Influence of Crystallography on Ferrite Nucleation at Austenite Grain-Boundary Faces, Edges, and Corners in a Co-15Fe Alloy

The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs (K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary.