Effect of light elements impurity on process of nickel crystallization near the triple interface of grain boundaries: a molecular dynamics simulation

Abstract

Molecular dynamics method was used to study the effect of impurities of light elements of carbon, nitrogen and oxygen on crystallization process near the triple interface of grain boundaries in nickel. Tilt boundaries with misorientation axis <111> were considered as the grain boundaries. Interactions of nickel atoms with each other were described by many-particle Clery-Rosato potential constructed within the framework of the tight binding model. To describe interactions of atoms of light elements impurities with nickel atoms and atoms of impurities with each other, Morse pair potentials were used. Calculation cell had a shape of cylinder, axis of which coincided with the line of triple interface and the axis of grain misorientation. Periodic boundary conditions were imposed along the cylinder axis, and the atoms on side surface of cylinder were motionless. To simulate crystallization, calculation cell was melted by heating to a temperature well above the melting temperature of nickel. After the simulated polycrystal become liquid, the thermostat was turned on and held at a constant temperature below the melting temperature. Rigid boundary conditions on the lateral surface of cylindrical calculation cell in this case simulated crystallization fronts from three crystallization centers. The area near the triple interface had crystallized the last. In this area, defects and free volume were concentrated. Presence of impurities led to a significant slowdown in the rate of crystallization. With introduction of 10 % of impurity atoms, the rate of motion of crystallization front decreased several times. The effect of impurities on crystallization rate was enhanced in C – N – O direction, which is due to difference in crystal lattice deformation caused by impurity atoms. The greater this deformation was, the stronger was impurity atoms inhibit crystallization front. Formation of aggregates at fairly high concentrations was typical for impurity carbon atoms. Crystallization front had impeded on these aggregates. The oxygen and nitrogen atoms did not form aggregates. However, due to distortions of crystal lattice caused by them, they also strongly slowed down the crystallization front.