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Languages : en
Pages : 0

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Category :
Languages : en
Pages : 16

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Radiation-induced amorphization is assessed. A rate-theory model is formulated wherein amorphous clusters are formed by the damage event These clusters are considered centers of expansion (CE), or excess-free-volume zones. Simultaneously, centers of compression (CC) are created in the material. The CCs are local regions of increased density that travel through the material as an elastic (e.g., acoustic) shock wave. The CEs can be annihilated upon contact with CCs (annihilation probability depends on height of the energy barrier), forming either a crystallized region indistinguishable from the host material, or a region with a slight disorientation (recrystallized grain). Recrystallized grains grow by the accumulation of additional CCs. Full amorphization is calculated on the basis of achieving a fuel volume fraction consistent with the close packing of spherical entities. Amorphization of a recrystallized grain is hindered by the grain boundary. Preirradiation of U3Si above the critical temperature for amorphization results in of nanometer-size grains. Subsequent reirradiation below the critical temperature shows that the material has developed a resistance to radiation-induced amorphization higher dose needed to amorphize the preirradiated samples than now preirradiated samples. In the model, it is assumed that grain boundaries act as effective defect sinks, and that enhanced defect annihilation is responsible for retarding amorphization at low temperature. The calculations have been validated against data from ion-irradiation experiments with U3Si. To obtain additional validation, the model has also been applied to the ion-induced motion of the interface between crystalline and amorphous phases of U3Si. Results of this analysis are compared to data and results of calculations for ion bombardment of Si.

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Languages : en
Pages : 9

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An alternative mechanism for the evolution of recrystallization nuclei is described for a model of irradiation-induced recrystallization of UO2 wherein the stored energy in the material is concentrated in a network of sinklike nuclei that diminish with dose due to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular dislocation structures that evolve relatively early in the irradiation period. A generalized theory of radiation-induced amorphization and crystallization, developed for intermetallic nuclear materials, is applied to UO2. The complicated kinetics involved in the formation of a cellular dislocation network are approximated by the formation and growth of subgrains due to the interaction of shock waves produced by fission- induced damage to the material.

Author: S. Watanabe
Publisher:
ISBN:
Category : Amorphization
Languages : en
Pages : 12

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Ion-irradiation induced amorphization and crystallization processes in TiNi shape memory alloys have been investigated. The primary concern is to clarify the relationship between the critical temperature for irradiation-induced amorphization and the thermal crystallization temperature. The critical temperature for irradiation-induced amorphization was 673K, however, the edge of the foil specimen crystallized during the irradiation at a lower temperature. The crystallization temperature of the specimen edge during irradiation decreased with increasing of incident ion energy. Thermal annealing experiments revealed that the thermal crystallization preferentially occurred at the specimen edge and the crystalline-amorphous (C-A) interface; The crystallization temperature of the preferential sites were 573K and 623K, respectively. These preferential nucleation indicate that the nucleation barrier dominates the thermal crystallization temperature; the critical temperature for amorphization is higher than the thermal crystallization temperature with no nucleation barrier.

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Category :
Languages : en
Pages :

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Author:
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Category :
Languages : en
Pages : 7

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A rate theory model is formulated wherein amorphous clusters are formed by a damage event. These clusters are considered centers of expansion (CEs), or excess-free-volume zones. Simultaneously, centers of compression (CCs) are created in the material. The CCs are local regions of increased density that travel through the material as an elastic (e.g., acoustic) shock wave. The CEs can be annihilated upon contact with a sufficient number of CCs, to form either a crystallized region indistinguishable from the host material, or a region with a slight disorientation (recrystallized grain). Recrystallized grains grow by the accumulation of additional CCs. Full amorphization is calculated on the basis of achieving a fuel volume fraction consistent with the close packing of spherical entities. Amorphization of a recrystallized grain is hindered by the presence of a grain boundary. Preirradiation of U3Si above the critical temperature for amorphization results in the formation of nanometer-size grains. In addition, subsequent reirradiation of these samples in the same ion flux at temperatures below the critical temperature shows that the material has developed a resistance to radiation-induced amorphization (i.e., a higher dose is needed to amorphous preirradiated samples than those that have not been preirradiated). In the model, it is assumed that grain boundaries act as effective defect sinks, and that enhanced defect annihilation is responsible for retarding amorphization below the critical temperature. The calculations have been validated against data from ion-irradiation experiments with U3Si. For appropriate values of the activation energy of thermal crystallization, the model predicts the evolution of a two phase microstructure consisting of nanocrystalline grains and amorphous clusters.

Author: John Richard Parsons
Publisher:
ISBN:
Category :
Languages : en
Pages : 104

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Author: Matthew Libera
Publisher: Mrs Proceedings
ISBN:
Category : Science
Languages : en
Pages : 782

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Papers presented during a symposium on crystallization, held in Boston, 1993.

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Category :
Languages : en
Pages : 40

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Proposed here is renewed support of a research program focused on interface motion and phase transformation during ion irradiation, with emphasis on elemental semiconductors. Broadly speaking, the aims of this program are to explore defect kinetics in amorphous and crystalline semiconductors, and to relate defect dynamics to interface motion and phase transformations. Over the last three years, we initiated a program under DOE support to explore crystallization and amorphization of elemental semiconductors under irradiation. This research has enabled new insights about the nature of defects in amorphous semiconductors and about microstructural evolution in the early stages of crystallization. In addition, we have demonstrated almost arbitrary control over the relative rates of crystal nucleation and crystal growth in silicon. As a result, the impinged grain microstructure of thin (100 nm) polycrystalline films crystallized under irradiation can be controlled with grain sizes ranging from a few nanometers to several micrometers, which may have interesting technological implications.

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Category :
Languages : en
Pages : 11

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Time-of-flight neutron powder diffraction studies of incrementally neutron-irradiated U3Si and U3Si2 have revealed details of progressive amorphization of bulk materials, crystalline transformation prior to amorphization, elastic strain and diffuse scattering resulting from scattering interference between crystalline and amorphous fractions. Density differences between amorphous and crystalline fractions give rise, respectively, to tensile and compressive strain in U3Si and U3Si2. Diffuse scattering associated with each Bragg peak shows crystallographic direction-dependent variation both in magnitude and displacement (relative to the Bragg position). A theoretical model describing this behavior relates the size of the amorphous zones, and the magnitude and displacement of the diffuse scattering contribution. After complete amorphization of U3Si and U3Si2, anneals to progressively higher temperatures generate gradual evolution of the short- to intermediate-range amorphous structures prior to re-crystallization.