2022 Volume 63 Issue 4 Pages 545-554
An improved combination of strength and ductility is generally a trade-off relationship, and it remains a major research topic in the field of structural materials. A bimodal grained microstructure, consisting of a coarse-grained region “core” and a surrounding fine-grained region “shell”, exhibits a good balance between strength and ductility. Therefore, the exact reason for the strengthening mechanism needs to be investigated. In the present study, we conducted nanomechanical characterization to evaluate the individual strengthening factors, including matrix strength (σ0), and grain boundary effect (k), in the Hall–Petch model for each region of the core and shell to clarify the strengthening mechanism in the bimodal grained microstructure. The nanoindentation technique was applied locally in the “grain interior” to evaluate σ0, and “on grain boundary” and “near grain boundary” to assess the grain boundary effect associated with the k value. The grain interior nanohardness was found to be higher in the core region than that in the shell, which is explained by the higher pre-existing dislocation density in the core region. The nanomechanical characterization of the “on grain boundary” and the “near grain boundary” regions show a higher barrier effect due to the grain boundary in the shell than that in the core, which is presumably dominated by the higher internal strain at the shell grain boundary. Furthermore, a Hall–Petch plot was constructed using nanohardness, Vickers hardness, and grain size to estimate the k value. The plot showed a higher k value in the shell, which is consistent with the higher strengthening effect of the shell grain boundary that is evaluated independently in the local region. Therefore, the macroscopic strength of the shell region is significantly affected by both grain boundary effect as well as fine grain size.
