Size dependence of lattice strain anisotropy in nickel and ceria under non-hydrostatic compression

Strain engineering under extreme non-hydrostatic compression provides a unique pathway to tune material properties, yet the evolution of fine structural parameters under such conditions remains unexplored. Here, we systematically investigate the elastic-plastic deformation behaviors of nickel and ceria (CeO₂) across varying grain sizes (8–200 nm) under non-hydrostatic compression up to ∼35 GPa using synchrotron-based radial X-ray diffraction in a diamond anvil cell. We find that the differential lattice aspect ratio in nickel plateaus at a relatively low pressure due to yielding, whereas ceria exhibits continuous elastic deformation with an increasing aspect ratio within the tested pressure range. Furthermore, as the grain size decreases to ∼8 nm, the maximum differential lattice aspect ratios of {200} plane in both nickel and ceria reach ∼3.2%. The lattice strain anisotropy experiences a multifold enhancement in nickel as the grain size reduces from ∼200 nm to ∼8 nm due to the suppression of dislocation slip. In contrast, this increase of anisotropy by reduction of grain size attenuates in ceria, a phenomenon attributed to the activation of nanoscale plasticity in nanoceramics. These size-dependent deformation mechanisms are further corroborated by microstructural evidence from transmission electron microscopy. Our results highlight the broad tunable parameter space in both metallic- and covalent-bonded nanomaterials via non-hydrostatic high-pressure strain engineering, and would help to understand the high-pressure strengthening effect in nanograined metals.