Ordering of γ-phase and formation of κ-phase in melt-quenched Fe-Al-C alloys and their magnetic structures

Nonequilibrium phases in melt-quenched Fe-Al-C alloys, austenite (γ), ordered austenite (γ′) and perovskite Fe3AlC(κ) were examined by X-ray diffraction, transmission electron microscopy, Mossbauer spectroscopy and computer simulation. As the result, it is confirmed that the second nearest local interaction of Al-C atomic pairs in austenite is strongly attractive (approx.30 kJ/mol) and the interaction is in favor of the formation of the κ phase. The κ phase is paramagnetic in the stoichiometric composition (Fe3AlC), in contrast to the ferromagnetic behavior (TCapprox.210°C) in the non-stoichiometric alloy(Fe3AlC0.64). It is concluded that the magnetic moment of iron atoms in the κ phase is mainly determined by the number (0, 1 or 2) of the nearest neighboring carbon atoms. It is also found that the equilibrium composition region of the κ phase at high temperatures (800approx.1200°C) is much extended compared with that shown in the previously reported phase diagram.