Atomic insights into effects of temperature and grain diameter on the micro-deformation mechanism, mechanical properties and sluggish diffusion of nanocrystalline high-entropy alloys

In this work, the effects of temperature and grain diameter on the micro- deformation mechanism, tensile properties, and sluggish diffusion of nanocrystalline Al_0.1CoCrFeNi HEAs during uniaxial tension have been studied by molecular dynamics (MD) simulations. The Hall-Petch (H-P) relation and inverse Hall-Petch relation still exist in nanocrystalline HEAs. At the case of grain diameter over a critical size (e.g., D_G>14.77 nm for 300 K), the deformation mechanism of HEAs is that the accumulation of dislocations at grain boundaries leads to the increase of HEAs strength, which conforms the H-P relation. At the case of grain diameter below a critical size (e.g., D_G<14.77 nm for 300 K), the migration of grain boundaries, amorphization of atoms as well as the rotation and merging of grains becomes the main deformation mechanism of HEAs, which weaken the strength of HEAs and conform the reverse H-P relation. The increase of temperature has a negative influence on the mechanical properties including Young's modulus, yield strength and flow stress of HEAs. The critical grain size that undergoes the H-P and reverse H-P relation transformation increases with the increase of temperature. An increase in grain boundary thickness and the appearance of a large number of discrete amorphous atoms in the grains can be observed at high temperatures. The shear strain of atoms at grain boundaries is larger than that of atoms in other regions, and high temperature promotes the increase of atomic shear strains in HEA, especially at grain boundaries. The lengths of all types of dislocation lines and dislocation densities tend to decrease with increasing temperature, and Shockley dislocations always dominate all other dislocations at 300–1200 K. The MSD results show that nanocrystalline HEA have good stability at 300–1200 K, and small-grained HEAs have higher MSD value and diffusion coefficient than those of large-grained HEAs at 1200 K. Moreover, at higher temperature (2500 K), the MSD values of HEAs with D_G= 7.4–23.45 nm all increases significantly, and the time-dependent curves basically overlap, indicating that the influence of grain boundaries on atomic diffusion can be ignored.