Creep Behaviors of Alloy 718 Type Ni-based Superalloys

Friday, February 28, 2025: 11:20 AM
Indian Wells I (Grand Hyatt Indian Wells Resort)
Prof. Satoru Kobayashi , Tokyo institute of technology, Meguro-ku, Tokyo, Japan
Alloy 718 is an important class of Nb-bearing Ni-based superalloys for high-temperature applications, such as compressor disks/blades and turbine disks in gas turbine systems, due to their high strength/toughness and good fabricability. The strength of Alloy 718 is derived from a fine/coherent precipitation of thermodynamically metastable D022 γ”-Ni3Nb precipitates in the fcc γ–Ni matrix. The metastable feature of the γ” precipitate, however, causes it to transform to the thermodynamically stable D0a δ-Ni3Nb structure at elevated temperatures, which results in the degradation of the mechanical properties, thereby limiting the service temperature of the alloy below ~650 °C.

Previous studies revealed an improved stability of γ” precipitates in “Ta-718” alloy where Nb is replaced by Ta in Alloy 718. In the present study, we investigated precipitation and creep behaviors in Ta-718 and Alloy 718 aiming at further understanding of 718-type alloy metallurgy and creep behaviors.

Hot forged Ta-718 and Alloy 718 bars were solution-treated at 1040-1050 °C and subsequently aged at 725 °C to introduce fine/coherent γ” precipitates. Creep tests were conducted at 700 °C under stress conditions of 400 and 500 MPa. It was found that while the minimum creep rates were comparable in the two specimens, accelerating creep rates were slower in Ta-718 than in Alloy 718 under the studied creep conditions. Microstructural observations on the creep specimens before and after the tests demonstrate that the size of γ” precipitates are finer in Ta-718 than in Alloy 718 throughout the creep tests and cutting of γ” precipitates occur significantly in Alloy 718 but less frequently in Ta-718. The observed creep behaviors will be discussed in more details in the paper.