Formation of Extra Vacancies in Nucleating γ Phase during δ-γ Massive-like Phase Transformation in Carbon Steel

Abstract

Possible formation of extra amount of Fe vacancies during the massive-like phase transformation of carbon steel has been examined by combinations of analytical modeling, atomistic simulations and phase-field modeling. Formation of a vacancy in a perfect crystal does not decrease its volume by as much as the volume of an atom, effectively increasing lattice constant per atom. Consequently, transformation strain due to solid-state transformation from BCC-structured δ phase to FCC-structured γ phase leads to formation of extra amount of Fe vacancies more than in thermal equilibrium. This is to relieve transformation strain due to nucleation and growth of γ phase, which would promotes the massive-like transformation. The extra vacancies may also promote diffusion of substitutional alloying elements across δ/γ interface or partitioning of the alloying elements during the massive-like transformation, at least in the nucleating γ phase and in the vicinity of δ/γ interface until transformation strain is relieved.