The formation mechanism(s), morphology, and crystallography of ferrite sideplates

Abstract

The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy in a high-purity Fe—0.12 wt pct C—3.3 wt pct Ni alloy isothermally transformed at temperatures of 550 °C, 600 °C, 650 °C, and 675 °C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and the allotriomorphs with which they are associated. These results indicate that “secondary sideplates” and the allotriomorphs from which they evolve arenot composed of monolithic single crystals formed by a morphological instability 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 presented here may be applicable to a number of other alloy systems as well.