On the nucleation of cementite on bainitic ferrite–austenite interphase boundaries

Among all possible variants of the Isaichev orientation relationship between cementite and ferrite, a single major cementite variant has been observed to appear in bainite. Interphase boundary nucleation of cementite on ferrite–austenite semi-coherent interfaces is considered a plausible reason for this observation. With the aid of known crystallographic relations and habit planes of the ferrite–cementite, ferrite–austenite and austenite–cementite phases, a model for cementite nucleation has been proposed. The interphase-boundary nucleus is assumed to form on a semi-coherent ferrite–austenite interface and to possess ferrite–cementite and austenite–cementite habits as two main facets of the nucleus. It is shown that interphase cementite nucleation will be viable if the energies of all facets of the nucleus are in the semi-coherent range.