LAUSR

Abstract The temperature elevation process by the molecular dynamics (MD) simulation was used to investigate the melting mechanism of PtPdRhCo and Pt41Pd18Rh5Co32 high-entropy alloy nanoparticles (HEA-NPs) with diameters of 5, 10, and 15 nm in the FCC arrangement. The second-nearest neighbor modified embedded-atom method (2NN MEAM) potential was used and the cross-element parameters were parametrized by the guaranteed convergence particle swarm optimization (GCPSO) based on the reference data prepared by the density functional theory (DFT) calculation. According to the MD results, three characteristic temperatures during the heating process are found, which include the surface local shear resistance temperature Tslsr, surface pre-melting temperature Tsm, and melting point Tm. By observing the distribution of atomic local shear strain ηiMises with the increasing temperature, the definition of Tslsr is first proposed, below which atoms with high local shear strains are blocked at the surface of HEA-NPs. As the system temperature is above Tslsr, the interior atoms near the surface begin to possess higher local shear strains and this influence continuously spread toward the core of HEA-NPs at higher temperatures. At Tsm, a certain amount of atoms within the thin surface shell are melted, leading to the first discontinuity of binding energy-temperature profile. At Tm, the HEA-NPs structure completely becomes liquid phase and the second discontinuity of binding energy-temperature profile can be found. Although the binding energies of these two HEA-NPs types of the same size are almost identical, the Tslsr, Tsm, and Tm values of PtPdRhCo are relatively higher than those of Pt41Pd18Rh5Co32. Each element type of PtPdRhCo shows a collective behavior during the heating process, resulting in a similar local structural evolution at different temperatures. For Pt41Pd18Rh5Co32, Co and Pd atoms tend to be distorted more easily than Pt and Rh atoms during the heating process, which can be regarded as a kind of defect. This is the main reason for relatively lower Tslsr, Tsm, and Tm values of Pt41Pd18Rh5Co32.