Scale-dependent structural evolution reveals the nanoscale-to-mesoscale mechanism of diameter-dependent strength degradation in carbon fibers.

The nanoscale structural mechanisms that lead to the failure of carbon fiber mechanical properties have not yet been fully elucidated. Here, we reveal the scale-dependent structural evolution underlying the nanoscale-to-mesoscale origin of strength loss in PAN-based carbon fibers. Utilizing an integrated synchrotron X-ray scattering framework (USAXS/MSAXS/SAXS) covering an unprecedented 2-1495 nm spatial range, coupled with μ-WAXS and AC-TEM, the hierarchical defect architecture is continuously characterized in both reciprocal and real space. The results demonstrate that the diameter effect is fundamentally governed by radial structural heterogeneity. With increasing fiber diameter, precursor-derived sub-micrometer low-density and structurally disordered regions preferentially accumulate within the core, intensifying skin-core heterogeneity and reducing the radial structural continuity of graphitic microcrystalline networks. High-temperature graphitization promotes carbon network reconstruction and enhances nanoscale structural ordering. At approximately 1500 °C, the low-density features undergo partial shrinkage and structural regularization, contributing to the attainment of peak tensile strength. However, during subsequent graphitization, sub-micrometer low-density regions and structural discontinuities undergo limited dimensional reduction and become the dominant structural factors governing tensile strength degradation. Therefore, the mechanical behavior of carbon fibers is governed not solely by graphitic ordering, but by the coupled evolution of microcrystalline structures, multiscale defects, and radial structural heterogeneity. This work establishes a multiscale structural framework linking hierarchical defect evolution with diameter-dependent mechanical limitations and provides new insights into the structural design of high-performance carbon fibers.