Ann M Kelly

Kinetics of Lamellar Decomposition Reactions in U-Nb Alloys

Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems [1]. DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions [7] are undesirable in U-Nb alloys, since they degrade corrosion resistance [5], ductility [8], and the shape-memory effect [9]. Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.

Magnetotransport and superconductivity of α-uranium

We have measured the electrical resistivity, magnetoresistance and Hall effect on several new single-crystal samples and one polycrystalline sample of α-U. The residual resistivity ratios of these samples vary from 13 to 315. Matthiessens law appears to hold above the onset of the charge-density wave phase transitions that begin near 43 K, but not below this temperature. Sharp features at all three charge-density wave transitions are observed and the effects of high magnetic fields on them are presented and discussed. The magnetoresistance is anisotropic, reaches 1000% at 2 K and 18 T and does not exhibit Kohler scaling. The Hall coefficient is positive, independent of magnetic field and slightly temperature dependent above about 40 K in agreement with earlier studies. Below 40 K the Hall coefficient changes sign as the temperature falls, varies with field and becomes much more strongly negative at the lowest temperatures than has been reported. Some of our results suggest that a spin-density wave may coexist with the charge-density wave states. Superconductivity is observed in two of our samples; we argue that it is intrinsic to α-U and suggest that it is consistent with a two-band model. Several parameters characterizing the transport and superconductivity of α-U are estimated.

Unraveling the Age Hardening Response in U-Nb Alloys

Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

Preparation of B4C/Al composites for image analysis

Abstract Composites made by infiltrating B 4 C networks with aluminum, or its alloys, are of interest for lightweight armor applications. Image analysis plays an important part in correlating the microstructures of such composites with their mechanical properties. Accurate image analysis requires a high degree of perfection in the metallographic preparation, which is particularly difficult where the phases present have such disparate hardnesses and reactivities. Two preparation procedures have been developed that produce adequate contrast and definition for analysis of key microstructural features.

Update on LANL DU U-10Mo Casting and on Characterization of Y-12 LEU Casting

This report presents an overview of LANL casting procedure and characterization of recent DU U-10 Mo castings and characterizes recent LEU U-10Mo casting from Y-12.

Dynamic Properties of Shock Loaded Thin Uranium Foils

A series of spall experiments has been completed with thin depleted uranium targets, nominally 0.1 mm thick. The first set of uranium spall targets was cut and ground to final thickness from electro-refined, high-purity, cast uranium. The second set was rolled to final thickness from low purity uranium. The impactors for these experiments were laser-launched 0.05-mm thick copper flyers, 3 mm in diameter. Laser energies were varied to yield a range of flyer impact velocities. This resulted in varying degrees of damage to the uranium spall targets, from deformation to complete spall or separation at the higher velocities. Dynamic measurements of the uranium target free surface velocities were obtained with dual velocity interferometers. Uranium targets were recovered and sectioned after testing. Free surface velocity profiles were similar for the two types of uranium, but spall strengths (estimated from the magnitude of the pull-back signal) are higher for the high-purity cast uranium. Velocity profiles and microstructural evidence of spall from the sectioned uranium targets are presented.