Insight into the nanoscale strengthening mechanism of polycrystalline iron implanted by Cr ions

The results of bi-crystalline models: (a1) the implantation models, (a2) the distribution of interstitial formation energy and (a3) the total number of point defects and their distribution at GB and in the bulk, and GB energy is plotted to assist analysis. The results of polycrystalline models: (b1) the MD models of ion implantation and nanoindentation; (b2) hardness; (b3) yield stress for different models with varying grain sizes. • The strengthening mechanisms of ion implantation for the polycrystalline iron are investigated from the microstructural perspective. • GB sites have much lower point defect formation energies than those in the bulk, and a GB with higher energy is more effective in generating and absorbing point defects. • Atoms tend to displace with a tendency of decreasing potential energy during ion implantation, promoting grain growth in nanocrystalline materials through grain rotation and GB migration. • The reverse Hall-Petch effect is caused by grain growth, which suppresses grain rotation and GB atoms movement, thereby transforming partial plastic deformation involving GBs into dislocations. • The implantation process affects grain size, the distribution of implanted atoms and defects, all of which collectively influence the mechanical properties. The strengthening mechanisms of ion implantation remain a challenge for the application on high-performance precision mechanical manufacturing. Based on the molecular dynamics (MD) method, the paper aims to explore the nanoscale mechanisms of the polycrystalline iron implanted by Cr ions. The microstructural evolution under implantation is analyzed, and the mechanical properties of different models under nanoindentation are studied. The results show that the ion implantation reduces surface roughness while enhancing both Young’s modulus and hardness. The increase in hardness arises from Cr atoms and interstitials obstructing dislocation propagation and the inverse Hall-Petch effect, which works by transforming the deformation from being grain boundary-dominated to dislocation-dominated. The role of grain boundaries (GBs) in ion implantation and deformation are analyzed by the molecular statics method and atomic displacement field. It is found that GB sites have lower point defect formation energy than the bulk and GB atoms first participate in plastic deformation. Additionally, the interactions between dislocations and GBs are investigated. This article provides an innovative research approach for exploring the nanoscale strengthening mechanisms of ion implantation by MD method.