The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformation of U-6Nb in the as-quenched state and after aging for two and eight hours at 473 K were completed to assess the influence of microstructural evolution on mechanical properties. With heating, large anisotropic changes in lattice parameter were observed followed by relaxation with time at the aging temperature. The lattice parameters return to nearly their initial values with cooling. The active plastic deformation mechanisms including, in order of occurrence, shape-memory de-twinning, mechanical twinning, and slip-mediated deformation do not change with prior aging. However, the resistance to motion of the as-quenched martensitic twin boundaries increases following aging, resulting in the observed increase in initial yield strength. (C) 2016 Elsevier B.V. All rights reserved.

A soft-phonon feature associated with the shape-memory transition in NiTi is observed in the phonon density of states (DOS) of the B2 phase of both NiTi and Ni50Ti47Fe3 (with Fe substituted for Ti) using inelastic neutron scattering. In both alloys, the feature softens with decreasing temperature, but the softening occurs about 100 K lower in the Fe-substituted alloy, indicating a decreased transition temperature. Electrical resistivity and magnetic susceptibility verify the decreased transition temperature but also show that the transition develops second-order-like behavior similar to that observed by others in Ni44Ti50Fe6 (with Fe substituted for Ni). First-principles calculations supported by Mossbauer spectroscopy and neutron diffraction indicate a double-defect scenario, where Fe occupies Ni sites and the displaced Ni occupies the empty Ti sites in the Ti-substituted alloys. A comparison between the current results for Ti-substituted alloys, and related experimental data for alloys featuring Fe substitution for Ni, indicates that the instability temperature is controlled by the number of Fe atoms occupying the Ni sites, while the second-order-like behavior is caused by the addition of the Ni antisite defects. We argue that this latter behavior results from percolated networks of interacting defects acting to frustrate the symmetry-breaking strains.

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100T(PM) is due to the Ni d minority-spin electrons. Below T(M) this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.

Particles formed during the reaction of cerium with hydrogen fractured into stacked plates with fully separated plate thicknesses averaging 100 nm and a finer partially separated thickness of 30 nm. The phonon density of states of these particles, measured using inelastic neutron scattering, showed a low-energy feature that could not be accounted for in the phonon-dispersion curves of bulk crystals but was similar to a feature predicted for the confinement of phonons in nanoplates. The shift of modes to lower energies indicates that excess vibrational entropy is created by the fracture. We argue that this excess entropy contributes to the observed fracture pattern by introducing a characteristically weak size for fracture.

Continuing the photoemission study begun with the work of Opeil et al. [Phys. Rev. B \textbf{73}, 165109 (2006)], in this paper we report results of an angle-resolved photoemission spectroscopy (ARPES) study performed on a high-quality single-crystal $\alpha$-uranium at 173 K. The absence of surface-reconstruction effects is verified using X-ray Laue and low-energy electron diffraction (LEED) patterns. We compare the ARPES intensity map with first-principles band structure calculations using a generalized gradient approximation (GGA) and we find good correlations with the calculated dispersion of the electronic bands.

An aging study of compositionally homogeneous U–5.6Nb and U–7.7Nb (wt.%) was undertaken to improve the understanding of the decomposition of metastable martensitic phases in the U–Nb system. Artificially aging these depleted (238U) uranium alloys at 373, 473, 523 and 573K for times up to 70 days resulted in significant age hardening. However, the microstructural changes giving rise to this hardening are subtle and elude standard light optical and electron microscopy techniques; this subtlety motivated the present study of aging-related changes by X-ray diffraction. A sealed tube X-ray diffractometer was used to record powder diffraction patterns from these aged polycrystalline U–Nb samples along with a CeO2 standard. Data were analyzed using GSAS for full-pattern Rietveld refinements. Planar defects as well as strain broadening appear to have caused unusual peak profiles, which makes data interpretation challenging. Lattice parameter and unit cell volume trends are presented. U–5.6Nb experienced a reversal in the b lattice parameter and unit cell volume during aging, which has both similarities and differences to aging in dilute U–Ti and U–Mo alloys. The longest-time aging unit cell volume trends of U–5.6Nb indicate Nb depletion of the matrix phase, while the corresponding behavior at all aging times in U–7.7Nb is indicative of Nb enrichment in its matrix phase.

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Recent experiments have indicated that the high-temperature properties of α-uranium may be strongly influenced by the formation of randomly distributed intrinsically localized vibrational modes, just a few atoms across in size. One observation was a loss of mechanical ductility that coincided with the formation of the intrinsically localized mode (ILM). Here, we consider this observation in more detail. In particular, we use the anisotropic thermal expansion behaviour to estimate the strains associated with each ILM and consider the implications for the forces between ILMs and the defects responsible for mechanical deformation. In the process we also suggest that an unusual transition from positive to negative thermal expansion along the [010] direction may be caused by the formation of ILMs.

Using a variety of thermodynamic measurements made in magnetic fields, we show evidence that the diffusionless transition (DT) in many shape-memory alloys is related to significant changes in the electronic structure. We investigate three alloys that show the shape-memory effect (In-24 at.% Tl, AuZn, and U-26 at.% Nb). We observe that the DT is significantly altered in these alloys by the application of a magnetic field. Specifically, the DT in InTl-24 at.% shows a decrease in the DT temperature with increasing magnetic field. Further investigations of AuZn were performed using an ultrasonic pulse-echo technique in magnetic fields up to 45 T. Quantum oscillations in the speed of the longitudinal sound waves propagating in the [110] direction indicated a strong acoustic de Haas-van Alphen-type effect and give information about part of the Fermi surface.

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Phonon dispersion curves were obtained from inelastic x-ray and neutron scattering measurements on alpha-uranium single crystals at temperatures from 298 to 573 K. Both measurements showed a softening and an abrupt loss of intensity in the longitudinal optic branch along [00zeta] above 450 K. Above the same temperature a new dynamical mode of comparable intensity emerges along the [01zeta] zone boundary with energy near the top of the phonon spectrum. The new mode forms without a structural transition but coincides with an anomaly in the mechanical deformation behavior. We argue that the mode is an intrinsically localized vibration and formed as a result of a strong electron-phonon interaction.

Valence-band ultraviolet photoemission spectroscopy (UPS) at 173 K and 6p core-level x-ray photoemission spectroscopy (XPS) at room temperature were performed on a high quality uranium single crystal. Significant agreement is found with first-principles electronic band-structure calculations, using a generalized gradient approximation (GGA). In addition, using low energy electron diffraction (LEED) for the (001) surface, we find a well-ordered orthorhombic crystallographic structure representative of the bulk material.

The gamma-->alpha isostructural transition in the Ce0.9-xLaxTh0.1 system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity or striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to 0 K with increasing La doping, and the transition changes from being first-order to continuous at a critical concentration, x(c) approximately 0.14. At the tricritical point, the coefficient of the linear T term in the specific heat gamma and the magnetic susceptibility increase rapidly near x(c) and approach large values at x=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. The Wilson ratio reaches a value above 2 for a narrow range of concentrations near x(c), where the specific heat and susceptibility vary most rapidly with the doping concentration.

Phonon densities of states were measured on pure uranium and solutions U-0.4% C, U-1.5% Si, and U-0.91% Fe (atomic) using inelastic neutron scattering. The solute atoms stiffened the phonons, resulting in large decreases in vibrational entropy. The vibrational entropy decrease for carbon was four times the configurational entropy increase, showing that the mixing entropy is not only negative but is dominated by vibrations. Comparison with single-crystal dispersion curves indicates that the phonon stiffening for all solutes involved the transverse optic branch propagating along (001) and displacing atoms along (010). The magnitudes of the changes were too large to be explained in terms of short-range force constant models but may originate with impurity pinning of collective modes associated with alpha-U's charge density wave transitions.

The shape memory effect (SME) has been reported in the uranium–niobium alloy system in the region of the phase diagram surrounding U–6.5wt.% Nb. In this regime, the material may have either an α″ monoclinic (U–6wt.% Nb), or γ0 tetragonal structure (U–7wt.% Nb) and is two phase near 6.5wt.% niobium. In situ neutron diffraction studies during uniaxial compressive loading of U–7wt.% Nb indicate that strain in the recoverable region is accommodated by both motion of existing twin boundaries within γ0-phase and stress-induced phase transformation from the γ0 to the α″ structure. The volume fraction of the γ0-phase decreases from 100% initially to ∼26% after 4% total strain and some reversion is observed on release. The initial stress state of the stress-induced α″ grains will be discussed as well as the load sharing between the two phases.