Correlation of grain boundary extra free volume with vacancy and solute segregation at grain boundaries: a case study for Al

Grain boundary extra free volume (GB EFV) can be considered as fundamental microstructural parameter for polycrystalline or nano-crystalline materials. Here, we present a systematic first principles study on a group of representative symmetric tilt grain boundaries of Al with various EFVs subjected to vacancy formation and Mg segregation. All grain boundaries were constructed using the coincident site lattice (CSL) and the structural unit (SU) models. It was found that the SU model is superior to the CSL in describing FCC-Al GBs, the same as we previously revealed for BCC-Fe. The predicted relation between GB misorientation angle and EFV, and the predicted EFV criteria for a stable GB, both agree with available experimental observations. Vacancy formation and Mg segregation show stronger preference to those GBs with high EFV values, due to the resultant high levels of atomic disorder. These findings not only provide a new, atomistic perspective on the significance of EFV, but also suggest a viable means of predicting GB properties based on direct experimental characterisation of GB EFVs.