Metastability of martensite and bainitic ferrite as carbon super-saturated structures

Both bainite and martensite reactions involve a sudden, ordered movement of iron atoms, which is accompanied by a given crystal correspondence, and a macroscopic shape strain of the transformed structure. Carbon supersaturation and tetragonality are not exclusive of martensite. A high level of carbon supersaturation in bainitic ferrite accompanied by a change in symmetry of the unit cell from cubic to tetragonal have been observed at low transformation temperatures confirming that, as martensite, bainitic ferrite grows without diffusion of carbon. It is therefore difficult to identify the morphological differences to discriminate metastable ferritic structures. In this work, comparable bainitic and martensitic structures were obtained and investigated by transmission electron microscopy (TEM) and atom probe tomography (APT) to identify all the subtle differences to discriminate both types of ferritic structures.