Formation of nanocrystalline δ-ZrHx in Zircoloy-4: Orientation relationship and twinning

An important aspect of materials phenomena during the service of structural Zr alloys for nuclear cladding components is associated with hydrogen picking, which leads to the formation of δ-hydrides ZrHx (x = 1.55 or 1.66), the structural evolution of which is essential to the mechanical stability of the cladding components. In this work, we report the formation of δ-hydrides due to accidental hydrogen pickup, using high-resolution transmission electron microscopy (HRTEM). The results showed that the habit plane for relatively large δ-hydrides was (0001)α, with the plate morphology. The morphology for regions with populous nano-sized hydrides is irregularly blocky, albeit being of the same orientation relationship between hydrides and matrix. Such remarkable morphological change is associated with multi-nucleation and twinning in the hydrides crystals in the nano-scale.