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Title: Macro-Segregation in Uranium-6wt%Niobium

  1. Los Alamos National Laboratory
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
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Resource Relation:
Conference: TMS 2015 Annual Meeting ; 2015-03-16 - 2015-03-16 ; Orlando, Florida, United States
Country of Publication:
United States
Materials Science(36)

Citation Formats

Aikin, Robert M. Jr. Macro-Segregation in Uranium-6wt%Niobium. United States: N. p., 2015. Web.
Aikin, Robert M. Jr. Macro-Segregation in Uranium-6wt%Niobium. United States.
Aikin, Robert M. Jr. 2015. "Macro-Segregation in Uranium-6wt%Niobium". United States. doi:.
title = {Macro-Segregation in Uranium-6wt%Niobium},
author = {Aikin, Robert M. Jr.},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2015,
month = 3

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  • Several years ago the SRF--community was unpleasantly surprised by the discovery that superconducting RF-cavities made from high purity niobium showed significant degradations of the Q-values when kept for longer periods of time at intermediate temperatures around 100 K. The first temperature map taken on such a degraded cavity showed a rather uniform distribution of the additional losses. This fact and the roughly 100 K holding temperature resulted in the hypothesis of precipitation of hydride phases in niobium. A large number of investigations in several laboratories followed this discovery and the results supported the initial explanation of hydride precipitation. It wasmore » experimentally verified that the Q-degradation could be avoided, if the cavities were quickly cooled down through the dangerous temperature region; hydrogen degassing at elevated temperatures eliminated the cavity deterioration, but subsequent extensive chemical surface treatment seemed to reverse the process. A summary of the recent experimental observations has been given, but the detrimental effect of hydrogen precipitation in niobium cavities has been known for many years. For large scale accelerator projects like CEBAF the cryogenic system might prefer certain cooldown cycles and it is important to know the cooling conditions under which the cavity performance is not effected. Such investigations were done in the past and have extended to other temperature regimes. The results and the analysis of these experiments are reported in the following based on a model of weak links between hydrogen segregates and the niobium matrix, which has been developed by one of the authors (JH) for high T{sub c} and classical superconductors.« less
  • The void-volume fraction in unalloyed Nb and Nb-base binary alloys containing either approx. 2.4 a/o Mo, Ti, Zr, Hf, Ni, Fe, V, or Ta has been determined following 3.0-MeV /sup 58/Ni/sup +/ ion irradiation at 1225 K to approx. 50 dpa. also, the thermal- and irradiation-induced segregation of these substitutional-atom solutes and the intrinsic, interstitial-O impurity (approx. 0.06 a/o) to the specimen surface and within the irradiation-damaged layer have been determined. The void-volume fraction and the segregation of solutes have been shown to be correlated by the solute diffusivity and the relative chemical affinity of the substitutional solutes for O.more » The correlated diffusion of vacancy defects and O atoms is postulated to explain the experimental results.« less
  • Because the distances between atoms vary with direction within a crystal, the properties of a crystal also vary with direction. Such property variations are called crystalline anisotropy, and the amount of anisotropy depends on the symmetry of the crystal structure. The alpha-phases of uranium depends on the symmetry of the crystal structure. The alpha-phases of uranium and uranium alloys are of orthorhombic symmetry and, thus, are highly anisotropic. Normally, uranium and uranium alloys consist of polycrystalline aggregates (grains) whose crystal axes are orientated randomly with respect to each other; however, mechanic deformation can align preferentially the crystal axes of themore » grains to form a textured structure. In particular, forward extrusion of the uranium-2.4 wt % niobium alloy aligns the a axis of the alpha-phase parallel to the extrusion direction and the b axis transverse to the extrusion direction. Axis-density distribution charts show that the texture of the uranium-2.4 wt % niobium alloy is a function of the extrusion ratio. The linear thermal expansion of the uranium-2.4 wt % niobium alloy is reduced transverse to the extrusion direction and is increased parallel to the extrusion direction. The effect of anisotropy on tensile properties also is discussed. It is concluded that the properties of mechanically deformed, alpha-phase uranium and uranium alloys depend on the texture.« less