Effect of the microstructure on the creep behavior of PM Udimet 720 superalloy—experiments and modeling

Powder metallurgy processed Udimet 720 is a high creep strength nickel-based superalloy considered for high temperature turbine disks for nuclear gas cooled reactors working under 700 °C. Both fine-grained and coarse-grained microstructures have been obtained by applying respectively a subsolvus or a supersolvus solution treatments, followed by ageing treatments. In both microstructures, the distribution of the strengthening γ′ precipitates has been characterized by transmission electron microscopy (TEM). The creep curves of the coarse-grained microstructure show the three usual creep stages. On the contrary, the creep curves of the fine-grained microstructure show a transition directly from primary to apparent tertiary creep without any obvious steady state. According to TEM analyses, Orowan loops surround Udimet 720 CR γ′ and U720 HS γ′ at high stress whereas U720 HS γ′ are sheared at low stress. To describe the behavior of the superalloy Udimet 720, a specific creep model is developed on the basis of McLean and Dyson models including physical damage parameters.