Twin boundary spacing and loading direction dependent tensile deformation of nano-twinned Al10(CrCoFeNi)90 high-entropy alloy: An atomic study

Experiments have shown that nanotwins play an important role in strengthening and toughening high-entropy alloys (HEAs), which are closely associated with twin boundary (TB) spacing. Understanding the TB spacing dependent deformation mechanisms is helpful to design strong and ductile HEAs. In this work, the TB spacing dependent tensile deformation of equiaxed-and columnar-grained Al10(CrCoFeNi)90 HEA with various TB spacings has been studied via molecular dynamics simulation. The results show that equiaxed-grained samples with nanotwins have higher strength than twin-free counterpart. De-twinning at small TB spacing softens equiaxed-grained samples, which is similar to that observed in nano-twinned Cu. However, for the columnar -grained samples, it is interesting to find that no softening occurs with decreasing TB spacing when the loading direction is parallel or vertical to TB. In the case of parallel loading, the main dislocation activities change from hairpin-to necklace-like dislocations with decreasing TB spacing, and this dislocation mode transformation leads to a slow-to-sharp-increase transition in strength. Our theoretical analysis indicates that the sharp increase in strength is related to the severe lattice distortion in HEAs which makes the de-pinning process of necklace-like dislocation unit difficult. In the case of vertical loading, the dominated dislocation motions shift from dislocation slipping in the twin and matrix regions to dislocation cutting through TBs as TB spacing de-creases from 6.2 to 0.62 nm, the latter of which leads to shear bands nucleation and expansion. Yet, the strength still shows a monotonous increase due to the increasing difficulty of dislocation nucleation in the initial plastic deformation that produces significant strengthening.