Modelling and simulations of local structural evolution and stability of bulk and nanoparticles of FeNi3 crystalline alloys

The local structural evolutions and stabilities of bulk and nanoparticles (NPs) of FeNi3 crystalline alloys with particle sizes of 2, 4 and 6 nm were studied using classical molecular dynamics (MD) simulation method combined with embedded atomic model (EAM) in Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The formation, evolution and stability of the structures were analyzed over a wide temperature range (300-1700 K) by calculating radial distribution functions (RDF), interatomic distances (ID), coordination numbers (CN), core-surface concentration profiles, Voronoi analysis and surface energies (SE) dependence on particle size and temperature. Structural analysis showed that all NPs deformed at high temperatures, but the deformation became negligible when the particle size increased to 6 nm, with 6 nm particles showing the most thermostable behavior. The CN of the 6 nm particle was calculated as 11.7 at 300 K and remained almost constant between 11.7 and 12 throughout the heat treatment. Voronoi analysis revealed that NPs with diameters of 4 and 6 nm have almost the same topological structure in the crystalline phase at 300 K, with the main polyhedra for cubic and spherical NPs consisting of pentagonal, hexagonal and rectangular facets corresponding to the truncated-octahedron and tetra-decahedron. It was found that the surface energy of spherical NPs tends to decrease up to 1300 K, indicating that the NPs become more stable at higher temperatures. This anomalous situation can be explained by the formation of higher atomic order and higher configuration density in the surface regions of NPs compared to the core regions, resulting in a decrease in the surface entropy of NPs.