High entropy alloys (HEAs) attract remarkable attention due to the excellent mechanical performance. However, the origins of their high strength and toughness compared with those of the traditional alloys are still hardly revealed. Here, using a microstructure-based constitutive model and molecular dynamics (MD) simulation, we investigate the unique mechanical behavior and microstructure evolution of FeCoCrNiCu HEAs during the indentation. Due to the interaction between the dislocation and solution, the high dislocation density in FeCoCrNiCu leads to strong work hardening. Plentiful slip systems are stimulated, leading to the good plasticity of FeCoCrNiCu. The plastic deformation of FeCoCrNiCu is basically affected by the motion of dislocation loops. The prismatic dislocation loops inside FeCoCrNiCu are formed by the dislocations with the Burgers vectors of $${a \over 6}\left[ {\bar 11\bar 2} \right]$$ a 6 [ 1 ¯ 1 2 ¯ ] and $${a \over 6}\left[ {1\bar 12} \right]$$ a 6 [ 1 1 ¯ 2 ] , which interact with each other, and then emit along the 〈111〉 slip direction. In addition, the mechanical properties of FeCoCrNiCu HEA can be predicted by constructing the microstructure-based constitutive model, which is identified according to the evolution of the dislocation density and the stress-strain curve. Strong dislocation strengthening and remarkable lattice distortion strengthening occur in the deformation process of FeCoCrNiCu, and improve the strength. Therefore, the origins of high strength and high toughness in FeCoCrNiCu HEAs come from lattice distortion strengthening and the more activable slip systems compared with Cu. These results accelerate the discovery of HEAs with excellent mechanical properties, and provide a valuable reference for the industrial application of HEAs.
The plastic-deformation behaviors of gradient nanotwinned (GNT) metallic multilayers are investigated at the nanoscale via molecular dynamics simulation. The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies (SFEs) during nanoindentation is revealed. The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag, Cu, to Al with SFE increasing. In addition, it is found that the GNT Ag and GNT Cu strengthen in case of a larger twin gradient based on more significant twin boundary (TB) strengthening and dislocation strengthening. But the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient. The softening mechanism is further analyzed theoretically. These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs, but also give a guideline to design the GNT metallic multilayers with required mechanical properties.
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