On potential core-shell morphologies of δ-hydride precipitates in zircaloy-2: A microstructural characterization approach by electron diffraction and energy-loss spectroscopy

The crystal structure of the interfacial areas of a δ-FCC water-quenched nano-hydride was characterized by electron energy-loss spectroscopy (EELS) and electron diffraction. EELS revealed ribbons with plasmon energy (PE) values of 17.4 ± 0.01 eV and 18.3 ± 0.01 eV (nominally characteristic of the ζ- and γ-hydride phases, respectively) in the interfacial area between the δ-core and α-Zr matrix. Electron diffraction patterns (DPs) obtained from the <2 1 ¯ 1 ¯ 0> axes of the interface contained reflections that could be indexed as {0001} reflections of the ζ-phase. Such ζ-type reflections, however, disappeared after tilting the interface away from the <2 1 ¯ 1 ¯ 0>-axes; implying that they originated from sources other than a hypothetical ζ-phase. Moreover, electron DPs obtained from multiple zone axes of the interface, did not show characteristic {110}/{112} superlattice reflections of the γ-phase. These results ruled out the existence of ζ- and γ-phases in the interface (down to the spatial resolution of the utilized techniques) despite the measured plasmon energy values. Subsequently, dielectric theory was utilized to clarify the origin of interfacial ribbons with PE values characteristic of the ζ- and γ-phases. Dielectric functions of the α-Zr and δ-core were extracted from the energy-loss spectra of the two phases, to simulate the energy-loss functions across the interface. Simulations suggested that the observed interfacial ribbons in EELS maps (with PE values of 17.4 ± 0.01 eV and 18.3 ± 0.01 eV) could have stemmed from the delocalized nature of plasmon vibration and the effect of interface on shifting the plasmon vibration frequency, but not necessarily from the existence of the ζ- and γ-phases.