Equiaxed Solidification of Metastable Ferrite in Fe–22mass%Mn–0.7mass%C–0.3mass%Ti Alloy Nucleating on Ti Carbonitride

This study demonstrates the effect of Ti addition on phase selection and subsequent ferrite-austenite transformation in Fe–22mass%Mn–0.7mass%C alloy, where the austenite is the primary phase in equilibrium. X-ray radiography revealed that the metastable ferrite nucleated as equiaxed grains in the completely melted specimen. During subsequent cooling, the metastable ferrite massively transformed into the austenite in the solid state, forming multiple austenite grains in each metastable ferrite grain. The ferrite-austenite transformation was immediately followed by the coarsening of multiple austenite grains within each former metastable ferrite grain. Typical austenite grain size ranged from 100 to 500 μm. In the specimen after the observation, titanium carbonitride (Ti(C, N)), which acts as heterogeneous nucleation agent for the ferrite, was presented and overlaid manganese spinel (MnAl₂O₄) or Al-Ti oxide. Because disregistry between such oxides and Ti(C, N) can be relatively low, the oxides facilitated the formation of Ti(C, N) in the melt. Regarding the formation of the oxides, it can be postulated that titanium oxides, as a deoxidation product, first combined with soluble Al, Mn, and O to form liquid Al–Mn–Ti oxides. During cooling, MnAl₂O₄ or Al–Ti oxide was supersaturated in liquid Al-Mn-Ti oxides, which subsequently crystallized and dispersed in the melt. Thus, titanium oxide serves as a precursor to a multistep reaction leading to the formation of Ti(C, N), and its fine dispersion in the melt allows us to control the austenite grain size in the as-cast microstructure through promoting the metastable ferrite nucleation followed by the ferrite-austenite transformation.