[en] The high linear attenuation coefficient of steel, uranium and uranium based alloys is associated with the small penetration depth of X-rays with the usual wavelength used for diffraction. Nevertheless, by using the proper surface preparation technique, it is possible of obtaining surfaces with bulk properties (free of residual mechanical microstrain). Taking advantage of the feasibility to obtain well prepared surfaces, extensive work has been conducted in studying XRD line broadening effects from flat polycrystalline samples of steel, uranium and uranium alloys

[en] Full Text: Molecular electronics, one of the major fields of the current effort in nano-science, may be de ed as the study of electronic behaviors, devices and applications that depend on the properties of matter at the molecular scale. If the miniaturization trend of microelectronic devices is to continue, elements such as transistors and contacts will soon shrink to single molecules. The promise of these new technological breakthroughs has been major driving force in this ld. Moreover, the consideration of molecular systems as electronic devices has raised new fundamental questions. In particular, while traditional quantum chemistry deals with electronically closed systems, we now face problems involving molecular systems that are open to their electronic environment, moreover, function in far from equilibrium situations. A generic molecular junction is made of two electrodes connected by a molecular spacer that takes the form of a molecular chain of varying length or a molecular layer of varying thickness. We use a simple nearest-neighbors tight-biding model with the non-equilibrium Green's function (NEGF) method to investigate and compare between a self-assembled monolayer (SAM), finite molecular layer (FML), and single molecule (SM) chemisorption to a surface of a metal substrate. In addition, we examine the difference in the transmission probability through a SAM, FML and SM sandwiched between two metallic electrodes. Dramatic differences are observed between the SM, FML and SAM density of electronic states and transmission functions. In addition, we analyze the effects of changing different physical parameters such as molecule-substrate interaction, molecule-molecule interactions, etc; interesting effects that pertain to the conduction properties of single molecules and molecular layers are observed. Intriguing results are attained when we investigate the commensurability of the SAM with the metallic surface

[en] Short communication

[en] In recent years considerable attention has been given to the study of microstructure and mechanical properties of dilute U-Ti alloys. These alloys can be strengthened by precipitation hardening which is produced by solution treating of the U-alloy in the γ phase field followed by quenching to room temperature with subsequent aging in the 300⁰C to 550⁰C range to obtain the desired mechanical properties. It is known that rapid cooling suppresses diffusional decomposition of the γ phase which transforms martensitically to a supersaturated α' uranium. The subsequent aging causes G.P. Zone formation in the α' martensite and the overaging results in precipitation reaction α' → α + δ (the δ phase has a structure of U/sub 2/Ti.)

[en] αUranium formed by the β -> α phase transformation is stressed due to the volume mismatch between the parent and the product phases. It has been suggested that sub grains formed in the a phase are the result of dislocation slip to relax these transformation stresses. This hypothesis is analyzed by combination of computational tools and transmission electron microscopy. Methods for the calculations of the texture and microstructure developed during plastic forming is utilized to this microscopic phenomena. The theory is required to describe the structures formed due to the activity and interactions of huge numbers of dislocations for a wide range of materials and processing conditions. Although dislocation theory can give good predictions regarding the general plastic properties of materials. it cannot be easily applied to plastic forming because of the complex interactions and the dense dislocation structures formed during such processes. For many years the cell formation and texture development of highly symmetric 'simple' materials were studied. The high crystallographic symmetry gives rise to an intrinsic ambiguity concerning the type of dislocations trapped at the cell walls. Our work demonstrates that low symmetry materials are favorable as model systems for the investigation of dislocation boundary formation due to the small number of slip systems composing every vertex of the yield surface. U-0.1%Cr samples were heat treated at 720 degree C (β phase) for 0.5 hours and then furnace cooled to room temperature. The samples were examined using optical and scanning electron microscopy followed by TEM characterization. Using single crystal plasticity theory. the yield surface of a-uranium was calculated. Than the stresses prevailing in a newly formed a particle in the β matrix were calculated by solving the elasto-plastic inclusion problem. Both results were utilized to predict the relative activity of each slip system during the growth of a-uranium. Finally it is shown how the subgrain-boundaries in the a-uranium can be constructed by the operative slip dislocations

[en] Titanium is a stabilizer of the high temperature γ phase in dilute U-Ti alloys. A variety of structures may be obtained when samples are cooled from the γ-phase region to room temperature at different rates and under different conditions. We have studied the decomposition modes of the γ phase of U-3.7 at.% Ti alloy, at several isothermal heat treatments followed by quenching to room temperature. SEM and XRD techniques were employed. (orig./TW)

[en] U₂Ti phase, which is also known as the δ-phase, is a stable intermetallic phase which has been found in the U-Ti system. It precipitates when a γ-uranium/titanium solid solution is decomposed by the eutectoid reaction with the structure related to the parent BCC W-type gamma structure. The structure of the δ-phase is based on the hexagonal unit cell of AlB₂ type, with the parameters a = 0.4828 nm and c = 0.2847 nm. It was established that the U₂Ti phase exists in a very restricted composition range. However, little is known about the effect of small additions of the second solute on the structure and stability of this phase in ternary systems. The present paper reports the observations of orthorhombic distortion of the δ-phase as the result of niobium addition in the U-0.8wt% Ti-2.3wt%Nb alloy

No abstract available

[en] Short communication

[en] The effect of plastic deformation on the structure of as-quenched U-4 at.% Ti alloy was studied by X-ray diffraction and transmission electron microscopy. The plastic deformation of water-quenched U-Ti martensite causes the diffusionless formation of the delta phase (U₂Ti) identified in the electron diffraction patterns. An atomistic model of the diffusionless α → δ transition is proposed. The model is based on the observation that during quenching a decomposition reaction occurs and the final martensite consists of alternating Ti-poor and Ti-rich regions with a modulation wavelength of about 15 nm. It is suggested that the Ti-rich regions are ordered with a U/Ti ration of 2:1. When the alloy is subjected to plastic deformation these Ti-rich regions transform diffusionlessly into the delta phase