A REVIEW OF X-RAY DIFFRACTION STUDIES IN URANIUM ALLOYS* Harry L. Yakel Metals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37830 The Physical Metallurgy of Uranium Alloys Conference Sponsored by the AEC Army Material and Mechanical Research Center, Vail, Colorado, Feb. 12-14, 1974. -NOTICE- This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy 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. *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. ,-r-f OF THIS DOCUV.i-!'" -,ID! ! if I \ '-'I ' ' A REVIEW OF X-RAY DIFFRACTION STUDIES IN URANIUM ALLOYS Harry L. Yakel Metals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37830 ABSTRACT Results of x-ray diffraction experiments investigating equilibrium structures of uranium-base substitutional alloys and routes of transitions between equilibrium structures are critically reviewed. In the first section data on equilibrium alpha, beta, and gamma alloys are presented together with a resume1 of work on the crystal structures of relevant stable intermetallic compounds. Since many important physical and mechanical properties of practical uranium alloys are consequences of the production of metastable phases during heat-treatment or forming operations, the second part of the review is principally concerned with x-ray diffraction studies of such nonequi- librium phases — their structure and their mode of formation. Particular attention is given to the sequence of transitional phases found in gamma- quenched and aged uranium alloys, depending on aging time and temperature. The influence of external stresses imposed on the alloys before and during the transitions producing these phases is described in terms of the pre- ferred orientations that may be observed by x-ray diffraction methods. In the final section comparisons are drawn between the general structural behavior of uranium alloys and that of other alloy systems based on metals with allotropies similar to that of uranium. INTRODUCTION SCOPE AND OUTLINE An attempt to fully describe all investigations of uranium and its alloys that have employed x-ray diffraction methods in some aspect is clearly beyond the scope of this conference and its proceedings. I shall therefore focus attention on work in which x-ray techniques have been used as a primary tool to determine atomic arrangements in equilibrium and nonequiiibrium uranium-rich metallic phases. Crystallographic studies of modes of deformation and twinning, measurements of preferred orienta- tion, and determinations of structural effects of radiation damage in these materials will be mentioned only as they have relevance to this main area of interest. * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. In the following section, I summarize our current knowledge of the structures of equilibrium phases of elemental uranium and some of its sub- stitutional alloys. The alloy phases are selected on the basis of their relationship to the stable uranium allotropes and to the transitional structures often encountered in uranium-rich systems. Experimental and theoretical attempts to follow the course of transformations between these equilibrium phases are also briefly reviewed. Discussion of the phase transformations leads quite naturally to a consideration of the occurrence and structure of nonequilibrium transition phases in uranium-rich alloys. I review x~ray diffraction studies of such phases with special emphasis on those concerned with structures closely related to the y (A2-type1) configuration of uranium. Finally, and by way of summary, I compare the structural features of both equilibrium and nonequilibrium phases found in uranium-base metallic alloys with corresponding data for systems based on metals with similar allotropy. From this comparison, areas of unresolved problems and fields for future work emerge. SOME GENERAL REMARKS ON X-RAY METHODS While the results of x-ray diffraction experiments on uranium and its alloys have proven indispensable *-o an understanding of the structural phenomena observed in the materials, certain general problems have arisen that must be considered in any objecti. assessment of the state of our knowledge. The first has to do with the natural chemical reactivity of uranium in the metallic state. This reactivity often ensures that x-ray diffraction experiments carried out much above room temperature in a less- than-perfect vacuum or inert atmosphere have really studied materials contaminated by undesired and sometimes unknown impurities. The effects of these contaminants on the details of crystal structures, thermal expan- sion of lattices, or routes of transformation are usually indeterminate and probably significant. A second problem concerns the high linear photoelectric absorption coefficients of uranium and uranium-rich alloys for commonly used x-ray energies. With a value2 of 153 cm2/gm for the mass absorption coefficient of uranium for Mo Ka x rays (A » 0.7107 A), and assuming a density in the range of 15 to 20 gm/cm3, one may expect 99% of the incident x-ray intensity to be absorbed in a surface layer only 15 to 25 urn thick. This implies that results of x-ray diffraction experiments on such materials will be quite sensitive to surface aberrations, a condition suggesting interesting applications if it is the surface whose structural state is desired but creating potential pitfalls if it is the bulk material about which we wish to know. The large absorption corrections that must be applied to observed diffracted intensities before they can be compared to values predicted from a model structure also represent a serious uncertainty in the precise deri- vation of detailed crystal structures of uranium-rich phases. Lastly, the multiplicity of crystallographic orientations usually found on taking a single crystal through a phase transformation, espe- cially on cooling, constitutes a problem that must be recognized in analyzing x-ray scattering from the resulting composite. This is true of many systems, not only uranium alloys, and the literature contains several examples of errors that may occur if the situation is treated improperly.3 The hazards are more fully discussed by Curzon, Luhman and Silcock.4 STRUCTURAL STUDIES OF EQUILIBRIUM PHASES URANIUM ALLOTROPES Alpha Uranium The structure of uranium metal at room temperature and pressure, designated a, is stable up5 to 667.3 ± 1.3°C and down6 to about -231°C (42 K). Its atomic arrangement was first correctly determined by Jacob and Warren,7 who analyzed powder x-ray diffraction data, and then con- firmed by Lukesh8 from single-crystal data. The orthorhombic unit cell dimensions have been reported many times;7»9"15 best values14 would seem to be a = 2.8536 A, b - 5.8698 A, a * 4.9555 A (all ±0.0001 A) as determined from single crystals using a Bond16 goniometer. Four uranium atoms are located in this cell. They occupy the 4(e), (0, y, 1/4) etc., positions of space group Cmam with only a single posi- tional parameter y to be determined from the observed diffraction intensi- ties. The value of this parameter at room temperature and pressure has also been reported often;',11-13,15,17-20 t^e t,est value from x-ray diffraction experiments11*15 is 0.1025, while a recent neutron diffraction study20 gave 0.1027. Both results have uncertainties of 0.0001. The structure of a-uranium based on these best parameters is shown in Fig. 1. It is a rather severe distortion of an ideal hexagonal close- packed arrangement (jb:a - 2.0570 rather than 1.7321, a:a - 1.7366 rather than 1.6330). Each uranium atom has four near neighbors, two at 2.754 A, two at 2.854 A; the other eight atoms that would complete its nearest- neighbor shell in a dose-packed metal are at distances of 3.263 A (4) and 3.342 A (4). These four interatomic distances, labelled d\, d23 d$3 and <£it, respectively, are included in Table 1 together with values for a variety of other a-like phases. Note that do and c?3 depend only on the unit cell dimensions, while d\ and d^ also depend on y. The coordination geometry is unusual. Imagining the central uranium atom at the center of a sphere, two close contacts lie in the polar direc- tions (±a, respectively) and two lie on the equator at an angular separa- tion of 128.2°. The coordination polyhedron is roughly a trigonal bipyramid with one base atom missing. The disparity between the four short and the eight long contacts has suggested a stronger, more "covalent" character for the former and a weaker, more "metallic" character for the latter.21 ORNL-DWG 74-761 cum BETH URANIUM (Sfll KTB NCPTUNIUH SLPHR URANIUM BLPHfi NEPTUNIUM Fig. 1. Drawings of Equilibrium Structures of Uranium and Neptunium. For all but (3-uranium, close neighbor contacts are indicated by bonds; in the 3-uranium drawing these describe hexagonally connected main layers, and thicker bonds indicate abnormally short interatomic contacts. Unit cell edges appear as thin lines. For 3-uranium, a-neptunium, and 3- neptunium c is nearly vertical; for a-neptunium a is directed to the right; for~<x-uranium b is nearly vertical and a nearly horizontal. See text for references to parameters used in construction. Table 1. Near-Neighbor Separations in Alpha-Like Phases Temperature Atomic Distance, A Corrugation Material Angle, Reference (K) di d2 d* d, <!> (deg) a-U 873 2.813 2.911 3.263 3.355 127.24 10 a-U 298 2.754 2.854 3.263 3.342 128.19 14 a-U 50 2.7423 2.8364 3.2581 3.3352 128.32 15 a-U 4.2 2.7432 2.8444 3.2609 3.3322 128.02 15 ao-U-3.25 at.