Indirect nucleation in phase transformations with symmetry reduction

Non-classical nucleation in a first-order phase transition with a symmetry reduction and multicomponent long-range order parameter is investigated. A particular case of cubic → tetragonal transformation is considered. Based on the stochastic field equation, a computational method of finding the atomic configuration of a critical nucleus without any a priori constraints on the nucleation path is proposed. Two-dimensional computer simulations are performed. It is demonstrated that a transformation need not go directly to the equilibrium state during nucleation. Under certain conditions the system prefers an ‘indirect’ nucleation developing along a path that has a lower nucleation barrier. The critical nucleus formed along this path has a transient nonequilibrium structure, which transforms into the equilibrium structure during the growth process. The relation between indirect nucleation and the structural mechanism of the interfacial energy optimization in the case of multicomponent long-range order parameter is discussed.