The Molecular Simulation Of Microstructure And Mechanical Properties Of Nanocrystalline Fe, Ni And Zr Metals

Nanocrystalline materials are polycrystals with mean grain size ranging from 1nm to 100nm. Due to its unique properties, great attention to nanocrystalline materials has been increased in past years. In the present paper, the initial structural models of nanocrystallites are constructed with Voronoi cell method. The microstructure and mechanical properties of nanocrystalline Fe, Ni and Zr are simulated with the molecular dynamic (MD) simulation and the analytic embedded-atom method (AEAM). The microstructures of nanocrystallites are analyzed with atomic energy method, radius distribution function and common-neighbor analysis technique.The simulation result of microstructures of nanocrystallite Fe, Ni and Zr reveal that the structure of grain boundary in nanocrystalline is almost independent on average grain size. With reducing grain size, the fraction of grain boundary increases and the lattice distortion in grain interior enhances, and the structural difference between grain boundary and grain interior diminishes gradually. As the bonds between grain-boundary atoms are weaker than atoms in grain interior, the cohesive energy of nanocrystalline materials is lower than general microcrystal and decreases with reducing average grain size. Affected by the lattice distortion in grain interior and the high proportion grain boundary, the density of nanocrystallite can't approach that of the perfect crystal.The simulation result of nanoparticle Fe and Ni indicate that the cohesive energy of nanoparticle is lower than that of nanocrystallite with the same grain size for the free surface of nanoparticle. Under the function of surface tension, the nanoparticles show lattice contraction, and with reducing the particle size, the contraction increases and the lattice constant of nanoparticle decreases.The uniaxial tension simulations of nanocrystalline Fe, Ni and Zr show that the elastic modulus of nanocrystallite with small average grain size is lower than the conventional microcrystal and decreases with reducing grain size. The mechanical strength of nanocrystalline decrease with grain size reducing and there exists a reverse Hall-Petch relation between the yield stress and the mean grain size. As known from the dislocation theory, dislocations seldom appear in the grain interior, and the plasticdeformation of nanocrystallite mainly carries out by the grain boundary sliding and grain rotation.The uniaxial compression simulation of nanocrystalline Fe indicates that the compressive deformation process of nanocrystallite can be described as three characteristic regimes: quasi-elastic deformation, plastic flow deformation and strain strengthening regimes. During the plastic flow deformation process, the nanocrystallite show very good compressive ductibility. As discussed in the tensile deformation, the deformation mechanisms in the compressive deformation of nanocrystallite are mainly from atomic sliding and rotation of grain boundary.