{011} Twinning in Fe-Ni-C martensites

The substructures of two Fe-Ni-C alloys that form platelike martensite have been studied by transmission electron microscopy (TEM). Planar features with a {011 } habit are observed, in addition to the usual {112 } transformation twins and arrays of screw dislocations. The results of diffraction-contrast experiments are consistent with these {011 } defects being fine twins within which the carbon atoms occupy a different octahedral interstitial sublattice from the surrounding matrix. In any given martensitic plate, this twinning appears to occur preferentially on the {011 } plane containing the same {111 } direction as the operative {112 } transformation-twinning variant. The possibility that {011 } twinning occurs during the martensitic transformation and/or when virgin martensite is heated from subambient temperatures is discussed in relation to crystallographic observables such as the martensitic habit plane and axial ratios.