Nucleation and variant selection during the α-γ-α phase transformation in microalloyed steel

This study addresses variant selection during the α-γ-α phase transformation in recrystallized, low-carbon, microalloyed steel. Since variant selection was found to occur during incipient stages of transformation, the investigation focused mainly on γ-grain nucleation from the α phase. To obtain comprehensive microstructure information, the sample was characterized by high-temperature electron backscatter diffraction in a scanning electron microscope and additional three-dimensional serial sectioning. It is shown that preferred nucleation at a grain boundary near a triple junction requires an orientation relationship close to the Kurdjumov-Sachs (K-S) correspondence to both adjacent α grains (nucleation rule I). Furthermore, a low disorientation of a {1 1 0}bcc crystallographic plane to the grain boundary plane favors nucleation (nucleation rule II). The results suggest that the nucleation of γ grains is favored at triple junctions by a low surface energy of the nucleus. A corresponding model is proposed. On this basis, variant selection of different ferrite orientations and the product textures of the complete transformation cycle were successfully predicted for a random grain boundary plane distribution.