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M. Rath, E. Povoden-Karadeniz, E. Kozeschnik:
"A thermodynamic description for the new η‐phase Ni6AlNb";
Poster: EUROMAT 2015, Warszawa; 2015-09-20 - 2015-09-24.

In Ni-base superalloys, η-phase (Ni3Ti) typically occurs after long-time exposure and in alloys with high Ti/Al ratio. In IN718 and Allvac 718Plus, which are precipitation-hardened nickel-base superalloys, the η-phase is not expected to occur due to the low Ti/Al ratio. However, a new plate-like phase (Ni3Al0.5Nb0.5) has been observed in 718Plus with the same hexagonal structure (D024) as η-phase (Ni3Ti) and a morphology and XRD pattern similar to the orthorhombic δ-phase (Ni3Nb) [1]. Thus, the new η-phase is difficult to distinguish from the δ-phase. Whereas several experimental studies have only reported the presence of δ-phase in 718Plus, recent investigations by high-resolution diffraction (XRD) have confirmed the presence of a stable new η-phase (Ni6AlNb) in 718Plus [2,3]. Based on these observations, we extend our original CALPHAD description of η-phase in the MatCalc database mc_ni to the Ni-Al-Nb side, in order to stabilize the Ni3Al0.5Nb0.5 η-phase. Since there is no experimental evidence of continuous mixing between Ni3Ti and Ni3Al0.5Nb0.5, we propose a miscibility gap among these variants of η-phase. The results of thermodynamic equilibrium calculations of η-phases in IN718 and 718Plus alloy are presented and discussed in view of the experimental evidence. Finally, an outlook on the future modeling activities concerning the new η-phase is given.

η-phase, superalloys, CALPHAD