The origins of segregation behaviors of solute atoms and their effect on the strength of α-Al//θ'-Al2Cu interfaces in Al-Cu alloys

In many alloy systems, the segregation and strengthening of the solute atoms are caused by mechanical and chemical contributions. To uncover the origins of segregation behaviors and strengthening behaviors of the solute atoms Cd, Si, Sc and Zr at (001)_α-Al//(001)_θ' and (010)_α-Al//(010)_θ' interfaces, first-principles calculations were conducted. Results show that the chemical contribution primarily dominates the oscillatory segregation behaviors of Cd, Si, Sc and Zr on the Al matrix side. The oscillatory segregation behaviors of Cd, Si, Sc and Zr on the θ' side are mainly governed by both chemical and mechanical contributions. The segregation tendency of Cd at the (001)_α-Al//(001)_θ' interface (or (010)_α-Al//(010)_θ' interface) throughout the platelets is small (or strong) because the charge accumulation between Cd and the host atoms is weak (or significant). The segregation trend of Sc (or Zr) on the Al matrix side at the (001)_α-Al//(001)_θ' and (010)_α-Al//(010)_θ' interfaces is strong, which is attributed to significant charge accumulation between Sc (or Zr) and the host atoms. Si exhibits a favorable segregation tendency on the θ' side at both the (001)_α-Al//(001)_θ' and (010)_α-Al//(010)_θ' interfaces, which is ascribed to significant charge accumulation between Si and the host atoms. With the increase of Si, Sc and Zr coverage, the segregation tendencies of Si, Sc and Zr enhance. The segregation tendency of Cd decreases with the increase of Cd coverage. The first-principles tensile test for the interface was conducted. The work of dislocation emission was computed. Results show that the strengthening effects of solute atoms on the interface are primarily dominated by the chemical contribution. Sc (or Zr) segregation leads to an increase in the strength of the interface, which is majorly attributed to a strong electronic interaction between Sc (or Zr) and the host atoms. Cd segregation causes a weakening effect on the interface because of the weak electronic interaction between Cd and the host atoms. The ductility of the (001)_α-Al//(001)_θ' interface with the Sc (or Zr) is more significant than that with the Cd (or Si). This work provides a strategy for improving the mechanical properties of the Al-Cu alloys.

Effects of local geometry distortion at the Al/Al2Cu interfaces on solute segregation

Interfaces in materials play a critical role in a wide range of functional properties. The heterophase semicoherent interface is one of the most difficult systems in density functional theory (DFT) simulations. We have studied the segregation energies of different solutes (Si, Mg and Zn) at the Al/Al2Cu (semi)coherent interfaces based on the linear combination of atomic orbitals (LCAO). Our DFT-LCAO computed segregation energies of solutes at the Al/Al2Cu coherent interface are in good agreement with the DFT-PAW calculations. However, at the Al/Al2Cu semicoherent interface, results from the LCAO calculations are significantly different to those obtained using the PAW method. The local geometry distortion in the semicoherent interface region significantly influences the solute energetics. Considering the distortion, the segregation energies of solutes well explain the trend of the experimentally measured solute concentrations, such as the higher solute Si concentration at the semicoherent interface than that at the coherent interface, and the similar Si concentration across the semicoherent interface. The lattice mismatch effect on the solute segregation energy is investigated as well. At low lattice mismatch (0.25%), the segregation energies of Si and Mg are almost compatible. Our results shed light on improving the theoretical predictions for interfaces to determine accurate functional properties.