Effect of HIP process on the micro-structural evolution of a nickel-based superalloy

A nickel-based superalloy (GTD-111 equivalent) with a directionally solidified grain structure designed for land-based gas turbines has been investigated using a hot isostatic press (HIP) with respect to the rejuvenation of degraded microstructure after a long-term operation. The invest-cast and heat-treated superalloy was intentionally degraded in an electrical furnace for simulation of the service operation, and treated in a HIP at 1190 °C and 120 MPa for 4 h. Four different HIP processes differentiated from each other whether the pressure was applied on heating and/or on cooling were tested. This work focused on the evolution of the microstructure and chemical composition in the dendrite core after the HIP treatment. From the experimental observations, the effect of the pressure on these micro-structural and compositional evolutions was considered in two ways: the retarded diffusion of the alloying elements under high pressure and the volume difference between γ′ precipitates and the γ matrix.