ABSTRACT
Nanostructured Guinier-Preston (GP) zones are critical for the strength of Al-Mg-Si(-Cu) aluminum alloys. However, there are controversial reports about the structure and growth mechanism of GP zones. In this study, we construct several atomic configurations of GP zones according to the previous research. Then first-principles calculations based on density functional theory were used to investigate the relatively stable atomic structure and GP-zones growth mechanism. The results show that on the (100) plane, GP zones consist of {MgSi} atomic layers without Al atoms, and the size tends to grow up to 2 nm. Along the (100) growth direction, even numbers of {MgSi} atomic layers are more energetically favorable and there exist Al atomic layers to relieve the lattice strain. {MgSi}2Al4 is the most energetically favorable GP-zones configuration, and the substitution sequence of Cu atoms in {MgSi}2Al4 during the aging process is Al â Si â Mg. The growth of GP zones is accompanied by the increase in Mg and Si solute atoms and the decrease in Al atoms. Point defects, such as Cu atoms and vacancies, exhibit different occupation tendencies in GP zones: Cu atoms tend to segregate in the Al layer near the GP zones, while vacancies tend to be captured by the GP zones.
ABSTRACT
The nanostructured ßâ³ precipitates are critical for the strength of Al-Mg-Si-(Cu) aluminum alloys. However, there are still controversial reports about the composition of Cu-containing ßâ³ phases. In this work, first-principles calculations based on density functional theory were used to investigate the composition, mechanical properties, and electronic structure of Cu-containing ßâ³ phases. The results predict that the Cu-containing ßâ³ precipitates with a stoichiometry of Mg4+xAl2-xCuSi4 (x = 0, 1) are energetically favorable. As the concentration of Cu atoms increases, Cu-containing ßâ³ phases with different compositions will appear, such as Mg4AlCu2Si4 and Mg4Cu3Si4. The replacement order of Cu atoms in ßâ³ phases can be summarized as one Si3/Al site â two Si3/Al sites â two Si3/Al sites and one Mg1 site. The calculated elastic constants of the considered ßâ³ phases suggest that they are all mechanically stable, and all ßâ³ phases are ductile. When Cu atoms replace Al atoms at Si3/Al sites in ßâ³ phases, the values of bulk modulus (B), shear modulus (G), and Young's modulus (E) all increase. The calculation of the phonon spectrum shows that Mg4+xAl2-xCuSi4 (x = 0, 1) are also dynamically stable. The electronic structure analysis shows that the bond between the Si atom and the Cu atom has a covalent like property. The incorporation of the Cu atom enhances the electron interaction between the Mg2 and the Si3 atom so that the Mg2 atom also joins the Si network, which may be one of the reasons why Cu atoms increase the structure stability of the ßâ³ phases.
