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At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and currently models that accurately represent such a solid-solid phase transformation coupled with the multi-phase plasticity likely under shock conditions do not exist. For this reason, the influence of peak shock stress on the retention of omega phase in Zr is explored in this study. In-situ VISAR measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. This understanding of the role of peak shock stress will be utilized to address physics to be encoded in our present macro-scale models.

Abstract: Irradiation-induced microstructural evolution in uranium-bearing delta-phase oxides of A6U1O12 (A=rare earth cations) were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Polycrystalline Y6U1O12, Gd6U1O12, Ho6U1O12, Yb6U1O12, and Lu6U1O12 samples were irradiated with 300keV Kr++ to a fluence of 2×1020 ions/m2 at cryogenic temperature (∼100K). The crystal structure of these compounds was determined to be an ordered, fluorite derivative structure, known as the delta-phase, a rhombohedral symmetry belonging to space group . Experimental results indicate that all these compounds are resistant to amorphization to a displacement damage dose of ∼60 displacements per atom. In these experiments, we sometimes observed an irradiation-induced order-to-disorder phase transformation, from an ordered rhombohedral to a disordered fluorite structure. [ABSTRACT FROM AUTHOR]

Abstract: Polycrystalline uranium-bearing compounds Y6U1O12, Gd6U1O12, Ho6U1O12, Yb6U1O12, and Lu6U1O12 samples were irradiated with various ions species (300keV Kr++, 400keV Ne++, and 100keV He+) at cryogenic temperature (∼100K), and the microstructures were examined following irradiation using grazing incidence X-ray diffraction and transmission electron microscopy. The pristine samples are characterized by an ordered, fluorite derivative structure, known as the delta phase. This structure possesses rhombohedral symmetry. Amorphization was not observed in any of the irradiated samples, even at the highest dose ∼65dpa (displacement per atom). On the other hand, some of these compounds experienced an order-to-disorder (O–D) phase transformation, from an ordered rhombohedral to a disordered fluorite structure, at ion doses between 2.5 and 65dpa, depending on ion irradiation species. Factors influencing the irradiation-induced O–D transformation tendencies of these compounds are discussed in terms of density functional theory calculations of the O–D transformation energies. [Copyright &y& Elsevier]

Abstract: Ion irradiation damage effects in delta (δ) Y6U1O12 were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Experimental results revealed no amorphization transformation occurs in Kr-ion irradiated Y6U1O12 to a maximum displacement damage dose of ∼50 displacements per atom at cryogenic temperature. Density functional theory calculations indicate that δ-Y6U1O12 possesses a relatively low cation antisite formation energy, which may help to explain the observed resistance of δ-Y6U1O12 to irradiation-induced amorphization of δ-Y6U1O12. [Copyright &y& Elsevier]