Time-resolved and insitu observation of δ-γ transformation during unidirectional solidification in Fe-C alloys

Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that a massive-like transformation, in which the γ phase was produced through the solid-solid transformation and partitioning of substitute elements such as Mn and Si at the δ-γ interface was negligible, was selected in the unidirectional solidification of 0.3 mass% C steel at a pulling rate of 50 μm/s. The massive-like transformation produced fine γ grains near the front of the δ-γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ-γ front and the advancing front of coarse γ grains was as short as 200 μm. Namely, the fine γ grains disappeared within 10 s owing to growth of coarse γ grains. In addition, the observation of the δ-γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like γ growth occurred at a growth velocity of 5 μm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.