Mechanical properties of (Ni, Fe)Cr2O4 polycrystal spinels studied by molecular dynamics simulations

The elastic moduli and mechanical properties at the onset of crack in nanocrystalline and nanoporous (Ni, Fe)Cr2O4 compounds with a spinel structure are investigated by molecular dynamics simulations. The polycrystalline structures generated contain nanograins from 2.5 to 30 nm in diameter. These structures are representative of the internal corrosion layer in nickel-based alloys. These simulations enabled us to establish the evolution of elastic moduli as a function of the composition, porosity, and grain size of the polycrystals. From this evolution, the initial database for the elastic properties of corrosion layers based on von Bertalanffy growth functions was determined. The onset of crack in polycrystals is also investigated via uniaxial tensile and shear deformation. Under shear deformation, flow stress as a function of grain size follows normal and inverse Hall-Petch regimes. The regime change occurs for grain sizes around 10 nm. For grain sizes under this threshold, shear banding involving collective translation and rotation of nanograins dominates the plastic deformation. For grain sizes greater than 10 nm, phase transition inside grains from a spinel to a post-spinel-like structure is observed as well. In that case, phase transition dominates the plastic deformation. Under uniaxial tensile deformation, intergranular decohesion occurs. The general law as a function of grain size for toughness, which is the material's capacity to absorb elastic and plastic energy prior to failure, is also established.