Grain and twin boundaries dependent mechanical behavior of FeCoCrNiCu high-entropy alloy

Abstract

The nanoindentation response of FeCoCrNiCu high-entropy alloy is revealed through molecular dynamics simulation. The influences of various grain sizes and twin lamellae thicknesses on the mechanical characteristics and plastic deformation are carefully studied. The result indicates that the indentation force and substrate hardness increase as the grain size increases from 55.16 Å to 114.46 Å and the twin layer thickness rises from 6.74 Å to 30.04 Å, which reveals the inverse Hall-Petch effect in the relationship between the material strength versus grain size and the twin lamellae thickness. The shear strain and residual stress are converged around the indentation regions and then enhanced deep into the substrate along the grain boundary and twin boundary. The presence of grain boundaries significantly affects the movement of the atoms, resulting in an asymmetric dispersion of the atomic displacement vector during the deformation. Moreover, the surface morphology reveals that the pile-up height generally increases with increasing the grain size and decreasing the twin lamellae spacing. The microstructural evolution suggests that the rotation of the grain related to grain boundary sliding and the migration of the grain boundary are the dominant mechanism in the deformation with grain size reduction. Moreover, the dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic, which significantly affects the plastic deformation of the material. The result also shows that the dislocation density tends to increase with increasing grain size and twin lamellae thickness. The figure shows the simulation model of FeCoCrNiCu high-entropy alloy (a), atomic displacement vectors of samples with different grain sizes (b), atomic configuration of the specimens with various twin lamellar thicknesses (c). ● The inverse Hall-Petch effect is recognized with the variation of grain size and twin lamellae thickness. ● The deformation mechanism of FeCoCrNiCu HEA is significantly dependent on the grain/twin boundaries. ● The rotation of grain related to grain boundary sliding and the grain boundary migration are dominant mechanism. ● The dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic.