THE FORMATION MECHANISM(S), MORPHOLOGY, AND CRYSTALLOGRAPHY OF FERRITE SIDEPLATES

The formation mechanism(s), morphology, and crystallography of seconda ry ferrite sideplates were investigated with transmission electron mic roscopy (TEM), scanning electron microscopy (SEM), electron backscatte r pattern (EBSP) analysis, and optical microscopy in a high-purity Fe- 0.12 wt pet C-3.3 wt pet Ni alloy isothermally transformed at temperat ures of 550 degrees C, 600 degrees C, 650 degrees C, and 675 degrees C . The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the f irst mechanism, primary sideplates form initially, followed by rapid l ateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an all otriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, re sulting in ferrite:ferrite grain boundaries and significant crystallog raphic misorientations between the sideplates and the allotriomorphs w ith which they are associated. These results indicate that ''secondary sideplates'' and the allotriomorphs from which they evolve are not co mposed of monolithic single crystals formed by a morphological instabi lity mechanism but are instead composed of multiple crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel suggest that the findings present ed here may be applicable to a number of other alloy systems as well.