  First-principles calculations were carried out to study the segregation
behavior of Mg, and Cu and their effect on the energy and mechanical properties
of different Al grain boundaries (GBs). Four symmetrical tilt GBs were selected
for study, namely {\Sigma}5[001](210) GB, {\Sigma}5[001](310) GB,
{\Sigma}9[110](221) GB, and {\Sigma}11[110](332) GB. The results show that both
Mg and Cu have a segregation tendency at the GBs, and the segregation tendency
of Cu is stronger than Mg. Mg is prone to form substitutional segregation at
the GBs, but Cu is more likely to segregate at the interstitial sites. The
segregation of Mg and Cu can reduce GB energy, and the GB energy continues to
decrease with the increase of the segregation concentration. First-principles
calculation tensile test shows that the segregation of Mg has a negative effect
on the strength of GBs, and the GB strength decreases with the increase of the
Mg concentration, while the GB strength was gradually enhanced with the
increase of the Cu concentration. The strength of {\Sigma}5(210) GB and
{\Sigma}9(221) GB are more sensitive to the segregation of solute atoms than
the other two GBs. By calculating the charge density and the density of states
of the pristine and the segregated GBs, it was found that the segregation of Mg
caused charge depletion and structure expansion at the GBs, while the
segregation of Cu increases the charge density of GBs and form new bonds with
the surrounding Al atoms. The results provide useful information for improving
the mechanical properties of materials by using the concept of GB segregation
engineering.
