Molecular Dynamics Study of the Effect of Carbon and Oxygen Impurities on the Velocity of the Crystallization Front in Nickel

The effect of carbon and oxygen impurities on the velocity of the crystallization front in nickel is studied using molecular dynamics simulation. Three different orientations of the front relative to the growing crystal were considered: (100), (110), and (111). Impurity atoms were introduced randomly over the entire volume of the computational cell. The impurity concentration varied from 0 to 7 at %. It was found that the introduction of impurity atoms in all cases significantly reduces the crystallization velocity, wherein oxygen atoms slow down the crystallization front more strongly than carbon atoms. The mechanism of deceleration of crystallization by impurity atoms is associated with two factors: deceleration of self-diffusion in a liquid metal due to the formation of relatively strong bonds between metal atoms and impurity atoms (for oxygen, this bond is stronger than that for carbon atoms), and distortion of the crystal lattice due to the dilatation effect around impurity atoms in a growing crystal (this effect is also higher for oxygen atoms). In the case of carbon impurity at sufficiently high concentrations (on the order of several percent), carbon atoms form aggregates, which are accumulations of several tens of carbon atoms in the metal matrix. The crystallization front lingered on these aggregates. During crystallization in the presence of oxygen impurities, aggregates are not observed. The orientation of the crystallization front influences the crystallization velocity: crystallization proceeds faster with the (100) orientation, and slower with the (111) orientation. This anisotropy of the velocity of the crystallization front is due to the difference in the free energies of the metal atom in the liquid phase and that “embedded” into the boundary of the growing crystal.