Delay of inverse Hall-Petch relationship of nanocrystalline Cu by modifying grain boundaries with coherent twins

In nanocrystalline metallic materials, with the decrease of grain size, the relationship between strength and grain size usually changes from the Hall-Petch (HP) to inverse Hall-Petch (IHP) relationship related to grain boundary (GB) characteristics. A series of nanocrystalline Cu samples with pre-specified grain orientations and different grain sizes were established, in which a part of GBs were replaced with coherent twin boundaries (CTBs). The mechanical behaviors of the samples with and without GB modification under tension were studied using molecular dynamics simulations. It was found that the HP and IHP relationships still work in the nanocrystalline Cu samples with GB modification, but the maximum strength is significantly improved, meanwhile the critical grain size for the transition from HP to IHP is reduced to 7.5 nm. The grain size effects on the flow stress were qualitatively analyzed based on the monitoring microstructure evolutions. The model dominated by GB was combined with that by intracrystalline CTB, which was further extended to quantitatively describe the grain size effect. It showed that the modification of GB could substantially delay the IHP effect, which provides a promising way for the design and optimization of the microstructure of high-performance nanocrystalline materials.