Impurity Effect of Light Elements on the Nickel Crystallization in the Triple Junction Region of Grain Boundaries: Molecular Dynamics Simulation

Abstract

The impurity effect of light elements (carbon, nitrogen, and oxygen) on nickel crystallization in the triple junction region of grain boundaries is studied by the molecular dynamics method. The tilt grain boundaries with disorientation axis 〈111〉 are considered as grain boundaries. The computational cell is a cylinder, the axis of which coincided with the triple junction and the disorientation axis of grains. Periodic boundary conditions are imposed along the cylinder axis; the atoms on the lateral surface of the cylinder are stationary. To simulate crystallization, the computational cell was melted by heating to a temperature significantly higher than the melting point of nickel. After the simulated polycrystal became liquid, the thermostat was turned on and the polycrystal was kept at a constant temperature less than melting point. In this case, the rigid boundary conditions on the lateral surface of a cylindrical computational cell simulated the crystallization fronts from three crystallization centers. The area near the triple junction crystallized in the last turn. Defects and free volume are concentrated in this area. The impurities significantly slowed down the crystallization rate. The addition of 10% impurity atoms slowed down the rate of the crystallization front by several times. The impurity effect on the crystallization rate increased in the direction C–N–O. This is due to the difference in the deformation of the crystal lattice caused by impurity atoms: the greater this deformation, the more impurity atoms slow down the crystallization front. Impurity carbon atoms formed aggregates of crystal grains at rather high concentrations. Crystallization front remained on these aggregates. Oxygen atoms and nitrogen atoms did not form aggregates; nevertheless, they distorted the crystal lattice and significantly slowed down the crystallization front.