Influence of Crystallography on Nucleation of Ferrite Precipitates at Austenite Grain Boundary Corners in a Co-15Fe Alloy

The nucleation of bcc ferrite precipitates at austenite grain corners in a Co-15Fe alloy was studied by serial sectioning coupled with electron backscatter diffraction (EBSD) analysis. Grain corners were identified by recombination of triple points and triangular annihilation, whereas quite a few precipitates were surrounded by more than four matrix grains when twins were counted as individual grains. More than 40% of corners composed all of high angle grain boundaries were vacant at an undercooling of similar to 60 degrees C from the gamma/(alpha+gamma) phase boundary. All the precipitates had K-S or N-W orientation relationship with at least one grain and a larger proportion of them had the OR with two and three grains. For half of vacant corners a hypothetical precipitate could have the OR with more than one grain. It is likely that not only the misorientations among the matrix grains, but also the orientations of the grain boundary planes have a major influence on nucleation potency even at grain corners.