Materials Science Forum Vols. 561-565

Abstract: Self-propagating high temperature (combustion) synthesis (SHS) is being used to develop several synthesis and processing routes for the next generation of ceramic nuclear fuels. These fuels are based on an actinide nitride within an inert matrix. The application of SHS is particularly important in the synthesis of americium (Am) based ceramics; since the rapid heating and cooling cycles used in this process will help to minimize vaporization loss of Am, which is a major problem in synthesizing Am-based ceramics. Manganese, praseodymium, and dysprosium are being used as physical and chemical surrogates for various actinides. Actinide nitride powders produced using auto-ignition combustion synthesis (AICS) are subsequently reacted with zirconium powder using SHS to produce a final fuel pellet. This paper will discuss the research to date on the synthesis of Am-N powders as well as the production of dense Zr-Am-N pellets as a model ceramic fuel system.

Abstract: Molecular dynamics (MD) methods are utilized to study the displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom (PKA) energies, Ep, from o.5 keV to 20 keV are considered at a temperature of 100 K and 600 K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of defect clusters within displacement cascades with and without substitutional helium atoms. Particularly, the number and size of helium-vacancy clusters generally increase with increasing helium concentration and PKA energy. However, the number of He-vacancy (He-V) clusters increases with increasing temperature, the mean size of He-V clusters is independent on temperature for the same He concentration and energy recoils.

Abstract: SrTiO3 crystals were implanted with 100 keV xenon (Xe+) ions at 673 or 1073 K up to 2.0 × 1020 ions m−2. Defect clusters formed in the ion-implanted samples were investigated with conventional and high-resolution transmission electron microscopy. Nanometer-sized clusters were formed in the samples. The clusters grew large in size after post-implantation annealing and with increasing the implantation dose. The clusters were faceted with {100}, or {110} of SrTiO3. Though the nano-sized clusters were expected to contain Xe atoms, they were not in crystalline state. The results suggest that even if the clusters contain Xe atoms, they also contain other point defects such as vacancies.

Abstract: The commercial material PM2000 is investigated as a representative for the material class of Oxide Dispersion Strengthened (ODS) steels. ODS steels are envisaged as substance of structural components in Very High Temperature Reactors (VHTR). The VHTR concept is considered by the Generation IV International Forum, an initiative researching the next generation of nuclear power plants. The chosen ODS is mainly used for non-nuclear applications. In order to justify the applicability of this material within a neutron irradiative environment, the evolution of radiation damage must be investigated. PM2000 samples are therefore exposed to 4He2+ irradiation at different temperatures. The potential structural change is measured as a function of the radiation parameters, using Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy on the yttrium K-edge and on the iron K-edge. A degradation of the dispersoids or the steel matrix would show the limitation of this material candidate.

Abstract: The formation of hydrides in zirconium alloy has been one of the essential matters of discussion to maintain mechanical strength of nuclear fuel cladding tubes. In this work, we examined the precipitation process of zirconium hydride by transmission electron microscopy under hydrogen ion irradiation. Zircaloy-4, which has been used extensively as nuclear fuel cladding, was irradiated with hydrogen ion at room temperature to achieve enough hydrogen concentration for precipitation. The growth of hydrides accompanied with dislocations around hydrides was observed under hydrogen implantation. The observed hydride was the γ-hydride phase with fct structure and the orientation relationship was <110>γ ||<1120>α as reported previously. As the hydride grew, the dislocations were generated gradually. This process can be explained using a ratchet mechanism suggested by Carpenter. The growth rate became lower according to the approach of other hydrides. This behavior is considered to be influenced by the strain field caused by other hydrides.

Abstract: The effects of neutron-irradiation near 80°C on the deformation behavior of hexagonal close packed (hcp) materials, zirconium and zircalloy-4, were investigated by transmission electron microscopy (TEM). Particular emphasis is placed on the deformation microstructure responsible for the changes in mechanical behavior. Neutron irradiation at low temperature up to 1 dpa induced a high number density of defect clusters, which resulted in irradiation-induced hardening. Dislocation channel deformation is observed for doses greater than 0.1 dpa, and is coincident with prompt plastic instability at yield. TEM analysis suggests that the loss of work hardening capacity in irradiated zirconium and zircaloy-4 at higher doses is mainly due to dislocation channels that are formed under a high local resolved shear stress, leading to the observed localized deformation.

Abstract: Effects of neutron irradiation and thermal aging on the tensile properties and Charpy impact properties of oxide dispersion strengthened (ODS) ferritic steels for advanced nuclear systems were investigated and discussed with the results of microstructural observation. After the neutron irradiation in JMTR, significant hardening after irradiation at 290 and 400 °C as well as thermal aging at 500 °C, while no effect was observed after irradiation at 600 °C. While the irradiation hardening was not accompanied by a reduction of total elongation in tensile tests, Charpy impact energy at room temperature was reduced after the irradiation. The hardening after the irradiation at 400 °C and the aging at 500 °C was probably due to the formation of Cr-rich phases. The irradiation hardening observed in the ODS ferritic steels irradiated at 290 °C was well explained by the formation of dislocation loops.

Abstract: Molybdenum has many prominent properties, such as high melting point, good thermal properties, and low erosion rate and so on, which make it promising candidate materials for plasma facing materials in the next fusion reactor. In this paper, Molybdenum coatings were deposited onto the oxygen-free copper substrates by atmospheric plasma spraying. The spraying parameters had been carefully selected. Different interlayers were induced between the substrate and the coating. SEM and XRD were used to investigate the photographs and compositions of these coatings. The micro-hardness and bonding strength were also tested. Thermal behaviors of the coatings were evaluated by thermal shock tests. The coatings with interlayers showed better resistance of thermal shock but lower bonding strength compared to coatings that without interlayers.

Abstract: A 2D cylindrical plate model has been established to study the distribution of residual stress of cold expansion hole under different interference values. In addition, the effects of material models on residual stress fields are considered also. Experiments are carried out to measure the residual stress of cold expansion hole and verify simulation results. FEM results show, with interference values increasing, the higher residual radial and circumferential stresses are obtained. At same interference value, the residual stress of Hardening Material( HM ) model is much larger than that of Elastic Perfectly Plastic Material( EPPM ) model.

Abstract: A modified Cellular Automaton model was presented to simulate the evolution of dendritic microstructure in low pressure die casting of Al-Si Alloy, which accounted for the heterogeneous nucleation, the solute redistribution both in liquid and solid, the interface curvature and the growth anisotropy during solidification. The free growth of single equiaxed dendrite and the evolution of multi dendrites with various crystallographic orientations were predicted. The variation of the dendrite tip velocity and local solute concentration at the solid/liquid interface were analyzed. The grain morphology of aircraft turbine wheel casting at different specimen points were predicted and compared with experimental results.