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M. Rath:
"Modeling of grain boundary precipitation in 718-type Nickel-base superalloys";
Talk: X. seminar of Central European PhD students in Material Science, Kosice; 2016-04-04 - 2016-04-06.

Nickel-base superalloys are complex materials consisting of several precipitation phases, like the intermetallic phases γ´ and γ´´ as strengthening precipitates and δ or η phases as grain boundary precipitates. In Ni-base superalloys, the η-phase (Ni3Ti) typically occurs only after long-time exposure and in alloys with high Ti/Al ratio. In 718-type 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). 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. A new thermodynamic model description is proposed which is applicable for kinetic simulations of η and η* evolution in multi-component Ni-base superalloys. 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.