Fundamental study of nonclassical nucleation mechanisms in iron

Nucleation is the re-arrangement of a small number of atoms in the structure of a material leading to a new phase. According to the classical nucleation theory, a nucleus will grow if there is an energetically favourable balance between the stability of the newly formed structure and the energy costs associated to the formation of strains and new phase boundary. However, due to their atomic and dynamic nature, nucleation processes are difficult to observe and analyse experimentally. In this work, atomic mechanisms and thermodynamics of the homogeneous nucleation of BCC phase in FCC iron have been analysed by molecular dynamics simulations. The study shows that atomic system circumvents the high energy barrier for homogeneous nucleation that would occur according to the classical nucleation theory by opting for alternative, nonclassical nucleation processes, namely coalescence of subcritical clusters and stepwise nucleation. These observations show the potential of nonclassical nucleation mechanisms in metals.