Grain boundary engineering can improve the thermal stability and accelerate densification of nanoscale grain structure. In this study, dopant elements (Mg, Fe, Ni, and Y) were deliberately selected to enable grain boundary segregation and formation of amorphous grain boundary complexions in Al alloys fabricated using a powder metallurgy approach. The alloys showed grain sizes <60 nm and densities >99% after hot pressing, with full density connected directly to the complexion formation. In addition to grain boundary segregation and complexion formation, microstructural characterization revealed nanorod precipitates with a core-shell structure at grain boundaries, with the chemistry of the shell being the same as that of amorphous complexions. Moreover, the nanorods were observed within intermetallic particles. Therefore, a diffusion pathway of the dopants is identified, which first segregate to grain boundaries, then form amorphous complexions, subsequently diffuse to the nanorod shell, and finally combine with Al to form intermetallic phases.