Grain size-dependent nano-cutting behavior of polycrystalline α-Fe: insights from molecular dynamics simulations

Polycrystalline α-Fe is widely used in advanced engineering applications, yet the influence of grain size on its deformation and surface formation during nanoscale machining remains insufficiently understood. This study investigates the grain-size-dependent nano-cutting behavior of polycrystalline α-Fe. The results show that grain refinement strengthens grain-boundary constraints, leading to higher cutting resistance and more localized deformation, whereas relatively coarse grains provide greater space for intragranular dislocation accommodation and result in better surface quality under the present cutting conditions. These findings clarify the role of grain size in governing the nano-cutting response of polycrystalline α-Fe and provide guidance for microstructure selection in precision machining.

Classical molecular dynamics simulations were conducted for polycrystalline α-Fe workpieces with different grain sizes. Workpieces were generated by Voronoi tessellation in Atomsk, and the single-crystal diamond tool was treated as rigid. Fe-Fe and Fe-C interactions were described by an embedded-atom method potential and a Morse potential, respectively; C-C forces were not integrated. Models underwent conjugate-gradient energy minimization and NVT equilibration at 300 K. During cutting, Newtonian-region atoms were integrated under the NVE ensemble, while the thermostat layer maintained temperature. Simulations were performed in LAMMPS; atomic structures, dislocations, local crystal structures, stress, shear strain, and temperature were analyzed with OVITO.

Established nano-cutting models of polycrystalline α-Fe with different grain sizes. Grain refinement from 10.1 to 3.7 nm increased the average resultant cutting force by 26.0%. The average Newtonian-region temperature increased from 382.3 to 414.7 K with grain refinement. Dislocation accumulation was non-monotonic, reaching its maximum at d_g =6.4 nm. Grain refinement enhanced grain-boundary-mediated deformation and deteriorated the machined surface quality.