Molecular dynamics study of microscopic structures, phase transitions and dynamic crystallization in Ni nanoparticles

Using molecular dynamics simulations in conjunction with the quantum-corrected Sutton-Chen potential, we studied the influence of the heating and cooling rates, number of particles, temperature and relaxation time on the microscopic structure, phase transitions and dynamics of crystallization in four model systems containing N = 4000, 5324, 6912 and 8788 nickel atoms. The simulation results of a representative ensemble of 5324 Ni atoms have shown that the glass transition temperature, T-g, crystallization temperature, T-c, and melting temperature, T-m, are observed with heating and cooling rates in the range of Delta T/Delta t = 2 x 10(12) K s(-1) to 4 x 10(13) K s(-1), but T-g or T-c increases while T-m decreases with decreasing Delta T/Delta t. By applying Delta T/Delta t = 4 x 10(12) K s(-1) we found that after cooling from 2000 K down to 300 K the Ni atoms form nanoparticles for which the grain size follows the relation d proportional to N (1/3), and simultaneously the total potential energy of the investigated systems decreases linearly with the number of Ni atoms. With the help of common neighbor analysis, we detected the coexistence of amorphous and crystalline phases during the whole crystallization process. In the solid state, the dominant crystalline phase is characterized by the FCC and HCP structures, although a very small fraction of the BCC structure may occur at 300 K. It was established that the formation of the FCC structure is favored over the HCP one. In particular, lowering the temperature and increasing the relaxation time favour crystallization of the FCC lattice.