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.
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