Diamond nucleus doping induced non-uniform transition in polycrystalline graphite

Abstract

Mechanical behavior and phase transition mechanisms of nano-diamond (ND)-doped polycrystalline graphite (NG) heterostructures (NDG) are investigated using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Simulations cover a range of loading conditions, uniaxial compression, triaxial confinement and shear, to examine stress-driven structural evolution and localized deformation responses. ND inclusions act as internal stress modulators, impeding shear band propagation and facilitating site-specific sp${}^2$–sp${}^3$ transitions within NG domains. The emergence of metastable and twinned diamond structures is attributed to heterogeneous stress accumulation at NG-ND interfaces, with transformation efficiency governed by grain size, doping concentration, and pressure anisotropy. Under non-proportional triaxial loading, lateral confinement enhances structural stability and shifts the failure mode from plastic deformation to transformation-driven hardening. DFT results reveal that interfacial charge accumulation and out-of-plane lattice distortions reduce the energy threshold for sp${}^2$–sp${}^3$ hybridization. A comparative grain boundary (GB) model highlights the role of amorphous GBs in modulating local stress distributions and charge localization, corroborating MD-predicted non-uniform transformation fronts. Moreover, orbital-resolved analysis shows that shear promotes charge polarization and $p_x$/$p_y$ orbital localization, acting as electronic precursors to phase nucleation. These findings establish a multiscale modeling framework that connects mesoscale stress fields with atomistic transformation pathways, offering insight into the design of structurally robust carbon-based composites under extreme mechanical conditions.