Formation mechanism of hierarchical twins in the CoCrNi medium entropy alloy

The three-dimensional hierarchical twin network has been proved to be the source of the excellent strength-ductility combination in the CoCrNi medium entropy alloy. Revealing the formation mecha-nism of hierarchical twins, however, remains a challenge using either the post-mortem or the in-situ microstructural characterization. In this study, the atomistic formation mechanism of hierarchical twins was investigated using molecular dynamics simulations, with special focus on the effects of strain rate and deformation temperature. Compared to the primary twin boundaries kink-driven hierarchical twin-ning tendency in pure FCC metals, the chemical inhomogeneity in CoCrNi can reduce the necessary kink height to trigger conjugate twins (CTWs), fascinating the formation of twin networks. At room temper-ature, the plastic deformation is dominated by primary twins (PTWs) and conjugate slips at a relatively lower strain rate (e.g., 5 x 10 7 s -1). The hierarchical twins can be activated in cases of deforming at a higher strain rate (e.g., 2 x 10 8 s-1). Further increasing the strain rate (e.g., 1 x 10 10 s -1) leads to the phase-transformation induced plasticity. At cryogenic temperatures, the hierarchical twins are promoted within a large range of strain rates (e.g., 5 x 10 7 -1 x 10 10 s -1). A higher temperature leads to the synergy of CTWs and primary slips at a lower strain rate, but hierarchical twins at a higher strain rate. On this basis, a qualitative comparison and scalable trends between experiments and simulations were revealed. The present study would not only provide the basic understanding for the twinning behavior found ex-perimentally, but also contribute to the design of medium/high entropy alloys with excellent mechanical performances by tuning microstructures.(c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

In this study, the atomistic formation mechanism of the three-dimensional hierarchical twins in CoCrNi was investigated using molecular dynamics simulations. The effects of strain rate and deformation temperature were found to be important factors. Chemical inhomogeneity in CoCrNi can reduce the necessary kink height for conjugate twins, promoting the formation of twin networks. At room temperature, plastic deformation is dominated by primary twins and conjugate slips at a relatively lower strain rate. Higher strain rates activate hierarchical twins. At cryogenic temperatures, hierarchical twins are promoted within a large range of strain rates. The study provides insights into the twinning behavior and offers guidance for designing alloys with excellent mechanical performance by tuning microstructures.