Large-scale Molecular Dynamics Study on Evolution of Grain Boundary Groove of Iron

The kinetics of a solid-liquid interface of iron with a triple point is investigated by performing a large-scale molecular dynamics (MD) simulation. A grain boundary groove evolves at the triple point of a solid-liquid interface with a bcc Sigma 3(111) tilt grain boundary during solidification, whereas a solid-liquid interface with a twin boundary of small grain boundary energy is almost planar except in the vicinity of the triple point. The propagation velocity of the solid-liquid interface without a triple point is almost proportional to the degree of undercooling. On the other hand, that of the interface with the triple point deviates from linearity as the degree of undercooling decreases due to the balance between the driving force of the solid-liquid interface induced by undercooling and the pulling force excited by the grain boundary. Moreover, it is found that the equilibrium temperature at which the solid-liquid interface does not move is in close agreement with the curvature undercooling estimated by the Gibbs-Thomson effect. These insights are newly obtained by the large-scale MD simulation performed on a graphic processing unit (GPU), which achieves about 100 times the speed of our previous simulation performed on a single central processing unit (CPU).