Grain boundary segregation of solutes and associated plastic deformation mechanisms in nanocrystalline Al-Cu and Al-Mg alloys: A molecular dynamics study

Grain boundary (GB) solute segregation is an effective strategy for designing strong and stable nanocrystalline Al alloys. Cu and Mg are two well-known alloying elements in Al alloys, but their segregation behavior at extremely fine grain sizes and their effects on deformation mechanisms of nanocrystalline Al are still unclear. In this study, nanocrystalline Al-Cu and Al-Mg alloys models with a range of solute contents were constructed, and the tensile simulation of these models was carried out at different temperatures. The results show that both Cu and Mg tends to segregate at GBs and reduce the excess free volume (EFV) at GBs. Due to different segregation behavior, the GB fraction increases significantly after Cu segregation, whereas the GB fraction changes little after Mg segregation. At different simulated temperatures, the segregation of Cu and Mg at GBs increases the peak stress of nano- crystalline Al by inhibiting GB sliding. With the increase of solute content, solute segregation at GBs leads to a gradual increase trend in peak stress. The strengthening effect of Cu segregation is more obvious than that of Mg, which is related to the smaller EFV of Al-Cu alloys at GB and the greater difficulty of GB sliding. GB sliding is greatly promoted at high temperatures and the strengthening effect of Mg is much reduced. However, Al-Cu alloys still show a significant strengthening effect because Al-Cu alloys have a lower GB diffusion coefficient compared to Al-Mg alloys and are more effective in suppressing GB sliding.