Oxidation-assisted creep damage in a wrought nickel-based superalloy : experiments and modelling
Résumé
Creep damage investigation was carried out in wrought nickel-based superalloy Udimet 720LI in air at 850 °C using multiple cross-sections specimens in order to be able to make interrupted tests. In all tests conducted on this material, creep curves showed only tertiary stage and the surface connected intergranular cracking was found to be dominant in creep fracture. It was shown that γ′ precipitate coarsening occurs in the bulk of the specimens and obeys the LSW kinetics. Metallographic analysis led to the conclusion that creep does not alter oxidation, except at grain boundaries, where oxide spikes can be developed under creep. Therefore grain boundary oxidation was found to be creep strain-assisted. A constitutive model accounting for precipitate coarsening was proposed. The oxidation-assisted intergranular damage and the oxidation embrittlement of the microstructure elements phenomena were successfully described using the continuum damage mechanics and the local ductility exhaustion laws, respectively. Creep rupture and elongation curves were taken into account by the model with suitable accuracy, as well. The model was found able to figure the specimen's geometry variations effects on lifetime and creep elongation curves.