Determination of Phase Equilibria among δ-Fe, γ-Fe and Fe2M Phases in Fe-Cr-M (M: Hf, Ta) Ternary Systems

Phase equilibria among delta-Fe, gamma-Fe, and Fe2M phases in the Fe-Cr-M (M: Hf, Ta) ternary systems were determined using bulk alloys heat-treated at high temperatures. The final goal of this study is to develop novel ferritic heat resistant steels strengthened by precipitation of Fe2M phase on the eutectoid type reaction path: delta -> gamma + Fe2M. The phases present in heat-treated samples were identified by microstructural characterization and X-ray diffraction pattern analysis. The chemical compositions of the phases were analyzed by energy dispersive spectroscopy. A pseudo-eutectoid trough (delta -> gamma + Fe2M) exists at ~1220 degrees C at a Hf content of 0.1% and at ~1130 degrees C at a Ta content of 0.6% on the vertical section at a Cr content of 9.5% in each ternary system, respectively. A thermodynamic calculation with a database that reflects reported binary phase diagrams and the present study indicates that an increase in the Cr content decreases the temperature and the Hf/Ta contents of the pseudo-eutectoid troughs. The determined phase equilibria suggest that the supersaturation of Hf/Ta for the formation of gamma phase is higher in the Hf doped system than in the Ta doped system, which is probably an origin of a much slower kinetics of precipitation on the eutectoid path in the latter system.