Atomic-scale analysis of deformation behavior of face-centered cubic nanocrystalline high-entropy alloys with different grain sizes at high strain rates

In the present study, the microscopic deformation mechanism and tensile properties of Al_0.1CoCrFeNi nanocrystalline high-entropy alloys (HEAs) during uniaxial tension were studied through molecular dynamics simulation, and the dependence of mechanical properties on grain size and strain rate has been explored. As the tensile strain increases from 0 to 0.1, the atoms undergo a phase transformation from the face-centered cubic (FCC) structure into the hexagonal close-packed (HCP), body-centered cubic (BCC) and amorphous structure, and the dislocation density increases gradually due to dislocation annihilation and interaction. HEA grains undergo the transformation of the inverse Hall-Petch (H–P) relation into the H–P relation after exceeding one critical size of 14.77 nm at a strain rate of 5 × 10⁷/s, and this critical size increases with increasing strain rate. The microscopic deformation mechanism of the H–P relation is that the accumulation of dislocations at the grain boundaries leads to an increase in the strength of HEAs. In contrast, the inverse H–P relation is caused by the migration of grain boundaries, and the rotation and merging of HEA grains. The increase of strain rate positively affects the tensile properties of nanocrystalline HEAs, and the flow stress and yield strength exhibit sensitive dependence on the extremely high strain rates. In addition, the amorphization of atoms has become an important way for HEAs to deform plastically at high strain rates.