  The interactions between {\delta}-hydrides and plastic slip in a commercial
zirconium alloy, Zircaloy-4, under stress were studied using in situ secondary
electron microscope (SEM) micropillar compression tests of single crystal
samples and ex situ digital image correlation (DIC) macroscale tensile tests of
polycrystalline samples. The hydrides decorate near basal planes in
orientation, and for micropillars orientated for <a> basal slip localised shear
at the hydride-matrix interface is favoured over slip in {\alpha}-Zr matrix due
to a lower shear stress required. In contrast, for pillars oriented for <a>
prismatic slip the shear stress needed to trigger plastic slip within the
hydride is slightly higher than the critical resolved shear stress (CRSS) for
the <a> prismatic slip system. In this case, slip in the hydride is likely
achieved through <110>-type shear which is parallel to the activated <a>-type
shear in the parent matrix. At a longer lengthscale, these results are used to
inform polycrystalline samples analysed using high spatial resolution DIC. Here
localised interface shear remains to be a significant deformation path which
can both cause and be caused by matrix slip on planes closely-oriented to the
phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent
hydride-matrix interfaces can either result in plastic slip within the hydrides
or get arrested at the interfaces, generating local stress concentration.
Through these mechanisms, the presence of {\delta}-hydrides leads to enhanced
strain localisation in Zircaloy-4 early in the plastic regime.
