Electronic-instability-driven phase separation enables grain-boundary toughening in high-entropy carbides

High-entropy carbides (HECs) can exhibit enhanced phase stability, although elemental segregation and phase separation may occur under certain conditions. However, how intrinsic phase instability evolves under high pressure and how it can be exploited to regulate mechanical properties remain insufficiently understood. Here, (Hf_(1−X)/4Zr_(1−X)/4Nb_(1−X)/4Ta_(1−X)/4Co_X)C (X = 0.14, 0.18, and 0.20) was investigated by integrating first-principles calculations with high-pressure sintering. The decomposition Gibbs free energy remains negative from 0 to 6 GPa, indicating an intrinsic thermodynamic driving force for Co segregation. Electronic-structure analysis reveals that pressure amplifies the bonding asymmetry between the stable transition-metal–C framework and the comparatively weaker Co–C interactions, as evidenced by the opposite pressure-dependence change in their integrated crystal orbital Hamiltonian population values. This bonding asymmetry is accompanied by local structural rearrangement, with the first Co–Co radial distribution function (RDF) peak shifting from 3.14 to 3.10 Å and increasing in intensity from 15.97 to 24.50. Experimentally, Co segregates into a ductile FCC phase preferentially distributed along grain boundaries, accompanied by an increase in the low-angle grain boundary fraction from 20.2% to 37.9%, indicating a possible contribution to local strain accommodation. The resulting heterogeneous microstructure facilitates crack deflection and branching, leading to an increase in fracture toughness from 4.5 ± 0.2 to 5.0 ± 0.2 MPa·m^1/2. These findings demonstrate that pressure-driven electronic instability can be exploited to transform intrinsic phase instability into beneficial microstructural heterogeneity, providing a new strategy for toughening high-entropy ceramics.