Mobile dislocation mediated Hall-Petch and inverse Hall-Petch behaviors in nanocrystalline Al-doped boron carbide

The Hall-Petch and inverse Hall-Petch behaviors in nanocrystalline ceramics are not well understood because dislocation activities, which are important in metals, are usually limited in these materials. In this study, we use molecular dynamics simulations with a machine-learning force field to investigate the shear deformation behaviors of nanocrystalline Al-doped boron carbide (n-B-12-CAlC) as the grain size varies. We find a transition from the Hall-Petch to inverse Hall-Petch behavior in n-B-12-CAlC when the grain size reaches its critical value of 6.09 nm, with a maximum shear strength of 14.93 GPa. Interestingly, mobile dislocation nucleated from grain boundaries (GBs) is activated in n-B-12-CAlC due to the breakage of weakened C-Al chain bonds, which plays a significant role in its Hall-Petch behaviors. As the grain size decreases, the increasing GB regions homogenize the shear stress, which suppresses dislocation nucleation from GBs and strengthens the material, as observed in the conventional Hall-Petch relationship. However, with a further decrease in the grain size (< similar to 6 nm), the increasing GB regions provide more favorable sites and a higher possibility for dislocation nucleation, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. These findings shed light on deformation behaviors and mechanisms of nanocrystalline ceramics and provide an explanation for the transition from Hall-Petch to inverse Hall-Petch behaviors.