Interphase interfaces in NixCo1-xO/ZrO2(CaO) directionally solidified eutectics (DSEs) are modified by selective chemical reduction of the NixCo1-xO oxide phase. Scanning electron microscopy studies of such NixCo1-xO/ZrO2(CaO) DSEs show formation of a thin layer of metallic Ni(Co) interphase (similar to 100 nm) and interfacial porosity upon reduction at high temperatures. A significant change is observed in the mechanical behavior of the reduced NixCo1-xO/ZrO2(CaO) DSEs compared with its pristine counterpart, which is largely attributed to the formation of metallic Ni(Co) interphases. Indentation fracture studies indicate that the presence of metallic interphases in the DSEs enhance the stress energy release mechanism through plastic deformation, interfacial delamination, crack arrest, and crack bridging across the metallic interphase after reduction.