Formation mechanism and morphology change of K phase in Fe-6wt.%Al-2.1wt.%C alloy were studied by X-ray diffraction and transmission electron microscopy.
The broad maxima around Bragg$(200)_\gamma$peak were observed in the as rapidly solidified specimens by X-ray diffraction and approached to the main peak with increasing specimen thickness. Fine modulation image was observed by TE microscopy even in the most rapidly solidified specimen that we could obtain. Spinodal decomposition was confirmed by strain contrast change in TE images with respect to operating $\vec{g}$ vector. The modulation wavelengths measured in TE images increased with specimen thickness, which were in good agreement with X-ray investigations qualitatively. Plot of log λ(modulation wave-length) and log t(specimen thickness) has the slopes of about 0.101(1/9.9) at early stage and about 0.381(1/2.6) at later stage of decomposition. The difference between these experimental values and the ones calculated from LBM theory is thought to arise from continuous cooling of specimen. The existence of superlattice spot and the splitting of fundamental spot mean the formation of an ordered phase which has the same base structure as $\gamma$ phase but larger lattice parameter.
From all of these observations and considerations, it is concluded that $\gamma$ phase of Fe-6wt.%Al-2.1wt.%C alloy is separated into solute-rich and solute-poor region by spinodal decomposition during rapid solidification and ordering occurred in solute-rich region, resulting in K phase of $L`1_2$ structure.
The more specimen thickness increased, the more grain boundary and cell boundary reactions occurred. Grain boundary reaction is thought to consist of perovskite K phase and ferrite α phase, with lamellar like morphology.
The K phase particle size increased and its morphology changed cuboid to raft to plate with respect to aging at 800℃ by strain effect between particles.
