Submicrometer-scale molecular dynamics simulation of nucleation and solidification from undercooled melt: Linkage between empirical interpretation and atomistic nature

Abstract Nucleation and solidification from the undercooled iron melt are investigated from the atomistic point of view by large-scale molecular dynamics (MD) simulations up to 12 million atoms in systems of the submicrometer-scale. There exist some amount of atoms with icosahedral configuration in the undercooled iron melt and these atoms increase with decreasing temperature. It is expected that accumulation of atoms with icosahedral configuration in the initial β -relaxation regime of nucleation is the key to initiate the formation of bcc phase. On the other hand, mobility of atoms in the undercooled melt decreases drastically with decreasing temperature. These two competing factors in the atomistic scale are considered to derive a critical temperature at which nucleation rate becomes maximum, which agrees with a classical theory for homogeneous nucleation. Moreover, the Avrami exponents during solidification are approximately estimated to be close to 3 and 4 in two- and three-dimensional grain growths, respectively, which also agrees with empirical interpretation. Our novel approach utilizing the high parallel efficiency of the GPU supercomputer successfully links the empirical interpretation in metallurgy with the atomistic behavior of nucleation and solidification, which enlarges the application range of MD simulations for the study of structural materials.