Solid state transformations in Fe-Ni supercooled alloys

The effect of supercooled microstructures and composition in the solid state transformation of the Fe-Ni system was investigated utilizing transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Irrespective of the supercooling degree, Fe-Ni samples containing between 7 and 28 wt % Ni underwent a martensitic transformation above room temperature. The martensite morphology is superimposed on the sub-grain microstructure obtained by supercooling. Indications exist that alternative nucleation is activated if the samples are supercooled below the -T₀ curve. The impact of alternative nucleation on the martensitic transformation is discussed. Coherent precipitation was observed in Fe-33 wt % Ni alloys. The precipitate dimensions depend on the solidification path. The results are discussed in terms of current supercooling theories.