We developed a novel austenitic heat resistant steel, Fe-20Cr-30Ni-2Nb (at.%), strengthened by two intermetallic phases, Fe2Nb (TCP) and Ni3Nb (GCP), based on our phase diagram study on Fe-Ni-Nb ternary system at elevated temperatures. In this study, influence of Mo in the phase equilibria among the γ-Fe/TCP/GCP phases has been investigated in order to understand the Mo effect on phase stability of the intermetallic phases with respect to that of the γ phase. The alloys with compositions of Fe-(30~60)Ni-(5~15)Nb-(5~15)Mo (at.%) were prepared by arc melting, and then equilibrated at 1373 or 1473 K, followed by aging at 1073 K for up to 480 h. The two-phase region of γ+ε and γ+δ exists in Fe-rich and Ni-rich side, respectively, in the quaternary system. However, the liquid and hP24 (GCP) phases existing in the region of 60 at.% in Fe-Ni-Nb ternary system at 1473 K disappears by addition of 10 at.% Mo. Instead, in addition to a small amount of Fe5Mo3-R phase, Fe7(Nb,Mo)6-μ phase in equilibrium with γ-Fe phase appears. This indicates that Mo stabilizes the μ phase and thus leads to a transition peritectoid reaction of ε+δ→γ+μ. EPMA analysis revealed that Mo partitions more to ε (TCP) phase than γ phase, whereas it partitions more to γ phase than δ (GCP) phase. Note that the partition coefficient of Nb between ε and γ phases, kNbε/γ, and that of between δ and γ phases, kNbδ/γ, remain unchanged with the value larger than 1 regardless of the presence of Mo in solution. These results clearly indicate that Mo stabilizes ε phase against γ phase, but it destabilizes δ phase against γ phase. The results at 1373 K will be presented, and the temperature dependence of the phase stability will be discussed. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency).
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