Hall‐Petch to Inverse Hall‐Petch Transition in Nanocrystalline B ₄ C: Driven by Inter‐to‐Intragranular Amorphization

ABSTRACT

The transition from Hall‐Petch to inverse Hall‐Petch behaviors in ceramics is complex and remains poorly understood because of their limited plasticity. In the present study, we employ molecular dynamics simulations with a machine‐learning force field to investigate shear deformation behaviors and associated mechanisms of nanocrystalline boron carbide ( n ‐B ₄ C) with various grain sizes ranging from 3.46 to 12.12 nm. A transition from Hall‐Petch to inverse Hall‐Petch behaviors is identified in n ‐B ₄ C at a critical grain size of 8.34 nm, with the maximum shear strength of 19.55 GPa. Our results suggest that this transition is primarily driven by the competition between intergranular and intragranular amorphizations in n ‐B ₄ C. As the grain size decreases, the increasing grain boundary (GB) regions homogenize the shear stress, which suppresses intergranular amorphization and fracture, thus ultimately strengthening n ‐B ₄ C, as observed in the Hall‐Petch behavior. In contrast, with a further decrease of the grain size below the critical value (∼8.34 nm), the substantial increase in the volume fraction of GB regions inherently weakens the material and promotes GB sliding, which further facilitates the intragranular amorphization within interior grains, thereby significantly reducing the shear strength and triggering a transition into the inverse Hall‐Petch relationship. These findings provide an atomistic insight into the deformation mechanisms of n ‐B ₄ C and explain the transition from Hall‐Petch to inverse Hall‐Petch behaviors in strong covalent ceramics.