# Fabrication of Magnesium-Aluminum Composites under High-Pressure Torsion: Atomistic Simulation
## 3 Results
### Al-Mg under Tension  
In Figure 5, stress-strain curves with characteristic marks during tensile loading normal to the Al-Mg interface are presented for the tension of the initial sample (black curve, 1) and tension after compression at stages I (red curve, 2), II (green curve, 3), and III (blue curve, 4). Several pop-in events are observed on the stress-strain curves, which can be attributed to the release of strain energy accumulated during the deformation through defect activities. To analyze deformation mechanisms and strength of the composite, snapshots of the structure during tensile loading are presented for curves 1-3 in Figure 6, in accordance with CNA, and for curve 4 in Figure 7, in accordance with CNA and dislocation analysis. The first structural changes of the initial sample occurred at \(\varepsilon=0.03\) in the Mg part of the sample (see Figure 6a). Fracture is observed in the Mg part of the sample at \(\varepsilon=0.045\) (shown by a circle in Figure 6a). Close to this limit, dislocations appeared in Mg together with other structural defects. This result is quite expected since Mg is well known for having low strength. However, this result is important for further comparison. From Figure 6b, it can be seen that fracture of the sample compressed to stage I occurred at \(\varepsilon=0.15\) on the boundary between Al and Mg, where the mixed structure is obtained. This means that the interlayer part is even weaker than the Mg part (see also Figure 5, curves 1 and 2). At tensile strain \(\varepsilon=0.0\), there are two lattices in the MG part of the sample--HPC (red atoms) and BCC (blue atoms); however, after \(\varepsilon=0.06\), BCC phase almost disappears. It should be noted that a single HCP atom layer represents a twin  
Figure 5: Stress–strain curves during tension normal to the interface region after different initial compression strain: after \(\varepsilon_{xy}=0.5\) (2); \(\varepsilon_{xy}=1.0\) (3), and \(\varepsilon_{xy}=1.6\) (4). Figure 6: Snapshots of the structure in accordance with stress–strain curves shown in Figure 5: (**a**) tension of the initial sample; (**b**) tension of the sample after compression, stage I; (**c**) tension of the sample after compression, stage II. All atoms are colored by the CNA parameter, where HCP atoms are red, FCC atoms are green, BCC atoms are blue, and other atoms are gray. boundary, two adjacent HCP atom layers present an intrinsic stacking fault, and an FCC atom layer in the middle of two HCP atom layers stands for an intrinsic stacking fault. As in the experiments or other MD simulations, twinning is one of the effective mechanisms of deformation for Mg [41; 42]. From Figure 6c, it can be seen that fracture of the sample compressed to stage II occurred at \(\varepsilon=0.194\) in the Mg part of the sample, between two boundary regions. At \(\varepsilon\) = 0.047, the share of the BCC phase considerably decreases; however, the Mg part of the sample contains the BCC phase during the whole deformation process. Figure 7 presents a series of snapshots exhibiting the defect evolution during tension after stage III of compression corresponding to the characteristic points marked in the stress-strain curve 4 of Figure 5. This case is chosen for detailed analysis because the strength of the sample after stage III is the highest. The other atoms are colored grey, the HCP atoms are colored red, BCC atoms are colored blue, and the FCC atoms are colored green. In accordance with the OVITO dislocation extraction algorithm, the stair-rod dislocation lines are colored purple, the Hirth dislocation lines are colored yellow, Frank dislocation lines are colored light blue, the Perfect dislocation lines are colored blue, and the Shockley dislocation lines are colored green. After compression to \(\varepsilon\) = 0.16, the BCC phase is dominant in the Mg part of the sample. It should be mentioned that an even boundary region with mixed Al and Mg atoms is shown by green FCC lattice.