# Effect of Segregation on Deformation Behaviour of Nanoscale CoCrCuFeNi High-Entropy Alloy
## 2 Simulation Details  
The formation of GB segregations in metallic alloys is often observed during their solidification when they are cooled down from a high temperature and then held at a lower temperature [44]. However, due to limited spatial and temporal scales in atomistic simulations, it is difficult to capture the diffusion process in HEAs. The deformation mechanisms and effect of GB segregation in polycrystals are most appropriately studied using MD simulations for specific flat GBs with a certain geometry. Accordingly, in the current work, MD simulations of the shear deformation and stretching of the nanoscale CoCrCuFeNi HEA were carried out using bicrystals with tilt GBs.  
The MD simulations were implemented with the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software [45]. The visualization of the results was performed using Open Visualization Tools (OVITO) [46]. The CoCrCuFeNi HEA with a face-centered cubic structure was selected as the material of study [47]. Initially, the single-crystal model with orientations of [11; 12; 11; 13; 14; 15; 16; 17; 18; 19; 20] along the axes X, Y and Z, respectively, was created. Then, the bicrystal was obtained by rotating the upper and lower parts of the single crystal along the Z axis by \(\pm 15^{\circ}\), respectively (Figure 1a). Thus, the misorientation between the formed grains was set to be \(30^{\circ}\). We set a misorientation of \(30^{\circ}\) between grains to form a general high-angle GB. Solute atoms more readily form GB segregations at high-angle GBs rather than at low-angle GBs or special GBs with lower GB energy and free volume [43]. The dimensions of the created bicrystal were 17.6, 23.0, and 4.6 nm along the three orthogonal directions. The considered bicrystal model consisted of 156,000 atoms.  
The embedded-atom-method potential developed by [PERSON] and [PERSON]. [48] was used for the modelling. The integration of the atomic motion equations was carried out by the 4 th-order extrapolation scheme with the Verlet method. A timestep of 1 fs was set. The initial samples were subjected to energy minimization with the conjugate gradient method by iteratively adjusting the atom coordinates and were further equilibrated at the considered deformation temperature. The minimization convergence criterion for the energy is \(10^{-8}\) eV and for the force is \(10^{-10}\) eV/A. The structure optimization and subsequent deformation were modelled in the thermodynamic ensemble NPT (constant number of atoms in the simulation cell, as well as constant pressure and temperature) to control certain stress components at zero and maintain a constant temperature. The simulations were conducted at temperatures of 100 K and 300 K. Periodic boundary conditions were applied in all three orthogonal directions of the simulation cell. The tensile and shear loading were applied at a constant rate of \(10^{-8}\) s\({}^{-1}\). Such a high strain rate is acceptable [49; 50], considering the limited temporal scale of MD simulations.  
For analyzing the effect of GB segregation on the deformation behavior of the bicrystals under applied loading, the segregation of single element type (Co, Cr, Cu, Fe, and Ni) is introduced at the GB region. Thus, five different bicrystals were created depending on the type of the GB segregation. The thickness of the segregated layer was set to 1, 3, or 5 A. For the HEAs subject to the tension, a rectangular crack was additionally introduced into the middle of the GB of the bicrystals (Figure 1b). During the shear deformation, all stress components, except \(\sigma_{xy}\), were kept equal to zero, while for the tensile deformation, all the components except \(\sigma_{yy}\) were controlled to be zero. The results for the bicrystals with GB segregation were compared with those obtained for the case without any GB segregation.