Stress-induced influence on the evolution of γ′ phase in dendritic area during long-term stress aging at 900 °C in Ni-based superalloy K444

Abstract

The evolution of γ′ phase in the Ni-based superalloy K444 alloy is investigated after long-term aging for 5000 h (LTA5,000) at 900 °C under different stresses (0 MPa, 50 MPa, 65 MPa, 80 MPa, 100 MPa and 120 MPa). The role of applied stress in the LTA5,000 process is analyzed in detail from multiple aspects, including morphology, composition partition, the formation of TCP phase, lattice misfit (δ) and dislocations configuration. The results reveal that the stress promotes the γ′ phase directional coarsening when it exceeds the critical value. For this alloy, the rafting structure is obvious when the stress is higher than 80 MPa, while γ′ phases maintain cuboid shape when the stress is below 65 MPa. It is caused by the stress gradient in the γ channels which accelerates the γ′ forming elements diffuse from horizontal γ channels to the vertical channels and only sufficient stress can activate the directional diffusion behavior. Besides, the stress induces that Al and Ti strongly assign to γ′ phases and the concentration of Cr in γ matrix increases clearly. In addition, the σ-TCP is free until the applied stress raises to 120 MPa, it is attributed to the increase in Cr concentration in the γ matrix. Moreover, the value of |δ| increases as the applied stress increases, and reducing interfacial energy becomes the driving force for directional coarsening. Finally, the calculated results based on the threshold stress demonstrate that the dislocations climb over the γ′ particles within the range of 65 MPa ∼ 120 MPa, which is in agreement with the observed phenomena.