Influence of thermal exposure on the microstructural evolution and mechanical properties of a wrought Ni-base superalloy

Phase transformation and microstructural evolution of a wrought Ni-base superalloy Alloy 263 during long-term exposure to high temperatures have been studied. The influence of thermal degradation on tensile and creep properties of the alloy has been also investigated. Exposure of the alloy to 700 °C resulted in precipitation of small blocky type M23C6 carbides in the matrix. Acicular type M23C6 carbides also precipitated at the incoherent twin boundary and grew on the microtwins in front of the incoherent twin boundary. A plate-like η phase precipitated in the alloy with γ′ particle depletion zone around η plate after long-term exposure to 750 °C and above. Tensile strength of the alloy increased after exposure to 700 °C due to precipitation of M23C6 carbides. Tensile strength and creep rupture life decreased after exposure to 750 °C and above due to coarsening of γ′ particle and formation of η phase. Cracks nucleated at the interface between M23C6 carbide and matrix on the grain boundary and grew along the grain boundary irrespective of precipitation of η phase. Formation of η phase might change the deformation behavior, but it does not have influence on the fracture mode of the alloy.