The molecular dynamics (MD) simulation is used to explore the role of local face‐centered cubic (FCC) structure to the body‐centered cubic (BCC) polycrystalline Nb20.6Mo21.7Ta15.6W21.1V21.0 refractory high‐entropy alloy under plastic deformation. With the grain size of 25.3, 27.9, and 31.9 nm, the dislocation propagation within grains at strains larger than the ultimate strain is the dominant deformation mechanism. With the grain size of 20.1 and 15.6 nm, the dislocation propagation within the grain and the local structural transformation from BCC to the undefined type at the grain/grain boundary (GB) interface are two main deformation mechanisms. With the grain size of 10.0 and 5.2 nm, the local BCC structures of grain atoms significantly become the undefined type, which is the main deformation mechanism during the tensile process. The FCC fraction increases from yield strain to ultimate strain, and remains a FCC fraction of the highest value, indicating the role of local FCC structure is more significant for the cases without dislocation deformation mechanism. Once a large amount of BCC atoms is transformed into the undefined type, some BCC atoms need to transform into local FCC atoms for making their neighbor BCC atoms more local space to decrease the energy barrier for the BCC transformation to the undefined type.

The melting mechanism of single crystal and polycrystalline Nb 20.6 Mo 21.7 Ta 15.6 W 21.1 V 21.0 RHEAs was investigated by the molecular dynamics (MD) simulation using the 2NN MEAM potential. For the single crystal RHEA, the density profile displays an abrupt drop from 11.25 to 11.00 g/cm 3 at temperatures from 2910 to 2940 K, indicating all atoms begin significant local structural rearrangement. For polycrystalline RHEAs, a two-stage melting process is found. In the first melting stage, the melting of the grain boundary (GB) regions firstly occurs at the pre-melting temperature, which is relatively lower than the corresponding system-melting point. At the pre-melting temperature, most GB atoms have enough kinetic energies to leave their equilibrium positions, and then gradually induce the rearrangement of grain atoms close to GB. In the second melting stage at the melting point, most grain atoms have enough kinetic energies to rearrange, resulting in the chemical short-ranged order (CSRO) changes of all pairs.