Abstract: 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 mechanism 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 twinning 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 temperature, the plastic deformation is dominated by primary twins (PTWs) and conjugate slips at a relatively lower strain rate (e.g., 5 × 10⁷ s^-1). The hierarchical twins can be activated in cases of deforming at a higher strain rate (e.g., 2 × 10⁸ s^-1). Further increasing the strain rate (e.g., 1 × 10¹⁰ 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 × 10⁷-1 × 10¹⁰ 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 experimentally, but also contribute to the design of medium/high entropy alloys with excellent mechanical performances by tuning microstructures.

Key words: Hierarchical twin, Medium entropy alloy, Strain rate, Deformation temperature, Molecular dynamics simulation