Molecular dynamics study on crack propagation of Fe and Fe-Ni: Temperature, crystallographic orientation and elemental segregation

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• BCC Fe plasticity involves amorphous structures, phase transitions and point defects. • Defect-evolution statistics quantify ductile–brittle fracture transition. • Ni addition enhances plasticity; orientation and temperature govern effects. • Ni distribution tailors crack paths, potentially balancing strength and ductility. Fe and Fe-Ni alloys are crucial in extreme environments, yet the atomic-scale fracture mechanisms remain incompletely understood. Previous molecular dynamics (MD) studies have examined temperature, crystallographic orientation, or alloying individually, but the combined influence of these factors has not been systematically addressed. Here, we employ large-scale MD simulations to provide an integrated picture of crack propagation in Fe and Fe-Ni, with emphasis on the orientation–temperature coupling and contrasting effects of bulk versus grain-boundary Ni distribution. Our results reveal that Fe exhibits a distinctive interplay among phase transitions, point defects, and dislocation activity, which differs from FCC metals and is strongly dependent on temperature and crystallographic orientation. A quantitative criterion is further established to identify the brittle-to-ductile transition. Ni doping lowers unstable stacking-fault energy and modifies deformation pathways in an orientation-specific manner: Fe〈110〉-Ni exhibits enhanced toughness even at cryogenic conditions, whereas Fe〈100〉-Ni and Fe〈111〉-Ni localize deformation and embrittle. In polycrystals, bulk Ni addition promotes amorphization and ductility, while grain-boundary segregation suppresses void nucleation but drives phase-transition dominated fracture. This integrated framework reveals solute distribution and orientation–temperature coupling govern crack growth behavior, providing atomic-scale guidance for designing Fe-based alloys with an optimized strength–ductility balance.