Microstructural Evolution of Creep-Induced Cavities and Casting Porosities for a Damaged Ni-based Superalloy Under Various Hot Isostatic Pressing Conditions

The microstructural evolution of casting porosities and creep-induced cavities for a damaged nickel-based superalloy under different hot isostatic pressing (HIP) conditions was investigated in order to understand the effects of HIP parameters on the healing behavior of micropores. A number of small-sized creep cavities formed during long-term service and large-sized porosities formed during the casting process were observed. These microdefects were partially healed after treated at high temperature of 1100 °C combined with 150 MPa pressure for 2 h, together with the formation of the so-called concentrically oriented γ′ rafting structure. When HIP temperature was increased to 1150 and 1175 °C, both the amount and the size of the microdefects were decreased. The concentrically oriented γ′ rafting around creep cavities became more remarkable, and the primary γ′ denuded zone was also formed between the raft structure and the cavity. Energy-dispersive X-ray spectroscopy analysis revealed that the γ matrix solute atoms diffused toward the cavity under the concentration gradient, whereas the γ′-forming elements diffused in a negative direction. When increasing HIP temperature up to 1200 °C, the micropores were hardly observed, indicating that both casting porosities and creep-induced cavities had almost been healed. Meanwhile, the γ′ rafting structure disappeared since HIP temperature was beyond the γ′ solvus temperature. It is revealed by the experimental results that the atomic diffusion could mainly dominate the healing process of micropores.