Nucleation-controlled selection of metastable ferrite in solidification of Fe-22mass%Mn-0.7mass%C alloy

The competition between the ferrite and austenite for nucleation in the melt can result in various solidification sequences in the Fe-based alloy. This study demonstrates that the austenite solidification was initiated by metastable ferrite nucleation followed by ferrite-austenite transformation even in Fe-22mass%Mn-0.7mass%C, where the austenite is the primary phase in equilibrium. Time-resolved X-ray diffraction measurements were performed using a time-resolved X-ray tomography apparatus to identify the metastable ferrite nucleation followed by the austenite solidification. X-ray radiography was performed to observe the microstructure evolution through the metastable ferrite nucleation followed by the austenite solidification. The metastable ferrite nucleation was preferably selected when the completely melted specimen was cooled. During subsequent cooling, the ferrite massively transformed to the austenite in the solid state, and multiple austenite grains were produced in a single ferrite grain through ferrite-austenite transformation. The ferrite-austenite transformation was immediately followed by the coarsening of multiple austenite grains. When the ferrite-austenite transformation occurred in a semisolid state consisting of the ferrite and liquid phase, the liquid phase, which isolated the austenite grains, suppressed the coarsening of austenite grain. The typical austenite grain size ranged from 100 to 500 μm. Thus, the present results suggest that the ferrite-austenite transformation following the metastable ferrite nucleation has the potential to control the austenite grain size in as-cast microstructures.