Transformation-Mediated Kinetic Unfreezing in Mixed-Motif Amorphous Carbon

High-temperature softening severely limits the service reliability of amorphous solids under extreme conditions. Unlike conventional amorphous materials, mixed-motif amorphous carbon contains coexisting sp2- and sp3-rich phases, whose distinct mechanical and kinetic properties may couple the microstructural transformation with dynamical activation during heating. To elucidate how microstructural motifs govern the softening of amorphous carbon, we here conducted large-scale molecular dynamics simulations and kinetic statistical analyses of amorphous diaphite (a-DG), a mixed-motif amorphous carbon composed of a compliant disordered multilayer graphene (DMG) matrix and a rigid continuous random network (CRN) skeleton. We reveal a transformation-mediated kinetic unfreezing process in which the dynamical mismatch between coexisting sp2/sp3 motifs generates stress mismatch assistance for graphitization and connectivity reconstruction. The flexible DMG matrix provides local mobility and relaxation space, while the rigid CRN skeleton undergoes graphitization induced by mechanically heterogeneous sp2/sp3 interfaces. This transformation reconstructs graphitic connectivity, releases topological constraints, and ultimately activates global atomic mobility, leading to macroscopic softening. This mechanism points to a kinetic unfreezing scenario that differs from that of conventional amorphous solids, where the structural topology is largely preserved during the dynamic process. Our findings identify mixed-motif amorphous carbon as a representative platform for transformation-mediated kinetic unfreezing and provide a structural basis for tailoring the high-temperature service window of advanced carbon materials.