  Zircaloy-4 is used as a fuel cladding material for water reactors, as it has
good mechanical properties, corrosion resistance, and a low thermal neutron
absorption cross section. However, the mechanical performance of Zircaloy-4 can
be reduced during service due to hydrogen uptake and hydride formation. These
hydrides are brittle, and often reduce the strength and toughness of materials
as well as increase susceptibility to delayed hydride cracking (DHC). In this
work, large grain Zircaloy-4 with hydrides was prepared and then cross
sectioned using cryo-ion beam polishing, using plasma focused ion beam (pFIB)
and broad ion beam (BIB) approaches to enable the preparation of a very high
quality flat surface with no preferential etching of either the hydride or
zirconium metal (typically metallographic polishing preferentially removes
hydrides). Conventional and high angular resolution electron backscatter
diffraction (EBSD) analysis were then used to explore morphology, deformation
fields, and orientation relationships between the zirconium matrix and
hydrides. Four maps were collected for analysis which included hydrides near
grain boundaries: (a) where the hydride smoothly decorates across two of the
connecting boundaries near a triple junction; (b) where the hydride smoothly
decorates the boundary; (c) a mixture of smooth decoration of the interface and
protrusion into the grains; (d) fine scale hydride that protrudes into one
grain. This work highlights that incompatibility of the hydride within the
zirconium matrix is strongly linked to the orientation relationship of the
hydride and matrix, and the grain boundary character. These results may enable
enhanced understanding of the role of hydrides in fracture as well as
stress-induced hydride reorientation and DHC susceptibility.
