Atomistic insights into the mechanical behaviors of nanocrystalline FeNiCrCoCu high entropy alloy under tension and compression

To understand the mechanical behaviors of nanocrystalline FeNiCrCoCu high entropy alloy (HEA) from the atomic perspective, molecular dynamics simulations of tension and compression tests are conducted. The effects of grain size, strain rate, twin thickness, element composition, and environment temperature on the mechanical properties and deformation mechanisms are analyzed. It is found that the classical Hall-Petch (H-P) behavior of flow stress transforms to the inverse H-P relation when the grain size of the nanocrystalline FeNiCrCoCu HEA is below 12-14 nm. In the H-P regime, the emission and glide of partial dislocations are the ruling plastic activities. However, in the inverse H-P regime, the primary deformation behavior is grain rotation and grain boundary migration. Dislocation propagation, intrinsic/extrinsic stacking faults, Lomer-Cottrell locks, FCC to BCC to HCP phase transition, and twinning are observed under both compressive and tensile loads. The dislocation density is higher during compression deformation than during tension deformation, resulting in greater modulus and strength under compression. With increasing temperature, the dislocation activity weakens, while the amorphization intensifies. Additionally, the mechanical parameters, including Young's modulus, yield strength, flow stress, and peak stress, increase with increasing strain rate or decreasing temperature. These findings provide a guide for the design of FeNiCrCoCu HEA with desired mechanical performance.