Microstructural effects on shock-induced deformation behavior in CoCrNi medium-entropy alloy: A molecular dynamics study

The impact of chemical short-range order (SRO) and twin boundary (TB) structures on the deformation response under shock compression in CoCrNi medium-entropy alloy (MEA) was investigated using molecular dynamics (MD) simulation. Four microstructural configurations were considered, including random solid solution (RSS), short-range order (SRO), twin boundaries (Twin), and a coupling of SRO and TB (Coup). The results demonstrate that, in comparison to the random MEAs (RSS sample and Twin sample), those with the chemical SRO structure (SRO sample and Coup sample) exhibit a higher shock front zone ratio and an elevated Hugoniot elastic limit (HEL) at a low shock velocity (U_P = 1200 m/s). This improvement can be attributed to the chemical SRO structure, which increases the energy barrier for dislocation nucleation and propagation. Additionally, pre-existing TBs can also serve as barriers to dislocation movement. In random samples, amorphous clusters tend to initiate from Cr atoms, due to the weak bonding of Cr-Cr pairs. In contrast, in the samples with chemical SRO structure, the increased presence of strong Co-Cr bonding and reduced Cr-Cr bonding effectively raises the activation energy for amorphization. These local amorphous clusters provide an environment conducive to dislocation nucleation. Consequently, chemical SRO structures lead to increased resistance to dislocation nucleation, where the formation of Shockley Partial (SP) dislocation necessitates longer loading durations, with the nucleation sites situated at a greater distance from the surface. Furthermore, during shock compression in CoCrNi MEAs, SP dislocations preferentially nucleate in the Co-Cr clusters. In conclusion, the presence of chemical SRO structure enhances the shock resistance of the CoCrNi MEAs at lower shock velocities. However, the strengthening effect diminishes with increasing impact velocity and eventually becomes negligible.