Formation mechanism of bainitic ferrite and carbide

Superledges on the broad faces of bainitic ferrite plates have been observed with transmission electron microscope (TEM). The observed superledges, ranging from less than 1 to 24 nm in height, are imaged in three dimension by way of tilt operation under TEM. Also, an array of smaller superledges are observed to pile up in front of a secondary phase. Pileup of superledges in front of a barrier is indicative of the mobility of an individual superledge. The precipitation of carbide associated with bainitic ferrite is also studied. It is observed that a carbide of a wedgelike shape may exist in front of a superledge with its tip(i.e., thinner end) penetrating the austenite and its root (the other end) originating at α:γ boundary. This condition indicates that the observed carbides may nucleate at the austenite side of α:γ phase boundary and grow toward austenite matrix. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.