Crystallography of ferrite nucleation at austenite grain boundary in a low carbon steel

Orientation relationships of proeutectoid ferrite formed at an austenite grain boundary with respect to adjacent austenite grains were investigated by means of electron backscatter diffraction in an isothermally transformed Fe-1.5Mn-0.2C (mass%) alloy. A grain bounadry ferrite holds nearly the Kurdjumov-Sachs (K-S) orientation relationship with some small misorientation against at least one of the adjacent austenite grains. There is strong variant selection by the austenite grain boundary for ferrite nucleation. At a higher transformation temperature, the fraction of ferrites holding a near K-S relationship with respect to the opposite austenite grain is lower. As the transformation temperature becomes lower, the misorientation from the K-S relationship becomes smaller against the near K-S related austenite grain whereas the misorientation against the irrationally oriented austenite grain becomes larger.