Naoya Sasaki

Molecular Dynamics Study of Impurity Effects on Grain Boundary Grooving.

Grain boundary grooving in crystalline aluminum is simulated by computer molecular dynamics, and impurity effects are investigated. We use a Morse potential that includes equilibrium spacing, γA1, and potential well depth, |uA1| to characterize aluminum-aluminum interaction. We also use a two-body interatomic potential that includes equilibrium spacing, γm, and potential well depth, |umin| to characterize aluminum-impurity interaction. The simulations show that when γm is smaller than γA1 and when |umin| is close to |uA1| (with the relative difference smaller than 20%), grain boundary grooving is prevented. This effect is explained by a decrease in the ratio of grain boundary diffusion to surface diffusion. Diffusion coefficients obtained by these simulations show that impurities at the grain boundaries which satisfy the above conditions (e. g., copper) strengthen surface diffusion without strengthening grain boundary diffusion.

Molecular Dynamics Simulation of Shear Deformation of Bicrystalline Aluminum

We investigate the shear deformation of bicrystalline aluminum using a molecular dynamics simulation. The computational cell contains a [001](310)Σ=5 tilt grain boundary. In simulations, we use a Morse potential. The simulations show that when the strain rate is small, and when the temperature is high, the strain rate is proportional to the shear force and the Boltzmann factor. The activation energy for the deformation agrees well with the activation energy for grain-boundary sliding and migration induced by thermal activation. This means that diffusions of the same type occur in both cases. However, this type of diffusion does not seem to occur when the temperature is low. When the strain rate is sufficiently large, yielding is observed in the grain. The critical shear force becomes smaller as the temperature is raised.