Structural phase transformation and mechanical properties of metastable Al Σ9(221) grain boundaries with solutes segregation by first-principles calculation

Solute segregation in the grain boundary (GB) considerably impacts the stability and mechanical response of nanocrystalline alloys by altering the GB characteristics. In this study, first-principles calculations were employed to examine the effects of Mg and Cu segregation on the structure and mechanical response of multiple Al Σ9(221) GBs, including one ground-state GB structure (GB-I) and three metastable structures (GB-II, GB-III, and GB-IV). The calculated results reveal that both Mg and Cu segregation can lower GB energy, indicating a favored segregation trend of the solute atoms at the four GBs. GB energy decreases with the increasing solute concentration, and Cu has a more pronounced impact on decreasing GB energy compared to Mg. The structural unit analysis of GB shows that the solute segregation can lead to the transition of GB structure and can induce GB structural phase transformation. The GB strength calculations show that the ground-state Σ9(221) GB is weaker than that of the metastable GBs with higher GB energy, indicating that the GBs with lower energy do not necessarily have higher mechanical strength. The GB-I, GB-II, and GB-IV are weakened by the segregation of Mg, while the GB-I, GB-II, and GB-III are strengthened by the segregation of Cu atoms. The weakening and strengthening effects intensify as the increasing solute concentration because of the charge accumulation and depletion in the GB plane. The abnormal strengthening effects of Mg in GB-III and the weakening effects of Cu in GB-IV can be ascribed to the GB structural phase transformation induced by the GB segregation of solute atoms.