The melting mechanism of single crystal and polycrystalline Nb20.6Mo21.7Ta15.6W21.1V21.0 refractory high entropy alloys (RHEAs) were investigated by the molecular dynamics (MD) simulation using the second-nearest neighbor modified embedded-atom method (2NN MEAM) potential. For the single crystal RHEA, the density profile displays an abrupt drop from 11.25 to 11.00 g/cm3 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 changes of all pairs.

High-entropy alloys (HEAs) are one of newly developed alloy materials in the last two decades. Generally, HEA contains at least four or more major elements with a concentration between 5% and 35%, and the concentration of minor elements is lower than 5%. [1] For examples, four-element Nb 25 Mo 25 Ta 25 W 25 HEA has a single-phase body-centered cubic (BCC) crystal structure [2] and possesses high yield stress about 1058 MPa at room temperature and 405 MPa at 1600 °C, indicating that this HEA still has certain mechanical strength at high temperatures. In Li's study, [3] the glass fluxing method was used to fabricate CrFeNi HEA, and the yield strength is about 3 times higher than those by traditional casting methods. In Wang's study, [4] the effect of Al content on the phase transformation of Al x CoCrFeNi HEAs was discussed. When the aluminum content is low, the HEA tends to form the face-centered cubic (FCC) arrangement. At the higher aluminum content, the HEA tends to form a BCC structure.The most noticeable aspect of HEA is that they can maintain a single solid solution instead of intermetallic compounds under the composition of multiple elements. Because HEAs contain atoms of different sizes, the lattice distortion and local strain distributed within HEAs make it difficult for the dislocation to occur, leading to strengthening of the solid solution. [3] The lattice distortion of HEAs also leads to the scatterings of electrons and phonons, degrading the electronic and thermal conductivity of HEAs. [4] At extreme temperatures, some HEAs also have better thermal stability, because the structural changes of grain coarsening and recrystallization occur less readily. [5] Despite HEAs have many appealing properties, there are still technical areas that need to be overcome to synthesize multiple metals into alloys with a single solid solution.Several methods have been established to fabricate HEAs, such as arc melting technology and casting methods. Hsu synthesized AlCoCrFe x Mo 0.5 (x = 0.6-2.0 in molar ratio) Ni HEAs through arc smelting and casting method under the protection of argon, which uses Al, Co, Cr, Fe, Mo, and Ni materials with the purity over 99 wt%. By scanning electron microscope (SEM) and X-ray energy dispersive spectrometry, it was found these HEAs include the duplex BCC-body-centered tetragonal (BCT) structure and the fraction of BCC phase increases with the increasing iron content. [6] In Wen's study, the AlCoCrCuFeNi HEA was synthesized by the arc melting mixture of commercial pure metal in a Ti-gettered high-purity argon atmosphere. It can be found the structure of this HEA is composed of a major BCC arrangement and a minor FCC arrangement. [7] Using the

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