M. V. Nevitt

The elastic properties of monoclinic ZrO2

Abstract We have made ultrasonic and Brillouin scattering measurements at ambient temperature on small single crystals of monoclinic ZrO2. Using these data, we have established various features of the angular anisotropy in the acoustic longitudinal wave and shear wave velocities, and we have computed the elastic stiffness and compliance moduli. We observe shallow minima in the transverse shear wave velocity in directions parallel to the a and c crystallographic axes. We anticipate that acoustic shear wave softening will be observed in these directions when measurements are made at temperatures close to the monoclinic-tetragonal transformation.

Single-crystal elastic constants of Co-Al and Co-Fe spinels

The elastic stiffness moduli of single-crystals CoAl2O4, a normal spinel, and CoFe2O4, an inverse spinel, have been determined by an ultrasonic method. The measured values are: C11=290.5, C12=170.3 and C44=138.6 GPa for CoAl2O4, and C11=257.1, C12=150.0 and C44= 85.3 GPa for CoFe2O4. The elastic constants are compared with previously published data of other spinel single crystals and the results show that the cation valence and cation distribution have little influence on the elastic properties of the spinel materials.

Transformation temperatures and structures in uranium-fissium alloys

Abstract Solid state transformation temperatures and resulting structures are detailed for alloys of uranium with certain fission-product elements that might remain in a spent fuel from a fast reactor following pyrometallurgical refining. These elements, the aggregate of which is called “fissium” (Fs), are Zr, Nb, Mo, Tc, Ru, Rh and Pd. The phase relations can be related to those in the dominant U-Mo-Ru ternary system. The uranium gamma phase is stabilized down to 562° C, while the beta phase is entirely suppressed at high fissium contents. Certain crystallographic data are given and the minor phases that occur in the alloys are identified.

Fast reactor fuel alloys: Retrospective and prospective views

Abstract The relationship between the physical metallurgy of the EBR-II metallic fuel, U-5% Fs, and its performance in the reactor are described. An understanding of these relationships, along with the optimal matching of fuel properties to fuel-element design, have been essential in the 23 year successful utilization of the fuel. The knowledge and experience gained are being employed in the current development of a new metal fuel for a proposed advanced reactor.

Metallographic and X-ray observations of Pu-C alloys

Metallographic, x-ray diffraction, and incipient-melting techniques were used to investigate the Pu-C phase diagram below 50 at.% C and for temperatures between 500 and 1500 deg C. The variation of lattice parameter with composition for Pu--C alloys is presented and used to define portions of the low-carbon boundary of the PuC phase field. Unique microstructural characteristics are detailed for the epsilon Pu + PuC in equilibrium Pu/sub 3/C/sub 2/ peritectoid reaction, namely, the preference of the Pu/sub 3/C/sub 2/ phase to grow as platelets rather than as an interfacial envelope surrounding the PuC reactants. Some aspects of coring in cast alloys containing the monocarbide phase are discussed. (auth)

The U-Pu-C ternary phase diagram below 50 atomic percent carbon☆

Abstract The U-Pu-C ternary phase diagram for the composition range from 0 to 50 atom percent carbon and for temperatures below 635° C has been constructed from metallographic, X-ray diffraction, and melting range data. Isothermal sections at 400° C, 570° C and 635° C are presented and certain four phase reactions are outlined. Uranium monocarbide forms a complete series of solid solutions with plutonium monocarbide. All phase boundaries other than those of the monocarbide phase represent the sides of three-phase tie-triangles which have one side coincident with either the U-Pu or the Pu-C binary side and have the opposite apex touching the phase boundary of the monocarbide solid solution, (U, Pu)C. The metal: carbon stoichiometry of the monocarbide is influenced by the U: Pu ratio.

The uranium monocarbide-plutonium monocarbide system

Abstract The (U, Pu)C phase field of the U-Pu-C ternary system for temperatures below 635° C has been constructed from metallographic and X-ray diffraction data. Uranium monocarbide forms a complete series of solid solutions with plutonium monocarbide. The (U, Pu) C phase is stoichiometric as regards its carbon content in a composition range from 0 to 35 atomic percent Pu. With a further increase in Pu content it deviates from ideal stoichiometry toward greater metal-to-carbon ratios to form the defect (U, Pu) C phase. Pu expands the unit cell of the monocarbide over the composition range in which it retains its stoichiometric character. In the region of the defect structure the lattice parameter decreases with increasing Pu content, the extent of the decrease being dependent upon the C content. Metallographic observations on cast alloys show that a transition in the microstructures occurs at about 10 atomic percent Pu: Alloys containing less Pu show a homogeneous carbide phase while alloys of higher Pu content have a cored carbide phase.

Displacive Phase Transformation in Vanadium - Substituted Lanthanum Niobate

The displacive transformations in complex oxides of the type LaNb/sub 1-x/V/sub x/O/sub 4/ has been studied by x-ray diffraction and Raman scattering for 0 less than or equal to x < 0.3. X-ray diffraction results indicate that the transformation from the tetragonal high temperature structure (C/sub 4h//sup 6/) to the monoclinic low-temperature structure (C/sub 2h//sup 6/) is higher than first order and that the transformation temperature T/sub c/ is depressed significantly by V substitution. Raman scattering results show that the force constant between the nearest (Nb, V)O/sub 4/ tetrahedral units behave uniquely compared to others. It softens at T/sub c/ as a function of composition and it also softens as a function of temperature as T/sub c/ is approached from above. 10 references, 2 figures, 2 tables.

The effect of surface constraint and finite particle size on a continuous transformation

Etude par RX et microscopie electronique a balayage de deux echantillons polycristallins La(Nb 1−x V x )O 4 (x=0,2) prepares differemment et subissant un phenomene de coexistence de phase lors de la transformation polymorphique structure scheelite quadratique (I4 1 /a)→structure fergusonite monoclinique (I2/c) lorsque la temperature decroit.