In-situ study of the stress relaxation during aging of nickel-base superalloy forgings

Pierre-Emmanuel Aba-Perea, Philip Withers, T. Pirling, A Paradowska, D. Ma, Michael Preuss

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Nickel-base superalloys are conventionally aged in order to develop a favorable microstructure as well as to relieve residual stresses. Here neutron diffraction was used to follow the evolution of residual stress during aging in Inconel 718 and Udimet 720LI for the first time by a combination of in-situ and ex-situ measurements. First, the quench-induced stress profiles were determined for different geometries of Inconel 718 forgings confirming that the amplitude of stresses is not significantly affected by the thickness of the component. It was followed by in situ residual stress analysis using neutron diffraction during aging/annealing treatments at 650°C, 720°C and 750°C. Almost 90% of stress relaxation was found to occur primarily during heating to the aging temperature as a result of a combination of plasticity and early stage creep relaxation. Creep-like stress relaxation was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720°C and 750°C while no further stress relaxation occurred at 650°C. This means that a change in hold temperature might have a more immediate impact on stress relaxation than change in duration of heat treatment. The post-aging ex-situ measurements showed that a heat treatment at 750°C for 8h reduced the stresses by approximately 70% in Inconel 718. By comparison, when heat treating Udimet 720LI in the same way only a 20% stress reduction was observed, which is explained by the higher creep resistance of this alloy.
Original languageEnglish
JournalMetallurgical and Materials Transactions A - Physical Metallurgy and Materials Science
Early online date3 Jun 2019
Publication statusPublished - 2019


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