In-Situ Observation of the Growth of Massive Ferrite in Very Low-Carbon Fe-Mn and Ni Alloys

The growth of ferrite was observed in situ under confocal scanning laser microscope in Fe-Ni and Mn alloys containing less than 0.01 mass pct C. The advancements of α/γ boundary during cooling at 2 and 20 °C/s were measured in the paraequilibrium α single-phase region in Fe-2Ni and 2Mn alloys and near the paraequilibrium α/(α + γ) phase boundary in an Fe-5Ni alloy. The growth rates obtained from the boundary displacement vs time plots were almost constant and were considerably greater than those calculated under long-range carbon diffusion control, and thus, the ferrite growth is likely to have occurred in a massive manner, albeit a carbon spike may develop ahead of the boundary. The mobility of massive α/γ boundary was calculated to fall between those proposed by Hillert and Wits et al. irrespective of the assumption as to the formation of carbon spike in front of the boundary.