a '-Ce (high pressure form) and y ~Ce by Knietko . ) The sudden size decrease of Ce atoms in metastable Ce-Pu solid solutions, followed by a continuous further size decrease with increasing Pu content is believed to be due to the tendency of Ce atoms to increase their valence and to decrease in size when subjected to severe compressive strains, whether by cooling9, isostatic compression , or dilution with smaller Pu atoms, as in the present case, where the differences in size and valence of the elements are reduced by the y->a. ->ct transformation. In contrast to the sharp transition in Pu-Ce, a gradual change of the y-Ce valence state to a higher-valence state seems to occur in the Ce-Th alloys14" . An increase in the number of valence electrons of Ce upon compression is also found in the band structure calculations of Kmetko . The nature of the discontinuity at ~ 75 at. % Ce will be studied further by examining splats of different compositions and cooling rates to determine whether the free energy curves of y-Ce(Pu) and a . (Ce-Pu) intersect at this composition in a first - (or higher) order phase transition, or whether this is merely the composition at which the free energy curve of the supercooled liquid intersects that of a. at a supercooling temperature determined by the quenching process. In the latter case, this composition has significance only in terms of the free energy diagram of the super-cooled metastable alloys, which form constrained equilibria. Summary: Single-phase, metastable Ce-Pu alloys have been produced over the complete composition range by splat-cooling. Three f .c .c . Cu-Al type phases can be distinguished: yCe(Pu) solid solutions, a metastable phase designated aJCe(Pa) between ~ 20 and 75 at.% Ce, and 6Pu(Ce) solid solutions with lattice parameters which extrapolate to that of arCe„ As inferred from lattice parameter data, a sudden electronic transition from trivalent vCe to that of a higher valent a Ce occurs at ~ 75 at.% Ce. With increasing Pu content, a further, gradual Ce valence increase to the valence of a-Ce was observed. Acknowledgment The authors are grateful to D. L. Rohr for assisting with the ex-periments ard to A. M. Russell for his comments regarding the manu-script . One of us (BCG) acknowledges support by the AEC under Contract AT-(11-1)-D395 in the early stage of this work, and further support by ONR under Contract N14-68-A-207-3. References 1. F . H. Ellinger, C. C. Land and E. M. Cramer , Extractive and Physical Metallurgy of Platomum and Its Alloys , W. D. Wilkinson, E d . , Interscience, New York, p . 149 (1960). 2 . J . E . Selle and D. E. Et te r , Trans . Met Soc. AIME 230, 1000 (1964). 3 . F . H. Ellinger, W. N. Miner, D. R. O'Boyle and F . W. Schonfeld, Los Alamos Scientific Laboratory (USA) report LA-3870 (1968). 4. K. A. Gschneidner, J r . , R. O. Elliott a n d R . R. McDonald, J . Phys. Chem. Solids 23, 555 (1962). 5. K. A. Gschneidner, J r . , R. O. Elliott a n d R . R. McDonald, J . Phys. Chem. Solids 23_, 1191 (1902). 6. R. O. Elliott, A. M. Russell and B. C. Giessen, J . Mater. Sci. 8., 1325 (1973). 7. R. C. Rulil, B. C. Giessen, M. Cohen and N. J . Grant, Acta Met. 15_, 1693 (1967). 8. R. C. Ruhl and M. Cohen, Trans . Met. Soc. AIME 245., 241 (1968). 9. W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals Vol 1, Pergamon P r e s s , New York, p . 124 (1958). 10. T. X. MahypndB. C. Giessen, Unpublished Results (1974). 11. A. Kiriri, A. Tonejc, and A. Bonefacic, Scripta Metall, 3_, 943 (1969). 12. H. Jones andC. Soryanarayana, J . Mater. Sci. 8_, 705 (1973). 13. F . H. Ellinger, C. C. Land and V. O. Struebing, J . Nucl. Mat. 12_, 226 (1964). 14. R. T. Weiner, W. E. Freeth and G. V. Raynor, J . Inst. Metals 86, 185 (1957-58). 15. J . H. N. van Vucht, Phillips Res . Rep. 12_, 351 (1957). 16. D. S. Evans and G. V. Raynor, J . Nucl. Mat. 5 , 308 (1962). 17. K. A. Gschneidner, J r . andR. Smoluchowski, J . Less Common Met. 5 , 374 (1963). 18. C. J . McHargue and H. J . Yakel, J r . , Acta Met. 8_, 637 (1960). 19. W. H. Zachariasen, J . Inorg. Nucl. Chem. 35., 3487 (1973). 20. E. A. Kmetko, in Electronic Density of States. NBS Special Public, 323, L. H. Bennett, E d . , (1971) p . 67. 800 600 o o UJ {£. !5 ce UJ 0 . 5 UJ 400 200 Lv € y <«*s \ 8 ^ — "T* I ^ "1 1 1 +» 1 1 625 7 / L L+y-Ce U€*r -Ce _ © 8 * y 592 -Ce L*8-Ce "~ 720 •-'' S -f ' it-s' y 8-Ce Cct + S-Ce y-Ce \ \ \ \ \ \ \ 20 40 60 80 ATOMIC PERCENT CERIUM (a) EQUILIBRIUM PHASES 8 Pu(Ce) yCe(Pu) 100 lors.Vale»os • • l a nil) I • <*fc«raJ«*y yCe(Pu)-H METASTABLE PHASES AT 20° C (b) Fig. 1 5.2 T T o A Ellinger.et a I. • • Present work yCe(Pu) °S5.0 w a> £ o w o a. a> u 34.8 (5.161) a(Ce(Pu)-a ,(4.91) »(4.84) 20 40 60 80 Atomic Percent Cerium 100 Fig. 2 Table 1 Lattice Parameters of Ce-Pu Alloys All Lattice Parameters Refer to the Cu-Al Structure Type at 20°C Phase SPu(Ce) a^Ce-Pu) YCe(Pu) Composition Range (at.% Ce) 0 - ~ 20 ~ 20-75 75-100 „ * ** ao(A) • 4 - 6 4 * i 3 4.6421 '* 4.789* 4.84 4 .84 1 4.857 ± . 0031" 4.623 ~4 .838 4 .91 ~ 4.64 ~ 4 - 6 4 * 5. 030* 5.161 Composition (at.% Ce) 0 0 75 100 100 100 0 75 100 0 0 75 100 Terminal Phase 6-Pu 6-Pu -~ a - C e ~ a - C e ~ a - C e (~8-Pu) -a -Ce ~ 5-Pu ~ 6-Pu -v - C e Values corrected for vacancy effects using v-Ce as calibration, see text. ** I By extrapolation to composition given, except for 0!-Ce and v-Ce. Extrapolated from value for a-Ce in unalloyed Ce measured at 77° K., see text. 

Metastable FCC solid solutions in Ce--Pu alloys quenched rapidly from the melt

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Metastable FCC solid solutions in Ce--Pu alloys quenched rapidly from the melt

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