# Fabrication of Magnesium-Aluminum Composites under High-Pressure Torsion: Atomistic Simulation
## 3 Results  
### Composite Fabrication  
During deformation treatment, considerable mixing of Al and Mg atoms occurred through the interface. In Figure 3, changes of the atomic positions through Al-Mg interface for 0.0 < \(\varepsilon_{xy}\) < 1.6 are presented. Mg block in the model is shifted to the right by about 100 A for a clearance. As it can be seen, mostly, atomic mixing occurred during the first deformation stages \(\varepsilon_{xy}\) < 0.4. The first atomic movement occurred at \(\varepsilon_{xy}\) = 0.017 simultaneously with the appearance of base-centered cubic (BCC) lattice defects in the Mg part of the sample. Atomic positions between \(\varepsilon_{xy}\) = 0.027 and \(\varepsilon_{xy}\) = 0.4 are not presented since the continuous atomic movement occurred: Al atoms move towards the Mg part of the sample and vise versa. The process of atomic migration can be better described by the average and maximum distances of an atomic displacement in comparison with the initial position of the boundary, which is presented in Figure 4. The value of \(\Delta z\) is calculated as the average movement of the atoms from the initial position of the interface. At \(\varepsilon_{xy}\) = 0.017, Al atoms moved from the atomic planes closest to the interface for \(\Delta z_{max}\) = 2.2 A, and Mg atoms at the same strain moved for \(\Delta z_{max}\) = 2.3 A. Considerable displacement of Al (Mg) atoms inside Mg (Al) part  
Figure 2: Stress–strain curves during uniaxial compression normal to the Al–Mg interface combined with shear: (**a**) compressive strain and stress; (**b**) shear strain and stress; (**c**) the snapshots of the structure at three stages of deformation. The red color is for Mg atoms, the blue color is for Al atoms. of the sample occurred before \(\varepsilon_{xy}\)=0.4, with the average displacement for 4 A, while for \(0.04<\varepsilon_{xy}=1.6\), the average atomic displacement is about 2 A.  
Figure 4: Changes of the atomic positions \(\Delta z\) as the function of compression strain. (**a**) \(\Delta z_{av}\), (**b**) \(\Delta z_{max}\). Figure 3: Changes of the atomic positions through Al–Mg interface: (**a**) for \(0.0<\varepsilon_{xy}<0.4\) and (**b**) for \(0.4<\varepsilon_{xy}<1.6\). Different colors correspond to different deformation stages. Only part of the sample is presented. As it can be seen from Figure 3, both average distances and changes of the maximum atomic displacements can be described by the power function for both metals. It is well seen that \(\varepsilon=0.4\) is enough to obtain good mixing of the atoms near the interface. After \(\varepsilon=1.6\), compression strain begins to prevent mixing, since the structure near the boundary is strongly compressed. As it is found, Al atoms move towards the Mg part of the sample slightly better, which is connected with the difference of the lattices: Mg HPC lattice is not as densely packed as FCC Al lattice. However, it should be mentioned that the number of Mg atoms moving into the Al part is almost equal to the number of Al atoms moving into the Mg matrix. This can be explained by the similarity of the atomic radii of Mg and Al. The lack of difference in the melting point for Mg (923 K) and Al (933.5 K) is worth considering, which means the bonds in both metals are similarly stronger to fracture. For both metals, a symmetrical binding movement occurred during the deformation of the initial sample. In accordance with common-neighbor analysis (CNA), during the first stage of deformation (until \(\varepsilon_{xy}=0.4\)), the Al part of the sample preserves FCC lattice, and the Mg part of the sample preserves HCP lattice with the appearance of BCC lattice. After \(\varepsilon_{xy}=0.0015\), a considerable number of dislocations appeared in the Al part, about two times more than in Mg. Dislocations in the Mg part rapidly move and disappear during shear, while in Al, they change type but never entirely disappear. Numerous dislocations appeared on the interface during a mixture of Al and Mg atoms.