Atomic structure of the austenite-cementite interface of proeutectoid cementite plates

In a previous investigation, the atomic habit plane (or terrace plane) of p roeutectoid cementite plates in austenite was deduced to be (1 (1) over bar 3)(A)parallel to(101)(C) (where the subscript A indicates austenite and th e subscript C cementite), based on conventional transmission electron micro scopy and computer-aided atom matching studies. In the present study, high- resolution transmission electron microscopy (HRTEM) was employed in order t o determine directly the atomic structure and atomic habit plane of the cem entite-austenite interface corresponding to the broad faces of cementite pl ates. An Fe-1.3 wt%C-13 wt%Mn alloy was isothermally reacted at 650 degrees C for 100s in order to form cementite plates embedded in a completely reta ined austenite matrix. The HRTEM observations directly confirm the previous deductions that; firstly (1 (1) over bar 3)(A)parallel to(101)(C) is the t errace plane of proeutectoid cementite plates at the atomic level, secondly the cementite-austenite interfaces contain ledges with a [010](C)parallel to[110](A) line direction, and thirdly the ledges result in an average habi t plane of the cementite plates that is inclined with respect to the (101)C parallel to(1 (1) over bar 3)(A) terrace plane. The ledge plane connecting adjacent (1 (1) over bar 3)(A)parallel to(101)(C) terraces was determined to be (001)(C)parallel to((1) over bar 13)(A) and thus is not orthogonal to the terrace plane. A nearly perpendicular pair of edge misfit dislocations is associated with each ledge and terrace. One or both of these dislocatio ns would have to climb in order for the cementite-austenite interface in th e vicinity of these ledges to migrate. The ledges are intrinsic features of the interface, that is in most cases they do not result from intersections of faults or other defects with the cementite-austenite interface.