Characterization of austenite grain structure evolution through metastable ferrite nucleation and ferrite-austenite transformation in Fe-Mn-C austenitic steel using X-ray imaging techniques

Various microstructure evolution can occur during solidification in the Fe-based alloy due to the competition between the ferrite (δ) and austenite (γ) for nucleation in the undercooled melt. This study elucidates the preference for metastable δ phase nucleation even in Fe-22mass%Mn-0.7mass%C, where the γ phase is the primary phase in equilibrium, using time-resolved X-ray imaging techniques. X-ray radiography shows that the metastable δ phase preferably nucleates, followed by the massive-like transformation, resulting in the formation of multiple γ grain structure. Meanwhile, the multiple γ grain structure was occasionally formed without detecting the δ phase. Time-resolved X-ray diffractometry revealed that several γ grains had twin relationships in the multiple γ grain structure formed without detecting the δ phase. The twin relationship suggests that the metastable δ phase nucleated in advance, and multiple γ grains were produced by the massive-like transformation.