INTERFACIAL STRUCTURE OF THE BROAD FACES OF FERRITE PLATES

An investigation of the interfacial structure at the broad faces of ferrite plates embedded in retained austenite has been undertaken in an Fe-0.62 wt% C-2.0 wt% Si alloy by means of weakbeam dark-field TEM. These boundaries have been shown to be partially coherent and to contain single arrays of parallel misfit dislocations spaced ca. 15 to 25 Å apart and arrays of parallel structural ledges spaced ca. 22 to 90 Å apart. The dislocations are mixed in character and have lattice-type Burgers vectors lying in the atomic habit planes of the interface. The structural ledges are triatomic in height, with spacings and directions varying with the rotation of {111}γ relative to {110}α, about a common orthogonal axis. Data on apparent habit planes and orientation relationships were obtained with stereographic projection and trace analysis techniques. Comparisons of these data with those obtained by means of the interface modeling techniques described in the companion paper show good agreement. The misfit dislocation structure is always sessile with respect to glide out of the interface, whereas the lattice invariant deformation portion of a martensitic f.c.c. → b.c.c. transformation requires a glissile structure; hence, it is mechanistically impossible for ferrite plates to thicken by a martensitic mechanism. © 1979.