Seminars and Events

Experiment and thermodynamic assessment of the Al-Ni-Mo system

Date/Time: 17.09.2014. 10:30 a.m.
Place: Ruhr-Universität Bochum, Bochum, Germany

Jian Peng, Department of Thermophysics und Thermodynamics, Karlsruhe Institute of Technology, Karlsruhe

The intermetallic compound NiAl possesses attractive properties for high temperature structural applications, e.g. high melting point (1638°C), substantially lower density (5.7 g/cm3) than that of Ni-based superalloys, high oxidation resistance, and good thermal and electrical conductivity. However, besides its poor ductility and fracture toughness at room temperature, the use of NiAl as a structural material suffers mainly from low strength and creep resistance at temperatures above 600°C. Fortunately, recent advancements in directional solidification of NiAl-Mo eutectics enable to produce in-situ fiber reinforced composite (NiAl matrix and Mo fiber) with an unprecedented level of fiber alignment and regularity, and the mechanical properties are greatly improved compared to intermetallic compound NiAl. This composite has been demonstrated as a potential candidate used for as specific hot section components of turbine engines, blades or buckets and vanes as well as nozzles.

Although three versions of thermodynamic assessment for the Al-Ni-Mo system have been reported, it is still necessary to re-assess it due to several limitations of previous ones: a) In the earliest assessment, all compounds were described as stoichiometric compounds. b) The second assessment was found to be inconsistent with experimental phase equilibrium data at 800°C, limiting its utility in phase-field modeling. c) Despite of the more complete descriptions of Al-Ni-Mo system, most of the invariant reaction temperatures are inconsistent with the experimental data in the latest assessment. d) None of them can reproduce a reliable description for NiAl-Mo quasi-binary system. Moreover, important improvements, such as the modeling of the order-disorder transition of bcc A2 and bcc B2 phases, are published, but absent in Al-Ni-Mo system. Therefore, a more accurate thermodynamic descriptions for Al-Ni-Mo system, particularly for the NiAl-Mo quasi-binary system is needed, which can support the design of alloy microstructures, microstructure modeling and thermodynamic modelling of higher order systems.

In the present work, key experiments are carried out to describe the Ni-Al-Mo section more accurately and the experimental results are taken into account in the present assessment. A substitutional-solution model is used to describe liquid, fcc, and bcc phases, while sublattice models are used to describe intermetallic phases. More importantly, the disordered bcc A2 and ordered bcc B2 phases are also modelled with single Gibbs free energy function, as well as the disordered fcc and ordered L12 phases. Calculated phase diagrams are compared with the experimental data.

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