Microtwinning as a common mechanism for the martensitic and pearlitic transformations

Common model for austenite transformations to pearlite and martensite has been proposed. The model uses a topological operation which is a local flipping of interatomic bonds resulting in formation of the structural unit of the coherent twin boundary. Main structural unit of a coherent twin boundary along {1 1 3} in the FCC lattice is a trigonal prism which is also the main building unit for the cementite structure. The atomic structure of such multiple {1 1 3} twins coincides with the structure of multiple twins along {1 1 2} of the BCC lattice, so the FCC–BCC transformation route is the FCC twinning along {1 1 3} and detwinning of {1 1 2} BCC twins into BCC structure. Carbon atoms serve as stabilizers to the prismatic iron configurations thus forming the cementite structure during pearlite transformation. High density of microtwins in the rest austenite after pearlite transformation in 1.2C–4Mn steel and new orientation relationships between pearlite constituents and austenite have been observed by transmission electron microscopy in full consistency with the proposed model. The twinning–detwinning mechanism prescribes also parallelism between {1 1 3} FCC and {1 1 2} BCC for martensite transformation in full accordance with published data.

► Diagonal flipping in the coordination polyhedra was proposed as the main mechanism of polymorph transformation.
► Microtwinning of crystal lattice can be effected by diagonal flipping in polyhedra.
► Martensite and perlite transformation can be fulfilled by the similar microtwinning.
► Twinning of austenite during perlite transformation has been observed by electron microscopy, thus confirming the proposed model.
► Orientation relationships predicted by model are in accordance with the experiment.