مشخصات پژوهش

To obtain a fundamental understanding on the grain size effect on the diffusion kinetics in nanocrystal materials, molecular dynamics (MD) simulations were carried out on nanocrystal bcc iron samples with hexagonal shape grain and different grain sizes of 2.5, 4.5, 6.5, 8.5 and 10.5 nm. The second nearest- neighbor modified embedded atom method (2NN MEAM) interatomic potential was used to perform MD simulations. The effective (apparent) diffusivity in nanocrystal samples were calculated in the tem- perature range from 350 K to 1000 K and analyzed to clarify the dominant factors that affects diffusion. A temperature dependence of the diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that as the grain size of the nanocrystals decreases, the pre-exponential factor increases. However, the MD simulation results show that the activation energy is not affected signifi- cantly by increasing the grain size. Moreover, the results indicated that the Maxwell-Garnett’s equation provides a much better description of grain size dependence of effective diffusivity than the Hart’s equa- tion. The mechanism for the enhancement of effective diffusivity by decreasing grain size is discussed on the basis of mixing rule for lattice and grain boundary diffusion