On the real-time atomistic deformation of nano twinned CrCoFeNi high entropy alloy

Abstract

High entropy alloys (HEAs) holding several principal elements in high concentration have unprecedented combination properties. The design of strong and highly ductile HEAs has attracted extensive attention from researchers in the last decade, such as in mechanisms for inducing different types of phase and nano-sized precipitates. Since some HEAs have low stacking fault energy, nanotwins can form during the plastic deformation process or magnetron sputtering, resulting in enhanced mechanical properties due to the existence of twin boundaries. The addition of twin boundaries is implied to be a promising method in engineering HEAs. Understanding how these twin boundaries affect the mechanical properties of nanotwinned HEAs is the key to designing strong and ductile examples. In this study, we performed a large-scale molecular dynamic simulation to investigate the mechanical properties of HEAs with different twin boundary spacings at various temperatures. The results show that the strength of HEAs at all tested temperatures increases with decreasing twin boundary spacing until a lower critical value of 1.83 nm is reached, which is close to the experimental value (2 nm). The strength of the HEAs at all tested temperatures decreases as the twin boundary spacing is decreased further. The dislocation motion transitions at the critical twin boundary spacing. In the sample with a twin boundary spacing bigger than 1.83 nm, Shockley dislocations tend to intersect the twin boundaries and glide in the hardening modes; on the other hand, Shockley dislocations travel along the direction parallel to the twin boundaries in samples with a twin boundary spacing smaller than 1.83 nm, leading to detwinning and softening in the HEAs. The dislocation motion and entanglement at 1 K are respectively slower and stronger than those at 300 K; the grain boundary activity is more obvious at a higher temperature. A mechanistic theoretical model together with a Hall–Petch relationship is then proposed to consider the coupled twin boundary and temperature effect on the deformation of nanotwinned HEAs.